llvm-project/mlir/lib/Conversion/GPUCommon/GPUOpsLowering.cpp

//===- GPUOpsLowering.cpp - GPU FuncOp / ReturnOp lowering ----------------===//
//
// 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 "GPUOpsLowering.h"
#include "mlir/Dialect/StandardOps/IR/Ops.h"
#include "mlir/IR/Builders.h"
#include "llvm/Support/FormatVariadic.h"

using namespace mlir;

LogicalResult
GPUFuncOpLowering::matchAndRewrite(gpu::GPUFuncOp gpuFuncOp,
                                   ArrayRef<Value> operands,
                                   ConversionPatternRewriter &rewriter) const {
  assert(operands.empty() && "func op is not expected to have operands");
  Location loc = gpuFuncOp.getLoc();

  SmallVector<LLVM::GlobalOp, 3> workgroupBuffers;
  workgroupBuffers.reserve(gpuFuncOp.getNumWorkgroupAttributions());
  for (auto en : llvm::enumerate(gpuFuncOp.getWorkgroupAttributions())) {
    Value attribution = en.value();

    auto type = attribution.getType().dyn_cast<MemRefType>();
    assert(type && type.hasStaticShape() && "unexpected type in attribution");

    uint64_t numElements = type.getNumElements();

    auto elementType =
        typeConverter->convertType(type.getElementType()).template cast<Type>();
    auto arrayType = LLVM::LLVMArrayType::get(elementType, numElements);
    std::string name = std::string(
        llvm::formatv("__wg_{0}_{1}", gpuFuncOp.getName(), en.index()));
    auto globalOp = rewriter.create<LLVM::GlobalOp>(
        gpuFuncOp.getLoc(), arrayType, /*isConstant=*/false,
        LLVM::Linkage::Internal, name, /*value=*/Attribute(),
        gpu::GPUDialect::getWorkgroupAddressSpace());
    workgroupBuffers.push_back(globalOp);
  }

  // Rewrite the original GPU function to an LLVM function.
  auto funcType = typeConverter->convertType(gpuFuncOp.getType())
                      .template cast<LLVM::LLVMPointerType>()
                      .getElementType();

  // Remap proper input types.
  TypeConverter::SignatureConversion signatureConversion(
      gpuFuncOp.front().getNumArguments());
  getTypeConverter()->convertFunctionSignature(
      gpuFuncOp.getType(), /*isVariadic=*/false, signatureConversion);

  // Create the new function operation. Only copy those attributes that are
  // not specific to function modeling.
  SmallVector<NamedAttribute, 4> attributes;
  for (const auto &attr : gpuFuncOp.getAttrs()) {
    if (attr.first == SymbolTable::getSymbolAttrName() ||
        attr.first == impl::getTypeAttrName() ||
        attr.first == gpu::GPUFuncOp::getNumWorkgroupAttributionsAttrName())
      continue;
    attributes.push_back(attr);
  }
  // Add a dialect specific kernel attribute in addition to GPU kernel
  // attribute. The former is necessary for further translation while the
  // latter is expected by gpu.launch_func.
  if (gpuFuncOp.isKernel())
    attributes.emplace_back(kernelAttributeName, rewriter.getUnitAttr());
  auto llvmFuncOp = rewriter.create<LLVM::LLVMFuncOp>(
      gpuFuncOp.getLoc(), gpuFuncOp.getName(), funcType,
      LLVM::Linkage::External, attributes);

  {
    // Insert operations that correspond to converted workgroup and private
    // memory attributions to the body of the function. This must operate on
    // the original function, before the body region is inlined in the new
    // function to maintain the relation between block arguments and the
    // parent operation that assigns their semantics.
    OpBuilder::InsertionGuard guard(rewriter);

    // Rewrite workgroup memory attributions to addresses of global buffers.
    rewriter.setInsertionPointToStart(&gpuFuncOp.front());
    unsigned numProperArguments = gpuFuncOp.getNumArguments();
    auto i32Type = IntegerType::get(rewriter.getContext(), 32);

    Value zero = nullptr;
    if (!workgroupBuffers.empty())
      zero = rewriter.create<LLVM::ConstantOp>(loc, i32Type,
                                               rewriter.getI32IntegerAttr(0));
    for (auto en : llvm::enumerate(workgroupBuffers)) {
      LLVM::GlobalOp global = en.value();
      Value address = rewriter.create<LLVM::AddressOfOp>(loc, global);
      auto elementType =
          global.getType().cast<LLVM::LLVMArrayType>().getElementType();
      Value memory = rewriter.create<LLVM::GEPOp>(
          loc, LLVM::LLVMPointerType::get(elementType, global.addr_space()),
          address, ArrayRef<Value>{zero, zero});

      // Build a memref descriptor pointing to the buffer to plug with the
      // existing memref infrastructure. This may use more registers than
      // otherwise necessary given that memref sizes are fixed, but we can try
      // and canonicalize that away later.
      Value attribution = gpuFuncOp.getWorkgroupAttributions()[en.index()];
      auto type = attribution.getType().cast<MemRefType>();
      auto descr = MemRefDescriptor::fromStaticShape(
          rewriter, loc, *getTypeConverter(), type, memory);
      signatureConversion.remapInput(numProperArguments + en.index(), descr);
    }

    // Rewrite private memory attributions to alloca'ed buffers.
    unsigned numWorkgroupAttributions = gpuFuncOp.getNumWorkgroupAttributions();
    auto int64Ty = IntegerType::get(rewriter.getContext(), 64);
    for (auto en : llvm::enumerate(gpuFuncOp.getPrivateAttributions())) {
      Value attribution = en.value();
      auto type = attribution.getType().cast<MemRefType>();
      assert(type && type.hasStaticShape() && "unexpected type in attribution");

      // Explicitly drop memory space when lowering private memory
      // attributions since NVVM models it as `alloca`s in the default
      // memory space and does not support `alloca`s with addrspace(5).
      auto ptrType = LLVM::LLVMPointerType::get(
          typeConverter->convertType(type.getElementType())
              .template cast<Type>(),
          allocaAddrSpace);
      Value numElements = rewriter.create<LLVM::ConstantOp>(
          gpuFuncOp.getLoc(), int64Ty,
          rewriter.getI64IntegerAttr(type.getNumElements()));
      Value allocated = rewriter.create<LLVM::AllocaOp>(
          gpuFuncOp.getLoc(), ptrType, numElements, /*alignment=*/0);
      auto descr = MemRefDescriptor::fromStaticShape(
          rewriter, loc, *getTypeConverter(), type, allocated);
      signatureConversion.remapInput(
          numProperArguments + numWorkgroupAttributions + en.index(), descr);
    }
  }

  // Move the region to the new function, update the entry block signature.
  rewriter.inlineRegionBefore(gpuFuncOp.getBody(), llvmFuncOp.getBody(),
                              llvmFuncOp.end());
  if (failed(rewriter.convertRegionTypes(&llvmFuncOp.getBody(), *typeConverter,
                                         &signatureConversion)))
    return failure();

  rewriter.eraseOp(gpuFuncOp);
  return success();
}