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 #include "llvm/Transforms/Utils/MisExpect.h"
  30
  31 using namespace llvm;
  32
  33 #define DEBUG_TYPE "lower-expect-intrinsic"
  34
  35 STATISTIC(ExpectIntrinsicsHandled,
  36           "Number of 'expect' intrinsic instructions handled");
  37
  38 // These default values are chosen to represent an extremely skewed outcome for
  39 // a condition, but they leave some room for interpretation by later passes.
  40 //
  41 // If the documentation for __builtin_expect() was made explicit that it should
  42 // only be used in extreme cases, we could make this ratio higher. As it stands,
  43 // programmers may be using __builtin_expect() / llvm.expect to annotate that a
  44 // branch is likely or unlikely to be taken.
  45 //
  46 // There is a known dependency on this ratio in CodeGenPrepare when transforming
  47 // 'select' instructions. It may be worthwhile to hoist these values to some
  48 // shared space, so they can be used directly by other passes.
  49
  50 static cl::opt<uint32_t> LikelyBranchWeight(
  51     "likely-branch-weight", cl::Hidden, cl::init(2000),
  52     cl::desc("Weight of the branch likely to be taken (default = 2000)"));
  53 static cl::opt<uint32_t> UnlikelyBranchWeight(
  54     "unlikely-branch-weight", cl::Hidden, cl::init(1),
  55     cl::desc("Weight of the branch unlikely to be taken (default = 1)"));
  56
  57 static bool handleSwitchExpect(SwitchInst &SI) {
  58   CallInst *CI = dyn_cast<CallInst>(SI.getCondition());
  59   if (!CI)
  60     return false;
  61
  62   Function *Fn = CI->getCalledFunction();
  63   if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
  64     return false;
  65
  66   Value *ArgValue = CI->getArgOperand(0);
  67   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
  68   if (!ExpectedValue)
  69     return false;
  70
  71   SwitchInst::CaseHandle Case = *SI.findCaseValue(ExpectedValue);
  72   unsigned n = SI.getNumCases(); // +1 for default case.
  73   SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeight);
  74
  75   uint64_t Index = (Case == *SI.case_default()) ? 0 : Case.getCaseIndex() + 1;
  76   Weights[Index] = LikelyBranchWeight;
  77
  78   SI.setMetadata(
  79       LLVMContext::MD_misexpect,
  80       MDBuilder(CI->getContext())
  81           .createMisExpect(Index, LikelyBranchWeight, UnlikelyBranchWeight));
  82
  83   SI.setCondition(ArgValue);
  84   misexpect::checkFrontendInstrumentation(SI);
  85
  86   SI.setMetadata(LLVMContext::MD_prof,
  87                  MDBuilder(CI->getContext()).createBranchWeights(Weights));
  88
  89   return true;
  90 }
  91
  92 /// Handler for PHINodes that define the value argument to an
  93 /// @llvm.expect call.
  94 ///
  95 /// If the operand of the phi has a constant value and it 'contradicts'
  96 /// with the expected value of phi def, then the corresponding incoming
  97 /// edge of the phi is unlikely to be taken. Using that information,
  98 /// the branch probability info for the originating branch can be inferred.
  99 static void handlePhiDef(CallInst *Expect) {
 100   Value &Arg = *Expect->getArgOperand(0);
 101   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(Expect->getArgOperand(1));
 102   if (!ExpectedValue)
 103     return;
 104   const APInt &ExpectedPhiValue = ExpectedValue->getValue();
 105
 106   // Walk up in backward a list of instructions that
 107   // have 'copy' semantics by 'stripping' the copies
 108   // until a PHI node or an instruction of unknown kind
 109   // is reached. Negation via xor is also handled.
 110   //
 111   //       C = PHI(...);
 112   //       B = C;
 113   //       A = B;
 114   //       D = __builtin_expect(A, 0);
 115   //
 116   Value *V = &Arg;
 117   SmallVector<Instruction *, 4> Operations;
 118   while (!isa<PHINode>(V)) {
 119     if (ZExtInst *ZExt = dyn_cast<ZExtInst>(V)) {
 120       V = ZExt->getOperand(0);
 121       Operations.push_back(ZExt);
 122       continue;
 123     }
 124
 125     if (SExtInst *SExt = dyn_cast<SExtInst>(V)) {
 126       V = SExt->getOperand(0);
 127       Operations.push_back(SExt);
 128       continue;
 129     }
 130
 131     BinaryOperator *BinOp = dyn_cast<BinaryOperator>(V);
 132     if (!BinOp || BinOp->getOpcode() != Instruction::Xor)
 133       return;
 134
 135     ConstantInt *CInt = dyn_cast<ConstantInt>(BinOp->getOperand(1));
 136     if (!CInt)
 137       return;
 138
 139     V = BinOp->getOperand(0);
 140     Operations.push_back(BinOp);
 141   }
 142
 143   // Executes the recorded operations on input 'Value'.
 144   auto ApplyOperations = [&](const APInt &Value) {
 145     APInt Result = Value;
 146     for (auto Op : llvm::reverse(Operations)) {
 147       switch (Op->getOpcode()) {
 148       case Instruction::Xor:
 149         Result ^= cast<ConstantInt>(Op->getOperand(1))->getValue();
 150         break;
 151       case Instruction::ZExt:
 152         Result = Result.zext(Op->getType()->getIntegerBitWidth());
 153         break;
 154       case Instruction::SExt:
 155         Result = Result.sext(Op->getType()->getIntegerBitWidth());
 156         break;
 157       default:
 158         llvm_unreachable("Unexpected operation");
 159       }
 160     }
 161     return Result;
 162   };
 163
 164   auto *PhiDef = cast<PHINode>(V);
 165
 166   // Get the first dominating conditional branch of the operand
 167   // i's incoming block.
 168   auto GetDomConditional = [&](unsigned i) -> BranchInst * {
 169     BasicBlock *BB = PhiDef->getIncomingBlock(i);
 170     BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
 171     if (BI && BI->isConditional())
 172       return BI;
 173     BB = BB->getSinglePredecessor();
 174     if (!BB)
 175       return nullptr;
 176     BI = dyn_cast<BranchInst>(BB->getTerminator());
 177     if (!BI || BI->isUnconditional())
 178       return nullptr;
 179     return BI;
 180   };
 181
 182   // Now walk through all Phi operands to find phi oprerands with values
 183   // conflicting with the expected phi output value. Any such operand
 184   // indicates the incoming edge to that operand is unlikely.
 185   for (unsigned i = 0, e = PhiDef->getNumIncomingValues(); i != e; ++i) {
 186
 187     Value *PhiOpnd = PhiDef->getIncomingValue(i);
 188     ConstantInt *CI = dyn_cast<ConstantInt>(PhiOpnd);
 189     if (!CI)
 190       continue;
 191
 192     // Not an interesting case when IsUnlikely is false -- we can not infer
 193     // anything useful when the operand value matches the expected phi
 194     // output.
 195     if (ExpectedPhiValue == ApplyOperations(CI->getValue()))
 196       continue;
 197
 198     BranchInst *BI = GetDomConditional(i);
 199     if (!BI)
 200       continue;
 201
 202     MDBuilder MDB(PhiDef->getContext());
 203
 204     // There are two situations in which an operand of the PhiDef comes
 205     // from a given successor of a branch instruction BI.
 206     // 1) When the incoming block of the operand is the successor block;
 207     // 2) When the incoming block is BI's enclosing block and the
 208     // successor is the PhiDef's enclosing block.
 209     //
 210     // Returns true if the operand which comes from OpndIncomingBB
 211     // comes from outgoing edge of BI that leads to Succ block.
 212     auto *OpndIncomingBB = PhiDef->getIncomingBlock(i);
 213     auto IsOpndComingFromSuccessor = [&](BasicBlock *Succ) {
 214       if (OpndIncomingBB == Succ)
 215         // If this successor is the incoming block for this
 216         // Phi operand, then this successor does lead to the Phi.
 217         return true;
 218       if (OpndIncomingBB == BI->getParent() && Succ == PhiDef->getParent())
 219         // Otherwise, if the edge is directly from the branch
 220         // to the Phi, this successor is the one feeding this
 221         // Phi operand.
 222         return true;
 223       return false;
 224     };
 225
 226     if (IsOpndComingFromSuccessor(BI->getSuccessor(1)))
 227       BI->setMetadata(
 228           LLVMContext::MD_prof,
 229           MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight));
 230     else if (IsOpndComingFromSuccessor(BI->getSuccessor(0)))
 231       BI->setMetadata(
 232           LLVMContext::MD_prof,
 233           MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight));
 234   }
 235 }
 236
 237 // Handle both BranchInst and SelectInst.
 238 template <class BrSelInst> static bool handleBrSelExpect(BrSelInst &BSI) {
 239
 240   // Handle non-optimized IR code like:
 241   //   %expval = call i64 @llvm.expect.i64(i64 %conv1, i64 1)
 242   //   %tobool = icmp ne i64 %expval, 0
 243   //   br i1 %tobool, label %if.then, label %if.end
 244   //
 245   // Or the following simpler case:
 246   //   %expval = call i1 @llvm.expect.i1(i1 %cmp, i1 1)
 247   //   br i1 %expval, label %if.then, label %if.end
 248
 249   CallInst *CI;
 250
 251   ICmpInst *CmpI = dyn_cast<ICmpInst>(BSI.getCondition());
 252   CmpInst::Predicate Predicate;
 253   ConstantInt *CmpConstOperand = nullptr;
 254   if (!CmpI) {
 255     CI = dyn_cast<CallInst>(BSI.getCondition());
 256     Predicate = CmpInst::ICMP_NE;
 257   } else {
 258     Predicate = CmpI->getPredicate();
 259     if (Predicate != CmpInst::ICMP_NE && Predicate != CmpInst::ICMP_EQ)
 260       return false;
 261
 262     CmpConstOperand = dyn_cast<ConstantInt>(CmpI->getOperand(1));
 263     if (!CmpConstOperand)
 264       return false;
 265     CI = dyn_cast<CallInst>(CmpI->getOperand(0));
 266   }
 267
 268   if (!CI)
 269     return false;
 270
 271   uint64_t ValueComparedTo = 0;
 272   if (CmpConstOperand) {
 273     if (CmpConstOperand->getBitWidth() > 64)
 274       return false;
 275     ValueComparedTo = CmpConstOperand->getZExtValue();
 276   }
 277
 278   Function *Fn = CI->getCalledFunction();
 279   if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
 280     return false;
 281
 282   Value *ArgValue = CI->getArgOperand(0);
 283   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
 284   if (!ExpectedValue)
 285     return false;
 286
 287   MDBuilder MDB(CI->getContext());
 288   MDNode *Node;
 289   MDNode *ExpNode;
 290
 291   if ((ExpectedValue->getZExtValue() == ValueComparedTo) ==
 292       (Predicate == CmpInst::ICMP_EQ)) {
 293     Node = MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight);
 294     ExpNode = MDB.createMisExpect(0, LikelyBranchWeight, UnlikelyBranchWeight);
 295   } else {
 296     Node = MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight);
 297     ExpNode = MDB.createMisExpect(1, LikelyBranchWeight, UnlikelyBranchWeight);
 298   }
 299
 300   BSI.setMetadata(LLVMContext::MD_misexpect, ExpNode);
 301
 302   if (CmpI)
 303     CmpI->setOperand(0, ArgValue);
 304   else
 305     BSI.setCondition(ArgValue);
 306
 307   misexpect::checkFrontendInstrumentation(BSI);
 308
 309   BSI.setMetadata(LLVMContext::MD_prof, Node);
 310
 311   return true;
 312 }
 313
 314 static bool handleBranchExpect(BranchInst &BI) {
 315   if (BI.isUnconditional())
 316     return false;
 317
 318   return handleBrSelExpect<BranchInst>(BI);
 319 }
 320
 321 static bool lowerExpectIntrinsic(Function &F) {
 322   bool Changed = false;
 323
 324   for (BasicBlock &BB : F) {
 325     // Create "block_weights" metadata.
 326     if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator())) {
 327       if (handleBranchExpect(*BI))
 328         ExpectIntrinsicsHandled++;
 329     } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator())) {
 330       if (handleSwitchExpect(*SI))
 331         ExpectIntrinsicsHandled++;
 332     }
 333
 334     // Remove llvm.expect intrinsics. Iterate backwards in order
 335     // to process select instructions before the intrinsic gets
 336     // removed.
 337     for (auto BI = BB.rbegin(), BE = BB.rend(); BI != BE;) {
 338       Instruction *Inst = &*BI++;
 339       CallInst *CI = dyn_cast<CallInst>(Inst);
 340       if (!CI) {
 341         if (SelectInst *SI = dyn_cast<SelectInst>(Inst)) {
 342           if (handleBrSelExpect(*SI))
 343             ExpectIntrinsicsHandled++;
 344         }
 345         continue;
 346       }
 347
 348       Function *Fn = CI->getCalledFunction();
 349       if (Fn && Fn->getIntrinsicID() == Intrinsic::expect) {
 350         // Before erasing the llvm.expect, walk backward to find
 351         // phi that define llvm.expect's first arg, and
 352         // infer branch probability:
 353         handlePhiDef(CI);
 354         Value *Exp = CI->getArgOperand(0);
 355         CI->replaceAllUsesWith(Exp);
 356         CI->eraseFromParent();
 357         Changed = true;
 358       }
 359     }
 360   }
 361
 362   return Changed;
 363 }
 364
 365 PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F,
 366                                                 FunctionAnalysisManager &) {
 367   if (lowerExpectIntrinsic(F))
 368     return PreservedAnalyses::none();
 369
 370   return PreservedAnalyses::all();
 371 }
 372
 373 namespace {
 374 /// Legacy pass for lowering expect intrinsics out of the IR.
 375 ///
 376 /// When this pass is run over a function it uses expect intrinsics which feed
 377 /// branches and switches to provide branch weight metadata for those
 378 /// terminators. It then removes the expect intrinsics from the IR so the rest
 379 /// of the optimizer can ignore them.
 380 class LowerExpectIntrinsic : public FunctionPass {
 381 public:
 382   static char ID;
 383   LowerExpectIntrinsic() : FunctionPass(ID) {
 384     initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
 385   }
 386
 387   bool runOnFunction(Function &F) override { return lowerExpectIntrinsic(F); }
 388 };
 389 }
 390
 391 char LowerExpectIntrinsic::ID = 0;
 392 INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect",
 393                 "Lower 'expect' Intrinsics", false, false)
 394
 395 FunctionPass *llvm::createLowerExpectIntrinsicPass() {
 396   return new LowerExpectIntrinsic();
 397 }