MakeGuardsExplicit.cpp

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//===- MakeGuardsExplicit.cpp - Turn guard intrinsics into guard branches -===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This pass lowers the @llvm.experimental.guard intrinsic to the new form of
// guard represented as widenable explicit branch to the deopt block. The
// difference between this pass and LowerGuardIntrinsic is that after this pass
// the guard represented as intrinsic:
//
//   call void(i1, ...) @llvm.experimental.guard(i1 %old_cond) [ "deopt"() ]
//
// transforms to a guard represented as widenable explicit branch:
//
//   %widenable_cond = call i1 @llvm.experimental.widenable.condition()
//   br i1 (%old_cond & %widenable_cond), label %guarded, label %deopt
//
// Here:
//   - The semantics of @llvm.experimental.widenable.condition allows to replace
//     %widenable_cond with the construction (%widenable_cond & %any_other_cond)
//     without loss of correctness;
//   - %guarded is the lower part of old guard intrinsic's parent block split by
//     the intrinsic call;
//   - %deopt is a block containing a sole call to @llvm.experimental.deoptimize
//     intrinsic.
//
// Therefore, this branch preserves the property of widenability.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar/MakeGuardsExplicit.h"
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/GuardUtils.h"

using namespace llvm;

namespace {
struct MakeGuardsExplicitLegacyPass : public FunctionPass {
  static char ID;
  MakeGuardsExplicitLegacyPass() : FunctionPass(ID) {
    initializeMakeGuardsExplicitLegacyPassPass(*PassRegistry::getPassRegistry());
  }

  bool runOnFunction(Function &F) override;
};
}

static void turnToExplicitForm(CallInst *Guard, Function *DeoptIntrinsic) {
  // Replace the guard with an explicit branch (just like in GuardWidening).
  BasicBlock *BB = Guard->getParent();
  makeGuardControlFlowExplicit(DeoptIntrinsic, Guard);
  BranchInst *ExplicitGuard = cast<BranchInst>(BB->getTerminator());
  assert(ExplicitGuard->isConditional() && "Must be!");

  // We want the guard to be expressed as explicit control flow, but still be
  // widenable. For that, we add Widenable Condition intrinsic call to the
  // guard's condition.
  IRBuilder<> B(ExplicitGuard);
  auto *WidenableCondition =
      B.CreateIntrinsic(Intrinsic::experimental_widenable_condition,
                        {}, {}, nullptr, "widenable_cond");
  WidenableCondition->setCallingConv(Guard->getCallingConv());
  auto *NewCond =
      B.CreateAnd(ExplicitGuard->getCondition(), WidenableCondition);
  NewCond->setName("exiplicit_guard_cond");
  ExplicitGuard->setCondition(NewCond);
  Guard->eraseFromParent();
}

static bool explicifyGuards(Function &F) {
  // Check if we can cheaply rule out the possibility of not having any work to
  // do.
  auto *GuardDecl = F.getParent()->getFunction(
      Intrinsic::getName(Intrinsic::experimental_guard));
  if (!GuardDecl || GuardDecl->use_empty())
    return false;

  SmallVector<CallInst *, 8> GuardIntrinsics;
  for (auto &I : instructions(F))
    if (isGuard(&I))
      GuardIntrinsics.push_back(cast<CallInst>(&I));

  if (GuardIntrinsics.empty())
    return false;

  auto *DeoptIntrinsic = Intrinsic::getDeclaration(
      F.getParent(), Intrinsic::experimental_deoptimize, {F.getReturnType()});
  DeoptIntrinsic->setCallingConv(GuardDecl->getCallingConv());

  for (auto *Guard : GuardIntrinsics)
    turnToExplicitForm(Guard, DeoptIntrinsic);

  return true;
}

bool MakeGuardsExplicitLegacyPass::runOnFunction(Function &F) {
  return explicifyGuards(F);
}

char MakeGuardsExplicitLegacyPass::ID = 0;
INITIALIZE_PASS(MakeGuardsExplicitLegacyPass, "make-guards-explicit",
                "Lower the guard intrinsic to explicit control flow form",
                false, false)

PreservedAnalyses MakeGuardsExplicitPass::run(Function &F,
                                           FunctionAnalysisManager &) {
  if (explicifyGuards(F))
    return PreservedAnalyses::none();
  return PreservedAnalyses::all();
}