Imported Upstream version 5.18.0.179

Former-commit-id: 67aa10e65b237e1c4537630979ee99ebe1374215

external/llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp

@@ -0,0 +1,383 @@
+//===- LowerExpectIntrinsic.cpp - Lower expect intrinsic ------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass lowers the 'expect' intrinsic to LLVM metadata.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar/LowerExpectIntrinsic.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/iterator_range.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/MDBuilder.h"
+#include "llvm/IR/Metadata.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Transforms/Scalar.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "lower-expect-intrinsic"
+
+STATISTIC(ExpectIntrinsicsHandled,
+          "Number of 'expect' intrinsic instructions handled");
+
+// These default values are chosen to represent an extremely skewed outcome for
+// a condition, but they leave some room for interpretation by later passes.
+//
+// If the documentation for __builtin_expect() was made explicit that it should
+// only be used in extreme cases, we could make this ratio higher. As it stands,
+// programmers may be using __builtin_expect() / llvm.expect to annotate that a
+// branch is likely or unlikely to be taken.
+//
+// There is a known dependency on this ratio in CodeGenPrepare when transforming
+// 'select' instructions. It may be worthwhile to hoist these values to some
+// shared space, so they can be used directly by other passes.
+
+static cl::opt<uint32_t> LikelyBranchWeight(
+    "likely-branch-weight", cl::Hidden, cl::init(2000),
+    cl::desc("Weight of the branch likely to be taken (default = 2000)"));
+static cl::opt<uint32_t> UnlikelyBranchWeight(
+    "unlikely-branch-weight", cl::Hidden, cl::init(1),
+    cl::desc("Weight of the branch unlikely to be taken (default = 1)"));
+
+static bool handleSwitchExpect(SwitchInst &SI) {
+  CallInst *CI = dyn_cast<CallInst>(SI.getCondition());
+  if (!CI)
+    return false;
+
+  Function *Fn = CI->getCalledFunction();
+  if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
+    return false;
+
+  Value *ArgValue = CI->getArgOperand(0);
+  ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
+  if (!ExpectedValue)
+    return false;
+
+  SwitchInst::CaseHandle Case = *SI.findCaseValue(ExpectedValue);
+  unsigned n = SI.getNumCases(); // +1 for default case.
+  SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeight);
+
+  if (Case == *SI.case_default())
+    Weights[0] = LikelyBranchWeight;
+  else
+    Weights[Case.getCaseIndex() + 1] = LikelyBranchWeight;
+
+  SI.setMetadata(LLVMContext::MD_prof,
+                 MDBuilder(CI->getContext()).createBranchWeights(Weights));
+
+  SI.setCondition(ArgValue);
+  return true;
+}
+
+/// Handler for PHINodes that define the value argument to an
+/// @llvm.expect call.
+///
+/// If the operand of the phi has a constant value and it 'contradicts'
+/// with the expected value of phi def, then the corresponding incoming
+/// edge of the phi is unlikely to be taken. Using that information,
+/// the branch probability info for the originating branch can be inferred.
+static void handlePhiDef(CallInst *Expect) {
+  Value &Arg = *Expect->getArgOperand(0);
+  ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(Expect->getArgOperand(1));
+  if (!ExpectedValue)
+    return;
+  const APInt &ExpectedPhiValue = ExpectedValue->getValue();
+
+  // Walk up in backward a list of instructions that
+  // have 'copy' semantics by 'stripping' the copies
+  // until a PHI node or an instruction of unknown kind
+  // is reached. Negation via xor is also handled.
+  //
+  //       C = PHI(...);
+  //       B = C;
+  //       A = B;
+  //       D = __builtin_expect(A, 0);
+  //
+  Value *V = &Arg;
+  SmallVector<Instruction *, 4> Operations;
+  while (!isa<PHINode>(V)) {
+    if (ZExtInst *ZExt = dyn_cast<ZExtInst>(V)) {
+      V = ZExt->getOperand(0);
+      Operations.push_back(ZExt);
+      continue;
+    }
+
+    if (SExtInst *SExt = dyn_cast<SExtInst>(V)) {
+      V = SExt->getOperand(0);
+      Operations.push_back(SExt);
+      continue;
+    }
+
+    BinaryOperator *BinOp = dyn_cast<BinaryOperator>(V);
+    if (!BinOp || BinOp->getOpcode() != Instruction::Xor)
+      return;
+
+    ConstantInt *CInt = dyn_cast<ConstantInt>(BinOp->getOperand(1));
+    if (!CInt)
+      return;
+
+    V = BinOp->getOperand(0);
+    Operations.push_back(BinOp);
+  }
+
+  // Executes the recorded operations on input 'Value'.
+  auto ApplyOperations = [&](const APInt &Value) {
+    APInt Result = Value;
+    for (auto Op : llvm::reverse(Operations)) {
+      switch (Op->getOpcode()) {
+      case Instruction::Xor:
+        Result ^= cast<ConstantInt>(Op->getOperand(1))->getValue();
+        break;
+      case Instruction::ZExt:
+        Result = Result.zext(Op->getType()->getIntegerBitWidth());
+        break;
+      case Instruction::SExt:
+        Result = Result.sext(Op->getType()->getIntegerBitWidth());
+        break;
+      default:
+        llvm_unreachable("Unexpected operation");
+      }
+    }
+    return Result;
+  };
+
+  auto *PhiDef = dyn_cast<PHINode>(V);
+
+  // Get the first dominating conditional branch of the operand
+  // i's incoming block.
+  auto GetDomConditional = [&](unsigned i) -> BranchInst * {
+    BasicBlock *BB = PhiDef->getIncomingBlock(i);
+    BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
+    if (BI && BI->isConditional())
+      return BI;
+    BB = BB->getSinglePredecessor();
+    if (!BB)
+      return nullptr;
+    BI = dyn_cast<BranchInst>(BB->getTerminator());
+    if (!BI || BI->isUnconditional())
+      return nullptr;
+    return BI;
+  };
+
+  // Now walk through all Phi operands to find phi oprerands with values
+  // conflicting with the expected phi output value. Any such operand
+  // indicates the incoming edge to that operand is unlikely.
+  for (unsigned i = 0, e = PhiDef->getNumIncomingValues(); i != e; ++i) {
+
+    Value *PhiOpnd = PhiDef->getIncomingValue(i);
+    ConstantInt *CI = dyn_cast<ConstantInt>(PhiOpnd);
+    if (!CI)
+      continue;
+
+    // Not an interesting case when IsUnlikely is false -- we can not infer
+    // anything useful when the operand value matches the expected phi
+    // output.
+    if (ExpectedPhiValue == ApplyOperations(CI->getValue()))
+      continue;
+
+    BranchInst *BI = GetDomConditional(i);
+    if (!BI)
+      continue;
+
+    MDBuilder MDB(PhiDef->getContext());
+
+    // There are two situations in which an operand of the PhiDef comes
+    // from a given successor of a branch instruction BI.
+    // 1) When the incoming block of the operand is the successor block;
+    // 2) When the incoming block is BI's enclosing block and the
+    // successor is the PhiDef's enclosing block.
+    //
+    // Returns true if the operand which comes from OpndIncomingBB
+    // comes from outgoing edge of BI that leads to Succ block.
+    auto *OpndIncomingBB = PhiDef->getIncomingBlock(i);
+    auto IsOpndComingFromSuccessor = [&](BasicBlock *Succ) {
+      if (OpndIncomingBB == Succ)
+        // If this successor is the incoming block for this
+        // Phi operand, then this successor does lead to the Phi.
+        return true;
+      if (OpndIncomingBB == BI->getParent() && Succ == PhiDef->getParent())
+        // Otherwise, if the edge is directly from the branch
+        // to the Phi, this successor is the one feeding this
+        // Phi operand.
+        return true;
+      return false;
+    };
+
+    if (IsOpndComingFromSuccessor(BI->getSuccessor(1)))
+      BI->setMetadata(
+          LLVMContext::MD_prof,
+          MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight));
+    else if (IsOpndComingFromSuccessor(BI->getSuccessor(0)))
+      BI->setMetadata(
+          LLVMContext::MD_prof,
+          MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight));
+  }
+}
+
+// Handle both BranchInst and SelectInst.
+template <class BrSelInst> static bool handleBrSelExpect(BrSelInst &BSI) {
+
+  // Handle non-optimized IR code like:
+  //   %expval = call i64 @llvm.expect.i64(i64 %conv1, i64 1)
+  //   %tobool = icmp ne i64 %expval, 0
+  //   br i1 %tobool, label %if.then, label %if.end
+  //
+  // Or the following simpler case:
+  //   %expval = call i1 @llvm.expect.i1(i1 %cmp, i1 1)
+  //   br i1 %expval, label %if.then, label %if.end
+
+  CallInst *CI;
+
+  ICmpInst *CmpI = dyn_cast<ICmpInst>(BSI.getCondition());
+  CmpInst::Predicate Predicate;
+  ConstantInt *CmpConstOperand = nullptr;
+  if (!CmpI) {
+    CI = dyn_cast<CallInst>(BSI.getCondition());
+    Predicate = CmpInst::ICMP_NE;
+  } else {
+    Predicate = CmpI->getPredicate();
+    if (Predicate != CmpInst::ICMP_NE && Predicate != CmpInst::ICMP_EQ)
+      return false;
+
+    CmpConstOperand = dyn_cast<ConstantInt>(CmpI->getOperand(1));
+    if (!CmpConstOperand)
+      return false;
+    CI = dyn_cast<CallInst>(CmpI->getOperand(0));
+  }
+
+  if (!CI)
+    return false;
+
+  uint64_t ValueComparedTo = 0;
+  if (CmpConstOperand) {
+    if (CmpConstOperand->getBitWidth() > 64)
+      return false;
+    ValueComparedTo = CmpConstOperand->getZExtValue();
+  }
+
+  Function *Fn = CI->getCalledFunction();
+  if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
+    return false;
+
+  Value *ArgValue = CI->getArgOperand(0);
+  ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
+  if (!ExpectedValue)
+    return false;
+
+  MDBuilder MDB(CI->getContext());
+  MDNode *Node;
+
+  if ((ExpectedValue->getZExtValue() == ValueComparedTo) ==
+      (Predicate == CmpInst::ICMP_EQ))
+    Node = MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight);
+  else
+    Node = MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight);
+
+  BSI.setMetadata(LLVMContext::MD_prof, Node);
+
+  if (CmpI)
+    CmpI->setOperand(0, ArgValue);
+  else
+    BSI.setCondition(ArgValue);
+  return true;
+}
+
+static bool handleBranchExpect(BranchInst &BI) {
+  if (BI.isUnconditional())
+    return false;
+
+  return handleBrSelExpect<BranchInst>(BI);
+}
+
+static bool lowerExpectIntrinsic(Function &F) {
+  bool Changed = false;
+
+  for (BasicBlock &BB : F) {
+    // Create "block_weights" metadata.
+    if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator())) {
+      if (handleBranchExpect(*BI))
+        ExpectIntrinsicsHandled++;
+    } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator())) {
+      if (handleSwitchExpect(*SI))
+        ExpectIntrinsicsHandled++;
+    }
+
+    // Remove llvm.expect intrinsics. Iterate backwards in order
+    // to process select instructions before the intrinsic gets
+    // removed.
+    for (auto BI = BB.rbegin(), BE = BB.rend(); BI != BE;) {
+      Instruction *Inst = &*BI++;
+      CallInst *CI = dyn_cast<CallInst>(Inst);
+      if (!CI) {
+        if (SelectInst *SI = dyn_cast<SelectInst>(Inst)) {
+          if (handleBrSelExpect(*SI))
+            ExpectIntrinsicsHandled++;
+        }
+        continue;
+      }
+
+      Function *Fn = CI->getCalledFunction();
+      if (Fn && Fn->getIntrinsicID() == Intrinsic::expect) {
+        // Before erasing the llvm.expect, walk backward to find
+        // phi that define llvm.expect's first arg, and
+        // infer branch probability:
+        handlePhiDef(CI);
+        Value *Exp = CI->getArgOperand(0);
+        CI->replaceAllUsesWith(Exp);
+        CI->eraseFromParent();
+        Changed = true;
+      }
+    }
+  }
+
+  return Changed;
+}
+
+PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F,
+                                                FunctionAnalysisManager &) {
+  if (lowerExpectIntrinsic(F))
+    return PreservedAnalyses::none();
+
+  return PreservedAnalyses::all();
+}
+
+namespace {
+/// \brief Legacy pass for lowering expect intrinsics out of the IR.
+///
+/// When this pass is run over a function it uses expect intrinsics which feed
+/// branches and switches to provide branch weight metadata for those
+/// terminators. It then removes the expect intrinsics from the IR so the rest
+/// of the optimizer can ignore them.
+class LowerExpectIntrinsic : public FunctionPass {
+public:
+  static char ID;
+  LowerExpectIntrinsic() : FunctionPass(ID) {
+    initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnFunction(Function &F) override { return lowerExpectIntrinsic(F); }
+};
+}
+
+char LowerExpectIntrinsic::ID = 0;
+INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect",
+                "Lower 'expect' Intrinsics", false, false)
+
+FunctionPass *llvm::createLowerExpectIntrinsicPass() {
+  return new LowerExpectIntrinsic();
+}