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[mlir][vector] Add mask elimination transform (llvm#99314)
This adds a new transform `eliminateVectorMasks()` which aims at removing scalable `vector.create_masks` that will be all-true at runtime. It attempts to do this by simply pattern-matching the mask operands (similar to some canonicalizations), if that does not lead to an answer (is all-true? yes/no), then value bounds analysis will be used to find the lower bound of the unknown operands. If the lower bound is >= to the corresponding mask vector type dim, then that dimension of the mask is all true. Note that the pattern matching prevents expensive value-bounds analysis in cases where the mask won't be all true. For example: ```mlir %mask = vector.create_mask %dynamicValue, %c2 : vector<8x4xi1> ``` From looking at `%c2` we can tell this is not going to be an all-true mask, so we don't need to run the value-bounds analysis for `%dynamicValue` (and can exit the transform early). Note: Eliminating create_masks here means replacing them with all-true constants (which will then lead to the masks folding away).
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118
mlir/lib/Dialect/Vector/Transforms/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 | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
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| #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" | ||
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| using namespace mlir; | ||
| using namespace mlir::vector; | ||
| namespace { | ||
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| /// 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(); | ||
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| struct UnknownMaskDim { | ||
| size_t position; | ||
| Value dimSize; | ||
| }; | ||
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| // Loop over the CreateMaskOp operands and collect unknown dims (i.e. dims | ||
| // that are not obviously constant). If any constant dimension is not all-true | ||
| // bail out early (as this transform only trying to resolve all-true masks). | ||
| // This avoids doing value-bounds anaylis in cases like: | ||
| // `%mask = vector.create_mask %dynamicValue, %c2 : vector<8x4xi1>` | ||
| // ...where it is known the mask is not all-true by looking at `%c2`. | ||
| 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}); | ||
| } | ||
| } | ||
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| for (auto [i, dimSize] : unknownDims) { | ||
| // Compute the lower bound for the unknown dimension (i.e. the smallest | ||
| // value it could be). | ||
| FailureOr<ConstantOrScalableBound> dimLowerBound = | ||
| vector::ScalableValueBoundsConstraintSet::computeScalableBound( | ||
| dimSize, {}, vscaleRange.vscaleMin, vscaleRange.vscaleMax, | ||
| presburger::BoundType::LB); | ||
| if (failed(dimLowerBound)) | ||
| return failure(); | ||
| auto dimLowerBoundSize = dimLowerBound->getSize(); | ||
| if (failed(dimLowerBoundSize)) | ||
| return failure(); | ||
| if (dimLowerBoundSize->scalable) { | ||
| // 1. The lower bound, LB, is scalable. If LB is < the mask dim size then | ||
| // this dim is not all-true. | ||
| if (dimLowerBoundSize->baseSize < maskTypeDimSizes[i]) | ||
| return failure(); | ||
| } else { | ||
| // 2. The lower bound, LB, is a constant. | ||
| // - If the mask dim size is scalable then this dim is not all-true. | ||
| if (maskTypeDimScalableFlags[i]) | ||
| return failure(); | ||
| // - If LB < the _fixed-size_ mask dim size then this dim is not all-true. | ||
| if (dimLowerBoundSize->baseSize < maskTypeDimSizes[i]) | ||
| return failure(); | ||
| } | ||
| } | ||
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| // 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<vector::ConstantMaskOp>( | ||
| createMaskOp.getLoc(), maskType, ConstantMaskKind::AllTrue); | ||
| rewriter.replaceAllUsesWith(createMaskOp, allTrue); | ||
| return success(); | ||
| } | ||
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| } // namespace | ||
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| namespace mlir::vector { | ||
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| 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; | ||
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| OpBuilder::InsertionGuard g(rewriter); | ||
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| // Build worklist so we can safely insert new ops in | ||
| // `resolveAllTrueCreateMaskOp()`. | ||
| SmallVector<vector::CreateMaskOp> worklist; | ||
| function.walk([&](vector::CreateMaskOp createMaskOp) { | ||
| worklist.push_back(createMaskOp); | ||
| }); | ||
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| rewriter.setInsertionPointToStart(&function.front()); | ||
| for (auto mask : worklist) | ||
| (void)resolveAllTrueCreateMaskOp(rewriter, mask, *vscaleRange); | ||
| } | ||
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| } // namespace mlir::vector |
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