llvm-project/llvm/lib/Target/SPIRV/SPIRVCBufferAccess.cpp

//===- SPIRVCBufferAccess.cpp - Translate CBuffer Loads ---------*- 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
//
//===----------------------------------------------------------------------===//
//
// This pass replaces all accesses to constant buffer global variables with
// accesses to the proper SPIR-V resource.
//
// The pass operates as follows:
// 1. It finds all constant buffers by looking for the `!hlsl.cbs` metadata.
// 2. For each cbuffer, it finds the global variable holding the resource handle
//    and the global variables for each of the cbuffer's members.
// 3. For each member variable, it creates a call to the
//    `llvm.spv.resource.getpointer` intrinsic. This intrinsic takes the
//    resource handle and the member's index within the cbuffer as arguments.
//    The result is a pointer to that member within the SPIR-V resource.
// 4. It then replaces all uses of the original member global variable with the
//    pointer returned by the `getpointer` intrinsic. This effectively retargets
//    all loads and GEPs to the new resource pointer.
// 5. Finally, it cleans up by deleting the original global variables and the
//    `!hlsl.cbs` metadata.
//
// This approach allows subsequent passes, like SPIRVEmitIntrinsics, to
// correctly handle GEPs that operate on the result of the `getpointer` call,
// folding them into a single OpAccessChain instruction.
//
//===----------------------------------------------------------------------===//

#include "SPIRVCBufferAccess.h"
#include "SPIRV.h"
#include "llvm/Frontend/HLSL/CBuffer.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/IntrinsicsSPIRV.h"
#include "llvm/IR/Module.h"

#define DEBUG_TYPE "spirv-cbuffer-access"
using namespace llvm;

// Finds the single instruction that defines the resource handle. This is
// typically a call to `llvm.spv.resource.handlefrombinding`.
static Instruction *findHandleDef(GlobalVariable *HandleVar) {
  for (User *U : HandleVar->users()) {
    if (auto *SI = dyn_cast<StoreInst>(U)) {
      if (auto *I = dyn_cast<Instruction>(SI->getValueOperand())) {
        return I;
      }
    }
  }
  return nullptr;
}

static bool replaceCBufferAccesses(Module &M) {
  std::optional<hlsl::CBufferMetadata> CBufMD = hlsl::CBufferMetadata::get(M);
  if (!CBufMD)
    return false;

  for (const hlsl::CBufferMapping &Mapping : *CBufMD) {
    Instruction *HandleDef = findHandleDef(Mapping.Handle);
    if (!HandleDef) {
      report_fatal_error("Could not find handle definition for cbuffer: " +
                         Mapping.Handle->getName());
    }

    // The handle definition should dominate all uses of the cbuffer members.
    // We'll insert our getpointer calls right after it.
    IRBuilder<> Builder(HandleDef->getNextNode());

    for (uint32_t Index = 0; Index < Mapping.Members.size(); ++Index) {
      GlobalVariable *MemberGV = Mapping.Members[Index].GV;
      if (MemberGV->use_empty()) {
        continue;
      }

      // Create the getpointer intrinsic call.
      Value *IndexVal = Builder.getInt32(Index);
      Type *PtrType = MemberGV->getType();
      Value *GetPointerCall = Builder.CreateIntrinsic(
          PtrType, Intrinsic::spv_resource_getpointer, {HandleDef, IndexVal});

      // We cannot use replaceAllUsesWith here because some uses may be
      // ConstantExprs, which cannot be replaced with non-constants.
      SmallVector<User *, 4> Users(MemberGV->users());
      for (User *U : Users) {
        U->replaceUsesOfWith(MemberGV, GetPointerCall);
      }
    }
  }

  // Now that all uses are replaced, clean up the globals and metadata.
  for (const hlsl::CBufferMapping &Mapping : *CBufMD) {
    for (const auto &Member : Mapping.Members) {
      Member.GV->eraseFromParent();
    }
    // Erase the stores to the handle variable before erasing the handle itself.
    SmallVector<Instruction *, 4> HandleStores;
    for (User *U : Mapping.Handle->users()) {
      if (auto *SI = dyn_cast<StoreInst>(U)) {
        HandleStores.push_back(SI);
      }
    }
    for (Instruction *I : HandleStores) {
      I->eraseFromParent();
    }
    Mapping.Handle->eraseFromParent();
  }

  CBufMD->eraseFromModule();
  return true;
}

PreservedAnalyses SPIRVCBufferAccess::run(Module &M,
                                          ModuleAnalysisManager &AM) {
  if (replaceCBufferAccesses(M)) {
    return PreservedAnalyses::none();
  }
  return PreservedAnalyses::all();
}

namespace {
class SPIRVCBufferAccessLegacy : public ModulePass {
public:
  bool runOnModule(Module &M) override { return replaceCBufferAccesses(M); }
  StringRef getPassName() const override { return "SPIRV CBuffer Access"; }
  SPIRVCBufferAccessLegacy() : ModulePass(ID) {}

  static char ID; // Pass identification.
};
char SPIRVCBufferAccessLegacy::ID = 0;
} // end anonymous namespace

INITIALIZE_PASS(SPIRVCBufferAccessLegacy, DEBUG_TYPE, "SPIRV CBuffer Access",
                false, false)

ModulePass *llvm::createSPIRVCBufferAccessLegacyPass() {
  return new SPIRVCBufferAccessLegacy();
}