CoroEarly.cpp

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//===- CoroEarly.cpp - Coroutine Early Function Pass ----------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
 
#include "llvm/Transforms/Coroutines/CoroEarly.h"
#include "CoroInternal.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Module.h"
#include "llvm/Transforms/Coroutines/CoroShape.h"
 
using namespace llvm;
 
#define DEBUG_TYPE "coro-early"
 
namespace {
// Created on demand if the coro-early pass has work to do.
class Lowerer : public coro::LowererBase {
  IRBuilder<> Builder;
  PointerType *const AnyResumeFnPtrTy;
 
  void lowerResumeOrDestroy(CallBase &CB, CoroSubFnInst::ResumeKind);
  void lowerCoroPromise(CoroPromiseInst *Intrin);
  void lowerCoroDone(IntrinsicInst *II);
  void hidePromiseAlloca(CoroIdInst *CoroId, CoroBeginInst *CoroBegin);
 
public:
  Lowerer(Module &M)
      : LowererBase(M), Builder(Context),
        AnyResumeFnPtrTy(PointerType::getUnqual(Context)) {}
  void lowerEarlyIntrinsics(Function &F);
};
} // namespace
 
// Replace a direct call to coro.resume or coro.destroy with an indirect call to
// an address returned by coro.subfn.addr intrinsic. This is done so that
// CGPassManager recognizes devirtualization when CoroElide pass replaces a call
// to coro.subfn.addr with an appropriate function address.
void Lowerer::lowerResumeOrDestroy(CallBase &CB,
                                   CoroSubFnInst::ResumeKind Index) {
  Value *ResumeAddr = makeSubFnCall(CB.getArgOperand(0), Index, &CB);
  CB.setCalledOperand(ResumeAddr);
  CB.setCallingConv(CallingConv::Fast);
}
 
// Coroutine promise field is always at the fixed offset from the beginning of
// the coroutine frame. i8* coro.promise(i8*, i1 from) intrinsic adds an offset
// to a passed pointer to move from coroutine frame to coroutine promise and
// vice versa. Since we don't know exactly which coroutine frame it is, we build
// a coroutine frame mock up starting with two function pointers, followed by a
// properly aligned coroutine promise field.
// TODO: Handle the case when coroutine promise alloca has align override.
void Lowerer::lowerCoroPromise(CoroPromiseInst *Intrin) {
  Value *Operand = Intrin->getArgOperand(0);
  Align Alignment = Intrin->getAlignment();
  Type *Int8Ty = Builder.getInt8Ty();
 
  auto *SampleStruct =
      StructType::get(Context, {AnyResumeFnPtrTy, AnyResumeFnPtrTy, Int8Ty});
  const DataLayout &DL = TheModule.getDataLayout();
  int64_t Offset = alignTo(
      DL.getStructLayout(SampleStruct)->getElementOffset(2), Alignment);
  if (Intrin->isFromPromise())
    Offset = -Offset;
 
  Builder.SetInsertPoint(Intrin);
  Value *Replacement =
      Builder.CreateConstInBoundsGEP1_32(Int8Ty, Operand, Offset);
 
  Intrin->replaceAllUsesWith(Replacement);
  Intrin->eraseFromParent();
}
 
// When a coroutine reaches final suspend point, it zeros out ResumeFnAddr in
// the coroutine frame (it is UB to resume from a final suspend point).
// The llvm.coro.done intrinsic is used to check whether a coroutine is
// suspended at the final suspend point or not.
void Lowerer::lowerCoroDone(IntrinsicInst *II) {
  Value *Operand = II->getArgOperand(0);
 
  // ResumeFnAddr is the first pointer sized element of the coroutine frame.
  static_assert(coro::Shape::SwitchFieldIndex::Resume == 0,
                "resume function not at offset zero");
  auto *FrameTy = Int8Ptr;
 
  Builder.SetInsertPoint(II);
  auto *Load = Builder.CreateLoad(FrameTy, Operand);
  auto *Cond = Builder.CreateICmpEQ(Load, NullPtr);
 
  II->replaceAllUsesWith(Cond);
  II->eraseFromParent();
}
 
// Later middle-end passes will assume promise alloca dead after coroutine
// suspend, leading to misoptimizations. We hide promise alloca using
// coro.promise and will lower it back to alloca at CoroSplit.
void Lowerer::hidePromiseAlloca(CoroIdInst *CoroId, CoroBeginInst *CoroBegin) {
  auto *PA = CoroId->getPromise();
  if (!PA || !CoroBegin)
    return;
  Builder.SetInsertPoint(*CoroBegin->getInsertionPointAfterDef());
 
  auto *Alignment = Builder.getInt32(PA->getAlign().value());
  auto *FromPromise = Builder.getInt1(false);
  SmallVector<Value *, 3> Arg{CoroBegin, Alignment, FromPromise};
  auto *PI = Builder.CreateIntrinsic(
      Builder.getPtrTy(), Intrinsic::coro_promise, Arg, {}, "promise.addr");
  PI->setCannotDuplicate();
  // Remove lifetime markers, as these are only allowed on allocas.
  for (User *U : make_early_inc_range(PA->users())) {
    auto *I = cast<Instruction>(U);
    if (I->isLifetimeStartOrEnd())
      I->eraseFromParent();
  }
  PA->replaceUsesWithIf(PI, [CoroId](Use &U) {
    bool IsBitcast = U == U.getUser()->stripPointerCasts();
    bool IsCoroId = U.getUser() == CoroId;
    return !IsBitcast && !IsCoroId;
  });
}
 
// Prior to CoroSplit, calls to coro.begin needs to be marked as NoDuplicate,
// as CoroSplit assumes there is exactly one coro.begin. After CoroSplit,
// NoDuplicate attribute will be removed from coro.begin otherwise, it will
// interfere with inlining.
static void setCannotDuplicate(CoroIdInst *CoroId) {
  for (User *U : CoroId->users())
    if (auto *CB = dyn_cast<CoroBeginInst>(U))
      CB->setCannotDuplicate();
}
 
void Lowerer::lowerEarlyIntrinsics(Function &F) {
  CoroIdInst *CoroId = nullptr;
  CoroBeginInst *CoroBegin = nullptr;
  SmallVector<CoroFreeInst *, 4> CoroFrees;
  bool HasCoroSuspend = false;
  for (Instruction &I : llvm::make_early_inc_range(instructions(F))) {
    auto *CB = dyn_cast<CallBase>(&I);
    if (!CB)
      continue;
 
    switch (CB->getIntrinsicID()) {
      default:
        continue;
      case Intrinsic::coro_begin:
      case Intrinsic::coro_begin_custom_abi:
        if (CoroBegin)
          report_fatal_error(
              "coroutine should have exactly one defining @llvm.coro.begin");
        CoroBegin = cast<CoroBeginInst>(&I);
        break;
      case Intrinsic::coro_free:
        CoroFrees.push_back(cast<CoroFreeInst>(&I));
        break;
      case Intrinsic::coro_suspend:
        // Make sure that final suspend point is not duplicated as CoroSplit
        // pass expects that there is at most one final suspend point.
        if (cast<CoroSuspendInst>(&I)->isFinal())
          CB->setCannotDuplicate();
        HasCoroSuspend = true;
        break;
      case Intrinsic::coro_end_async:
      case Intrinsic::coro_end:
        // Make sure that fallthrough coro.end is not duplicated as CoroSplit
        // pass expects that there is at most one fallthrough coro.end.
        if (cast<AnyCoroEndInst>(&I)->isFallthrough())
          CB->setCannotDuplicate();
        break;
      case Intrinsic::coro_id:
        if (auto *CII = cast<CoroIdInst>(&I)) {
          if (CII->getInfo().isPreSplit()) {
            assert(F.isPresplitCoroutine() &&
                   "The frontend uses Switch-Resumed ABI should emit "
                   "\"presplitcoroutine\" attribute for the coroutine.");
            setCannotDuplicate(CII);
            CII->setCoroutineSelf();
            CoroId = cast<CoroIdInst>(&I);
          }
        }
        break;
      case Intrinsic::coro_id_retcon:
      case Intrinsic::coro_id_retcon_once:
      case Intrinsic::coro_id_async:
        F.setPresplitCoroutine();
        break;
      case Intrinsic::coro_resume:
        lowerResumeOrDestroy(*CB, CoroSubFnInst::ResumeIndex);
        break;
      case Intrinsic::coro_destroy:
        lowerResumeOrDestroy(*CB, CoroSubFnInst::DestroyIndex);
        break;
      case Intrinsic::coro_promise:
        lowerCoroPromise(cast<CoroPromiseInst>(&I));
        break;
      case Intrinsic::coro_done:
        lowerCoroDone(cast<IntrinsicInst>(&I));
        break;
    }
  }
 
  if (CoroId) {
    // Make sure that all CoroFree reference the coro.id intrinsic.
    // Token type is not exposed through coroutine C/C++ builtins to plain C, so
    // we allow specifying none and fixing it up here.
    for (CoroFreeInst *CF : CoroFrees)
      CF->setArgOperand(0, CoroId);
 
    hidePromiseAlloca(CoroId, CoroBegin);
  }
 
  // Coroutine suspention could potentially lead to any argument modified
  // outside of the function, hence arguments should not have noalias
  // attributes.
  if (HasCoroSuspend)
    for (Argument &A : F.args())
      if (A.hasNoAliasAttr())
        A.removeAttr(Attribute::NoAlias);
}
 
static bool declaresCoroEarlyIntrinsics(const Module &M) {
  // coro_suspend omitted as it is overloaded.
  return coro::declaresIntrinsics(
      M, {Intrinsic::coro_id, Intrinsic::coro_id_retcon,
          Intrinsic::coro_id_retcon_once, Intrinsic::coro_id_async,
          Intrinsic::coro_destroy, Intrinsic::coro_done, Intrinsic::coro_end,
          Intrinsic::coro_end_async, Intrinsic::coro_free,
          Intrinsic::coro_promise, Intrinsic::coro_resume});
}
 
PreservedAnalyses CoroEarlyPass::run(Module &M, ModuleAnalysisManager &) {
  if (!declaresCoroEarlyIntrinsics(M))
    return PreservedAnalyses::all();
 
  Lowerer L(M);
  for (auto &F : M)
    L.lowerEarlyIntrinsics(F);
 
  PreservedAnalyses PA;
  PA.preserveSet<CFGAnalyses>();
  return PA;
}