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[llvm/stm8.git] / lib / Transforms / Scalar / LoopUnswitch.cpp
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1 //===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass transforms loops that contain branches on loop-invariant conditions
11 // to have multiple loops. For example, it turns the left into the right code:
13 // for (...) if (lic)
14 // A for (...)
15 // if (lic) A; B; C
16 // B else
17 // C for (...)
18 // A; C
20 // This can increase the size of the code exponentially (doubling it every time
21 // a loop is unswitched) so we only unswitch if the resultant code will be
22 // smaller than a threshold.
24 // This pass expects LICM to be run before it to hoist invariant conditions out
25 // of the loop, to make the unswitching opportunity obvious.
27 //===----------------------------------------------------------------------===//
29 #define DEBUG_TYPE "loop-unswitch"
30 #include "llvm/Transforms/Scalar.h"
31 #include "llvm/Constants.h"
32 #include "llvm/DerivedTypes.h"
33 #include "llvm/Function.h"
34 #include "llvm/Instructions.h"
35 #include "llvm/Analysis/InlineCost.h"
36 #include "llvm/Analysis/InstructionSimplify.h"
37 #include "llvm/Analysis/LoopInfo.h"
38 #include "llvm/Analysis/LoopPass.h"
39 #include "llvm/Analysis/Dominators.h"
40 #include "llvm/Analysis/ScalarEvolution.h"
41 #include "llvm/Transforms/Utils/Cloning.h"
42 #include "llvm/Transforms/Utils/Local.h"
43 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
44 #include "llvm/ADT/Statistic.h"
45 #include "llvm/ADT/SmallPtrSet.h"
46 #include "llvm/ADT/STLExtras.h"
47 #include "llvm/Support/CommandLine.h"
48 #include "llvm/Support/Debug.h"
49 #include "llvm/Support/raw_ostream.h"
50 #include <algorithm>
51 #include <set>
52 using namespace llvm;
54 STATISTIC(NumBranches, "Number of branches unswitched");
55 STATISTIC(NumSwitches, "Number of switches unswitched");
56 STATISTIC(NumSelects , "Number of selects unswitched");
57 STATISTIC(NumTrivial , "Number of unswitches that are trivial");
58 STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
60 // The specific value of 50 here was chosen based only on intuition and a
61 // few specific examples.
62 static cl::opt<unsigned>
63 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
64 cl::init(50), cl::Hidden);
66 namespace {
67 class LoopUnswitch : public LoopPass {
68 LoopInfo *LI; // Loop information
69 LPPassManager *LPM;
71 // LoopProcessWorklist - Used to check if second loop needs processing
72 // after RewriteLoopBodyWithConditionConstant rewrites first loop.
73 std::vector<Loop*> LoopProcessWorklist;
74 SmallPtrSet<Value *,8> UnswitchedVals;
76 bool OptimizeForSize;
77 bool redoLoop;
79 Loop *currentLoop;
80 DominatorTree *DT;
81 BasicBlock *loopHeader;
82 BasicBlock *loopPreheader;
84 // LoopBlocks contains all of the basic blocks of the loop, including the
85 // preheader of the loop, the body of the loop, and the exit blocks of the
86 // loop, in that order.
87 std::vector<BasicBlock*> LoopBlocks;
88 // NewBlocks contained cloned copy of basic blocks from LoopBlocks.
89 std::vector<BasicBlock*> NewBlocks;
91 public:
92 static char ID; // Pass ID, replacement for typeid
93 explicit LoopUnswitch(bool Os = false) :
94 LoopPass(ID), OptimizeForSize(Os), redoLoop(false),
95 currentLoop(NULL), DT(NULL), loopHeader(NULL),
96 loopPreheader(NULL) {
97 initializeLoopUnswitchPass(*PassRegistry::getPassRegistry());
100 bool runOnLoop(Loop *L, LPPassManager &LPM);
101 bool processCurrentLoop();
103 /// This transformation requires natural loop information & requires that
104 /// loop preheaders be inserted into the CFG.
106 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
107 AU.addRequiredID(LoopSimplifyID);
108 AU.addPreservedID(LoopSimplifyID);
109 AU.addRequired<LoopInfo>();
110 AU.addPreserved<LoopInfo>();
111 AU.addRequiredID(LCSSAID);
112 AU.addPreservedID(LCSSAID);
113 AU.addPreserved<DominatorTree>();
114 AU.addPreserved<ScalarEvolution>();
117 private:
119 virtual void releaseMemory() {
120 UnswitchedVals.clear();
123 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist,
124 /// remove it.
125 void RemoveLoopFromWorklist(Loop *L) {
126 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(),
127 LoopProcessWorklist.end(), L);
128 if (I != LoopProcessWorklist.end())
129 LoopProcessWorklist.erase(I);
132 void initLoopData() {
133 loopHeader = currentLoop->getHeader();
134 loopPreheader = currentLoop->getLoopPreheader();
137 /// Split all of the edges from inside the loop to their exit blocks.
138 /// Update the appropriate Phi nodes as we do so.
139 void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks);
141 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val);
142 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
143 BasicBlock *ExitBlock);
144 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
146 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
147 Constant *Val, bool isEqual);
149 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
150 BasicBlock *TrueDest,
151 BasicBlock *FalseDest,
152 Instruction *InsertPt);
154 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
155 void RemoveBlockIfDead(BasicBlock *BB,
156 std::vector<Instruction*> &Worklist, Loop *l);
157 void RemoveLoopFromHierarchy(Loop *L);
158 bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0,
159 BasicBlock **LoopExit = 0);
163 char LoopUnswitch::ID = 0;
164 INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops",
165 false, false)
166 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
167 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
168 INITIALIZE_PASS_DEPENDENCY(LCSSA)
169 INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops",
170 false, false)
172 Pass *llvm::createLoopUnswitchPass(bool Os) {
173 return new LoopUnswitch(Os);
176 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
177 /// invariant in the loop, or has an invariant piece, return the invariant.
178 /// Otherwise, return null.
179 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
180 // We can never unswitch on vector conditions.
181 if (Cond->getType()->isVectorTy())
182 return 0;
184 // Constants should be folded, not unswitched on!
185 if (isa<Constant>(Cond)) return 0;
187 // TODO: Handle: br (VARIANT|INVARIANT).
189 // Hoist simple values out.
190 if (L->makeLoopInvariant(Cond, Changed))
191 return Cond;
193 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
194 if (BO->getOpcode() == Instruction::And ||
195 BO->getOpcode() == Instruction::Or) {
196 // If either the left or right side is invariant, we can unswitch on this,
197 // which will cause the branch to go away in one loop and the condition to
198 // simplify in the other one.
199 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
200 return LHS;
201 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
202 return RHS;
205 return 0;
208 bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
209 LI = &getAnalysis<LoopInfo>();
210 LPM = &LPM_Ref;
211 DT = getAnalysisIfAvailable<DominatorTree>();
212 currentLoop = L;
213 Function *F = currentLoop->getHeader()->getParent();
214 bool Changed = false;
215 do {
216 assert(currentLoop->isLCSSAForm(*DT));
217 redoLoop = false;
218 Changed |= processCurrentLoop();
219 } while(redoLoop);
221 if (Changed) {
222 // FIXME: Reconstruct dom info, because it is not preserved properly.
223 if (DT)
224 DT->runOnFunction(*F);
226 return Changed;
229 /// processCurrentLoop - Do actual work and unswitch loop if possible
230 /// and profitable.
231 bool LoopUnswitch::processCurrentLoop() {
232 bool Changed = false;
233 LLVMContext &Context = currentLoop->getHeader()->getContext();
235 // Loop over all of the basic blocks in the loop. If we find an interior
236 // block that is branching on a loop-invariant condition, we can unswitch this
237 // loop.
238 for (Loop::block_iterator I = currentLoop->block_begin(),
239 E = currentLoop->block_end(); I != E; ++I) {
240 TerminatorInst *TI = (*I)->getTerminator();
241 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
242 // If this isn't branching on an invariant condition, we can't unswitch
243 // it.
244 if (BI->isConditional()) {
245 // See if this, or some part of it, is loop invariant. If so, we can
246 // unswitch on it if we desire.
247 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
248 currentLoop, Changed);
249 if (LoopCond && UnswitchIfProfitable(LoopCond,
250 ConstantInt::getTrue(Context))) {
251 ++NumBranches;
252 return true;
255 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
256 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
257 currentLoop, Changed);
258 if (LoopCond && SI->getNumCases() > 1) {
259 // Find a value to unswitch on:
260 // FIXME: this should chose the most expensive case!
261 Constant *UnswitchVal = SI->getCaseValue(1);
262 // Do not process same value again and again.
263 if (!UnswitchedVals.insert(UnswitchVal))
264 continue;
266 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) {
267 ++NumSwitches;
268 return true;
273 // Scan the instructions to check for unswitchable values.
274 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
275 BBI != E; ++BBI)
276 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
277 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
278 currentLoop, Changed);
279 if (LoopCond && UnswitchIfProfitable(LoopCond,
280 ConstantInt::getTrue(Context))) {
281 ++NumSelects;
282 return true;
286 return Changed;
289 /// isTrivialLoopExitBlock - Check to see if all paths from BB exit the
290 /// loop with no side effects (including infinite loops).
292 /// If true, we return true and set ExitBB to the block we
293 /// exit through.
295 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
296 BasicBlock *&ExitBB,
297 std::set<BasicBlock*> &Visited) {
298 if (!Visited.insert(BB).second) {
299 // Already visited. Without more analysis, this could indicate an infinte loop.
300 return false;
301 } else if (!L->contains(BB)) {
302 // Otherwise, this is a loop exit, this is fine so long as this is the
303 // first exit.
304 if (ExitBB != 0) return false;
305 ExitBB = BB;
306 return true;
309 // Otherwise, this is an unvisited intra-loop node. Check all successors.
310 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
311 // Check to see if the successor is a trivial loop exit.
312 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
313 return false;
316 // Okay, everything after this looks good, check to make sure that this block
317 // doesn't include any side effects.
318 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
319 if (I->mayHaveSideEffects())
320 return false;
322 return true;
325 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally
326 /// leads to an exit from the specified loop, and has no side-effects in the
327 /// process. If so, return the block that is exited to, otherwise return null.
328 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
329 std::set<BasicBlock*> Visited;
330 Visited.insert(L->getHeader()); // Branches to header make infinite loops.
331 BasicBlock *ExitBB = 0;
332 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
333 return ExitBB;
334 return 0;
337 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
338 /// trivial: that is, that the condition controls whether or not the loop does
339 /// anything at all. If this is a trivial condition, unswitching produces no
340 /// code duplications (equivalently, it produces a simpler loop and a new empty
341 /// loop, which gets deleted).
343 /// If this is a trivial condition, return true, otherwise return false. When
344 /// returning true, this sets Cond and Val to the condition that controls the
345 /// trivial condition: when Cond dynamically equals Val, the loop is known to
346 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when
347 /// Cond == Val.
349 bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
350 BasicBlock **LoopExit) {
351 BasicBlock *Header = currentLoop->getHeader();
352 TerminatorInst *HeaderTerm = Header->getTerminator();
353 LLVMContext &Context = Header->getContext();
355 BasicBlock *LoopExitBB = 0;
356 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
357 // If the header block doesn't end with a conditional branch on Cond, we
358 // can't handle it.
359 if (!BI->isConditional() || BI->getCondition() != Cond)
360 return false;
362 // Check to see if a successor of the branch is guaranteed to
363 // exit through a unique exit block without having any
364 // side-effects. If so, determine the value of Cond that causes it to do
365 // this.
366 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
367 BI->getSuccessor(0)))) {
368 if (Val) *Val = ConstantInt::getTrue(Context);
369 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
370 BI->getSuccessor(1)))) {
371 if (Val) *Val = ConstantInt::getFalse(Context);
373 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
374 // If this isn't a switch on Cond, we can't handle it.
375 if (SI->getCondition() != Cond) return false;
377 // Check to see if a successor of the switch is guaranteed to go to the
378 // latch block or exit through a one exit block without having any
379 // side-effects. If so, determine the value of Cond that causes it to do
380 // this. Note that we can't trivially unswitch on the default case.
381 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
382 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
383 SI->getSuccessor(i)))) {
384 // Okay, we found a trivial case, remember the value that is trivial.
385 if (Val) *Val = SI->getCaseValue(i);
386 break;
390 // If we didn't find a single unique LoopExit block, or if the loop exit block
391 // contains phi nodes, this isn't trivial.
392 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
393 return false; // Can't handle this.
395 if (LoopExit) *LoopExit = LoopExitBB;
397 // We already know that nothing uses any scalar values defined inside of this
398 // loop. As such, we just have to check to see if this loop will execute any
399 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
400 // part of the loop that the code *would* execute. We already checked the
401 // tail, check the header now.
402 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
403 if (I->mayHaveSideEffects())
404 return false;
405 return true;
408 /// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
409 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
410 /// unswitch the loop, reprocess the pieces, then return true.
411 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val) {
413 initLoopData();
415 // If LoopSimplify was unable to form a preheader, don't do any unswitching.
416 if (!loopPreheader)
417 return false;
419 Function *F = loopHeader->getParent();
421 Constant *CondVal = 0;
422 BasicBlock *ExitBlock = 0;
423 if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
424 // If the condition is trivial, always unswitch. There is no code growth
425 // for this case.
426 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock);
427 return true;
430 // Check to see if it would be profitable to unswitch current loop.
432 // Do not do non-trivial unswitch while optimizing for size.
433 if (OptimizeForSize || F->hasFnAttr(Attribute::OptimizeForSize))
434 return false;
436 // FIXME: This is overly conservative because it does not take into
437 // consideration code simplification opportunities and code that can
438 // be shared by the resultant unswitched loops.
439 CodeMetrics Metrics;
440 for (Loop::block_iterator I = currentLoop->block_begin(),
441 E = currentLoop->block_end();
442 I != E; ++I)
443 Metrics.analyzeBasicBlock(*I);
445 // Limit the number of instructions to avoid causing significant code
446 // expansion, and the number of basic blocks, to avoid loops with
447 // large numbers of branches which cause loop unswitching to go crazy.
448 // This is a very ad-hoc heuristic.
449 if (Metrics.NumInsts > Threshold ||
450 Metrics.NumBlocks * 5 > Threshold ||
451 Metrics.containsIndirectBr || Metrics.isRecursive) {
452 DEBUG(dbgs() << "NOT unswitching loop %"
453 << currentLoop->getHeader()->getName() << ", cost too high: "
454 << currentLoop->getBlocks().size() << "\n");
455 return false;
458 UnswitchNontrivialCondition(LoopCond, Val, currentLoop);
459 return true;
462 /// CloneLoop - Recursively clone the specified loop and all of its children,
463 /// mapping the blocks with the specified map.
464 static Loop *CloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM,
465 LoopInfo *LI, LPPassManager *LPM) {
466 Loop *New = new Loop();
467 LPM->insertLoop(New, PL);
469 // Add all of the blocks in L to the new loop.
470 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
471 I != E; ++I)
472 if (LI->getLoopFor(*I) == L)
473 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), LI->getBase());
475 // Add all of the subloops to the new loop.
476 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
477 CloneLoop(*I, New, VM, LI, LPM);
479 return New;
482 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
483 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
484 /// code immediately before InsertPt.
485 void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
486 BasicBlock *TrueDest,
487 BasicBlock *FalseDest,
488 Instruction *InsertPt) {
489 // Insert a conditional branch on LIC to the two preheaders. The original
490 // code is the true version and the new code is the false version.
491 Value *BranchVal = LIC;
492 if (!isa<ConstantInt>(Val) ||
493 Val->getType() != Type::getInt1Ty(LIC->getContext()))
494 BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val, "tmp");
495 else if (Val != ConstantInt::getTrue(Val->getContext()))
496 // We want to enter the new loop when the condition is true.
497 std::swap(TrueDest, FalseDest);
499 // Insert the new branch.
500 BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt);
502 // If either edge is critical, split it. This helps preserve LoopSimplify
503 // form for enclosing loops.
504 SplitCriticalEdge(BI, 0, this);
505 SplitCriticalEdge(BI, 1, this);
508 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
509 /// condition in it (a cond branch from its header block to its latch block,
510 /// where the path through the loop that doesn't execute its body has no
511 /// side-effects), unswitch it. This doesn't involve any code duplication, just
512 /// moving the conditional branch outside of the loop and updating loop info.
513 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
514 Constant *Val,
515 BasicBlock *ExitBlock) {
516 DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
517 << loopHeader->getName() << " [" << L->getBlocks().size()
518 << " blocks] in Function " << L->getHeader()->getParent()->getName()
519 << " on cond: " << *Val << " == " << *Cond << "\n");
521 // First step, split the preheader, so that we know that there is a safe place
522 // to insert the conditional branch. We will change loopPreheader to have a
523 // conditional branch on Cond.
524 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this);
526 // Now that we have a place to insert the conditional branch, create a place
527 // to branch to: this is the exit block out of the loop that we should
528 // short-circuit to.
530 // Split this block now, so that the loop maintains its exit block, and so
531 // that the jump from the preheader can execute the contents of the exit block
532 // without actually branching to it (the exit block should be dominated by the
533 // loop header, not the preheader).
534 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
535 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this);
537 // Okay, now we have a position to branch from and a position to branch to,
538 // insert the new conditional branch.
539 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
540 loopPreheader->getTerminator());
541 LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L);
542 loopPreheader->getTerminator()->eraseFromParent();
544 // We need to reprocess this loop, it could be unswitched again.
545 redoLoop = true;
547 // Now that we know that the loop is never entered when this condition is a
548 // particular value, rewrite the loop with this info. We know that this will
549 // at least eliminate the old branch.
550 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
551 ++NumTrivial;
554 /// SplitExitEdges - Split all of the edges from inside the loop to their exit
555 /// blocks. Update the appropriate Phi nodes as we do so.
556 void LoopUnswitch::SplitExitEdges(Loop *L,
557 const SmallVector<BasicBlock *, 8> &ExitBlocks){
559 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
560 BasicBlock *ExitBlock = ExitBlocks[i];
561 SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
562 pred_end(ExitBlock));
563 SplitBlockPredecessors(ExitBlock, Preds.data(), Preds.size(),
564 ".us-lcssa", this);
568 /// UnswitchNontrivialCondition - We determined that the loop is profitable
569 /// to unswitch when LIC equal Val. Split it into loop versions and test the
570 /// condition outside of either loop. Return the loops created as Out1/Out2.
571 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
572 Loop *L) {
573 Function *F = loopHeader->getParent();
574 DEBUG(dbgs() << "loop-unswitch: Unswitching loop %"
575 << loopHeader->getName() << " [" << L->getBlocks().size()
576 << " blocks] in Function " << F->getName()
577 << " when '" << *Val << "' == " << *LIC << "\n");
579 if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>())
580 SE->forgetLoop(L);
582 LoopBlocks.clear();
583 NewBlocks.clear();
585 // First step, split the preheader and exit blocks, and add these blocks to
586 // the LoopBlocks list.
587 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this);
588 LoopBlocks.push_back(NewPreheader);
590 // We want the loop to come after the preheader, but before the exit blocks.
591 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
593 SmallVector<BasicBlock*, 8> ExitBlocks;
594 L->getUniqueExitBlocks(ExitBlocks);
596 // Split all of the edges from inside the loop to their exit blocks. Update
597 // the appropriate Phi nodes as we do so.
598 SplitExitEdges(L, ExitBlocks);
600 // The exit blocks may have been changed due to edge splitting, recompute.
601 ExitBlocks.clear();
602 L->getUniqueExitBlocks(ExitBlocks);
604 // Add exit blocks to the loop blocks.
605 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
607 // Next step, clone all of the basic blocks that make up the loop (including
608 // the loop preheader and exit blocks), keeping track of the mapping between
609 // the instructions and blocks.
610 NewBlocks.reserve(LoopBlocks.size());
611 ValueToValueMapTy VMap;
612 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
613 BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], VMap, ".us", F);
614 NewBlocks.push_back(NewBB);
615 VMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping.
616 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L);
619 // Splice the newly inserted blocks into the function right before the
620 // original preheader.
621 F->getBasicBlockList().splice(NewPreheader, F->getBasicBlockList(),
622 NewBlocks[0], F->end());
624 // Now we create the new Loop object for the versioned loop.
625 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), VMap, LI, LPM);
626 Loop *ParentLoop = L->getParentLoop();
627 if (ParentLoop) {
628 // Make sure to add the cloned preheader and exit blocks to the parent loop
629 // as well.
630 ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase());
633 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
634 BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[i]]);
635 // The new exit block should be in the same loop as the old one.
636 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
637 ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase());
639 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
640 "Exit block should have been split to have one successor!");
641 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
643 // If the successor of the exit block had PHI nodes, add an entry for
644 // NewExit.
645 PHINode *PN;
646 for (BasicBlock::iterator I = ExitSucc->begin(); isa<PHINode>(I); ++I) {
647 PN = cast<PHINode>(I);
648 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
649 ValueToValueMapTy::iterator It = VMap.find(V);
650 if (It != VMap.end()) V = It->second;
651 PN->addIncoming(V, NewExit);
655 // Rewrite the code to refer to itself.
656 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
657 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
658 E = NewBlocks[i]->end(); I != E; ++I)
659 RemapInstruction(I, VMap,RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
661 // Rewrite the original preheader to select between versions of the loop.
662 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
663 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
664 "Preheader splitting did not work correctly!");
666 // Emit the new branch that selects between the two versions of this loop.
667 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
668 LPM->deleteSimpleAnalysisValue(OldBR, L);
669 OldBR->eraseFromParent();
671 LoopProcessWorklist.push_back(NewLoop);
672 redoLoop = true;
674 // Keep a WeakVH holding onto LIC. If the first call to RewriteLoopBody
675 // deletes the instruction (for example by simplifying a PHI that feeds into
676 // the condition that we're unswitching on), we don't rewrite the second
677 // iteration.
678 WeakVH LICHandle(LIC);
680 // Now we rewrite the original code to know that the condition is true and the
681 // new code to know that the condition is false.
682 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
684 // It's possible that simplifying one loop could cause the other to be
685 // changed to another value or a constant. If its a constant, don't simplify
686 // it.
687 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
688 LICHandle && !isa<Constant>(LICHandle))
689 RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true);
692 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
693 /// specified.
694 static void RemoveFromWorklist(Instruction *I,
695 std::vector<Instruction*> &Worklist) {
696 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
697 Worklist.end(), I);
698 while (WI != Worklist.end()) {
699 unsigned Offset = WI-Worklist.begin();
700 Worklist.erase(WI);
701 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
705 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
706 /// program, replacing all uses with V and update the worklist.
707 static void ReplaceUsesOfWith(Instruction *I, Value *V,
708 std::vector<Instruction*> &Worklist,
709 Loop *L, LPPassManager *LPM) {
710 DEBUG(dbgs() << "Replace with '" << *V << "': " << *I);
712 // Add uses to the worklist, which may be dead now.
713 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
714 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
715 Worklist.push_back(Use);
717 // Add users to the worklist which may be simplified now.
718 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
719 UI != E; ++UI)
720 Worklist.push_back(cast<Instruction>(*UI));
721 LPM->deleteSimpleAnalysisValue(I, L);
722 RemoveFromWorklist(I, Worklist);
723 I->replaceAllUsesWith(V);
724 I->eraseFromParent();
725 ++NumSimplify;
728 /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
729 /// information, and remove any dead successors it has.
731 void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
732 std::vector<Instruction*> &Worklist,
733 Loop *L) {
734 if (pred_begin(BB) != pred_end(BB)) {
735 // This block isn't dead, since an edge to BB was just removed, see if there
736 // are any easy simplifications we can do now.
737 if (BasicBlock *Pred = BB->getSinglePredecessor()) {
738 // If it has one pred, fold phi nodes in BB.
739 while (isa<PHINode>(BB->begin()))
740 ReplaceUsesOfWith(BB->begin(),
741 cast<PHINode>(BB->begin())->getIncomingValue(0),
742 Worklist, L, LPM);
744 // If this is the header of a loop and the only pred is the latch, we now
745 // have an unreachable loop.
746 if (Loop *L = LI->getLoopFor(BB))
747 if (loopHeader == BB && L->contains(Pred)) {
748 // Remove the branch from the latch to the header block, this makes
749 // the header dead, which will make the latch dead (because the header
750 // dominates the latch).
751 LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L);
752 Pred->getTerminator()->eraseFromParent();
753 new UnreachableInst(BB->getContext(), Pred);
755 // The loop is now broken, remove it from LI.
756 RemoveLoopFromHierarchy(L);
758 // Reprocess the header, which now IS dead.
759 RemoveBlockIfDead(BB, Worklist, L);
760 return;
763 // If pred ends in a uncond branch, add uncond branch to worklist so that
764 // the two blocks will get merged.
765 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
766 if (BI->isUnconditional())
767 Worklist.push_back(BI);
769 return;
772 DEBUG(dbgs() << "Nuking dead block: " << *BB);
774 // Remove the instructions in the basic block from the worklist.
775 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
776 RemoveFromWorklist(I, Worklist);
778 // Anything that uses the instructions in this basic block should have their
779 // uses replaced with undefs.
780 // If I is not void type then replaceAllUsesWith undef.
781 // This allows ValueHandlers and custom metadata to adjust itself.
782 if (!I->getType()->isVoidTy())
783 I->replaceAllUsesWith(UndefValue::get(I->getType()));
786 // If this is the edge to the header block for a loop, remove the loop and
787 // promote all subloops.
788 if (Loop *BBLoop = LI->getLoopFor(BB)) {
789 if (BBLoop->getLoopLatch() == BB)
790 RemoveLoopFromHierarchy(BBLoop);
793 // Remove the block from the loop info, which removes it from any loops it
794 // was in.
795 LI->removeBlock(BB);
798 // Remove phi node entries in successors for this block.
799 TerminatorInst *TI = BB->getTerminator();
800 SmallVector<BasicBlock*, 4> Succs;
801 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
802 Succs.push_back(TI->getSuccessor(i));
803 TI->getSuccessor(i)->removePredecessor(BB);
806 // Unique the successors, remove anything with multiple uses.
807 array_pod_sort(Succs.begin(), Succs.end());
808 Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end());
810 // Remove the basic block, including all of the instructions contained in it.
811 LPM->deleteSimpleAnalysisValue(BB, L);
812 BB->eraseFromParent();
813 // Remove successor blocks here that are not dead, so that we know we only
814 // have dead blocks in this list. Nondead blocks have a way of becoming dead,
815 // then getting removed before we revisit them, which is badness.
817 for (unsigned i = 0; i != Succs.size(); ++i)
818 if (pred_begin(Succs[i]) != pred_end(Succs[i])) {
819 // One exception is loop headers. If this block was the preheader for a
820 // loop, then we DO want to visit the loop so the loop gets deleted.
821 // We know that if the successor is a loop header, that this loop had to
822 // be the preheader: the case where this was the latch block was handled
823 // above and headers can only have two predecessors.
824 if (!LI->isLoopHeader(Succs[i])) {
825 Succs.erase(Succs.begin()+i);
826 --i;
830 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
831 RemoveBlockIfDead(Succs[i], Worklist, L);
834 /// RemoveLoopFromHierarchy - We have discovered that the specified loop has
835 /// become unwrapped, either because the backedge was deleted, or because the
836 /// edge into the header was removed. If the edge into the header from the
837 /// latch block was removed, the loop is unwrapped but subloops are still alive,
838 /// so they just reparent loops. If the loops are actually dead, they will be
839 /// removed later.
840 void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) {
841 LPM->deleteLoopFromQueue(L);
842 RemoveLoopFromWorklist(L);
845 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
846 // the value specified by Val in the specified loop, or we know it does NOT have
847 // that value. Rewrite any uses of LIC or of properties correlated to it.
848 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
849 Constant *Val,
850 bool IsEqual) {
851 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
853 // FIXME: Support correlated properties, like:
854 // for (...)
855 // if (li1 < li2)
856 // ...
857 // if (li1 > li2)
858 // ...
860 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
861 // selects, switches.
862 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
863 std::vector<Instruction*> Worklist;
864 LLVMContext &Context = Val->getContext();
867 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
868 // in the loop with the appropriate one directly.
869 if (IsEqual || (isa<ConstantInt>(Val) &&
870 Val->getType()->isIntegerTy(1))) {
871 Value *Replacement;
872 if (IsEqual)
873 Replacement = Val;
874 else
875 Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
876 !cast<ConstantInt>(Val)->getZExtValue());
878 for (unsigned i = 0, e = Users.size(); i != e; ++i)
879 if (Instruction *U = cast<Instruction>(Users[i])) {
880 if (!L->contains(U))
881 continue;
882 U->replaceUsesOfWith(LIC, Replacement);
883 Worklist.push_back(U);
885 SimplifyCode(Worklist, L);
886 return;
889 // Otherwise, we don't know the precise value of LIC, but we do know that it
890 // is certainly NOT "Val". As such, simplify any uses in the loop that we
891 // can. This case occurs when we unswitch switch statements.
892 for (unsigned i = 0, e = Users.size(); i != e; ++i) {
893 Instruction *U = cast<Instruction>(Users[i]);
894 if (!L->contains(U))
895 continue;
897 Worklist.push_back(U);
899 // TODO: We could do other simplifications, for example, turning
900 // 'icmp eq LIC, Val' -> false.
902 // If we know that LIC is not Val, use this info to simplify code.
903 SwitchInst *SI = dyn_cast<SwitchInst>(U);
904 if (SI == 0 || !isa<ConstantInt>(Val)) continue;
906 unsigned DeadCase = SI->findCaseValue(cast<ConstantInt>(Val));
907 if (DeadCase == 0) continue; // Default case is live for multiple values.
909 // Found a dead case value. Don't remove PHI nodes in the
910 // successor if they become single-entry, those PHI nodes may
911 // be in the Users list.
913 // FIXME: This is a hack. We need to keep the successor around
914 // and hooked up so as to preserve the loop structure, because
915 // trying to update it is complicated. So instead we preserve the
916 // loop structure and put the block on a dead code path.
917 BasicBlock *Switch = SI->getParent();
918 SplitEdge(Switch, SI->getSuccessor(DeadCase), this);
919 // Compute the successors instead of relying on the return value
920 // of SplitEdge, since it may have split the switch successor
921 // after PHI nodes.
922 BasicBlock *NewSISucc = SI->getSuccessor(DeadCase);
923 BasicBlock *OldSISucc = *succ_begin(NewSISucc);
924 // Create an "unreachable" destination.
925 BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
926 Switch->getParent(),
927 OldSISucc);
928 new UnreachableInst(Context, Abort);
929 // Force the new case destination to branch to the "unreachable"
930 // block while maintaining a (dead) CFG edge to the old block.
931 NewSISucc->getTerminator()->eraseFromParent();
932 BranchInst::Create(Abort, OldSISucc,
933 ConstantInt::getTrue(Context), NewSISucc);
934 // Release the PHI operands for this edge.
935 for (BasicBlock::iterator II = NewSISucc->begin();
936 PHINode *PN = dyn_cast<PHINode>(II); ++II)
937 PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
938 UndefValue::get(PN->getType()));
939 // Tell the domtree about the new block. We don't fully update the
940 // domtree here -- instead we force it to do a full recomputation
941 // after the pass is complete -- but we do need to inform it of
942 // new blocks.
943 if (DT)
944 DT->addNewBlock(Abort, NewSISucc);
947 SimplifyCode(Worklist, L);
950 /// SimplifyCode - Okay, now that we have simplified some instructions in the
951 /// loop, walk over it and constant prop, dce, and fold control flow where
952 /// possible. Note that this is effectively a very simple loop-structure-aware
953 /// optimizer. During processing of this loop, L could very well be deleted, so
954 /// it must not be used.
956 /// FIXME: When the loop optimizer is more mature, separate this out to a new
957 /// pass.
959 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
960 while (!Worklist.empty()) {
961 Instruction *I = Worklist.back();
962 Worklist.pop_back();
964 // Simple DCE.
965 if (isInstructionTriviallyDead(I)) {
966 DEBUG(dbgs() << "Remove dead instruction '" << *I);
968 // Add uses to the worklist, which may be dead now.
969 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
970 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
971 Worklist.push_back(Use);
972 LPM->deleteSimpleAnalysisValue(I, L);
973 RemoveFromWorklist(I, Worklist);
974 I->eraseFromParent();
975 ++NumSimplify;
976 continue;
979 // See if instruction simplification can hack this up. This is common for
980 // things like "select false, X, Y" after unswitching made the condition be
981 // 'false'.
982 if (Value *V = SimplifyInstruction(I, 0, DT))
983 if (LI->replacementPreservesLCSSAForm(I, V)) {
984 ReplaceUsesOfWith(I, V, Worklist, L, LPM);
985 continue;
988 // Special case hacks that appear commonly in unswitched code.
989 if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
990 if (BI->isUnconditional()) {
991 // If BI's parent is the only pred of the successor, fold the two blocks
992 // together.
993 BasicBlock *Pred = BI->getParent();
994 BasicBlock *Succ = BI->getSuccessor(0);
995 BasicBlock *SinglePred = Succ->getSinglePredecessor();
996 if (!SinglePred) continue; // Nothing to do.
997 assert(SinglePred == Pred && "CFG broken");
999 DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- "
1000 << Succ->getName() << "\n");
1002 // Resolve any single entry PHI nodes in Succ.
1003 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1004 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM);
1006 // Move all of the successor contents from Succ to Pred.
1007 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1008 Succ->end());
1009 LPM->deleteSimpleAnalysisValue(BI, L);
1010 BI->eraseFromParent();
1011 RemoveFromWorklist(BI, Worklist);
1013 // If Succ has any successors with PHI nodes, update them to have
1014 // entries coming from Pred instead of Succ.
1015 Succ->replaceAllUsesWith(Pred);
1017 // Remove Succ from the loop tree.
1018 LI->removeBlock(Succ);
1019 LPM->deleteSimpleAnalysisValue(Succ, L);
1020 Succ->eraseFromParent();
1021 ++NumSimplify;
1022 continue;
1025 if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){
1026 // Conditional branch. Turn it into an unconditional branch, then
1027 // remove dead blocks.
1028 continue; // FIXME: Enable.
1030 DEBUG(dbgs() << "Folded branch: " << *BI);
1031 BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue());
1032 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue());
1033 DeadSucc->removePredecessor(BI->getParent(), true);
1034 Worklist.push_back(BranchInst::Create(LiveSucc, BI));
1035 LPM->deleteSimpleAnalysisValue(BI, L);
1036 BI->eraseFromParent();
1037 RemoveFromWorklist(BI, Worklist);
1038 ++NumSimplify;
1040 RemoveBlockIfDead(DeadSucc, Worklist, L);
1042 continue;