MaskedloadToLoad.cpp

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//===- MaskedloadToLoad.cpp - Lowers maskedload to load -------===//
//
// 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 "mlir/Dialect/AMDGPU/Transforms/Passes.h"
 
#include "mlir/Dialect/AMDGPU/IR/AMDGPUDialect.h"
#include "mlir/Dialect/Arith/IR/Arith.h"
#include "mlir/Dialect/MemRef/IR/MemRef.h"
#include "mlir/Dialect/MemRef/Utils/MemRefUtils.h"
#include "mlir/Dialect/SCF/IR/SCF.h"
#include "mlir/Dialect/Vector/IR/VectorOps.h"
#include "mlir/IR/BuiltinTypes.h"
#include "mlir/IR/OpDefinition.h"
#include "mlir/IR/PatternMatch.h"
#include "mlir/IR/TypeUtilities.h"
#include "mlir/Support/LogicalResult.h"
#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
#include "llvm/Support/MathExtras.h"
 
namespace mlir::amdgpu {
#define GEN_PASS_DEF_AMDGPUMASKEDLOADTOLOADPASS
#include "mlir/Dialect/AMDGPU/Transforms/Passes.h.inc"
} // namespace mlir::amdgpu
 
using namespace mlir;
using namespace mlir::amdgpu;
 
/// This pattern supports lowering of: `vector.maskedload` to `vector.load`
/// and `arith.select` if the memref is in buffer address space.
static LogicalResult hasBufferAddressSpace(Type type) {
  auto memRefType = dyn_cast<MemRefType>(type);
  if (!memRefType)
    return failure();
 
  Attribute addrSpace = memRefType.getMemorySpace();
  if (!isa_and_nonnull<amdgpu::AddressSpaceAttr>(addrSpace))
    return failure();
 
  if (dyn_cast<amdgpu::AddressSpaceAttr>(addrSpace).getValue() !=
      amdgpu::AddressSpace::FatRawBuffer)
    return failure();
 
  return success();
}
 
static Value createVectorLoadForMaskedLoad(OpBuilder &builder, Location loc,
                                           vector::MaskedLoadOp maskedOp,
                                           bool passthru) {
  VectorType vectorType = maskedOp.getVectorType();
  Value load = vector::LoadOp::create(
      builder, loc, vectorType, maskedOp.getBase(), maskedOp.getIndices());
  if (passthru)
    load = arith::SelectOp::create(builder, loc, vectorType, maskedOp.getMask(),
                                   load, maskedOp.getPassThru());
  return load;
}
 
/// Check if the given value comes from a broadcasted i1 condition.
static FailureOr<Value> matchFullMask(OpBuilder &b, Value val) {
  auto broadcastOp = val.getDefiningOp<vector::BroadcastOp>();
  if (!broadcastOp)
    return failure();
  if (isa<VectorType>(broadcastOp.getSourceType()))
    return failure();
  return broadcastOp.getSource();
}
 
static constexpr char kMaskedloadNeedsMask[] =
    "amdgpu.buffer_maskedload_needs_mask";
 
namespace {
 
struct MaskedLoadLowering final : OpRewritePattern<vector::MaskedLoadOp> {
  using OpRewritePattern::OpRewritePattern;
 
  LogicalResult matchAndRewrite(vector::MaskedLoadOp maskedOp,
                                PatternRewriter &rewriter) const override {
    if (maskedOp->hasDiscardableAttr(kMaskedloadNeedsMask))
      return rewriter.notifyMatchFailure(maskedOp, "already rewritten");
 
    if (failed(hasBufferAddressSpace(maskedOp.getBase().getType()))) {
      return rewriter.notifyMatchFailure(
          maskedOp, "isn't a load from a fat buffer resource");
    }
 
    // Check if this is either a full inbounds load or an empty, oob load. If
    // so, take the fast path and don't generate an if condition, because we
    // know doing the oob load is always safe.
    if (succeeded(matchFullMask(rewriter, maskedOp.getMask()))) {
      Value load = createVectorLoadForMaskedLoad(rewriter, maskedOp.getLoc(),
                                                 maskedOp, /*passthru=*/true);
      rewriter.replaceOp(maskedOp, load);
      return success();
    }
 
    Location loc = maskedOp.getLoc();
    Value src = maskedOp.getBase();
 
    VectorType vectorType = maskedOp.getVectorType();
    int64_t vectorSize = vectorType.getNumElements();
    int64_t elementBitWidth = vectorType.getElementTypeBitWidth();
    SmallVector<OpFoldResult> indices = maskedOp.getIndices();
 
    auto stridedMetadata =
        memref::ExtractStridedMetadataOp::create(rewriter, loc, src);
    SmallVector<OpFoldResult> strides =
        stridedMetadata.getConstifiedMixedStrides();
    SmallVector<OpFoldResult> sizes = stridedMetadata.getConstifiedMixedSizes();
    OpFoldResult offset = stridedMetadata.getConstifiedMixedOffset();
    memref::LinearizedMemRefInfo linearizedInfo;
    OpFoldResult linearizedIndices;
    std::tie(linearizedInfo, linearizedIndices) =
        memref::getLinearizedMemRefOffsetAndSize(rewriter, loc, elementBitWidth,
                                                 elementBitWidth, offset, sizes,
                                                 strides, indices);
 
    // delta = bufferSize - linearizedOffset
    Value vectorSizeOffset =
        arith::ConstantIndexOp::create(rewriter, loc, vectorSize);
    Value linearIndex =
        getValueOrCreateConstantIndexOp(rewriter, loc, linearizedIndices);
    Value totalSize = getValueOrCreateConstantIndexOp(
        rewriter, loc, linearizedInfo.linearizedSize);
    Value delta = arith::SubIOp::create(rewriter, loc, totalSize, linearIndex);
 
    // 1) check if delta < vectorSize
    Value isOutofBounds = arith::CmpIOp::create(
        rewriter, loc, arith::CmpIPredicate::ult, delta, vectorSizeOffset);
 
    // 2) check if (detla % elements_per_word != 0)
    Value elementsPerWord = arith::ConstantIndexOp::create(
        rewriter, loc, llvm::divideCeil(32, elementBitWidth));
    Value isNotWordAligned = arith::CmpIOp::create(
        rewriter, loc, arith::CmpIPredicate::ne,
        arith::RemUIOp::create(rewriter, loc, delta, elementsPerWord),
        arith::ConstantIndexOp::create(rewriter, loc, 0));
 
    // We take the fallback of maskedload default lowering only it is both
    // out-of-bounds and not word aligned. The fallback ensures correct results
    // when loading at the boundary of the buffer since buffer load returns
    // inconsistent zeros for the whole word when boundary is crossed.
    Value ifCondition =
        arith::AndIOp::create(rewriter, loc, isOutofBounds, isNotWordAligned);
 
    auto thenBuilder = [&](OpBuilder &builder, Location loc) {
      Operation *read = builder.clone(*maskedOp.getOperation());
      read->setDiscardableAttr(kMaskedloadNeedsMask, builder.getUnitAttr());
      Value readResult = read->getResult(0);
      scf::YieldOp::create(builder, loc, readResult);
    };
 
    auto elseBuilder = [&](OpBuilder &builder, Location loc) {
      Value res = createVectorLoadForMaskedLoad(builder, loc, maskedOp,
                                                /*passthru=*/true);
      scf::YieldOp::create(rewriter, loc, res);
    };
 
    auto ifOp =
        scf::IfOp::create(rewriter, loc, ifCondition, thenBuilder, elseBuilder);
 
    rewriter.replaceOp(maskedOp, ifOp);
 
    return success();
  }
};
 
struct FullMaskedLoadToConditionalLoad
    : OpRewritePattern<vector::MaskedLoadOp> {
  using OpRewritePattern::OpRewritePattern;
 
  LogicalResult matchAndRewrite(vector::MaskedLoadOp loadOp,
                                PatternRewriter &rewriter) const override {
    if (succeeded(hasBufferAddressSpace(loadOp.getBase().getType())))
      return rewriter.notifyMatchFailure(
          loadOp, "buffer loads are handled by a more specialized pattern");
 
    FailureOr<Value> maybeCond = matchFullMask(rewriter, loadOp.getMask());
    if (failed(maybeCond)) {
      return rewriter.notifyMatchFailure(loadOp,
                                         "isn't loading a broadcasted scalar");
    }
 
    Value cond = maybeCond.value();
    auto trueBuilder = [&](OpBuilder &builder, Location loc) {
      Value res = createVectorLoadForMaskedLoad(builder, loc, loadOp,
                                                /*passthru=*/false);
      scf::YieldOp::create(rewriter, loc, res);
    };
    auto falseBuilder = [&](OpBuilder &builder, Location loc) {
      scf::YieldOp::create(rewriter, loc, loadOp.getPassThru());
    };
    auto ifOp = scf::IfOp::create(rewriter, loadOp.getLoc(), cond, trueBuilder,
                                  falseBuilder);
    rewriter.replaceOp(loadOp, ifOp);
    return success();
  }
};
 
struct FullMaskedStoreToConditionalStore
    : OpRewritePattern<vector::MaskedStoreOp> {
  using OpRewritePattern::OpRewritePattern;
 
  LogicalResult matchAndRewrite(vector::MaskedStoreOp storeOp,
                                PatternRewriter &rewriter) const override {
    // A condition-free implementation of fully masked stores requires
    // 1) an accessor for the num_records field on buffer resources/fat pointers
    // 2) knowledge that said field will always be set accurately - that is,
    // that writes to x < num_records of offset wouldn't trap, which is
    // something a pattern user would need to assert or we'd need to prove.
    //
    // Therefore, conditional stores to buffers still go down this path at
    // present.
 
    FailureOr<Value> maybeCond = matchFullMask(rewriter, storeOp.getMask());
    if (failed(maybeCond)) {
      return failure();
    }
    Value cond = maybeCond.value();
 
    auto trueBuilder = [&](OpBuilder &builder, Location loc) {
      vector::StoreOp::create(rewriter, loc, storeOp.getValueToStore(),
                              storeOp.getBase(), storeOp.getIndices());
      scf::YieldOp::create(rewriter, loc);
    };
    auto ifOp =
        scf::IfOp::create(rewriter, storeOp.getLoc(), cond, trueBuilder);
    rewriter.replaceOp(storeOp, ifOp);
    return success();
  }
};
 
} // namespace
 
void mlir::amdgpu::populateAmdgpuMaskedloadToLoadPatterns(
    RewritePatternSet &patterns, PatternBenefit benefit) {
  patterns.add<MaskedLoadLowering, FullMaskedLoadToConditionalLoad,
               FullMaskedStoreToConditionalStore>(patterns.getContext(),
                                                  benefit);
}
 
struct AmdgpuMaskedloadToLoadPass final
    : amdgpu::impl::AmdgpuMaskedloadToLoadPassBase<AmdgpuMaskedloadToLoadPass> {
  void runOnOperation() override {
    RewritePatternSet patterns(&getContext());
    populateAmdgpuMaskedloadToLoadPatterns(patterns);
    if (failed(applyPatternsGreedily(getOperation(), std::move(patterns)))) {
      return signalPassFailure();
    }
  }
};