GuardUtils.cpp

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//===-- GuardUtils.cpp - Utils for work with guards -------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
// Utils that are used to perform transformations related to guards and their
// conditions.
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/GuardUtils.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"

using namespace llvm;

static cl::opt<uint32_t> PredicatePassBranchWeight(
    "guards-predicate-pass-branch-weight", cl::Hidden, cl::init(1 << 20),
    cl::desc("The probability of a guard failing is assumed to be the "
             "reciprocal of this value (default = 1 << 20)"));

void llvm::makeGuardControlFlowExplicit(Function *DeoptIntrinsic,
                                        CallInst *Guard) {
  OperandBundleDef DeoptOB(*Guard->getOperandBundle(LLVMContext::OB_deopt));
  SmallVector<Value *, 4> Args(std::next(Guard->arg_begin()), Guard->arg_end());

  auto *CheckBB = Guard->getParent();
  auto *DeoptBlockTerm =
      SplitBlockAndInsertIfThen(Guard->getArgOperand(0), Guard, true);

  auto *CheckBI = cast<BranchInst>(CheckBB->getTerminator());

  // SplitBlockAndInsertIfThen inserts control flow that branches to
  // DeoptBlockTerm if the condition is true.  We want the opposite.
  CheckBI->swapSuccessors();

  CheckBI->getSuccessor(0)->setName("guarded");
  CheckBI->getSuccessor(1)->setName("deopt");

  if (auto *MD = Guard->getMetadata(LLVMContext::MD_make_implicit))
    CheckBI->setMetadata(LLVMContext::MD_make_implicit, MD);

  MDBuilder MDB(Guard->getContext());
  CheckBI->setMetadata(LLVMContext::MD_prof,
                       MDB.createBranchWeights(PredicatePassBranchWeight, 1));

  IRBuilder<> B(DeoptBlockTerm);
  auto *DeoptCall = B.CreateCall(DeoptIntrinsic, Args, {DeoptOB}, "");

  if (DeoptIntrinsic->getReturnType()->isVoidTy()) {
    B.CreateRetVoid();
  } else {
    DeoptCall->setName("deoptcall");
    B.CreateRet(DeoptCall);
  }

  DeoptCall->setCallingConv(Guard->getCallingConv());
  DeoptBlockTerm->eraseFromParent();
}