[Mlir-commits] [mlir] [mlir][vector] Add mask elimination transform (PR #99314)

[Mehdi Amini]

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@@ -0,0 +1,117 @@
+#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);
+  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);
----------------
joker-eph wrote:

This statement as a generality does not seem correct: a lot of mutation are perfectly fine to do while walking the IR, it all depends what you do. 
I would say that if you need a worklist, you should document why the delayed modification is needed.

https://github.com/llvm/llvm-project/pull/99314