SPIRVLowerMemmove.cpp

//===- SPIRVLowerMemmove.cpp - Lower llvm.memmove to llvm.memcpys ---------===//
//
//                     The LLVM/SPIRV Translator
//
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//===----------------------------------------------------------------------===//
//
// This file implements lowering llvm.memmove into several llvm.memcpys.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "spvmemmove"

#include "SPIRVInternal.h"
#include "libSPIRV/SPIRVDebug.h"

#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Utils/LowerMemIntrinsics.h"

using namespace llvm;
using namespace SPIRV;

namespace SPIRV {

class SPIRVLowerMemmoveBase {
public:
  SPIRVLowerMemmoveBase() : Context(nullptr) {}

  void LowerMemMoveInst(MemMoveInst &I) {
    // There is no direct equivalent of @llvm.memmove in SPIR-V and the closest
    // instructions are 'OpCopyMemory' and 'OpCopyMemorySized'.
    //
    // 'OpCopyMemory' does not accept amount of bytes to copy and infers that
    // from type which is being copied; also it only allows to copy value of a
    // particular type to pointer pointing to the same type.
    //
    // 'OpCopyMemorySized' is closer to @llvm.memmove, because it actually
    // copies bytes, but unlike memove it is not explicitly specified whether it
    // supports overlapping source and destination. Therefore, we replace
    // memmove with two 'OpCopyMemorySized' instructions: the first one copies
    // bytes from source to a temporary location, the second one copies bytes
    // from that temporary location to the destination.
    IRBuilder<> Builder(I.getParent());
    Builder.SetInsertPoint(&I);

    auto *Length = cast<ConstantInt>(I.getLength());
    auto *AllocaTy = ArrayType::get(IntegerType::getInt8Ty(*Context),
                                    Length->getZExtValue());
    MaybeAlign SrcAlign = I.getSourceAlign();

    auto *Alloca = Builder.CreateAlloca(AllocaTy);
    if (SrcAlign.hasValue())
      Alloca->setAlignment(SrcAlign.getValue());

    // FIXME: Do we need to pass the size of alloca here? From LangRef:
    // > The first argument is a constant integer representing the size of the
    // > object, or -1 if it is variable sized.
    //
    // https://llvm.org/docs/LangRef.html#llvm-lifetime-start-intrinsic
    Builder.CreateLifetimeStart(Alloca);
    Builder.CreateMemCpy(Alloca, SrcAlign, I.getRawSource(), SrcAlign, Length,
                         I.isVolatile());

    auto *SecondCpy =
        Builder.CreateMemCpy(I.getRawDest(), I.getDestAlign(), Alloca, SrcAlign,
                             Length, I.isVolatile());
    Builder.CreateLifetimeEnd(Alloca);

    SecondCpy->takeName(&I);
    I.replaceAllUsesWith(SecondCpy);
    I.dropAllReferences();
    I.eraseFromParent();
  }

  bool expandMemMoveIntrinsicUses(Function &F) {
    bool Changed = false;

    for (User *U : make_early_inc_range(F.users())) {
      MemMoveInst *Inst = cast<MemMoveInst>(U);
      if (!isa<ConstantInt>(Inst->getLength())) {
        expandMemMoveAsLoop(Inst);
        Inst->eraseFromParent();
      } else {
        LowerMemMoveInst(*Inst);
      }
      Changed = true;
    }
    return Changed;
  }
  bool runLowerMemmove(Module &M) {
    Context = &M.getContext();
    bool Changed = false;

    for (Function &F : M) {
      if (!F.isDeclaration())
        continue;

      if (F.getIntrinsicID() == Intrinsic::memmove)
        Changed |= expandMemMoveIntrinsicUses(F);
    }

    verifyRegularizationPass(M, "SPIRVLowerMemmove");
    return Changed;
  }

private:
  LLVMContext *Context;
};

class SPIRVLowerMemmovePass : public llvm::PassInfoMixin<SPIRVLowerMemmovePass>,
                              public SPIRVLowerMemmoveBase {
public:
  llvm::PreservedAnalyses run(llvm::Module &M,
                              llvm::ModuleAnalysisManager &MAM) {
    return runLowerMemmove(M) ? llvm::PreservedAnalyses::none()
                              : llvm::PreservedAnalyses::all();
  }
};

class SPIRVLowerMemmoveLegacy : public ModulePass,
                                public SPIRVLowerMemmoveBase {
public:
  SPIRVLowerMemmoveLegacy() : ModulePass(ID) {
    initializeSPIRVLowerMemmoveLegacyPass(*PassRegistry::getPassRegistry());
  }
  bool runOnModule(Module &M) override { return runLowerMemmove(M); }

  static char ID;
};

char SPIRVLowerMemmoveLegacy::ID = 0;
} // namespace SPIRV

INITIALIZE_PASS(SPIRVLowerMemmoveLegacy, "spvmemmove",
                "Lower llvm.memmove into llvm.memcpy", false, false)

ModulePass *llvm::createSPIRVLowerMemmoveLegacy() {
  return new SPIRVLowerMemmoveLegacy();
}