1 //===- JumpThreading.cpp - Thread control through conditional blocks ------===//
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //===----------------------------------------------------------------------===//
9 // This file implements the Jump Threading pass.
11 //===----------------------------------------------------------------------===//
13 #include "llvm/Transforms/Scalar/JumpThreading.h"
14 #include "llvm/ADT/DenseMap.h"
15 #include "llvm/ADT/DenseSet.h"
16 #include "llvm/ADT/Optional.h"
17 #include "llvm/ADT/STLExtras.h"
18 #include "llvm/ADT/SmallPtrSet.h"
19 #include "llvm/ADT/SmallVector.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/Analysis/AliasAnalysis.h"
22 #include "llvm/Analysis/BlockFrequencyInfo.h"
23 #include "llvm/Analysis/BranchProbabilityInfo.h"
24 #include "llvm/Analysis/CFG.h"
25 #include "llvm/Analysis/ConstantFolding.h"
26 #include "llvm/Analysis/DomTreeUpdater.h"
27 #include "llvm/Analysis/GlobalsModRef.h"
28 #include "llvm/Analysis/GuardUtils.h"
29 #include "llvm/Analysis/InstructionSimplify.h"
30 #include "llvm/Analysis/LazyValueInfo.h"
31 #include "llvm/Analysis/Loads.h"
32 #include "llvm/Analysis/LoopInfo.h"
33 #include "llvm/Analysis/TargetLibraryInfo.h"
34 #include "llvm/Analysis/ValueTracking.h"
35 #include "llvm/IR/BasicBlock.h"
36 #include "llvm/IR/CFG.h"
37 #include "llvm/IR/Constant.h"
38 #include "llvm/IR/ConstantRange.h"
39 #include "llvm/IR/Constants.h"
40 #include "llvm/IR/DataLayout.h"
41 #include "llvm/IR/Dominators.h"
42 #include "llvm/IR/Function.h"
43 #include "llvm/IR/InstrTypes.h"
44 #include "llvm/IR/Instruction.h"
45 #include "llvm/IR/Instructions.h"
46 #include "llvm/IR/IntrinsicInst.h"
47 #include "llvm/IR/Intrinsics.h"
48 #include "llvm/IR/LLVMContext.h"
49 #include "llvm/IR/MDBuilder.h"
50 #include "llvm/IR/Metadata.h"
51 #include "llvm/IR/Module.h"
52 #include "llvm/IR/PassManager.h"
53 #include "llvm/IR/PatternMatch.h"
54 #include "llvm/IR/Type.h"
55 #include "llvm/IR/Use.h"
56 #include "llvm/IR/User.h"
57 #include "llvm/IR/Value.h"
58 #include "llvm/Pass.h"
59 #include "llvm/Support/BlockFrequency.h"
60 #include "llvm/Support/BranchProbability.h"
61 #include "llvm/Support/Casting.h"
62 #include "llvm/Support/CommandLine.h"
63 #include "llvm/Support/Debug.h"
64 #include "llvm/Support/raw_ostream.h"
65 #include "llvm/Transforms/Scalar.h"
66 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
67 #include "llvm/Transforms/Utils/Cloning.h"
68 #include "llvm/Transforms/Utils/Local.h"
69 #include "llvm/Transforms/Utils/SSAUpdater.h"
70 #include "llvm/Transforms/Utils/ValueMapper.h"
80 using namespace jumpthreading
;
82 #define DEBUG_TYPE "jump-threading"
84 STATISTIC(NumThreads
, "Number of jumps threaded");
85 STATISTIC(NumFolds
, "Number of terminators folded");
86 STATISTIC(NumDupes
, "Number of branch blocks duplicated to eliminate phi");
88 static cl::opt
<unsigned>
89 BBDuplicateThreshold("jump-threading-threshold",
90 cl::desc("Max block size to duplicate for jump threading"),
91 cl::init(6), cl::Hidden
);
93 static cl::opt
<unsigned>
94 ImplicationSearchThreshold(
95 "jump-threading-implication-search-threshold",
96 cl::desc("The number of predecessors to search for a stronger "
97 "condition to use to thread over a weaker condition"),
98 cl::init(3), cl::Hidden
);
100 static cl::opt
<bool> PrintLVIAfterJumpThreading(
101 "print-lvi-after-jump-threading",
102 cl::desc("Print the LazyValueInfo cache after JumpThreading"), cl::init(false),
105 static cl::opt
<bool> ThreadAcrossLoopHeaders(
106 "jump-threading-across-loop-headers",
107 cl::desc("Allow JumpThreading to thread across loop headers, for testing"),
108 cl::init(false), cl::Hidden
);
113 /// This pass performs 'jump threading', which looks at blocks that have
114 /// multiple predecessors and multiple successors. If one or more of the
115 /// predecessors of the block can be proven to always jump to one of the
116 /// successors, we forward the edge from the predecessor to the successor by
117 /// duplicating the contents of this block.
119 /// An example of when this can occur is code like this:
126 /// In this case, the unconditional branch at the end of the first if can be
127 /// revectored to the false side of the second if.
128 class JumpThreading
: public FunctionPass
{
129 JumpThreadingPass Impl
;
132 static char ID
; // Pass identification
134 JumpThreading(int T
= -1) : FunctionPass(ID
), Impl(T
) {
135 initializeJumpThreadingPass(*PassRegistry::getPassRegistry());
138 bool runOnFunction(Function
&F
) override
;
140 void getAnalysisUsage(AnalysisUsage
&AU
) const override
{
141 AU
.addRequired
<DominatorTreeWrapperPass
>();
142 AU
.addPreserved
<DominatorTreeWrapperPass
>();
143 AU
.addRequired
<AAResultsWrapperPass
>();
144 AU
.addRequired
<LazyValueInfoWrapperPass
>();
145 AU
.addPreserved
<LazyValueInfoWrapperPass
>();
146 AU
.addPreserved
<GlobalsAAWrapperPass
>();
147 AU
.addRequired
<TargetLibraryInfoWrapperPass
>();
150 void releaseMemory() override
{ Impl
.releaseMemory(); }
153 } // end anonymous namespace
155 char JumpThreading::ID
= 0;
157 INITIALIZE_PASS_BEGIN(JumpThreading
, "jump-threading",
158 "Jump Threading", false, false)
159 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass
)
160 INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass
)
161 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass
)
162 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass
)
163 INITIALIZE_PASS_END(JumpThreading
, "jump-threading",
164 "Jump Threading", false, false)
166 // Public interface to the Jump Threading pass
167 FunctionPass
*llvm::createJumpThreadingPass(int Threshold
) {
168 return new JumpThreading(Threshold
);
171 JumpThreadingPass::JumpThreadingPass(int T
) {
172 BBDupThreshold
= (T
== -1) ? BBDuplicateThreshold
: unsigned(T
);
175 // Update branch probability information according to conditional
176 // branch probability. This is usually made possible for cloned branches
177 // in inline instances by the context specific profile in the caller.
189 // cond = PN([true, %A], [..., %B]); // PHI node
192 // ... // P(cond == true) = 1%
195 // Here we know that when block A is taken, cond must be true, which means
196 // P(cond == true | A) = 1
198 // Given that P(cond == true) = P(cond == true | A) * P(A) +
199 // P(cond == true | B) * P(B)
201 // P(cond == true ) = P(A) + P(cond == true | B) * P(B)
204 // P(A) is less than P(cond == true), i.e.
205 // P(t == true) <= P(cond == true)
207 // In other words, if we know P(cond == true) is unlikely, we know
208 // that P(t == true) is also unlikely.
210 static void updatePredecessorProfileMetadata(PHINode
*PN
, BasicBlock
*BB
) {
211 BranchInst
*CondBr
= dyn_cast
<BranchInst
>(BB
->getTerminator());
215 BranchProbability BP
;
216 uint64_t TrueWeight
, FalseWeight
;
217 if (!CondBr
->extractProfMetadata(TrueWeight
, FalseWeight
))
220 // Returns the outgoing edge of the dominating predecessor block
221 // that leads to the PhiNode's incoming block:
222 auto GetPredOutEdge
=
223 [](BasicBlock
*IncomingBB
,
224 BasicBlock
*PhiBB
) -> std::pair
<BasicBlock
*, BasicBlock
*> {
225 auto *PredBB
= IncomingBB
;
226 auto *SuccBB
= PhiBB
;
227 SmallPtrSet
<BasicBlock
*, 16> Visited
;
229 BranchInst
*PredBr
= dyn_cast
<BranchInst
>(PredBB
->getTerminator());
230 if (PredBr
&& PredBr
->isConditional())
231 return {PredBB
, SuccBB
};
232 Visited
.insert(PredBB
);
233 auto *SinglePredBB
= PredBB
->getSinglePredecessor();
235 return {nullptr, nullptr};
237 // Stop searching when SinglePredBB has been visited. It means we see
238 // an unreachable loop.
239 if (Visited
.count(SinglePredBB
))
240 return {nullptr, nullptr};
243 PredBB
= SinglePredBB
;
247 for (unsigned i
= 0, e
= PN
->getNumIncomingValues(); i
!= e
; ++i
) {
248 Value
*PhiOpnd
= PN
->getIncomingValue(i
);
249 ConstantInt
*CI
= dyn_cast
<ConstantInt
>(PhiOpnd
);
251 if (!CI
|| !CI
->getType()->isIntegerTy(1))
254 BP
= (CI
->isOne() ? BranchProbability::getBranchProbability(
255 TrueWeight
, TrueWeight
+ FalseWeight
)
256 : BranchProbability::getBranchProbability(
257 FalseWeight
, TrueWeight
+ FalseWeight
));
259 auto PredOutEdge
= GetPredOutEdge(PN
->getIncomingBlock(i
), BB
);
260 if (!PredOutEdge
.first
)
263 BasicBlock
*PredBB
= PredOutEdge
.first
;
264 BranchInst
*PredBr
= dyn_cast
<BranchInst
>(PredBB
->getTerminator());
268 uint64_t PredTrueWeight
, PredFalseWeight
;
269 // FIXME: We currently only set the profile data when it is missing.
270 // With PGO, this can be used to refine even existing profile data with
271 // context information. This needs to be done after more performance
273 if (PredBr
->extractProfMetadata(PredTrueWeight
, PredFalseWeight
))
276 // We can not infer anything useful when BP >= 50%, because BP is the
277 // upper bound probability value.
278 if (BP
>= BranchProbability(50, 100))
281 SmallVector
<uint32_t, 2> Weights
;
282 if (PredBr
->getSuccessor(0) == PredOutEdge
.second
) {
283 Weights
.push_back(BP
.getNumerator());
284 Weights
.push_back(BP
.getCompl().getNumerator());
286 Weights
.push_back(BP
.getCompl().getNumerator());
287 Weights
.push_back(BP
.getNumerator());
289 PredBr
->setMetadata(LLVMContext::MD_prof
,
290 MDBuilder(PredBr
->getParent()->getContext())
291 .createBranchWeights(Weights
));
295 /// runOnFunction - Toplevel algorithm.
296 bool JumpThreading::runOnFunction(Function
&F
) {
299 auto TLI
= &getAnalysis
<TargetLibraryInfoWrapperPass
>().getTLI(F
);
300 // Get DT analysis before LVI. When LVI is initialized it conditionally adds
301 // DT if it's available.
302 auto DT
= &getAnalysis
<DominatorTreeWrapperPass
>().getDomTree();
303 auto LVI
= &getAnalysis
<LazyValueInfoWrapperPass
>().getLVI();
304 auto AA
= &getAnalysis
<AAResultsWrapperPass
>().getAAResults();
305 DomTreeUpdater
DTU(*DT
, DomTreeUpdater::UpdateStrategy::Lazy
);
306 std::unique_ptr
<BlockFrequencyInfo
> BFI
;
307 std::unique_ptr
<BranchProbabilityInfo
> BPI
;
308 bool HasProfileData
= F
.hasProfileData();
309 if (HasProfileData
) {
310 LoopInfo LI
{DominatorTree(F
)};
311 BPI
.reset(new BranchProbabilityInfo(F
, LI
, TLI
));
312 BFI
.reset(new BlockFrequencyInfo(F
, *BPI
, LI
));
315 bool Changed
= Impl
.runImpl(F
, TLI
, LVI
, AA
, &DTU
, HasProfileData
,
316 std::move(BFI
), std::move(BPI
));
317 if (PrintLVIAfterJumpThreading
) {
318 dbgs() << "LVI for function '" << F
.getName() << "':\n";
319 LVI
->printLVI(F
, *DT
, dbgs());
324 PreservedAnalyses
JumpThreadingPass::run(Function
&F
,
325 FunctionAnalysisManager
&AM
) {
326 auto &TLI
= AM
.getResult
<TargetLibraryAnalysis
>(F
);
327 // Get DT analysis before LVI. When LVI is initialized it conditionally adds
328 // DT if it's available.
329 auto &DT
= AM
.getResult
<DominatorTreeAnalysis
>(F
);
330 auto &LVI
= AM
.getResult
<LazyValueAnalysis
>(F
);
331 auto &AA
= AM
.getResult
<AAManager
>(F
);
332 DomTreeUpdater
DTU(DT
, DomTreeUpdater::UpdateStrategy::Lazy
);
334 std::unique_ptr
<BlockFrequencyInfo
> BFI
;
335 std::unique_ptr
<BranchProbabilityInfo
> BPI
;
336 if (F
.hasProfileData()) {
337 LoopInfo LI
{DominatorTree(F
)};
338 BPI
.reset(new BranchProbabilityInfo(F
, LI
, &TLI
));
339 BFI
.reset(new BlockFrequencyInfo(F
, *BPI
, LI
));
342 bool Changed
= runImpl(F
, &TLI
, &LVI
, &AA
, &DTU
, HasProfileData
,
343 std::move(BFI
), std::move(BPI
));
346 return PreservedAnalyses::all();
347 PreservedAnalyses PA
;
348 PA
.preserve
<GlobalsAA
>();
349 PA
.preserve
<DominatorTreeAnalysis
>();
350 PA
.preserve
<LazyValueAnalysis
>();
354 bool JumpThreadingPass::runImpl(Function
&F
, TargetLibraryInfo
*TLI_
,
355 LazyValueInfo
*LVI_
, AliasAnalysis
*AA_
,
356 DomTreeUpdater
*DTU_
, bool HasProfileData_
,
357 std::unique_ptr
<BlockFrequencyInfo
> BFI_
,
358 std::unique_ptr
<BranchProbabilityInfo
> BPI_
) {
359 LLVM_DEBUG(dbgs() << "Jump threading on function '" << F
.getName() << "'\n");
366 // When profile data is available, we need to update edge weights after
367 // successful jump threading, which requires both BPI and BFI being available.
368 HasProfileData
= HasProfileData_
;
369 auto *GuardDecl
= F
.getParent()->getFunction(
370 Intrinsic::getName(Intrinsic::experimental_guard
));
371 HasGuards
= GuardDecl
&& !GuardDecl
->use_empty();
372 if (HasProfileData
) {
373 BPI
= std::move(BPI_
);
374 BFI
= std::move(BFI_
);
377 // JumpThreading must not processes blocks unreachable from entry. It's a
378 // waste of compute time and can potentially lead to hangs.
379 SmallPtrSet
<BasicBlock
*, 16> Unreachable
;
380 assert(DTU
&& "DTU isn't passed into JumpThreading before using it.");
381 assert(DTU
->hasDomTree() && "JumpThreading relies on DomTree to proceed.");
382 DominatorTree
&DT
= DTU
->getDomTree();
384 if (!DT
.isReachableFromEntry(&BB
))
385 Unreachable
.insert(&BB
);
387 if (!ThreadAcrossLoopHeaders
)
390 bool EverChanged
= false;
395 if (Unreachable
.count(&BB
))
397 while (ProcessBlock(&BB
)) // Thread all of the branches we can over BB.
399 // Stop processing BB if it's the entry or is now deleted. The following
400 // routines attempt to eliminate BB and locating a suitable replacement
401 // for the entry is non-trivial.
402 if (&BB
== &F
.getEntryBlock() || DTU
->isBBPendingDeletion(&BB
))
405 if (pred_empty(&BB
)) {
406 // When ProcessBlock makes BB unreachable it doesn't bother to fix up
407 // the instructions in it. We must remove BB to prevent invalid IR.
408 LLVM_DEBUG(dbgs() << " JT: Deleting dead block '" << BB
.getName()
409 << "' with terminator: " << *BB
.getTerminator()
411 LoopHeaders
.erase(&BB
);
412 LVI
->eraseBlock(&BB
);
413 DeleteDeadBlock(&BB
, DTU
);
418 // ProcessBlock doesn't thread BBs with unconditional TIs. However, if BB
419 // is "almost empty", we attempt to merge BB with its sole successor.
420 auto *BI
= dyn_cast
<BranchInst
>(BB
.getTerminator());
421 if (BI
&& BI
->isUnconditional() &&
422 // The terminator must be the only non-phi instruction in BB.
423 BB
.getFirstNonPHIOrDbg()->isTerminator() &&
424 // Don't alter Loop headers and latches to ensure another pass can
425 // detect and transform nested loops later.
426 !LoopHeaders
.count(&BB
) && !LoopHeaders
.count(BI
->getSuccessor(0)) &&
427 TryToSimplifyUncondBranchFromEmptyBlock(&BB
, DTU
)) {
428 // BB is valid for cleanup here because we passed in DTU. F remains
429 // BB's parent until a DTU->getDomTree() event.
430 LVI
->eraseBlock(&BB
);
434 EverChanged
|= Changed
;
438 // Flush only the Dominator Tree.
444 // Replace uses of Cond with ToVal when safe to do so. If all uses are
445 // replaced, we can remove Cond. We cannot blindly replace all uses of Cond
446 // because we may incorrectly replace uses when guards/assumes are uses of
447 // of `Cond` and we used the guards/assume to reason about the `Cond` value
448 // at the end of block. RAUW unconditionally replaces all uses
449 // including the guards/assumes themselves and the uses before the
451 static void ReplaceFoldableUses(Instruction
*Cond
, Value
*ToVal
) {
452 assert(Cond
->getType() == ToVal
->getType());
453 auto *BB
= Cond
->getParent();
454 // We can unconditionally replace all uses in non-local blocks (i.e. uses
455 // strictly dominated by BB), since LVI information is true from the
457 replaceNonLocalUsesWith(Cond
, ToVal
);
458 for (Instruction
&I
: reverse(*BB
)) {
459 // Reached the Cond whose uses we are trying to replace, so there are no
463 // We only replace uses in instructions that are guaranteed to reach the end
464 // of BB, where we know Cond is ToVal.
465 if (!isGuaranteedToTransferExecutionToSuccessor(&I
))
467 I
.replaceUsesOfWith(Cond
, ToVal
);
469 if (Cond
->use_empty() && !Cond
->mayHaveSideEffects())
470 Cond
->eraseFromParent();
473 /// Return the cost of duplicating a piece of this block from first non-phi
474 /// and before StopAt instruction to thread across it. Stop scanning the block
475 /// when exceeding the threshold. If duplication is impossible, returns ~0U.
476 static unsigned getJumpThreadDuplicationCost(BasicBlock
*BB
,
478 unsigned Threshold
) {
479 assert(StopAt
->getParent() == BB
&& "Not an instruction from proper BB?");
480 /// Ignore PHI nodes, these will be flattened when duplication happens.
481 BasicBlock::const_iterator
I(BB
->getFirstNonPHI());
483 // FIXME: THREADING will delete values that are just used to compute the
484 // branch, so they shouldn't count against the duplication cost.
487 if (BB
->getTerminator() == StopAt
) {
488 // Threading through a switch statement is particularly profitable. If this
489 // block ends in a switch, decrease its cost to make it more likely to
491 if (isa
<SwitchInst
>(StopAt
))
494 // The same holds for indirect branches, but slightly more so.
495 if (isa
<IndirectBrInst
>(StopAt
))
499 // Bump the threshold up so the early exit from the loop doesn't skip the
500 // terminator-based Size adjustment at the end.
503 // Sum up the cost of each instruction until we get to the terminator. Don't
504 // include the terminator because the copy won't include it.
506 for (; &*I
!= StopAt
; ++I
) {
508 // Stop scanning the block if we've reached the threshold.
509 if (Size
> Threshold
)
512 // Debugger intrinsics don't incur code size.
513 if (isa
<DbgInfoIntrinsic
>(I
)) continue;
515 // If this is a pointer->pointer bitcast, it is free.
516 if (isa
<BitCastInst
>(I
) && I
->getType()->isPointerTy())
519 // Bail out if this instruction gives back a token type, it is not possible
520 // to duplicate it if it is used outside this BB.
521 if (I
->getType()->isTokenTy() && I
->isUsedOutsideOfBlock(BB
))
524 // All other instructions count for at least one unit.
527 // Calls are more expensive. If they are non-intrinsic calls, we model them
528 // as having cost of 4. If they are a non-vector intrinsic, we model them
529 // as having cost of 2 total, and if they are a vector intrinsic, we model
530 // them as having cost 1.
531 if (const CallInst
*CI
= dyn_cast
<CallInst
>(I
)) {
532 if (CI
->cannotDuplicate() || CI
->isConvergent())
533 // Blocks with NoDuplicate are modelled as having infinite cost, so they
534 // are never duplicated.
536 else if (!isa
<IntrinsicInst
>(CI
))
538 else if (!CI
->getType()->isVectorTy())
543 return Size
> Bonus
? Size
- Bonus
: 0;
546 /// FindLoopHeaders - We do not want jump threading to turn proper loop
547 /// structures into irreducible loops. Doing this breaks up the loop nesting
548 /// hierarchy and pessimizes later transformations. To prevent this from
549 /// happening, we first have to find the loop headers. Here we approximate this
550 /// by finding targets of backedges in the CFG.
552 /// Note that there definitely are cases when we want to allow threading of
553 /// edges across a loop header. For example, threading a jump from outside the
554 /// loop (the preheader) to an exit block of the loop is definitely profitable.
555 /// It is also almost always profitable to thread backedges from within the loop
556 /// to exit blocks, and is often profitable to thread backedges to other blocks
557 /// within the loop (forming a nested loop). This simple analysis is not rich
558 /// enough to track all of these properties and keep it up-to-date as the CFG
559 /// mutates, so we don't allow any of these transformations.
560 void JumpThreadingPass::FindLoopHeaders(Function
&F
) {
561 SmallVector
<std::pair
<const BasicBlock
*,const BasicBlock
*>, 32> Edges
;
562 FindFunctionBackedges(F
, Edges
);
564 for (const auto &Edge
: Edges
)
565 LoopHeaders
.insert(Edge
.second
);
568 /// getKnownConstant - Helper method to determine if we can thread over a
569 /// terminator with the given value as its condition, and if so what value to
570 /// use for that. What kind of value this is depends on whether we want an
571 /// integer or a block address, but an undef is always accepted.
572 /// Returns null if Val is null or not an appropriate constant.
573 static Constant
*getKnownConstant(Value
*Val
, ConstantPreference Preference
) {
577 // Undef is "known" enough.
578 if (UndefValue
*U
= dyn_cast
<UndefValue
>(Val
))
581 if (Preference
== WantBlockAddress
)
582 return dyn_cast
<BlockAddress
>(Val
->stripPointerCasts());
584 return dyn_cast
<ConstantInt
>(Val
);
587 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
588 /// if we can infer that the value is a known ConstantInt/BlockAddress or undef
589 /// in any of our predecessors. If so, return the known list of value and pred
590 /// BB in the result vector.
592 /// This returns true if there were any known values.
593 bool JumpThreadingPass::ComputeValueKnownInPredecessorsImpl(
594 Value
*V
, BasicBlock
*BB
, PredValueInfo
&Result
,
595 ConstantPreference Preference
,
596 DenseSet
<std::pair
<Value
*, BasicBlock
*>> &RecursionSet
,
598 // This method walks up use-def chains recursively. Because of this, we could
599 // get into an infinite loop going around loops in the use-def chain. To
600 // prevent this, keep track of what (value, block) pairs we've already visited
601 // and terminate the search if we loop back to them
602 if (!RecursionSet
.insert(std::make_pair(V
, BB
)).second
)
605 // If V is a constant, then it is known in all predecessors.
606 if (Constant
*KC
= getKnownConstant(V
, Preference
)) {
607 for (BasicBlock
*Pred
: predecessors(BB
))
608 Result
.push_back(std::make_pair(KC
, Pred
));
610 return !Result
.empty();
613 // If V is a non-instruction value, or an instruction in a different block,
614 // then it can't be derived from a PHI.
615 Instruction
*I
= dyn_cast
<Instruction
>(V
);
616 if (!I
|| I
->getParent() != BB
) {
618 // Okay, if this is a live-in value, see if it has a known value at the end
619 // of any of our predecessors.
621 // FIXME: This should be an edge property, not a block end property.
622 /// TODO: Per PR2563, we could infer value range information about a
623 /// predecessor based on its terminator.
625 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
626 // "I" is a non-local compare-with-a-constant instruction. This would be
627 // able to handle value inequalities better, for example if the compare is
628 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
629 // Perhaps getConstantOnEdge should be smart enough to do this?
631 if (DTU
->hasPendingDomTreeUpdates())
635 for (BasicBlock
*P
: predecessors(BB
)) {
636 // If the value is known by LazyValueInfo to be a constant in a
637 // predecessor, use that information to try to thread this block.
638 Constant
*PredCst
= LVI
->getConstantOnEdge(V
, P
, BB
, CxtI
);
639 if (Constant
*KC
= getKnownConstant(PredCst
, Preference
))
640 Result
.push_back(std::make_pair(KC
, P
));
643 return !Result
.empty();
646 /// If I is a PHI node, then we know the incoming values for any constants.
647 if (PHINode
*PN
= dyn_cast
<PHINode
>(I
)) {
648 if (DTU
->hasPendingDomTreeUpdates())
652 for (unsigned i
= 0, e
= PN
->getNumIncomingValues(); i
!= e
; ++i
) {
653 Value
*InVal
= PN
->getIncomingValue(i
);
654 if (Constant
*KC
= getKnownConstant(InVal
, Preference
)) {
655 Result
.push_back(std::make_pair(KC
, PN
->getIncomingBlock(i
)));
657 Constant
*CI
= LVI
->getConstantOnEdge(InVal
,
658 PN
->getIncomingBlock(i
),
660 if (Constant
*KC
= getKnownConstant(CI
, Preference
))
661 Result
.push_back(std::make_pair(KC
, PN
->getIncomingBlock(i
)));
665 return !Result
.empty();
668 // Handle Cast instructions. Only see through Cast when the source operand is
669 // PHI or Cmp to save the compilation time.
670 if (CastInst
*CI
= dyn_cast
<CastInst
>(I
)) {
671 Value
*Source
= CI
->getOperand(0);
672 if (!isa
<PHINode
>(Source
) && !isa
<CmpInst
>(Source
))
674 ComputeValueKnownInPredecessorsImpl(Source
, BB
, Result
, Preference
,
679 // Convert the known values.
680 for (auto &R
: Result
)
681 R
.first
= ConstantExpr::getCast(CI
->getOpcode(), R
.first
, CI
->getType());
686 // Handle some boolean conditions.
687 if (I
->getType()->getPrimitiveSizeInBits() == 1) {
688 assert(Preference
== WantInteger
&& "One-bit non-integer type?");
690 // X & false -> false
691 if (I
->getOpcode() == Instruction::Or
||
692 I
->getOpcode() == Instruction::And
) {
693 PredValueInfoTy LHSVals
, RHSVals
;
695 ComputeValueKnownInPredecessorsImpl(I
->getOperand(0), BB
, LHSVals
,
696 WantInteger
, RecursionSet
, CxtI
);
697 ComputeValueKnownInPredecessorsImpl(I
->getOperand(1), BB
, RHSVals
,
698 WantInteger
, RecursionSet
, CxtI
);
700 if (LHSVals
.empty() && RHSVals
.empty())
703 ConstantInt
*InterestingVal
;
704 if (I
->getOpcode() == Instruction::Or
)
705 InterestingVal
= ConstantInt::getTrue(I
->getContext());
707 InterestingVal
= ConstantInt::getFalse(I
->getContext());
709 SmallPtrSet
<BasicBlock
*, 4> LHSKnownBBs
;
711 // Scan for the sentinel. If we find an undef, force it to the
712 // interesting value: x|undef -> true and x&undef -> false.
713 for (const auto &LHSVal
: LHSVals
)
714 if (LHSVal
.first
== InterestingVal
|| isa
<UndefValue
>(LHSVal
.first
)) {
715 Result
.emplace_back(InterestingVal
, LHSVal
.second
);
716 LHSKnownBBs
.insert(LHSVal
.second
);
718 for (const auto &RHSVal
: RHSVals
)
719 if (RHSVal
.first
== InterestingVal
|| isa
<UndefValue
>(RHSVal
.first
)) {
720 // If we already inferred a value for this block on the LHS, don't
722 if (!LHSKnownBBs
.count(RHSVal
.second
))
723 Result
.emplace_back(InterestingVal
, RHSVal
.second
);
726 return !Result
.empty();
729 // Handle the NOT form of XOR.
730 if (I
->getOpcode() == Instruction::Xor
&&
731 isa
<ConstantInt
>(I
->getOperand(1)) &&
732 cast
<ConstantInt
>(I
->getOperand(1))->isOne()) {
733 ComputeValueKnownInPredecessorsImpl(I
->getOperand(0), BB
, Result
,
734 WantInteger
, RecursionSet
, CxtI
);
738 // Invert the known values.
739 for (auto &R
: Result
)
740 R
.first
= ConstantExpr::getNot(R
.first
);
745 // Try to simplify some other binary operator values.
746 } else if (BinaryOperator
*BO
= dyn_cast
<BinaryOperator
>(I
)) {
747 assert(Preference
!= WantBlockAddress
748 && "A binary operator creating a block address?");
749 if (ConstantInt
*CI
= dyn_cast
<ConstantInt
>(BO
->getOperand(1))) {
750 PredValueInfoTy LHSVals
;
751 ComputeValueKnownInPredecessorsImpl(BO
->getOperand(0), BB
, LHSVals
,
752 WantInteger
, RecursionSet
, CxtI
);
754 // Try to use constant folding to simplify the binary operator.
755 for (const auto &LHSVal
: LHSVals
) {
756 Constant
*V
= LHSVal
.first
;
757 Constant
*Folded
= ConstantExpr::get(BO
->getOpcode(), V
, CI
);
759 if (Constant
*KC
= getKnownConstant(Folded
, WantInteger
))
760 Result
.push_back(std::make_pair(KC
, LHSVal
.second
));
764 return !Result
.empty();
767 // Handle compare with phi operand, where the PHI is defined in this block.
768 if (CmpInst
*Cmp
= dyn_cast
<CmpInst
>(I
)) {
769 assert(Preference
== WantInteger
&& "Compares only produce integers");
770 Type
*CmpType
= Cmp
->getType();
771 Value
*CmpLHS
= Cmp
->getOperand(0);
772 Value
*CmpRHS
= Cmp
->getOperand(1);
773 CmpInst::Predicate Pred
= Cmp
->getPredicate();
775 PHINode
*PN
= dyn_cast
<PHINode
>(CmpLHS
);
777 PN
= dyn_cast
<PHINode
>(CmpRHS
);
778 if (PN
&& PN
->getParent() == BB
) {
779 const DataLayout
&DL
= PN
->getModule()->getDataLayout();
780 // We can do this simplification if any comparisons fold to true or false.
782 if (DTU
->hasPendingDomTreeUpdates())
786 for (unsigned i
= 0, e
= PN
->getNumIncomingValues(); i
!= e
; ++i
) {
787 BasicBlock
*PredBB
= PN
->getIncomingBlock(i
);
790 LHS
= PN
->getIncomingValue(i
);
791 RHS
= CmpRHS
->DoPHITranslation(BB
, PredBB
);
793 LHS
= CmpLHS
->DoPHITranslation(BB
, PredBB
);
794 RHS
= PN
->getIncomingValue(i
);
796 Value
*Res
= SimplifyCmpInst(Pred
, LHS
, RHS
, {DL
});
798 if (!isa
<Constant
>(RHS
))
801 // getPredicateOnEdge call will make no sense if LHS is defined in BB.
802 auto LHSInst
= dyn_cast
<Instruction
>(LHS
);
803 if (LHSInst
&& LHSInst
->getParent() == BB
)
806 LazyValueInfo::Tristate
807 ResT
= LVI
->getPredicateOnEdge(Pred
, LHS
,
808 cast
<Constant
>(RHS
), PredBB
, BB
,
810 if (ResT
== LazyValueInfo::Unknown
)
812 Res
= ConstantInt::get(Type::getInt1Ty(LHS
->getContext()), ResT
);
815 if (Constant
*KC
= getKnownConstant(Res
, WantInteger
))
816 Result
.push_back(std::make_pair(KC
, PredBB
));
819 return !Result
.empty();
822 // If comparing a live-in value against a constant, see if we know the
823 // live-in value on any predecessors.
824 if (isa
<Constant
>(CmpRHS
) && !CmpType
->isVectorTy()) {
825 Constant
*CmpConst
= cast
<Constant
>(CmpRHS
);
827 if (!isa
<Instruction
>(CmpLHS
) ||
828 cast
<Instruction
>(CmpLHS
)->getParent() != BB
) {
829 if (DTU
->hasPendingDomTreeUpdates())
833 for (BasicBlock
*P
: predecessors(BB
)) {
834 // If the value is known by LazyValueInfo to be a constant in a
835 // predecessor, use that information to try to thread this block.
836 LazyValueInfo::Tristate Res
=
837 LVI
->getPredicateOnEdge(Pred
, CmpLHS
,
838 CmpConst
, P
, BB
, CxtI
? CxtI
: Cmp
);
839 if (Res
== LazyValueInfo::Unknown
)
842 Constant
*ResC
= ConstantInt::get(CmpType
, Res
);
843 Result
.push_back(std::make_pair(ResC
, P
));
846 return !Result
.empty();
849 // InstCombine can fold some forms of constant range checks into
850 // (icmp (add (x, C1)), C2). See if we have we have such a thing with
853 using namespace PatternMatch
;
856 ConstantInt
*AddConst
;
857 if (isa
<ConstantInt
>(CmpConst
) &&
858 match(CmpLHS
, m_Add(m_Value(AddLHS
), m_ConstantInt(AddConst
)))) {
859 if (!isa
<Instruction
>(AddLHS
) ||
860 cast
<Instruction
>(AddLHS
)->getParent() != BB
) {
861 if (DTU
->hasPendingDomTreeUpdates())
865 for (BasicBlock
*P
: predecessors(BB
)) {
866 // If the value is known by LazyValueInfo to be a ConstantRange in
867 // a predecessor, use that information to try to thread this
869 ConstantRange CR
= LVI
->getConstantRangeOnEdge(
870 AddLHS
, P
, BB
, CxtI
? CxtI
: cast
<Instruction
>(CmpLHS
));
871 // Propagate the range through the addition.
872 CR
= CR
.add(AddConst
->getValue());
874 // Get the range where the compare returns true.
875 ConstantRange CmpRange
= ConstantRange::makeExactICmpRegion(
876 Pred
, cast
<ConstantInt
>(CmpConst
)->getValue());
879 if (CmpRange
.contains(CR
))
880 ResC
= ConstantInt::getTrue(CmpType
);
881 else if (CmpRange
.inverse().contains(CR
))
882 ResC
= ConstantInt::getFalse(CmpType
);
886 Result
.push_back(std::make_pair(ResC
, P
));
889 return !Result
.empty();
894 // Try to find a constant value for the LHS of a comparison,
895 // and evaluate it statically if we can.
896 PredValueInfoTy LHSVals
;
897 ComputeValueKnownInPredecessorsImpl(I
->getOperand(0), BB
, LHSVals
,
898 WantInteger
, RecursionSet
, CxtI
);
900 for (const auto &LHSVal
: LHSVals
) {
901 Constant
*V
= LHSVal
.first
;
902 Constant
*Folded
= ConstantExpr::getCompare(Pred
, V
, CmpConst
);
903 if (Constant
*KC
= getKnownConstant(Folded
, WantInteger
))
904 Result
.push_back(std::make_pair(KC
, LHSVal
.second
));
907 return !Result
.empty();
911 if (SelectInst
*SI
= dyn_cast
<SelectInst
>(I
)) {
912 // Handle select instructions where at least one operand is a known constant
913 // and we can figure out the condition value for any predecessor block.
914 Constant
*TrueVal
= getKnownConstant(SI
->getTrueValue(), Preference
);
915 Constant
*FalseVal
= getKnownConstant(SI
->getFalseValue(), Preference
);
916 PredValueInfoTy Conds
;
917 if ((TrueVal
|| FalseVal
) &&
918 ComputeValueKnownInPredecessorsImpl(SI
->getCondition(), BB
, Conds
,
919 WantInteger
, RecursionSet
, CxtI
)) {
920 for (auto &C
: Conds
) {
921 Constant
*Cond
= C
.first
;
923 // Figure out what value to use for the condition.
925 if (ConstantInt
*CI
= dyn_cast
<ConstantInt
>(Cond
)) {
927 KnownCond
= CI
->isOne();
929 assert(isa
<UndefValue
>(Cond
) && "Unexpected condition value");
930 // Either operand will do, so be sure to pick the one that's a known
932 // FIXME: Do this more cleverly if both values are known constants?
933 KnownCond
= (TrueVal
!= nullptr);
936 // See if the select has a known constant value for this predecessor.
937 if (Constant
*Val
= KnownCond
? TrueVal
: FalseVal
)
938 Result
.push_back(std::make_pair(Val
, C
.second
));
941 return !Result
.empty();
945 // If all else fails, see if LVI can figure out a constant value for us.
946 if (DTU
->hasPendingDomTreeUpdates())
950 Constant
*CI
= LVI
->getConstant(V
, BB
, CxtI
);
951 if (Constant
*KC
= getKnownConstant(CI
, Preference
)) {
952 for (BasicBlock
*Pred
: predecessors(BB
))
953 Result
.push_back(std::make_pair(KC
, Pred
));
956 return !Result
.empty();
959 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
960 /// in an undefined jump, decide which block is best to revector to.
962 /// Since we can pick an arbitrary destination, we pick the successor with the
963 /// fewest predecessors. This should reduce the in-degree of the others.
964 static unsigned GetBestDestForJumpOnUndef(BasicBlock
*BB
) {
965 Instruction
*BBTerm
= BB
->getTerminator();
966 unsigned MinSucc
= 0;
967 BasicBlock
*TestBB
= BBTerm
->getSuccessor(MinSucc
);
968 // Compute the successor with the minimum number of predecessors.
969 unsigned MinNumPreds
= pred_size(TestBB
);
970 for (unsigned i
= 1, e
= BBTerm
->getNumSuccessors(); i
!= e
; ++i
) {
971 TestBB
= BBTerm
->getSuccessor(i
);
972 unsigned NumPreds
= pred_size(TestBB
);
973 if (NumPreds
< MinNumPreds
) {
975 MinNumPreds
= NumPreds
;
982 static bool hasAddressTakenAndUsed(BasicBlock
*BB
) {
983 if (!BB
->hasAddressTaken()) return false;
985 // If the block has its address taken, it may be a tree of dead constants
986 // hanging off of it. These shouldn't keep the block alive.
987 BlockAddress
*BA
= BlockAddress::get(BB
);
988 BA
->removeDeadConstantUsers();
989 return !BA
->use_empty();
992 /// ProcessBlock - If there are any predecessors whose control can be threaded
993 /// through to a successor, transform them now.
994 bool JumpThreadingPass::ProcessBlock(BasicBlock
*BB
) {
995 // If the block is trivially dead, just return and let the caller nuke it.
996 // This simplifies other transformations.
997 if (DTU
->isBBPendingDeletion(BB
) ||
998 (pred_empty(BB
) && BB
!= &BB
->getParent()->getEntryBlock()))
1001 // If this block has a single predecessor, and if that pred has a single
1002 // successor, merge the blocks. This encourages recursive jump threading
1003 // because now the condition in this block can be threaded through
1004 // predecessors of our predecessor block.
1005 if (BasicBlock
*SinglePred
= BB
->getSinglePredecessor()) {
1006 const Instruction
*TI
= SinglePred
->getTerminator();
1007 if (!TI
->isExceptionalTerminator() && TI
->getNumSuccessors() == 1 &&
1008 SinglePred
!= BB
&& !hasAddressTakenAndUsed(BB
)) {
1009 // If SinglePred was a loop header, BB becomes one.
1010 if (LoopHeaders
.erase(SinglePred
))
1011 LoopHeaders
.insert(BB
);
1013 LVI
->eraseBlock(SinglePred
);
1014 MergeBasicBlockIntoOnlyPred(BB
, DTU
);
1016 // Now that BB is merged into SinglePred (i.e. SinglePred Code followed by
1017 // BB code within one basic block `BB`), we need to invalidate the LVI
1018 // information associated with BB, because the LVI information need not be
1019 // true for all of BB after the merge. For example,
1020 // Before the merge, LVI info and code is as follows:
1021 // SinglePred: <LVI info1 for %p val>
1023 // call @exit() // need not transfer execution to successor.
1024 // assume(%p) // from this point on %p is true
1026 // BB: <LVI info2 for %p val, i.e. %p is true>
1030 // Note that this LVI info for blocks BB and SinglPred is correct for %p
1031 // (info2 and info1 respectively). After the merge and the deletion of the
1032 // LVI info1 for SinglePred. We have the following code:
1033 // BB: <LVI info2 for %p val>
1037 // %x = use of %p <-- LVI info2 is correct from here onwards.
1039 // LVI info2 for BB is incorrect at the beginning of BB.
1041 // Invalidate LVI information for BB if the LVI is not provably true for
1043 if (!isGuaranteedToTransferExecutionToSuccessor(BB
))
1044 LVI
->eraseBlock(BB
);
1049 if (TryToUnfoldSelectInCurrBB(BB
))
1052 // Look if we can propagate guards to predecessors.
1053 if (HasGuards
&& ProcessGuards(BB
))
1056 // What kind of constant we're looking for.
1057 ConstantPreference Preference
= WantInteger
;
1059 // Look to see if the terminator is a conditional branch, switch or indirect
1060 // branch, if not we can't thread it.
1062 Instruction
*Terminator
= BB
->getTerminator();
1063 if (BranchInst
*BI
= dyn_cast
<BranchInst
>(Terminator
)) {
1064 // Can't thread an unconditional jump.
1065 if (BI
->isUnconditional()) return false;
1066 Condition
= BI
->getCondition();
1067 } else if (SwitchInst
*SI
= dyn_cast
<SwitchInst
>(Terminator
)) {
1068 Condition
= SI
->getCondition();
1069 } else if (IndirectBrInst
*IB
= dyn_cast
<IndirectBrInst
>(Terminator
)) {
1070 // Can't thread indirect branch with no successors.
1071 if (IB
->getNumSuccessors() == 0) return false;
1072 Condition
= IB
->getAddress()->stripPointerCasts();
1073 Preference
= WantBlockAddress
;
1075 return false; // Must be an invoke or callbr.
1078 // Run constant folding to see if we can reduce the condition to a simple
1080 if (Instruction
*I
= dyn_cast
<Instruction
>(Condition
)) {
1082 ConstantFoldInstruction(I
, BB
->getModule()->getDataLayout(), TLI
);
1084 I
->replaceAllUsesWith(SimpleVal
);
1085 if (isInstructionTriviallyDead(I
, TLI
))
1086 I
->eraseFromParent();
1087 Condition
= SimpleVal
;
1091 // If the terminator is branching on an undef, we can pick any of the
1092 // successors to branch to. Let GetBestDestForJumpOnUndef decide.
1093 if (isa
<UndefValue
>(Condition
)) {
1094 unsigned BestSucc
= GetBestDestForJumpOnUndef(BB
);
1095 std::vector
<DominatorTree::UpdateType
> Updates
;
1097 // Fold the branch/switch.
1098 Instruction
*BBTerm
= BB
->getTerminator();
1099 Updates
.reserve(BBTerm
->getNumSuccessors());
1100 for (unsigned i
= 0, e
= BBTerm
->getNumSuccessors(); i
!= e
; ++i
) {
1101 if (i
== BestSucc
) continue;
1102 BasicBlock
*Succ
= BBTerm
->getSuccessor(i
);
1103 Succ
->removePredecessor(BB
, true);
1104 Updates
.push_back({DominatorTree::Delete
, BB
, Succ
});
1107 LLVM_DEBUG(dbgs() << " In block '" << BB
->getName()
1108 << "' folding undef terminator: " << *BBTerm
<< '\n');
1109 BranchInst::Create(BBTerm
->getSuccessor(BestSucc
), BBTerm
);
1110 BBTerm
->eraseFromParent();
1111 DTU
->applyUpdatesPermissive(Updates
);
1115 // If the terminator of this block is branching on a constant, simplify the
1116 // terminator to an unconditional branch. This can occur due to threading in
1118 if (getKnownConstant(Condition
, Preference
)) {
1119 LLVM_DEBUG(dbgs() << " In block '" << BB
->getName()
1120 << "' folding terminator: " << *BB
->getTerminator()
1123 ConstantFoldTerminator(BB
, true, nullptr, DTU
);
1127 Instruction
*CondInst
= dyn_cast
<Instruction
>(Condition
);
1129 // All the rest of our checks depend on the condition being an instruction.
1131 // FIXME: Unify this with code below.
1132 if (ProcessThreadableEdges(Condition
, BB
, Preference
, Terminator
))
1137 if (CmpInst
*CondCmp
= dyn_cast
<CmpInst
>(CondInst
)) {
1138 // If we're branching on a conditional, LVI might be able to determine
1139 // it's value at the branch instruction. We only handle comparisons
1140 // against a constant at this time.
1141 // TODO: This should be extended to handle switches as well.
1142 BranchInst
*CondBr
= dyn_cast
<BranchInst
>(BB
->getTerminator());
1143 Constant
*CondConst
= dyn_cast
<Constant
>(CondCmp
->getOperand(1));
1144 if (CondBr
&& CondConst
) {
1145 // We should have returned as soon as we turn a conditional branch to
1146 // unconditional. Because its no longer interesting as far as jump
1147 // threading is concerned.
1148 assert(CondBr
->isConditional() && "Threading on unconditional terminator");
1150 if (DTU
->hasPendingDomTreeUpdates())
1154 LazyValueInfo::Tristate Ret
=
1155 LVI
->getPredicateAt(CondCmp
->getPredicate(), CondCmp
->getOperand(0),
1157 if (Ret
!= LazyValueInfo::Unknown
) {
1158 unsigned ToRemove
= Ret
== LazyValueInfo::True
? 1 : 0;
1159 unsigned ToKeep
= Ret
== LazyValueInfo::True
? 0 : 1;
1160 BasicBlock
*ToRemoveSucc
= CondBr
->getSuccessor(ToRemove
);
1161 ToRemoveSucc
->removePredecessor(BB
, true);
1162 BranchInst
*UncondBr
=
1163 BranchInst::Create(CondBr
->getSuccessor(ToKeep
), CondBr
);
1164 UncondBr
->setDebugLoc(CondBr
->getDebugLoc());
1165 CondBr
->eraseFromParent();
1166 if (CondCmp
->use_empty())
1167 CondCmp
->eraseFromParent();
1168 // We can safely replace *some* uses of the CondInst if it has
1169 // exactly one value as returned by LVI. RAUW is incorrect in the
1170 // presence of guards and assumes, that have the `Cond` as the use. This
1171 // is because we use the guards/assume to reason about the `Cond` value
1172 // at the end of block, but RAUW unconditionally replaces all uses
1173 // including the guards/assumes themselves and the uses before the
1175 else if (CondCmp
->getParent() == BB
) {
1176 auto *CI
= Ret
== LazyValueInfo::True
?
1177 ConstantInt::getTrue(CondCmp
->getType()) :
1178 ConstantInt::getFalse(CondCmp
->getType());
1179 ReplaceFoldableUses(CondCmp
, CI
);
1181 DTU
->applyUpdatesPermissive(
1182 {{DominatorTree::Delete
, BB
, ToRemoveSucc
}});
1186 // We did not manage to simplify this branch, try to see whether
1187 // CondCmp depends on a known phi-select pattern.
1188 if (TryToUnfoldSelect(CondCmp
, BB
))
1193 if (SwitchInst
*SI
= dyn_cast
<SwitchInst
>(BB
->getTerminator()))
1194 if (TryToUnfoldSelect(SI
, BB
))
1197 // Check for some cases that are worth simplifying. Right now we want to look
1198 // for loads that are used by a switch or by the condition for the branch. If
1199 // we see one, check to see if it's partially redundant. If so, insert a PHI
1200 // which can then be used to thread the values.
1201 Value
*SimplifyValue
= CondInst
;
1202 if (CmpInst
*CondCmp
= dyn_cast
<CmpInst
>(SimplifyValue
))
1203 if (isa
<Constant
>(CondCmp
->getOperand(1)))
1204 SimplifyValue
= CondCmp
->getOperand(0);
1206 // TODO: There are other places where load PRE would be profitable, such as
1207 // more complex comparisons.
1208 if (LoadInst
*LoadI
= dyn_cast
<LoadInst
>(SimplifyValue
))
1209 if (SimplifyPartiallyRedundantLoad(LoadI
))
1212 // Before threading, try to propagate profile data backwards:
1213 if (PHINode
*PN
= dyn_cast
<PHINode
>(CondInst
))
1214 if (PN
->getParent() == BB
&& isa
<BranchInst
>(BB
->getTerminator()))
1215 updatePredecessorProfileMetadata(PN
, BB
);
1217 // Handle a variety of cases where we are branching on something derived from
1218 // a PHI node in the current block. If we can prove that any predecessors
1219 // compute a predictable value based on a PHI node, thread those predecessors.
1220 if (ProcessThreadableEdges(CondInst
, BB
, Preference
, Terminator
))
1223 // If this is an otherwise-unfoldable branch on a phi node in the current
1224 // block, see if we can simplify.
1225 if (PHINode
*PN
= dyn_cast
<PHINode
>(CondInst
))
1226 if (PN
->getParent() == BB
&& isa
<BranchInst
>(BB
->getTerminator()))
1227 return ProcessBranchOnPHI(PN
);
1229 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
1230 if (CondInst
->getOpcode() == Instruction::Xor
&&
1231 CondInst
->getParent() == BB
&& isa
<BranchInst
>(BB
->getTerminator()))
1232 return ProcessBranchOnXOR(cast
<BinaryOperator
>(CondInst
));
1234 // Search for a stronger dominating condition that can be used to simplify a
1235 // conditional branch leaving BB.
1236 if (ProcessImpliedCondition(BB
))
1242 bool JumpThreadingPass::ProcessImpliedCondition(BasicBlock
*BB
) {
1243 auto *BI
= dyn_cast
<BranchInst
>(BB
->getTerminator());
1244 if (!BI
|| !BI
->isConditional())
1247 Value
*Cond
= BI
->getCondition();
1248 BasicBlock
*CurrentBB
= BB
;
1249 BasicBlock
*CurrentPred
= BB
->getSinglePredecessor();
1252 auto &DL
= BB
->getModule()->getDataLayout();
1254 while (CurrentPred
&& Iter
++ < ImplicationSearchThreshold
) {
1255 auto *PBI
= dyn_cast
<BranchInst
>(CurrentPred
->getTerminator());
1256 if (!PBI
|| !PBI
->isConditional())
1258 if (PBI
->getSuccessor(0) != CurrentBB
&& PBI
->getSuccessor(1) != CurrentBB
)
1261 bool CondIsTrue
= PBI
->getSuccessor(0) == CurrentBB
;
1262 Optional
<bool> Implication
=
1263 isImpliedCondition(PBI
->getCondition(), Cond
, DL
, CondIsTrue
);
1265 BasicBlock
*KeepSucc
= BI
->getSuccessor(*Implication
? 0 : 1);
1266 BasicBlock
*RemoveSucc
= BI
->getSuccessor(*Implication
? 1 : 0);
1267 RemoveSucc
->removePredecessor(BB
);
1268 BranchInst
*UncondBI
= BranchInst::Create(KeepSucc
, BI
);
1269 UncondBI
->setDebugLoc(BI
->getDebugLoc());
1270 BI
->eraseFromParent();
1271 DTU
->applyUpdatesPermissive({{DominatorTree::Delete
, BB
, RemoveSucc
}});
1274 CurrentBB
= CurrentPred
;
1275 CurrentPred
= CurrentBB
->getSinglePredecessor();
1281 /// Return true if Op is an instruction defined in the given block.
1282 static bool isOpDefinedInBlock(Value
*Op
, BasicBlock
*BB
) {
1283 if (Instruction
*OpInst
= dyn_cast
<Instruction
>(Op
))
1284 if (OpInst
->getParent() == BB
)
1289 /// SimplifyPartiallyRedundantLoad - If LoadI is an obviously partially
1290 /// redundant load instruction, eliminate it by replacing it with a PHI node.
1291 /// This is an important optimization that encourages jump threading, and needs
1292 /// to be run interlaced with other jump threading tasks.
1293 bool JumpThreadingPass::SimplifyPartiallyRedundantLoad(LoadInst
*LoadI
) {
1294 // Don't hack volatile and ordered loads.
1295 if (!LoadI
->isUnordered()) return false;
1297 // If the load is defined in a block with exactly one predecessor, it can't be
1298 // partially redundant.
1299 BasicBlock
*LoadBB
= LoadI
->getParent();
1300 if (LoadBB
->getSinglePredecessor())
1303 // If the load is defined in an EH pad, it can't be partially redundant,
1304 // because the edges between the invoke and the EH pad cannot have other
1305 // instructions between them.
1306 if (LoadBB
->isEHPad())
1309 Value
*LoadedPtr
= LoadI
->getOperand(0);
1311 // If the loaded operand is defined in the LoadBB and its not a phi,
1312 // it can't be available in predecessors.
1313 if (isOpDefinedInBlock(LoadedPtr
, LoadBB
) && !isa
<PHINode
>(LoadedPtr
))
1316 // Scan a few instructions up from the load, to see if it is obviously live at
1317 // the entry to its block.
1318 BasicBlock::iterator
BBIt(LoadI
);
1320 if (Value
*AvailableVal
= FindAvailableLoadedValue(
1321 LoadI
, LoadBB
, BBIt
, DefMaxInstsToScan
, AA
, &IsLoadCSE
)) {
1322 // If the value of the load is locally available within the block, just use
1323 // it. This frequently occurs for reg2mem'd allocas.
1326 LoadInst
*NLoadI
= cast
<LoadInst
>(AvailableVal
);
1327 combineMetadataForCSE(NLoadI
, LoadI
, false);
1330 // If the returned value is the load itself, replace with an undef. This can
1331 // only happen in dead loops.
1332 if (AvailableVal
== LoadI
)
1333 AvailableVal
= UndefValue::get(LoadI
->getType());
1334 if (AvailableVal
->getType() != LoadI
->getType())
1335 AvailableVal
= CastInst::CreateBitOrPointerCast(
1336 AvailableVal
, LoadI
->getType(), "", LoadI
);
1337 LoadI
->replaceAllUsesWith(AvailableVal
);
1338 LoadI
->eraseFromParent();
1342 // Otherwise, if we scanned the whole block and got to the top of the block,
1343 // we know the block is locally transparent to the load. If not, something
1344 // might clobber its value.
1345 if (BBIt
!= LoadBB
->begin())
1348 // If all of the loads and stores that feed the value have the same AA tags,
1349 // then we can propagate them onto any newly inserted loads.
1351 LoadI
->getAAMetadata(AATags
);
1353 SmallPtrSet
<BasicBlock
*, 8> PredsScanned
;
1355 using AvailablePredsTy
= SmallVector
<std::pair
<BasicBlock
*, Value
*>, 8>;
1357 AvailablePredsTy AvailablePreds
;
1358 BasicBlock
*OneUnavailablePred
= nullptr;
1359 SmallVector
<LoadInst
*, 8> CSELoads
;
1361 // If we got here, the loaded value is transparent through to the start of the
1362 // block. Check to see if it is available in any of the predecessor blocks.
1363 for (BasicBlock
*PredBB
: predecessors(LoadBB
)) {
1364 // If we already scanned this predecessor, skip it.
1365 if (!PredsScanned
.insert(PredBB
).second
)
1368 BBIt
= PredBB
->end();
1369 unsigned NumScanedInst
= 0;
1370 Value
*PredAvailable
= nullptr;
1371 // NOTE: We don't CSE load that is volatile or anything stronger than
1372 // unordered, that should have been checked when we entered the function.
1373 assert(LoadI
->isUnordered() &&
1374 "Attempting to CSE volatile or atomic loads");
1375 // If this is a load on a phi pointer, phi-translate it and search
1376 // for available load/store to the pointer in predecessors.
1377 Value
*Ptr
= LoadedPtr
->DoPHITranslation(LoadBB
, PredBB
);
1378 PredAvailable
= FindAvailablePtrLoadStore(
1379 Ptr
, LoadI
->getType(), LoadI
->isAtomic(), PredBB
, BBIt
,
1380 DefMaxInstsToScan
, AA
, &IsLoadCSE
, &NumScanedInst
);
1382 // If PredBB has a single predecessor, continue scanning through the
1383 // single predecessor.
1384 BasicBlock
*SinglePredBB
= PredBB
;
1385 while (!PredAvailable
&& SinglePredBB
&& BBIt
== SinglePredBB
->begin() &&
1386 NumScanedInst
< DefMaxInstsToScan
) {
1387 SinglePredBB
= SinglePredBB
->getSinglePredecessor();
1389 BBIt
= SinglePredBB
->end();
1390 PredAvailable
= FindAvailablePtrLoadStore(
1391 Ptr
, LoadI
->getType(), LoadI
->isAtomic(), SinglePredBB
, BBIt
,
1392 (DefMaxInstsToScan
- NumScanedInst
), AA
, &IsLoadCSE
,
1397 if (!PredAvailable
) {
1398 OneUnavailablePred
= PredBB
;
1403 CSELoads
.push_back(cast
<LoadInst
>(PredAvailable
));
1405 // If so, this load is partially redundant. Remember this info so that we
1406 // can create a PHI node.
1407 AvailablePreds
.push_back(std::make_pair(PredBB
, PredAvailable
));
1410 // If the loaded value isn't available in any predecessor, it isn't partially
1412 if (AvailablePreds
.empty()) return false;
1414 // Okay, the loaded value is available in at least one (and maybe all!)
1415 // predecessors. If the value is unavailable in more than one unique
1416 // predecessor, we want to insert a merge block for those common predecessors.
1417 // This ensures that we only have to insert one reload, thus not increasing
1419 BasicBlock
*UnavailablePred
= nullptr;
1421 // If the value is unavailable in one of predecessors, we will end up
1422 // inserting a new instruction into them. It is only valid if all the
1423 // instructions before LoadI are guaranteed to pass execution to its
1424 // successor, or if LoadI is safe to speculate.
1425 // TODO: If this logic becomes more complex, and we will perform PRE insertion
1426 // farther than to a predecessor, we need to reuse the code from GVN's PRE.
1427 // It requires domination tree analysis, so for this simple case it is an
1429 if (PredsScanned
.size() != AvailablePreds
.size() &&
1430 !isSafeToSpeculativelyExecute(LoadI
))
1431 for (auto I
= LoadBB
->begin(); &*I
!= LoadI
; ++I
)
1432 if (!isGuaranteedToTransferExecutionToSuccessor(&*I
))
1435 // If there is exactly one predecessor where the value is unavailable, the
1436 // already computed 'OneUnavailablePred' block is it. If it ends in an
1437 // unconditional branch, we know that it isn't a critical edge.
1438 if (PredsScanned
.size() == AvailablePreds
.size()+1 &&
1439 OneUnavailablePred
->getTerminator()->getNumSuccessors() == 1) {
1440 UnavailablePred
= OneUnavailablePred
;
1441 } else if (PredsScanned
.size() != AvailablePreds
.size()) {
1442 // Otherwise, we had multiple unavailable predecessors or we had a critical
1443 // edge from the one.
1444 SmallVector
<BasicBlock
*, 8> PredsToSplit
;
1445 SmallPtrSet
<BasicBlock
*, 8> AvailablePredSet
;
1447 for (const auto &AvailablePred
: AvailablePreds
)
1448 AvailablePredSet
.insert(AvailablePred
.first
);
1450 // Add all the unavailable predecessors to the PredsToSplit list.
1451 for (BasicBlock
*P
: predecessors(LoadBB
)) {
1452 // If the predecessor is an indirect goto, we can't split the edge.
1454 if (isa
<IndirectBrInst
>(P
->getTerminator()) ||
1455 isa
<CallBrInst
>(P
->getTerminator()))
1458 if (!AvailablePredSet
.count(P
))
1459 PredsToSplit
.push_back(P
);
1462 // Split them out to their own block.
1463 UnavailablePred
= SplitBlockPreds(LoadBB
, PredsToSplit
, "thread-pre-split");
1466 // If the value isn't available in all predecessors, then there will be
1467 // exactly one where it isn't available. Insert a load on that edge and add
1468 // it to the AvailablePreds list.
1469 if (UnavailablePred
) {
1470 assert(UnavailablePred
->getTerminator()->getNumSuccessors() == 1 &&
1471 "Can't handle critical edge here!");
1472 LoadInst
*NewVal
= new LoadInst(
1473 LoadI
->getType(), LoadedPtr
->DoPHITranslation(LoadBB
, UnavailablePred
),
1474 LoadI
->getName() + ".pr", false, MaybeAlign(LoadI
->getAlignment()),
1475 LoadI
->getOrdering(), LoadI
->getSyncScopeID(),
1476 UnavailablePred
->getTerminator());
1477 NewVal
->setDebugLoc(LoadI
->getDebugLoc());
1479 NewVal
->setAAMetadata(AATags
);
1481 AvailablePreds
.push_back(std::make_pair(UnavailablePred
, NewVal
));
1484 // Now we know that each predecessor of this block has a value in
1485 // AvailablePreds, sort them for efficient access as we're walking the preds.
1486 array_pod_sort(AvailablePreds
.begin(), AvailablePreds
.end());
1488 // Create a PHI node at the start of the block for the PRE'd load value.
1489 pred_iterator PB
= pred_begin(LoadBB
), PE
= pred_end(LoadBB
);
1490 PHINode
*PN
= PHINode::Create(LoadI
->getType(), std::distance(PB
, PE
), "",
1492 PN
->takeName(LoadI
);
1493 PN
->setDebugLoc(LoadI
->getDebugLoc());
1495 // Insert new entries into the PHI for each predecessor. A single block may
1496 // have multiple entries here.
1497 for (pred_iterator PI
= PB
; PI
!= PE
; ++PI
) {
1498 BasicBlock
*P
= *PI
;
1499 AvailablePredsTy::iterator I
=
1500 llvm::lower_bound(AvailablePreds
, std::make_pair(P
, (Value
*)nullptr));
1502 assert(I
!= AvailablePreds
.end() && I
->first
== P
&&
1503 "Didn't find entry for predecessor!");
1505 // If we have an available predecessor but it requires casting, insert the
1506 // cast in the predecessor and use the cast. Note that we have to update the
1507 // AvailablePreds vector as we go so that all of the PHI entries for this
1508 // predecessor use the same bitcast.
1509 Value
*&PredV
= I
->second
;
1510 if (PredV
->getType() != LoadI
->getType())
1511 PredV
= CastInst::CreateBitOrPointerCast(PredV
, LoadI
->getType(), "",
1512 P
->getTerminator());
1514 PN
->addIncoming(PredV
, I
->first
);
1517 for (LoadInst
*PredLoadI
: CSELoads
) {
1518 combineMetadataForCSE(PredLoadI
, LoadI
, true);
1521 LoadI
->replaceAllUsesWith(PN
);
1522 LoadI
->eraseFromParent();
1527 /// FindMostPopularDest - The specified list contains multiple possible
1528 /// threadable destinations. Pick the one that occurs the most frequently in
1531 FindMostPopularDest(BasicBlock
*BB
,
1532 const SmallVectorImpl
<std::pair
<BasicBlock
*,
1533 BasicBlock
*>> &PredToDestList
) {
1534 assert(!PredToDestList
.empty());
1536 // Determine popularity. If there are multiple possible destinations, we
1537 // explicitly choose to ignore 'undef' destinations. We prefer to thread
1538 // blocks with known and real destinations to threading undef. We'll handle
1539 // them later if interesting.
1540 DenseMap
<BasicBlock
*, unsigned> DestPopularity
;
1541 for (const auto &PredToDest
: PredToDestList
)
1542 if (PredToDest
.second
)
1543 DestPopularity
[PredToDest
.second
]++;
1545 if (DestPopularity
.empty())
1548 // Find the most popular dest.
1549 DenseMap
<BasicBlock
*, unsigned>::iterator DPI
= DestPopularity
.begin();
1550 BasicBlock
*MostPopularDest
= DPI
->first
;
1551 unsigned Popularity
= DPI
->second
;
1552 SmallVector
<BasicBlock
*, 4> SamePopularity
;
1554 for (++DPI
; DPI
!= DestPopularity
.end(); ++DPI
) {
1555 // If the popularity of this entry isn't higher than the popularity we've
1556 // seen so far, ignore it.
1557 if (DPI
->second
< Popularity
)
1559 else if (DPI
->second
== Popularity
) {
1560 // If it is the same as what we've seen so far, keep track of it.
1561 SamePopularity
.push_back(DPI
->first
);
1563 // If it is more popular, remember it.
1564 SamePopularity
.clear();
1565 MostPopularDest
= DPI
->first
;
1566 Popularity
= DPI
->second
;
1570 // Okay, now we know the most popular destination. If there is more than one
1571 // destination, we need to determine one. This is arbitrary, but we need
1572 // to make a deterministic decision. Pick the first one that appears in the
1574 if (!SamePopularity
.empty()) {
1575 SamePopularity
.push_back(MostPopularDest
);
1576 Instruction
*TI
= BB
->getTerminator();
1577 for (unsigned i
= 0; ; ++i
) {
1578 assert(i
!= TI
->getNumSuccessors() && "Didn't find any successor!");
1580 if (!is_contained(SamePopularity
, TI
->getSuccessor(i
)))
1583 MostPopularDest
= TI
->getSuccessor(i
);
1588 // Okay, we have finally picked the most popular destination.
1589 return MostPopularDest
;
1592 bool JumpThreadingPass::ProcessThreadableEdges(Value
*Cond
, BasicBlock
*BB
,
1593 ConstantPreference Preference
,
1594 Instruction
*CxtI
) {
1595 // If threading this would thread across a loop header, don't even try to
1597 if (LoopHeaders
.count(BB
))
1600 PredValueInfoTy PredValues
;
1601 if (!ComputeValueKnownInPredecessors(Cond
, BB
, PredValues
, Preference
, CxtI
))
1604 assert(!PredValues
.empty() &&
1605 "ComputeValueKnownInPredecessors returned true with no values");
1607 LLVM_DEBUG(dbgs() << "IN BB: " << *BB
;
1608 for (const auto &PredValue
: PredValues
) {
1609 dbgs() << " BB '" << BB
->getName()
1610 << "': FOUND condition = " << *PredValue
.first
1611 << " for pred '" << PredValue
.second
->getName() << "'.\n";
1614 // Decide what we want to thread through. Convert our list of known values to
1615 // a list of known destinations for each pred. This also discards duplicate
1616 // predecessors and keeps track of the undefined inputs (which are represented
1617 // as a null dest in the PredToDestList).
1618 SmallPtrSet
<BasicBlock
*, 16> SeenPreds
;
1619 SmallVector
<std::pair
<BasicBlock
*, BasicBlock
*>, 16> PredToDestList
;
1621 BasicBlock
*OnlyDest
= nullptr;
1622 BasicBlock
*MultipleDestSentinel
= (BasicBlock
*)(intptr_t)~0ULL;
1623 Constant
*OnlyVal
= nullptr;
1624 Constant
*MultipleVal
= (Constant
*)(intptr_t)~0ULL;
1626 for (const auto &PredValue
: PredValues
) {
1627 BasicBlock
*Pred
= PredValue
.second
;
1628 if (!SeenPreds
.insert(Pred
).second
)
1629 continue; // Duplicate predecessor entry.
1631 Constant
*Val
= PredValue
.first
;
1634 if (isa
<UndefValue
>(Val
))
1636 else if (BranchInst
*BI
= dyn_cast
<BranchInst
>(BB
->getTerminator())) {
1637 assert(isa
<ConstantInt
>(Val
) && "Expecting a constant integer");
1638 DestBB
= BI
->getSuccessor(cast
<ConstantInt
>(Val
)->isZero());
1639 } else if (SwitchInst
*SI
= dyn_cast
<SwitchInst
>(BB
->getTerminator())) {
1640 assert(isa
<ConstantInt
>(Val
) && "Expecting a constant integer");
1641 DestBB
= SI
->findCaseValue(cast
<ConstantInt
>(Val
))->getCaseSuccessor();
1643 assert(isa
<IndirectBrInst
>(BB
->getTerminator())
1644 && "Unexpected terminator");
1645 assert(isa
<BlockAddress
>(Val
) && "Expecting a constant blockaddress");
1646 DestBB
= cast
<BlockAddress
>(Val
)->getBasicBlock();
1649 // If we have exactly one destination, remember it for efficiency below.
1650 if (PredToDestList
.empty()) {
1654 if (OnlyDest
!= DestBB
)
1655 OnlyDest
= MultipleDestSentinel
;
1656 // It possible we have same destination, but different value, e.g. default
1657 // case in switchinst.
1659 OnlyVal
= MultipleVal
;
1662 // If the predecessor ends with an indirect goto, we can't change its
1663 // destination. Same for CallBr.
1664 if (isa
<IndirectBrInst
>(Pred
->getTerminator()) ||
1665 isa
<CallBrInst
>(Pred
->getTerminator()))
1668 PredToDestList
.push_back(std::make_pair(Pred
, DestBB
));
1671 // If all edges were unthreadable, we fail.
1672 if (PredToDestList
.empty())
1675 // If all the predecessors go to a single known successor, we want to fold,
1676 // not thread. By doing so, we do not need to duplicate the current block and
1677 // also miss potential opportunities in case we dont/cant duplicate.
1678 if (OnlyDest
&& OnlyDest
!= MultipleDestSentinel
) {
1679 if (BB
->hasNPredecessors(PredToDestList
.size())) {
1680 bool SeenFirstBranchToOnlyDest
= false;
1681 std::vector
<DominatorTree::UpdateType
> Updates
;
1682 Updates
.reserve(BB
->getTerminator()->getNumSuccessors() - 1);
1683 for (BasicBlock
*SuccBB
: successors(BB
)) {
1684 if (SuccBB
== OnlyDest
&& !SeenFirstBranchToOnlyDest
) {
1685 SeenFirstBranchToOnlyDest
= true; // Don't modify the first branch.
1687 SuccBB
->removePredecessor(BB
, true); // This is unreachable successor.
1688 Updates
.push_back({DominatorTree::Delete
, BB
, SuccBB
});
1692 // Finally update the terminator.
1693 Instruction
*Term
= BB
->getTerminator();
1694 BranchInst::Create(OnlyDest
, Term
);
1695 Term
->eraseFromParent();
1696 DTU
->applyUpdatesPermissive(Updates
);
1698 // If the condition is now dead due to the removal of the old terminator,
1700 if (auto *CondInst
= dyn_cast
<Instruction
>(Cond
)) {
1701 if (CondInst
->use_empty() && !CondInst
->mayHaveSideEffects())
1702 CondInst
->eraseFromParent();
1703 // We can safely replace *some* uses of the CondInst if it has
1704 // exactly one value as returned by LVI. RAUW is incorrect in the
1705 // presence of guards and assumes, that have the `Cond` as the use. This
1706 // is because we use the guards/assume to reason about the `Cond` value
1707 // at the end of block, but RAUW unconditionally replaces all uses
1708 // including the guards/assumes themselves and the uses before the
1710 else if (OnlyVal
&& OnlyVal
!= MultipleVal
&&
1711 CondInst
->getParent() == BB
)
1712 ReplaceFoldableUses(CondInst
, OnlyVal
);
1718 // Determine which is the most common successor. If we have many inputs and
1719 // this block is a switch, we want to start by threading the batch that goes
1720 // to the most popular destination first. If we only know about one
1721 // threadable destination (the common case) we can avoid this.
1722 BasicBlock
*MostPopularDest
= OnlyDest
;
1724 if (MostPopularDest
== MultipleDestSentinel
) {
1725 // Remove any loop headers from the Dest list, ThreadEdge conservatively
1726 // won't process them, but we might have other destination that are eligible
1727 // and we still want to process.
1728 erase_if(PredToDestList
,
1729 [&](const std::pair
<BasicBlock
*, BasicBlock
*> &PredToDest
) {
1730 return LoopHeaders
.count(PredToDest
.second
) != 0;
1733 if (PredToDestList
.empty())
1736 MostPopularDest
= FindMostPopularDest(BB
, PredToDestList
);
1739 // Now that we know what the most popular destination is, factor all
1740 // predecessors that will jump to it into a single predecessor.
1741 SmallVector
<BasicBlock
*, 16> PredsToFactor
;
1742 for (const auto &PredToDest
: PredToDestList
)
1743 if (PredToDest
.second
== MostPopularDest
) {
1744 BasicBlock
*Pred
= PredToDest
.first
;
1746 // This predecessor may be a switch or something else that has multiple
1747 // edges to the block. Factor each of these edges by listing them
1748 // according to # occurrences in PredsToFactor.
1749 for (BasicBlock
*Succ
: successors(Pred
))
1751 PredsToFactor
.push_back(Pred
);
1754 // If the threadable edges are branching on an undefined value, we get to pick
1755 // the destination that these predecessors should get to.
1756 if (!MostPopularDest
)
1757 MostPopularDest
= BB
->getTerminator()->
1758 getSuccessor(GetBestDestForJumpOnUndef(BB
));
1760 // Ok, try to thread it!
1761 return ThreadEdge(BB
, PredsToFactor
, MostPopularDest
);
1764 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1765 /// a PHI node in the current block. See if there are any simplifications we
1766 /// can do based on inputs to the phi node.
1767 bool JumpThreadingPass::ProcessBranchOnPHI(PHINode
*PN
) {
1768 BasicBlock
*BB
= PN
->getParent();
1770 // TODO: We could make use of this to do it once for blocks with common PHI
1772 SmallVector
<BasicBlock
*, 1> PredBBs
;
1775 // If any of the predecessor blocks end in an unconditional branch, we can
1776 // *duplicate* the conditional branch into that block in order to further
1777 // encourage jump threading and to eliminate cases where we have branch on a
1778 // phi of an icmp (branch on icmp is much better).
1779 for (unsigned i
= 0, e
= PN
->getNumIncomingValues(); i
!= e
; ++i
) {
1780 BasicBlock
*PredBB
= PN
->getIncomingBlock(i
);
1781 if (BranchInst
*PredBr
= dyn_cast
<BranchInst
>(PredBB
->getTerminator()))
1782 if (PredBr
->isUnconditional()) {
1783 PredBBs
[0] = PredBB
;
1784 // Try to duplicate BB into PredBB.
1785 if (DuplicateCondBranchOnPHIIntoPred(BB
, PredBBs
))
1793 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
1794 /// a xor instruction in the current block. See if there are any
1795 /// simplifications we can do based on inputs to the xor.
1796 bool JumpThreadingPass::ProcessBranchOnXOR(BinaryOperator
*BO
) {
1797 BasicBlock
*BB
= BO
->getParent();
1799 // If either the LHS or RHS of the xor is a constant, don't do this
1801 if (isa
<ConstantInt
>(BO
->getOperand(0)) ||
1802 isa
<ConstantInt
>(BO
->getOperand(1)))
1805 // If the first instruction in BB isn't a phi, we won't be able to infer
1806 // anything special about any particular predecessor.
1807 if (!isa
<PHINode
>(BB
->front()))
1810 // If this BB is a landing pad, we won't be able to split the edge into it.
1814 // If we have a xor as the branch input to this block, and we know that the
1815 // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1816 // the condition into the predecessor and fix that value to true, saving some
1817 // logical ops on that path and encouraging other paths to simplify.
1819 // This copies something like this:
1822 // %X = phi i1 [1], [%X']
1823 // %Y = icmp eq i32 %A, %B
1824 // %Z = xor i1 %X, %Y
1829 // %Y = icmp ne i32 %A, %B
1832 PredValueInfoTy XorOpValues
;
1834 if (!ComputeValueKnownInPredecessors(BO
->getOperand(0), BB
, XorOpValues
,
1836 assert(XorOpValues
.empty());
1837 if (!ComputeValueKnownInPredecessors(BO
->getOperand(1), BB
, XorOpValues
,
1843 assert(!XorOpValues
.empty() &&
1844 "ComputeValueKnownInPredecessors returned true with no values");
1846 // Scan the information to see which is most popular: true or false. The
1847 // predecessors can be of the set true, false, or undef.
1848 unsigned NumTrue
= 0, NumFalse
= 0;
1849 for (const auto &XorOpValue
: XorOpValues
) {
1850 if (isa
<UndefValue
>(XorOpValue
.first
))
1851 // Ignore undefs for the count.
1853 if (cast
<ConstantInt
>(XorOpValue
.first
)->isZero())
1859 // Determine which value to split on, true, false, or undef if neither.
1860 ConstantInt
*SplitVal
= nullptr;
1861 if (NumTrue
> NumFalse
)
1862 SplitVal
= ConstantInt::getTrue(BB
->getContext());
1863 else if (NumTrue
!= 0 || NumFalse
!= 0)
1864 SplitVal
= ConstantInt::getFalse(BB
->getContext());
1866 // Collect all of the blocks that this can be folded into so that we can
1867 // factor this once and clone it once.
1868 SmallVector
<BasicBlock
*, 8> BlocksToFoldInto
;
1869 for (const auto &XorOpValue
: XorOpValues
) {
1870 if (XorOpValue
.first
!= SplitVal
&& !isa
<UndefValue
>(XorOpValue
.first
))
1873 BlocksToFoldInto
.push_back(XorOpValue
.second
);
1876 // If we inferred a value for all of the predecessors, then duplication won't
1877 // help us. However, we can just replace the LHS or RHS with the constant.
1878 if (BlocksToFoldInto
.size() ==
1879 cast
<PHINode
>(BB
->front()).getNumIncomingValues()) {
1881 // If all preds provide undef, just nuke the xor, because it is undef too.
1882 BO
->replaceAllUsesWith(UndefValue::get(BO
->getType()));
1883 BO
->eraseFromParent();
1884 } else if (SplitVal
->isZero()) {
1885 // If all preds provide 0, replace the xor with the other input.
1886 BO
->replaceAllUsesWith(BO
->getOperand(isLHS
));
1887 BO
->eraseFromParent();
1889 // If all preds provide 1, set the computed value to 1.
1890 BO
->setOperand(!isLHS
, SplitVal
);
1896 // Try to duplicate BB into PredBB.
1897 return DuplicateCondBranchOnPHIIntoPred(BB
, BlocksToFoldInto
);
1900 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1901 /// predecessor to the PHIBB block. If it has PHI nodes, add entries for
1902 /// NewPred using the entries from OldPred (suitably mapped).
1903 static void AddPHINodeEntriesForMappedBlock(BasicBlock
*PHIBB
,
1904 BasicBlock
*OldPred
,
1905 BasicBlock
*NewPred
,
1906 DenseMap
<Instruction
*, Value
*> &ValueMap
) {
1907 for (PHINode
&PN
: PHIBB
->phis()) {
1908 // Ok, we have a PHI node. Figure out what the incoming value was for the
1910 Value
*IV
= PN
.getIncomingValueForBlock(OldPred
);
1912 // Remap the value if necessary.
1913 if (Instruction
*Inst
= dyn_cast
<Instruction
>(IV
)) {
1914 DenseMap
<Instruction
*, Value
*>::iterator I
= ValueMap
.find(Inst
);
1915 if (I
!= ValueMap
.end())
1919 PN
.addIncoming(IV
, NewPred
);
1923 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1924 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1925 /// across BB. Transform the IR to reflect this change.
1926 bool JumpThreadingPass::ThreadEdge(BasicBlock
*BB
,
1927 const SmallVectorImpl
<BasicBlock
*> &PredBBs
,
1928 BasicBlock
*SuccBB
) {
1929 // If threading to the same block as we come from, we would infinite loop.
1931 LLVM_DEBUG(dbgs() << " Not threading across BB '" << BB
->getName()
1932 << "' - would thread to self!\n");
1936 // If threading this would thread across a loop header, don't thread the edge.
1937 // See the comments above FindLoopHeaders for justifications and caveats.
1938 if (LoopHeaders
.count(BB
) || LoopHeaders
.count(SuccBB
)) {
1940 bool BBIsHeader
= LoopHeaders
.count(BB
);
1941 bool SuccIsHeader
= LoopHeaders
.count(SuccBB
);
1942 dbgs() << " Not threading across "
1943 << (BBIsHeader
? "loop header BB '" : "block BB '") << BB
->getName()
1944 << "' to dest " << (SuccIsHeader
? "loop header BB '" : "block BB '")
1945 << SuccBB
->getName() << "' - it might create an irreducible loop!\n";
1950 unsigned JumpThreadCost
=
1951 getJumpThreadDuplicationCost(BB
, BB
->getTerminator(), BBDupThreshold
);
1952 if (JumpThreadCost
> BBDupThreshold
) {
1953 LLVM_DEBUG(dbgs() << " Not threading BB '" << BB
->getName()
1954 << "' - Cost is too high: " << JumpThreadCost
<< "\n");
1958 // And finally, do it! Start by factoring the predecessors if needed.
1960 if (PredBBs
.size() == 1)
1961 PredBB
= PredBBs
[0];
1963 LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs
.size()
1964 << " common predecessors.\n");
1965 PredBB
= SplitBlockPreds(BB
, PredBBs
, ".thr_comm");
1968 // And finally, do it!
1969 LLVM_DEBUG(dbgs() << " Threading edge from '" << PredBB
->getName()
1970 << "' to '" << SuccBB
->getName()
1971 << "' with cost: " << JumpThreadCost
1972 << ", across block:\n " << *BB
<< "\n");
1974 if (DTU
->hasPendingDomTreeUpdates())
1978 LVI
->threadEdge(PredBB
, BB
, SuccBB
);
1980 // We are going to have to map operands from the original BB block to the new
1981 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
1982 // account for entry from PredBB.
1983 DenseMap
<Instruction
*, Value
*> ValueMapping
;
1985 BasicBlock
*NewBB
= BasicBlock::Create(BB
->getContext(),
1986 BB
->getName()+".thread",
1987 BB
->getParent(), BB
);
1988 NewBB
->moveAfter(PredBB
);
1990 // Set the block frequency of NewBB.
1991 if (HasProfileData
) {
1993 BFI
->getBlockFreq(PredBB
) * BPI
->getEdgeProbability(PredBB
, BB
);
1994 BFI
->setBlockFreq(NewBB
, NewBBFreq
.getFrequency());
1997 BasicBlock::iterator BI
= BB
->begin();
1998 // Clone the phi nodes of BB into NewBB. The resulting phi nodes are trivial,
1999 // since NewBB only has one predecessor, but SSAUpdater might need to rewrite
2000 // the operand of the cloned phi.
2001 for (; PHINode
*PN
= dyn_cast
<PHINode
>(BI
); ++BI
) {
2002 PHINode
*NewPN
= PHINode::Create(PN
->getType(), 1, PN
->getName(), NewBB
);
2003 NewPN
->addIncoming(PN
->getIncomingValueForBlock(PredBB
), PredBB
);
2004 ValueMapping
[PN
] = NewPN
;
2007 // Clone the non-phi instructions of BB into NewBB, keeping track of the
2008 // mapping and using it to remap operands in the cloned instructions.
2009 for (; !BI
->isTerminator(); ++BI
) {
2010 Instruction
*New
= BI
->clone();
2011 New
->setName(BI
->getName());
2012 NewBB
->getInstList().push_back(New
);
2013 ValueMapping
[&*BI
] = New
;
2015 // Remap operands to patch up intra-block references.
2016 for (unsigned i
= 0, e
= New
->getNumOperands(); i
!= e
; ++i
)
2017 if (Instruction
*Inst
= dyn_cast
<Instruction
>(New
->getOperand(i
))) {
2018 DenseMap
<Instruction
*, Value
*>::iterator I
= ValueMapping
.find(Inst
);
2019 if (I
!= ValueMapping
.end())
2020 New
->setOperand(i
, I
->second
);
2024 // We didn't copy the terminator from BB over to NewBB, because there is now
2025 // an unconditional jump to SuccBB. Insert the unconditional jump.
2026 BranchInst
*NewBI
= BranchInst::Create(SuccBB
, NewBB
);
2027 NewBI
->setDebugLoc(BB
->getTerminator()->getDebugLoc());
2029 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
2030 // PHI nodes for NewBB now.
2031 AddPHINodeEntriesForMappedBlock(SuccBB
, BB
, NewBB
, ValueMapping
);
2033 // Update the terminator of PredBB to jump to NewBB instead of BB. This
2034 // eliminates predecessors from BB, which requires us to simplify any PHI
2036 Instruction
*PredTerm
= PredBB
->getTerminator();
2037 for (unsigned i
= 0, e
= PredTerm
->getNumSuccessors(); i
!= e
; ++i
)
2038 if (PredTerm
->getSuccessor(i
) == BB
) {
2039 BB
->removePredecessor(PredBB
, true);
2040 PredTerm
->setSuccessor(i
, NewBB
);
2043 // Enqueue required DT updates.
2044 DTU
->applyUpdatesPermissive({{DominatorTree::Insert
, NewBB
, SuccBB
},
2045 {DominatorTree::Insert
, PredBB
, NewBB
},
2046 {DominatorTree::Delete
, PredBB
, BB
}});
2048 // If there were values defined in BB that are used outside the block, then we
2049 // now have to update all uses of the value to use either the original value,
2050 // the cloned value, or some PHI derived value. This can require arbitrary
2051 // PHI insertion, of which we are prepared to do, clean these up now.
2052 SSAUpdater SSAUpdate
;
2053 SmallVector
<Use
*, 16> UsesToRename
;
2055 for (Instruction
&I
: *BB
) {
2056 // Scan all uses of this instruction to see if their uses are no longer
2057 // dominated by the previous def and if so, record them in UsesToRename.
2058 // Also, skip phi operands from PredBB - we'll remove them anyway.
2059 for (Use
&U
: I
.uses()) {
2060 Instruction
*User
= cast
<Instruction
>(U
.getUser());
2061 if (PHINode
*UserPN
= dyn_cast
<PHINode
>(User
)) {
2062 if (UserPN
->getIncomingBlock(U
) == BB
)
2064 } else if (User
->getParent() == BB
)
2067 UsesToRename
.push_back(&U
);
2070 // If there are no uses outside the block, we're done with this instruction.
2071 if (UsesToRename
.empty())
2073 LLVM_DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I
<< "\n");
2075 // We found a use of I outside of BB. Rename all uses of I that are outside
2076 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
2077 // with the two values we know.
2078 SSAUpdate
.Initialize(I
.getType(), I
.getName());
2079 SSAUpdate
.AddAvailableValue(BB
, &I
);
2080 SSAUpdate
.AddAvailableValue(NewBB
, ValueMapping
[&I
]);
2082 while (!UsesToRename
.empty())
2083 SSAUpdate
.RewriteUse(*UsesToRename
.pop_back_val());
2084 LLVM_DEBUG(dbgs() << "\n");
2087 // At this point, the IR is fully up to date and consistent. Do a quick scan
2088 // over the new instructions and zap any that are constants or dead. This
2089 // frequently happens because of phi translation.
2090 SimplifyInstructionsInBlock(NewBB
, TLI
);
2092 // Update the edge weight from BB to SuccBB, which should be less than before.
2093 UpdateBlockFreqAndEdgeWeight(PredBB
, BB
, NewBB
, SuccBB
);
2095 // Threaded an edge!
2100 /// Create a new basic block that will be the predecessor of BB and successor of
2101 /// all blocks in Preds. When profile data is available, update the frequency of
2103 BasicBlock
*JumpThreadingPass::SplitBlockPreds(BasicBlock
*BB
,
2104 ArrayRef
<BasicBlock
*> Preds
,
2105 const char *Suffix
) {
2106 SmallVector
<BasicBlock
*, 2> NewBBs
;
2108 // Collect the frequencies of all predecessors of BB, which will be used to
2109 // update the edge weight of the result of splitting predecessors.
2110 DenseMap
<BasicBlock
*, BlockFrequency
> FreqMap
;
2112 for (auto Pred
: Preds
)
2113 FreqMap
.insert(std::make_pair(
2114 Pred
, BFI
->getBlockFreq(Pred
) * BPI
->getEdgeProbability(Pred
, BB
)));
2116 // In the case when BB is a LandingPad block we create 2 new predecessors
2117 // instead of just one.
2118 if (BB
->isLandingPad()) {
2119 std::string NewName
= std::string(Suffix
) + ".split-lp";
2120 SplitLandingPadPredecessors(BB
, Preds
, Suffix
, NewName
.c_str(), NewBBs
);
2122 NewBBs
.push_back(SplitBlockPredecessors(BB
, Preds
, Suffix
));
2125 std::vector
<DominatorTree::UpdateType
> Updates
;
2126 Updates
.reserve((2 * Preds
.size()) + NewBBs
.size());
2127 for (auto NewBB
: NewBBs
) {
2128 BlockFrequency
NewBBFreq(0);
2129 Updates
.push_back({DominatorTree::Insert
, NewBB
, BB
});
2130 for (auto Pred
: predecessors(NewBB
)) {
2131 Updates
.push_back({DominatorTree::Delete
, Pred
, BB
});
2132 Updates
.push_back({DominatorTree::Insert
, Pred
, NewBB
});
2133 if (HasProfileData
) // Update frequencies between Pred -> NewBB.
2134 NewBBFreq
+= FreqMap
.lookup(Pred
);
2136 if (HasProfileData
) // Apply the summed frequency to NewBB.
2137 BFI
->setBlockFreq(NewBB
, NewBBFreq
.getFrequency());
2140 DTU
->applyUpdatesPermissive(Updates
);
2144 bool JumpThreadingPass::doesBlockHaveProfileData(BasicBlock
*BB
) {
2145 const Instruction
*TI
= BB
->getTerminator();
2146 assert(TI
->getNumSuccessors() > 1 && "not a split");
2148 MDNode
*WeightsNode
= TI
->getMetadata(LLVMContext::MD_prof
);
2152 MDString
*MDName
= cast
<MDString
>(WeightsNode
->getOperand(0));
2153 if (MDName
->getString() != "branch_weights")
2156 // Ensure there are weights for all of the successors. Note that the first
2157 // operand to the metadata node is a name, not a weight.
2158 return WeightsNode
->getNumOperands() == TI
->getNumSuccessors() + 1;
2161 /// Update the block frequency of BB and branch weight and the metadata on the
2162 /// edge BB->SuccBB. This is done by scaling the weight of BB->SuccBB by 1 -
2163 /// Freq(PredBB->BB) / Freq(BB->SuccBB).
2164 void JumpThreadingPass::UpdateBlockFreqAndEdgeWeight(BasicBlock
*PredBB
,
2167 BasicBlock
*SuccBB
) {
2168 if (!HasProfileData
)
2171 assert(BFI
&& BPI
&& "BFI & BPI should have been created here");
2173 // As the edge from PredBB to BB is deleted, we have to update the block
2175 auto BBOrigFreq
= BFI
->getBlockFreq(BB
);
2176 auto NewBBFreq
= BFI
->getBlockFreq(NewBB
);
2177 auto BB2SuccBBFreq
= BBOrigFreq
* BPI
->getEdgeProbability(BB
, SuccBB
);
2178 auto BBNewFreq
= BBOrigFreq
- NewBBFreq
;
2179 BFI
->setBlockFreq(BB
, BBNewFreq
.getFrequency());
2181 // Collect updated outgoing edges' frequencies from BB and use them to update
2182 // edge probabilities.
2183 SmallVector
<uint64_t, 4> BBSuccFreq
;
2184 for (BasicBlock
*Succ
: successors(BB
)) {
2185 auto SuccFreq
= (Succ
== SuccBB
)
2186 ? BB2SuccBBFreq
- NewBBFreq
2187 : BBOrigFreq
* BPI
->getEdgeProbability(BB
, Succ
);
2188 BBSuccFreq
.push_back(SuccFreq
.getFrequency());
2191 uint64_t MaxBBSuccFreq
=
2192 *std::max_element(BBSuccFreq
.begin(), BBSuccFreq
.end());
2194 SmallVector
<BranchProbability
, 4> BBSuccProbs
;
2195 if (MaxBBSuccFreq
== 0)
2196 BBSuccProbs
.assign(BBSuccFreq
.size(),
2197 {1, static_cast<unsigned>(BBSuccFreq
.size())});
2199 for (uint64_t Freq
: BBSuccFreq
)
2200 BBSuccProbs
.push_back(
2201 BranchProbability::getBranchProbability(Freq
, MaxBBSuccFreq
));
2202 // Normalize edge probabilities so that they sum up to one.
2203 BranchProbability::normalizeProbabilities(BBSuccProbs
.begin(),
2207 // Update edge probabilities in BPI.
2208 for (int I
= 0, E
= BBSuccProbs
.size(); I
< E
; I
++)
2209 BPI
->setEdgeProbability(BB
, I
, BBSuccProbs
[I
]);
2211 // Update the profile metadata as well.
2213 // Don't do this if the profile of the transformed blocks was statically
2214 // estimated. (This could occur despite the function having an entry
2215 // frequency in completely cold parts of the CFG.)
2217 // In this case we don't want to suggest to subsequent passes that the
2218 // calculated weights are fully consistent. Consider this graph:
2233 // Assuming the blocks check_* all compare the same value against 1, 2 and 3,
2234 // the overall probabilities are inconsistent; the total probability that the
2235 // value is either 1, 2 or 3 is 150%.
2237 // As a consequence if we thread eq_1 -> check_2 to check_3, check_2->check_3
2238 // becomes 0%. This is even worse if the edge whose probability becomes 0% is
2239 // the loop exit edge. Then based solely on static estimation we would assume
2240 // the loop was extremely hot.
2242 // FIXME this locally as well so that BPI and BFI are consistent as well. We
2243 // shouldn't make edges extremely likely or unlikely based solely on static
2245 if (BBSuccProbs
.size() >= 2 && doesBlockHaveProfileData(BB
)) {
2246 SmallVector
<uint32_t, 4> Weights
;
2247 for (auto Prob
: BBSuccProbs
)
2248 Weights
.push_back(Prob
.getNumerator());
2250 auto TI
= BB
->getTerminator();
2252 LLVMContext::MD_prof
,
2253 MDBuilder(TI
->getParent()->getContext()).createBranchWeights(Weights
));
2257 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
2258 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
2259 /// If we can duplicate the contents of BB up into PredBB do so now, this
2260 /// improves the odds that the branch will be on an analyzable instruction like
2262 bool JumpThreadingPass::DuplicateCondBranchOnPHIIntoPred(
2263 BasicBlock
*BB
, const SmallVectorImpl
<BasicBlock
*> &PredBBs
) {
2264 assert(!PredBBs
.empty() && "Can't handle an empty set");
2266 // If BB is a loop header, then duplicating this block outside the loop would
2267 // cause us to transform this into an irreducible loop, don't do this.
2268 // See the comments above FindLoopHeaders for justifications and caveats.
2269 if (LoopHeaders
.count(BB
)) {
2270 LLVM_DEBUG(dbgs() << " Not duplicating loop header '" << BB
->getName()
2271 << "' into predecessor block '" << PredBBs
[0]->getName()
2272 << "' - it might create an irreducible loop!\n");
2276 unsigned DuplicationCost
=
2277 getJumpThreadDuplicationCost(BB
, BB
->getTerminator(), BBDupThreshold
);
2278 if (DuplicationCost
> BBDupThreshold
) {
2279 LLVM_DEBUG(dbgs() << " Not duplicating BB '" << BB
->getName()
2280 << "' - Cost is too high: " << DuplicationCost
<< "\n");
2284 // And finally, do it! Start by factoring the predecessors if needed.
2285 std::vector
<DominatorTree::UpdateType
> Updates
;
2287 if (PredBBs
.size() == 1)
2288 PredBB
= PredBBs
[0];
2290 LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs
.size()
2291 << " common predecessors.\n");
2292 PredBB
= SplitBlockPreds(BB
, PredBBs
, ".thr_comm");
2294 Updates
.push_back({DominatorTree::Delete
, PredBB
, BB
});
2296 // Okay, we decided to do this! Clone all the instructions in BB onto the end
2298 LLVM_DEBUG(dbgs() << " Duplicating block '" << BB
->getName()
2299 << "' into end of '" << PredBB
->getName()
2300 << "' to eliminate branch on phi. Cost: "
2301 << DuplicationCost
<< " block is:" << *BB
<< "\n");
2303 // Unless PredBB ends with an unconditional branch, split the edge so that we
2304 // can just clone the bits from BB into the end of the new PredBB.
2305 BranchInst
*OldPredBranch
= dyn_cast
<BranchInst
>(PredBB
->getTerminator());
2307 if (!OldPredBranch
|| !OldPredBranch
->isUnconditional()) {
2308 BasicBlock
*OldPredBB
= PredBB
;
2309 PredBB
= SplitEdge(OldPredBB
, BB
);
2310 Updates
.push_back({DominatorTree::Insert
, OldPredBB
, PredBB
});
2311 Updates
.push_back({DominatorTree::Insert
, PredBB
, BB
});
2312 Updates
.push_back({DominatorTree::Delete
, OldPredBB
, BB
});
2313 OldPredBranch
= cast
<BranchInst
>(PredBB
->getTerminator());
2316 // We are going to have to map operands from the original BB block into the
2317 // PredBB block. Evaluate PHI nodes in BB.
2318 DenseMap
<Instruction
*, Value
*> ValueMapping
;
2320 BasicBlock::iterator BI
= BB
->begin();
2321 for (; PHINode
*PN
= dyn_cast
<PHINode
>(BI
); ++BI
)
2322 ValueMapping
[PN
] = PN
->getIncomingValueForBlock(PredBB
);
2323 // Clone the non-phi instructions of BB into PredBB, keeping track of the
2324 // mapping and using it to remap operands in the cloned instructions.
2325 for (; BI
!= BB
->end(); ++BI
) {
2326 Instruction
*New
= BI
->clone();
2328 // Remap operands to patch up intra-block references.
2329 for (unsigned i
= 0, e
= New
->getNumOperands(); i
!= e
; ++i
)
2330 if (Instruction
*Inst
= dyn_cast
<Instruction
>(New
->getOperand(i
))) {
2331 DenseMap
<Instruction
*, Value
*>::iterator I
= ValueMapping
.find(Inst
);
2332 if (I
!= ValueMapping
.end())
2333 New
->setOperand(i
, I
->second
);
2336 // If this instruction can be simplified after the operands are updated,
2337 // just use the simplified value instead. This frequently happens due to
2339 if (Value
*IV
= SimplifyInstruction(
2341 {BB
->getModule()->getDataLayout(), TLI
, nullptr, nullptr, New
})) {
2342 ValueMapping
[&*BI
] = IV
;
2343 if (!New
->mayHaveSideEffects()) {
2348 ValueMapping
[&*BI
] = New
;
2351 // Otherwise, insert the new instruction into the block.
2352 New
->setName(BI
->getName());
2353 PredBB
->getInstList().insert(OldPredBranch
->getIterator(), New
);
2354 // Update Dominance from simplified New instruction operands.
2355 for (unsigned i
= 0, e
= New
->getNumOperands(); i
!= e
; ++i
)
2356 if (BasicBlock
*SuccBB
= dyn_cast
<BasicBlock
>(New
->getOperand(i
)))
2357 Updates
.push_back({DominatorTree::Insert
, PredBB
, SuccBB
});
2361 // Check to see if the targets of the branch had PHI nodes. If so, we need to
2362 // add entries to the PHI nodes for branch from PredBB now.
2363 BranchInst
*BBBranch
= cast
<BranchInst
>(BB
->getTerminator());
2364 AddPHINodeEntriesForMappedBlock(BBBranch
->getSuccessor(0), BB
, PredBB
,
2366 AddPHINodeEntriesForMappedBlock(BBBranch
->getSuccessor(1), BB
, PredBB
,
2369 // If there were values defined in BB that are used outside the block, then we
2370 // now have to update all uses of the value to use either the original value,
2371 // the cloned value, or some PHI derived value. This can require arbitrary
2372 // PHI insertion, of which we are prepared to do, clean these up now.
2373 SSAUpdater SSAUpdate
;
2374 SmallVector
<Use
*, 16> UsesToRename
;
2375 for (Instruction
&I
: *BB
) {
2376 // Scan all uses of this instruction to see if it is used outside of its
2377 // block, and if so, record them in UsesToRename.
2378 for (Use
&U
: I
.uses()) {
2379 Instruction
*User
= cast
<Instruction
>(U
.getUser());
2380 if (PHINode
*UserPN
= dyn_cast
<PHINode
>(User
)) {
2381 if (UserPN
->getIncomingBlock(U
) == BB
)
2383 } else if (User
->getParent() == BB
)
2386 UsesToRename
.push_back(&U
);
2389 // If there are no uses outside the block, we're done with this instruction.
2390 if (UsesToRename
.empty())
2393 LLVM_DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I
<< "\n");
2395 // We found a use of I outside of BB. Rename all uses of I that are outside
2396 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
2397 // with the two values we know.
2398 SSAUpdate
.Initialize(I
.getType(), I
.getName());
2399 SSAUpdate
.AddAvailableValue(BB
, &I
);
2400 SSAUpdate
.AddAvailableValue(PredBB
, ValueMapping
[&I
]);
2402 while (!UsesToRename
.empty())
2403 SSAUpdate
.RewriteUse(*UsesToRename
.pop_back_val());
2404 LLVM_DEBUG(dbgs() << "\n");
2407 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
2409 BB
->removePredecessor(PredBB
, true);
2411 // Remove the unconditional branch at the end of the PredBB block.
2412 OldPredBranch
->eraseFromParent();
2413 DTU
->applyUpdatesPermissive(Updates
);
2419 // Pred is a predecessor of BB with an unconditional branch to BB. SI is
2420 // a Select instruction in Pred. BB has other predecessors and SI is used in
2421 // a PHI node in BB. SI has no other use.
2422 // A new basic block, NewBB, is created and SI is converted to compare and
2423 // conditional branch. SI is erased from parent.
2424 void JumpThreadingPass::UnfoldSelectInstr(BasicBlock
*Pred
, BasicBlock
*BB
,
2425 SelectInst
*SI
, PHINode
*SIUse
,
2427 // Expand the select.
2436 BranchInst
*PredTerm
= cast
<BranchInst
>(Pred
->getTerminator());
2437 BasicBlock
*NewBB
= BasicBlock::Create(BB
->getContext(), "select.unfold",
2438 BB
->getParent(), BB
);
2439 // Move the unconditional branch to NewBB.
2440 PredTerm
->removeFromParent();
2441 NewBB
->getInstList().insert(NewBB
->end(), PredTerm
);
2442 // Create a conditional branch and update PHI nodes.
2443 BranchInst::Create(NewBB
, BB
, SI
->getCondition(), Pred
);
2444 SIUse
->setIncomingValue(Idx
, SI
->getFalseValue());
2445 SIUse
->addIncoming(SI
->getTrueValue(), NewBB
);
2447 // The select is now dead.
2448 SI
->eraseFromParent();
2449 DTU
->applyUpdatesPermissive({{DominatorTree::Insert
, NewBB
, BB
},
2450 {DominatorTree::Insert
, Pred
, NewBB
}});
2452 // Update any other PHI nodes in BB.
2453 for (BasicBlock::iterator BI
= BB
->begin();
2454 PHINode
*Phi
= dyn_cast
<PHINode
>(BI
); ++BI
)
2456 Phi
->addIncoming(Phi
->getIncomingValueForBlock(Pred
), NewBB
);
2459 bool JumpThreadingPass::TryToUnfoldSelect(SwitchInst
*SI
, BasicBlock
*BB
) {
2460 PHINode
*CondPHI
= dyn_cast
<PHINode
>(SI
->getCondition());
2462 if (!CondPHI
|| CondPHI
->getParent() != BB
)
2465 for (unsigned I
= 0, E
= CondPHI
->getNumIncomingValues(); I
!= E
; ++I
) {
2466 BasicBlock
*Pred
= CondPHI
->getIncomingBlock(I
);
2467 SelectInst
*PredSI
= dyn_cast
<SelectInst
>(CondPHI
->getIncomingValue(I
));
2469 // The second and third condition can be potentially relaxed. Currently
2470 // the conditions help to simplify the code and allow us to reuse existing
2471 // code, developed for TryToUnfoldSelect(CmpInst *, BasicBlock *)
2472 if (!PredSI
|| PredSI
->getParent() != Pred
|| !PredSI
->hasOneUse())
2475 BranchInst
*PredTerm
= dyn_cast
<BranchInst
>(Pred
->getTerminator());
2476 if (!PredTerm
|| !PredTerm
->isUnconditional())
2479 UnfoldSelectInstr(Pred
, BB
, PredSI
, CondPHI
, I
);
2485 /// TryToUnfoldSelect - Look for blocks of the form
2491 /// %p = phi [%a, %bb1] ...
2495 /// And expand the select into a branch structure if one of its arms allows %c
2496 /// to be folded. This later enables threading from bb1 over bb2.
2497 bool JumpThreadingPass::TryToUnfoldSelect(CmpInst
*CondCmp
, BasicBlock
*BB
) {
2498 BranchInst
*CondBr
= dyn_cast
<BranchInst
>(BB
->getTerminator());
2499 PHINode
*CondLHS
= dyn_cast
<PHINode
>(CondCmp
->getOperand(0));
2500 Constant
*CondRHS
= cast
<Constant
>(CondCmp
->getOperand(1));
2502 if (!CondBr
|| !CondBr
->isConditional() || !CondLHS
||
2503 CondLHS
->getParent() != BB
)
2506 for (unsigned I
= 0, E
= CondLHS
->getNumIncomingValues(); I
!= E
; ++I
) {
2507 BasicBlock
*Pred
= CondLHS
->getIncomingBlock(I
);
2508 SelectInst
*SI
= dyn_cast
<SelectInst
>(CondLHS
->getIncomingValue(I
));
2510 // Look if one of the incoming values is a select in the corresponding
2512 if (!SI
|| SI
->getParent() != Pred
|| !SI
->hasOneUse())
2515 BranchInst
*PredTerm
= dyn_cast
<BranchInst
>(Pred
->getTerminator());
2516 if (!PredTerm
|| !PredTerm
->isUnconditional())
2519 // Now check if one of the select values would allow us to constant fold the
2520 // terminator in BB. We don't do the transform if both sides fold, those
2521 // cases will be threaded in any case.
2522 if (DTU
->hasPendingDomTreeUpdates())
2526 LazyValueInfo::Tristate LHSFolds
=
2527 LVI
->getPredicateOnEdge(CondCmp
->getPredicate(), SI
->getOperand(1),
2528 CondRHS
, Pred
, BB
, CondCmp
);
2529 LazyValueInfo::Tristate RHSFolds
=
2530 LVI
->getPredicateOnEdge(CondCmp
->getPredicate(), SI
->getOperand(2),
2531 CondRHS
, Pred
, BB
, CondCmp
);
2532 if ((LHSFolds
!= LazyValueInfo::Unknown
||
2533 RHSFolds
!= LazyValueInfo::Unknown
) &&
2534 LHSFolds
!= RHSFolds
) {
2535 UnfoldSelectInstr(Pred
, BB
, SI
, CondLHS
, I
);
2542 /// TryToUnfoldSelectInCurrBB - Look for PHI/Select or PHI/CMP/Select in the
2543 /// same BB in the form
2545 /// %p = phi [false, %bb1], [true, %bb2], [false, %bb3], [true, %bb4], ...
2546 /// %s = select %p, trueval, falseval
2551 /// %p = phi [0, %bb1], [1, %bb2], [0, %bb3], [1, %bb4], ...
2553 /// %s = select %c, trueval, falseval
2555 /// And expand the select into a branch structure. This later enables
2556 /// jump-threading over bb in this pass.
2558 /// Using the similar approach of SimplifyCFG::FoldCondBranchOnPHI(), unfold
2559 /// select if the associated PHI has at least one constant. If the unfolded
2560 /// select is not jump-threaded, it will be folded again in the later
2562 bool JumpThreadingPass::TryToUnfoldSelectInCurrBB(BasicBlock
*BB
) {
2563 // If threading this would thread across a loop header, don't thread the edge.
2564 // See the comments above FindLoopHeaders for justifications and caveats.
2565 if (LoopHeaders
.count(BB
))
2568 for (BasicBlock::iterator BI
= BB
->begin();
2569 PHINode
*PN
= dyn_cast
<PHINode
>(BI
); ++BI
) {
2570 // Look for a Phi having at least one constant incoming value.
2571 if (llvm::all_of(PN
->incoming_values(),
2572 [](Value
*V
) { return !isa
<ConstantInt
>(V
); }))
2575 auto isUnfoldCandidate
= [BB
](SelectInst
*SI
, Value
*V
) {
2576 // Check if SI is in BB and use V as condition.
2577 if (SI
->getParent() != BB
)
2579 Value
*Cond
= SI
->getCondition();
2580 return (Cond
&& Cond
== V
&& Cond
->getType()->isIntegerTy(1));
2583 SelectInst
*SI
= nullptr;
2584 for (Use
&U
: PN
->uses()) {
2585 if (ICmpInst
*Cmp
= dyn_cast
<ICmpInst
>(U
.getUser())) {
2586 // Look for a ICmp in BB that compares PN with a constant and is the
2587 // condition of a Select.
2588 if (Cmp
->getParent() == BB
&& Cmp
->hasOneUse() &&
2589 isa
<ConstantInt
>(Cmp
->getOperand(1 - U
.getOperandNo())))
2590 if (SelectInst
*SelectI
= dyn_cast
<SelectInst
>(Cmp
->user_back()))
2591 if (isUnfoldCandidate(SelectI
, Cmp
->use_begin()->get())) {
2595 } else if (SelectInst
*SelectI
= dyn_cast
<SelectInst
>(U
.getUser())) {
2596 // Look for a Select in BB that uses PN as condition.
2597 if (isUnfoldCandidate(SelectI
, U
.get())) {
2606 // Expand the select.
2608 SplitBlockAndInsertIfThen(SI
->getCondition(), SI
, false);
2609 BasicBlock
*SplitBB
= SI
->getParent();
2610 BasicBlock
*NewBB
= Term
->getParent();
2611 PHINode
*NewPN
= PHINode::Create(SI
->getType(), 2, "", SI
);
2612 NewPN
->addIncoming(SI
->getTrueValue(), Term
->getParent());
2613 NewPN
->addIncoming(SI
->getFalseValue(), BB
);
2614 SI
->replaceAllUsesWith(NewPN
);
2615 SI
->eraseFromParent();
2616 // NewBB and SplitBB are newly created blocks which require insertion.
2617 std::vector
<DominatorTree::UpdateType
> Updates
;
2618 Updates
.reserve((2 * SplitBB
->getTerminator()->getNumSuccessors()) + 3);
2619 Updates
.push_back({DominatorTree::Insert
, BB
, SplitBB
});
2620 Updates
.push_back({DominatorTree::Insert
, BB
, NewBB
});
2621 Updates
.push_back({DominatorTree::Insert
, NewBB
, SplitBB
});
2622 // BB's successors were moved to SplitBB, update DTU accordingly.
2623 for (auto *Succ
: successors(SplitBB
)) {
2624 Updates
.push_back({DominatorTree::Delete
, BB
, Succ
});
2625 Updates
.push_back({DominatorTree::Insert
, SplitBB
, Succ
});
2627 DTU
->applyUpdatesPermissive(Updates
);
2633 /// Try to propagate a guard from the current BB into one of its predecessors
2634 /// in case if another branch of execution implies that the condition of this
2635 /// guard is always true. Currently we only process the simplest case that
2640 /// br i1 %cond, label %T1, label %F1
2646 /// %condGuard = ...
2647 /// call void(i1, ...) @llvm.experimental.guard( i1 %condGuard )[ "deopt"() ]
2649 /// And cond either implies condGuard or !condGuard. In this case all the
2650 /// instructions before the guard can be duplicated in both branches, and the
2651 /// guard is then threaded to one of them.
2652 bool JumpThreadingPass::ProcessGuards(BasicBlock
*BB
) {
2653 using namespace PatternMatch
;
2655 // We only want to deal with two predecessors.
2656 BasicBlock
*Pred1
, *Pred2
;
2657 auto PI
= pred_begin(BB
), PE
= pred_end(BB
);
2669 // Try to thread one of the guards of the block.
2670 // TODO: Look up deeper than to immediate predecessor?
2671 auto *Parent
= Pred1
->getSinglePredecessor();
2672 if (!Parent
|| Parent
!= Pred2
->getSinglePredecessor())
2675 if (auto *BI
= dyn_cast
<BranchInst
>(Parent
->getTerminator()))
2677 if (isGuard(&I
) && ThreadGuard(BB
, cast
<IntrinsicInst
>(&I
), BI
))
2683 /// Try to propagate the guard from BB which is the lower block of a diamond
2684 /// to one of its branches, in case if diamond's condition implies guard's
2686 bool JumpThreadingPass::ThreadGuard(BasicBlock
*BB
, IntrinsicInst
*Guard
,
2688 assert(BI
->getNumSuccessors() == 2 && "Wrong number of successors?");
2689 assert(BI
->isConditional() && "Unconditional branch has 2 successors?");
2690 Value
*GuardCond
= Guard
->getArgOperand(0);
2691 Value
*BranchCond
= BI
->getCondition();
2692 BasicBlock
*TrueDest
= BI
->getSuccessor(0);
2693 BasicBlock
*FalseDest
= BI
->getSuccessor(1);
2695 auto &DL
= BB
->getModule()->getDataLayout();
2696 bool TrueDestIsSafe
= false;
2697 bool FalseDestIsSafe
= false;
2699 // True dest is safe if BranchCond => GuardCond.
2700 auto Impl
= isImpliedCondition(BranchCond
, GuardCond
, DL
);
2702 TrueDestIsSafe
= true;
2704 // False dest is safe if !BranchCond => GuardCond.
2705 Impl
= isImpliedCondition(BranchCond
, GuardCond
, DL
, /* LHSIsTrue */ false);
2707 FalseDestIsSafe
= true;
2710 if (!TrueDestIsSafe
&& !FalseDestIsSafe
)
2713 BasicBlock
*PredUnguardedBlock
= TrueDestIsSafe
? TrueDest
: FalseDest
;
2714 BasicBlock
*PredGuardedBlock
= FalseDestIsSafe
? TrueDest
: FalseDest
;
2716 ValueToValueMapTy UnguardedMapping
, GuardedMapping
;
2717 Instruction
*AfterGuard
= Guard
->getNextNode();
2718 unsigned Cost
= getJumpThreadDuplicationCost(BB
, AfterGuard
, BBDupThreshold
);
2719 if (Cost
> BBDupThreshold
)
2721 // Duplicate all instructions before the guard and the guard itself to the
2722 // branch where implication is not proved.
2723 BasicBlock
*GuardedBlock
= DuplicateInstructionsInSplitBetween(
2724 BB
, PredGuardedBlock
, AfterGuard
, GuardedMapping
, *DTU
);
2725 assert(GuardedBlock
&& "Could not create the guarded block?");
2726 // Duplicate all instructions before the guard in the unguarded branch.
2727 // Since we have successfully duplicated the guarded block and this block
2728 // has fewer instructions, we expect it to succeed.
2729 BasicBlock
*UnguardedBlock
= DuplicateInstructionsInSplitBetween(
2730 BB
, PredUnguardedBlock
, Guard
, UnguardedMapping
, *DTU
);
2731 assert(UnguardedBlock
&& "Could not create the unguarded block?");
2732 LLVM_DEBUG(dbgs() << "Moved guard " << *Guard
<< " to block "
2733 << GuardedBlock
->getName() << "\n");
2734 // Some instructions before the guard may still have uses. For them, we need
2735 // to create Phi nodes merging their copies in both guarded and unguarded
2736 // branches. Those instructions that have no uses can be just removed.
2737 SmallVector
<Instruction
*, 4> ToRemove
;
2738 for (auto BI
= BB
->begin(); &*BI
!= AfterGuard
; ++BI
)
2739 if (!isa
<PHINode
>(&*BI
))
2740 ToRemove
.push_back(&*BI
);
2742 Instruction
*InsertionPoint
= &*BB
->getFirstInsertionPt();
2743 assert(InsertionPoint
&& "Empty block?");
2744 // Substitute with Phis & remove.
2745 for (auto *Inst
: reverse(ToRemove
)) {
2746 if (!Inst
->use_empty()) {
2747 PHINode
*NewPN
= PHINode::Create(Inst
->getType(), 2);
2748 NewPN
->addIncoming(UnguardedMapping
[Inst
], UnguardedBlock
);
2749 NewPN
->addIncoming(GuardedMapping
[Inst
], GuardedBlock
);
2750 NewPN
->insertBefore(InsertionPoint
);
2751 Inst
->replaceAllUsesWith(NewPN
);
2753 Inst
->eraseFromParent();