lib/Transforms/Scalar/LowerExpectIntrinsic.cpp

   1 //===- LowerExpectIntrinsic.cpp - Lower expect intrinsic ------------------===//
   2 //
   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
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This pass lowers the 'expect' intrinsic to LLVM metadata.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/Transforms/Scalar/LowerExpectIntrinsic.h"
  14 #include "llvm/ADT/SmallVector.h"
  15 #include "llvm/ADT/Statistic.h"
  16 #include "llvm/ADT/iterator_range.h"
  17 #include "llvm/IR/BasicBlock.h"
  18 #include "llvm/IR/Constants.h"
  19 #include "llvm/IR/Function.h"
  20 #include "llvm/IR/Instructions.h"
  21 #include "llvm/IR/Intrinsics.h"
  22 #include "llvm/IR/LLVMContext.h"
  23 #include "llvm/IR/MDBuilder.h"
  24 #include "llvm/IR/Metadata.h"
  25 #include "llvm/Pass.h"
  26 #include "llvm/Support/CommandLine.h"
  27 #include "llvm/Support/Debug.h"
  28 #include "llvm/Transforms/Scalar.h"
  29
  30 using namespace llvm;
  31
  32 #define DEBUG_TYPE "lower-expect-intrinsic"
  33
  34 STATISTIC(ExpectIntrinsicsHandled,
  35           "Number of 'expect' intrinsic instructions handled");
  36
  37 // These default values are chosen to represent an extremely skewed outcome for
  38 // a condition, but they leave some room for interpretation by later passes.
  39 //
  40 // If the documentation for __builtin_expect() was made explicit that it should
  41 // only be used in extreme cases, we could make this ratio higher. As it stands,
  42 // programmers may be using __builtin_expect() / llvm.expect to annotate that a
  43 // branch is likely or unlikely to be taken.
  44 //
  45 // There is a known dependency on this ratio in CodeGenPrepare when transforming
  46 // 'select' instructions. It may be worthwhile to hoist these values to some
  47 // shared space, so they can be used directly by other passes.
  48
  49 static cl::opt<uint32_t> LikelyBranchWeight(
  50     "likely-branch-weight", cl::Hidden, cl::init(2000),
  51     cl::desc("Weight of the branch likely to be taken (default = 2000)"));
  52 static cl::opt<uint32_t> UnlikelyBranchWeight(
  53     "unlikely-branch-weight", cl::Hidden, cl::init(1),
  54     cl::desc("Weight of the branch unlikely to be taken (default = 1)"));
  55
  56 static bool handleSwitchExpect(SwitchInst &SI) {
  57   CallInst *CI = dyn_cast<CallInst>(SI.getCondition());
  58   if (!CI)
  59     return false;
  60
  61   Function *Fn = CI->getCalledFunction();
  62   if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
  63     return false;
  64
  65   Value *ArgValue = CI->getArgOperand(0);
  66   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
  67   if (!ExpectedValue)
  68     return false;
  69
  70   SwitchInst::CaseHandle Case = *SI.findCaseValue(ExpectedValue);
  71   unsigned n = SI.getNumCases(); // +1 for default case.
  72   SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeight);
  73
  74   if (Case == *SI.case_default())
  75     Weights[0] = LikelyBranchWeight;
  76   else
  77     Weights[Case.getCaseIndex() + 1] = LikelyBranchWeight;
  78
  79   SI.setMetadata(LLVMContext::MD_prof,
  80                  MDBuilder(CI->getContext()).createBranchWeights(Weights));
  81
  82   SI.setCondition(ArgValue);
  83   return true;
  84 }
  85
  86 /// Handler for PHINodes that define the value argument to an
  87 /// @llvm.expect call.
  88 ///
  89 /// If the operand of the phi has a constant value and it 'contradicts'
  90 /// with the expected value of phi def, then the corresponding incoming
  91 /// edge of the phi is unlikely to be taken. Using that information,
  92 /// the branch probability info for the originating branch can be inferred.
  93 static void handlePhiDef(CallInst *Expect) {
  94   Value &Arg = *Expect->getArgOperand(0);
  95   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(Expect->getArgOperand(1));
  96   if (!ExpectedValue)
  97     return;
  98   const APInt &ExpectedPhiValue = ExpectedValue->getValue();
  99
 100   // Walk up in backward a list of instructions that
 101   // have 'copy' semantics by 'stripping' the copies
 102   // until a PHI node or an instruction of unknown kind
 103   // is reached. Negation via xor is also handled.
 104   //
 105   //       C = PHI(...);
 106   //       B = C;
 107   //       A = B;
 108   //       D = __builtin_expect(A, 0);
 109   //
 110   Value *V = &Arg;
 111   SmallVector<Instruction *, 4> Operations;
 112   while (!isa<PHINode>(V)) {
 113     if (ZExtInst *ZExt = dyn_cast<ZExtInst>(V)) {
 114       V = ZExt->getOperand(0);
 115       Operations.push_back(ZExt);
 116       continue;
 117     }
 118
 119     if (SExtInst *SExt = dyn_cast<SExtInst>(V)) {
 120       V = SExt->getOperand(0);
 121       Operations.push_back(SExt);
 122       continue;
 123     }
 124
 125     BinaryOperator *BinOp = dyn_cast<BinaryOperator>(V);
 126     if (!BinOp || BinOp->getOpcode() != Instruction::Xor)
 127       return;
 128
 129     ConstantInt *CInt = dyn_cast<ConstantInt>(BinOp->getOperand(1));
 130     if (!CInt)
 131       return;
 132
 133     V = BinOp->getOperand(0);
 134     Operations.push_back(BinOp);
 135   }
 136
 137   // Executes the recorded operations on input 'Value'.
 138   auto ApplyOperations = [&](const APInt &Value) {
 139     APInt Result = Value;
 140     for (auto Op : llvm::reverse(Operations)) {
 141       switch (Op->getOpcode()) {
 142       case Instruction::Xor:
 143         Result ^= cast<ConstantInt>(Op->getOperand(1))->getValue();
 144         break;
 145       case Instruction::ZExt:
 146         Result = Result.zext(Op->getType()->getIntegerBitWidth());
 147         break;
 148       case Instruction::SExt:
 149         Result = Result.sext(Op->getType()->getIntegerBitWidth());
 150         break;
 151       default:
 152         llvm_unreachable("Unexpected operation");
 153       }
 154     }
 155     return Result;
 156   };
 157
 158   auto *PhiDef = dyn_cast<PHINode>(V);
 159
 160   // Get the first dominating conditional branch of the operand
 161   // i's incoming block.
 162   auto GetDomConditional = [&](unsigned i) -> BranchInst * {
 163     BasicBlock *BB = PhiDef->getIncomingBlock(i);
 164     BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
 165     if (BI && BI->isConditional())
 166       return BI;
 167     BB = BB->getSinglePredecessor();
 168     if (!BB)
 169       return nullptr;
 170     BI = dyn_cast<BranchInst>(BB->getTerminator());
 171     if (!BI || BI->isUnconditional())
 172       return nullptr;
 173     return BI;
 174   };
 175
 176   // Now walk through all Phi operands to find phi oprerands with values
 177   // conflicting with the expected phi output value. Any such operand
 178   // indicates the incoming edge to that operand is unlikely.
 179   for (unsigned i = 0, e = PhiDef->getNumIncomingValues(); i != e; ++i) {
 180
 181     Value *PhiOpnd = PhiDef->getIncomingValue(i);
 182     ConstantInt *CI = dyn_cast<ConstantInt>(PhiOpnd);
 183     if (!CI)
 184       continue;
 185
 186     // Not an interesting case when IsUnlikely is false -- we can not infer
 187     // anything useful when the operand value matches the expected phi
 188     // output.
 189     if (ExpectedPhiValue == ApplyOperations(CI->getValue()))
 190       continue;
 191
 192     BranchInst *BI = GetDomConditional(i);
 193     if (!BI)
 194       continue;
 195
 196     MDBuilder MDB(PhiDef->getContext());
 197
 198     // There are two situations in which an operand of the PhiDef comes
 199     // from a given successor of a branch instruction BI.
 200     // 1) When the incoming block of the operand is the successor block;
 201     // 2) When the incoming block is BI's enclosing block and the
 202     // successor is the PhiDef's enclosing block.
 203     //
 204     // Returns true if the operand which comes from OpndIncomingBB
 205     // comes from outgoing edge of BI that leads to Succ block.
 206     auto *OpndIncomingBB = PhiDef->getIncomingBlock(i);
 207     auto IsOpndComingFromSuccessor = [&](BasicBlock *Succ) {
 208       if (OpndIncomingBB == Succ)
 209         // If this successor is the incoming block for this
 210         // Phi operand, then this successor does lead to the Phi.
 211         return true;
 212       if (OpndIncomingBB == BI->getParent() && Succ == PhiDef->getParent())
 213         // Otherwise, if the edge is directly from the branch
 214         // to the Phi, this successor is the one feeding this
 215         // Phi operand.
 216         return true;
 217       return false;
 218     };
 219
 220     if (IsOpndComingFromSuccessor(BI->getSuccessor(1)))
 221       BI->setMetadata(
 222           LLVMContext::MD_prof,
 223           MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight));
 224     else if (IsOpndComingFromSuccessor(BI->getSuccessor(0)))
 225       BI->setMetadata(
 226           LLVMContext::MD_prof,
 227           MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight));
 228   }
 229 }
 230
 231 // Handle both BranchInst and SelectInst.
 232 template <class BrSelInst> static bool handleBrSelExpect(BrSelInst &BSI) {
 233
 234   // Handle non-optimized IR code like:
 235   //   %expval = call i64 @llvm.expect.i64(i64 %conv1, i64 1)
 236   //   %tobool = icmp ne i64 %expval, 0
 237   //   br i1 %tobool, label %if.then, label %if.end
 238   //
 239   // Or the following simpler case:
 240   //   %expval = call i1 @llvm.expect.i1(i1 %cmp, i1 1)
 241   //   br i1 %expval, label %if.then, label %if.end
 242
 243   CallInst *CI;
 244
 245   ICmpInst *CmpI = dyn_cast<ICmpInst>(BSI.getCondition());
 246   CmpInst::Predicate Predicate;
 247   ConstantInt *CmpConstOperand = nullptr;
 248   if (!CmpI) {
 249     CI = dyn_cast<CallInst>(BSI.getCondition());
 250     Predicate = CmpInst::ICMP_NE;
 251   } else {
 252     Predicate = CmpI->getPredicate();
 253     if (Predicate != CmpInst::ICMP_NE && Predicate != CmpInst::ICMP_EQ)
 254       return false;
 255
 256     CmpConstOperand = dyn_cast<ConstantInt>(CmpI->getOperand(1));
 257     if (!CmpConstOperand)
 258       return false;
 259     CI = dyn_cast<CallInst>(CmpI->getOperand(0));
 260   }
 261
 262   if (!CI)
 263     return false;
 264
 265   uint64_t ValueComparedTo = 0;
 266   if (CmpConstOperand) {
 267     if (CmpConstOperand->getBitWidth() > 64)
 268       return false;
 269     ValueComparedTo = CmpConstOperand->getZExtValue();
 270   }
 271
 272   Function *Fn = CI->getCalledFunction();
 273   if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
 274     return false;
 275
 276   Value *ArgValue = CI->getArgOperand(0);
 277   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
 278   if (!ExpectedValue)
 279     return false;
 280
 281   MDBuilder MDB(CI->getContext());
 282   MDNode *Node;
 283
 284   if ((ExpectedValue->getZExtValue() == ValueComparedTo) ==
 285       (Predicate == CmpInst::ICMP_EQ))
 286     Node = MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight);
 287   else
 288     Node = MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight);
 289
 290   BSI.setMetadata(LLVMContext::MD_prof, Node);
 291
 292   if (CmpI)
 293     CmpI->setOperand(0, ArgValue);
 294   else
 295     BSI.setCondition(ArgValue);
 296   return true;
 297 }
 298
 299 static bool handleBranchExpect(BranchInst &BI) {
 300   if (BI.isUnconditional())
 301     return false;
 302
 303   return handleBrSelExpect<BranchInst>(BI);
 304 }
 305
 306 static bool lowerExpectIntrinsic(Function &F) {
 307   bool Changed = false;
 308
 309   for (BasicBlock &BB : F) {
 310     // Create "block_weights" metadata.
 311     if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator())) {
 312       if (handleBranchExpect(*BI))
 313         ExpectIntrinsicsHandled++;
 314     } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator())) {
 315       if (handleSwitchExpect(*SI))
 316         ExpectIntrinsicsHandled++;
 317     }
 318
 319     // Remove llvm.expect intrinsics. Iterate backwards in order
 320     // to process select instructions before the intrinsic gets
 321     // removed.
 322     for (auto BI = BB.rbegin(), BE = BB.rend(); BI != BE;) {
 323       Instruction *Inst = &*BI++;
 324       CallInst *CI = dyn_cast<CallInst>(Inst);
 325       if (!CI) {
 326         if (SelectInst *SI = dyn_cast<SelectInst>(Inst)) {
 327           if (handleBrSelExpect(*SI))
 328             ExpectIntrinsicsHandled++;
 329         }
 330         continue;
 331       }
 332
 333       Function *Fn = CI->getCalledFunction();
 334       if (Fn && Fn->getIntrinsicID() == Intrinsic::expect) {
 335         // Before erasing the llvm.expect, walk backward to find
 336         // phi that define llvm.expect's first arg, and
 337         // infer branch probability:
 338         handlePhiDef(CI);
 339         Value *Exp = CI->getArgOperand(0);
 340         CI->replaceAllUsesWith(Exp);
 341         CI->eraseFromParent();
 342         Changed = true;
 343       }
 344     }
 345   }
 346
 347   return Changed;
 348 }
 349
 350 PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F,
 351                                                 FunctionAnalysisManager &) {
 352   if (lowerExpectIntrinsic(F))
 353     return PreservedAnalyses::none();
 354
 355   return PreservedAnalyses::all();
 356 }
 357
 358 namespace {
 359 /// Legacy pass for lowering expect intrinsics out of the IR.
 360 ///
 361 /// When this pass is run over a function it uses expect intrinsics which feed
 362 /// branches and switches to provide branch weight metadata for those
 363 /// terminators. It then removes the expect intrinsics from the IR so the rest
 364 /// of the optimizer can ignore them.
 365 class LowerExpectIntrinsic : public FunctionPass {
 366 public:
 367   static char ID;
 368   LowerExpectIntrinsic() : FunctionPass(ID) {
 369     initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
 370   }
 371
 372   bool runOnFunction(Function &F) override { return lowerExpectIntrinsic(F); }
 373 };
 374 }
 375
 376 char LowerExpectIntrinsic::ID = 0;
 377 INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect",
 378                 "Lower 'expect' Intrinsics", false, false)
 379
 380 FunctionPass *llvm::createLowerExpectIntrinsicPass() {
 381   return new LowerExpectIntrinsic();
 382 }