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VectorMaskElimination.cpp
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VectorMaskElimination.cpp
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//===- VectorMaskElimination.cpp - Eliminate Vector Masks -----------------===//
//
// 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/Arith/IR/Arith.h"
#include "mlir/Dialect/Utils/StaticValueUtils.h"
#include "mlir/Dialect/Vector/IR/ScalableValueBoundsConstraintSet.h"
#include "mlir/Dialect/Vector/Transforms/VectorRewritePatterns.h"
#include "mlir/Dialect/Vector/Transforms/VectorTransforms.h"
#include "mlir/Interfaces/FunctionInterfaces.h"
using namespace mlir;
using namespace mlir::vector;
namespace {
/// If `value` is a constant multiple of `vector.vscale` return the multiplier.
std::optional<int64_t> getConstantVscaleMultiplier(Value value) {
if (value.getDefiningOp<vector::VectorScaleOp>())
return 1;
auto mul = value.getDefiningOp<arith::MulIOp>();
if (!mul)
return {};
auto lhs = mul.getLhs();
auto rhs = mul.getRhs();
if (lhs.getDefiningOp<vector::VectorScaleOp>())
return getConstantIntValue(rhs);
if (rhs.getDefiningOp<vector::VectorScaleOp>())
return getConstantIntValue(lhs);
return {};
}
/// Attempts to resolve a (scalable) CreateMaskOp to an all-true constant mask.
/// All-true masks can then be eliminated by simple folds.
LogicalResult resolveAllTrueCreateMaskOp(IRRewriter &rewriter,
vector::CreateMaskOp createMaskOp,
VscaleRange vscaleRange) {
auto maskType = createMaskOp.getVectorType();
auto maskTypeDimScalableFlags = maskType.getScalableDims();
auto maskTypeDimSizes = maskType.getShape();
struct UnknownMaskDim {
size_t position;
Value dimSize;
};
// Check for any dims that could be (partially) false before doing the more
// expensive value bounds computations.
SmallVector<UnknownMaskDim> unknownDims;
for (auto [i, dimSize] : llvm::enumerate(createMaskOp.getOperands())) {
if (auto intSize = getConstantIntValue(dimSize)) {
// Mask not all-true for this dim.
if (maskTypeDimScalableFlags[i] || intSize < maskTypeDimSizes[i])
return failure();
} else if (auto vscaleMultiplier = getConstantVscaleMultiplier(dimSize)) {
// Mask not all-true for this dim.
if (vscaleMultiplier < maskTypeDimSizes[i])
return failure();
} else {
// Unknown (without further analysis).
unknownDims.push_back(UnknownMaskDim{i, dimSize});
}
}
for (auto [i, dimSize] : unknownDims) {
// Compute the lower bound for the unknown dimension (i.e. the smallest
// value it could be).
auto lowerBound =
vector::ScalableValueBoundsConstraintSet::computeScalableBound(
dimSize, {}, vscaleRange.vscaleMin, vscaleRange.vscaleMax,
presburger::BoundType::LB);
if (failed(lowerBound))
return failure();
auto boundSize = lowerBound->getSize();
if (failed(boundSize))
return failure();
if (boundSize->scalable) {
// If the lower bound is scalable and >= to the mask dim size then this
// dim is all-true.
if (boundSize->baseSize < maskTypeDimSizes[i])
return failure();
} else {
// If the lower bound is a constant and >= to the _fixed-size_ mask dim
// size then this dim is all-true.
if (maskTypeDimScalableFlags[i])
return failure();
if (boundSize->baseSize < maskTypeDimSizes[i])
return failure();
}
}
// Replace createMaskOp with an all-true constant. This should result in the
// mask being removed in most cases (as xfer ops + vector.mask have folds to
// remove all-true masks).
auto allTrue = rewriter.create<arith::ConstantOp>(
createMaskOp.getLoc(), maskType, DenseElementsAttr::get(maskType, true));
rewriter.replaceAllUsesWith(createMaskOp, allTrue);
return success();
}
} // namespace
namespace mlir::vector {
void eliminateVectorMasks(IRRewriter &rewriter, FunctionOpInterface function,
std::optional<VscaleRange> vscaleRange) {
// TODO: Support fixed-size case. This is less likely to be useful as for
// fixed-size code dimensions are all static so masks tend to fold away.
if (!vscaleRange)
return;
OpBuilder::InsertionGuard g(rewriter);
// Build worklist so we can safely insert new ops in
// `resolveAllTrueCreateMaskOp()`.
SmallVector<vector::CreateMaskOp> worklist;
function.walk([&](vector::CreateMaskOp createMaskOp) {
worklist.push_back(createMaskOp);
});
rewriter.setInsertionPointToStart(&function.front());
for (auto mask : worklist)
(void)resolveAllTrueCreateMaskOp(rewriter, mask, *vscaleRange);
}
} // namespace mlir::vector