Padding.cpp

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//===- Padding.cpp - Padding of Linalg ops --------------------------------===//
//
// 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/Linalg/Transforms/Transforms.h"
 
#include "mlir/Dialect/Affine/IR/AffineOps.h"
#include "mlir/Dialect/Bufferization/IR/Bufferization.h"
#include "mlir/Dialect/Complex/IR/Complex.h"
#include "mlir/Dialect/Linalg/IR/Linalg.h"
#include "mlir/Dialect/Tensor/IR/Tensor.h"
#include "mlir/Interfaces/ValueBoundsOpInterface.h"
 
#define DEBUG_TYPE "linalg-padding"
 
using namespace mlir;
using namespace mlir::linalg;
 
#define DBGS() (llvm::dbgs() << "[" DEBUG_TYPE << "]: ")
#define DBGSNL() (llvm::dbgs() << "\n")
 
namespace {
/// Helper class for storing padding information.
struct PaddingInfo {
  PaddingInfo(int64_t padToMultipleOf = 1, OpFoldResult size = {})
      : padToMultipleOf(padToMultipleOf), size(size) {}
  /// Pad the tensor to a multiple of.
  int64_t padToMultipleOf = 1;
  /// The size used for padding.
  OpFoldResult size = {};
};
 
/// Helper class for storing and computing the padded shape.
struct PaddedShape {
  /// Initializes the shape information and on success it returns whether the
  /// shape of the operand will change. Returns failure if the operand cannot be
  /// padded.
  FailureOr<bool> initialize(linalg::LinalgOp opToPad, OpOperand *opOperand,
                             const LinalgPaddingOptions &options);
 
  /// Computs the padded shape.
  void computePadding(OpBuilder &builder, Value operand);
 
  /// Returns the new tensor type.
  RankedTensorType getType(Type elemTy) {
    return RankedTensorType::get(shape, elemTy);
  }
 
  SmallVector<Value> dynDims;
 
private:
  SmallVector<int64_t> shape;
  DenseMap<int64_t, PaddingInfo> dimToInfo;
};
} // namespace
 
FailureOr<bool> PaddedShape::initialize(linalg::LinalgOp opToPad,
                                        OpOperand *opOperand,
                                        const LinalgPaddingOptions &options) {
  AffineMap indexingMap = opToPad.getMatchingIndexingMap(opOperand);
 
  // Initialize the padded shape.
  llvm::append_range(shape, opToPad.getShape(opOperand));
 
  // Collect the shape dimensions that are a function of "paddingDimensions",
  // along with the multiple that they should be padded to ("1" if none).
  bool alreadyHasRequestedShape = true;
  for (const auto &dimEn : enumerate(options.paddingDimensions)) {
    for (const auto &en : enumerate(indexingMap.getResults())) {
      if (en.value().isFunctionOfDim(dimEn.value())) {
        PaddingInfo paddingInfo;
        int64_t dimSize = shape[en.index()];
        if (options.padToMultipleOf.has_value()) {
          paddingInfo.padToMultipleOf =
              (*options.padToMultipleOf)[dimEn.index()];
        } else {
          paddingInfo.padToMultipleOf = 1;
        }
 
        // Check if the user provided a size in the options.
        paddingInfo.size =
            options.getSizeToPadTo(opOperand->getOperandNumber(), en.index());
 
        // Set the padding info.
        dimToInfo[en.index()] = paddingInfo;
        if (ShapedType::isDynamic(dimSize) ||
            dimSize % paddingInfo.padToMultipleOf != 0 ||
            !paddingInfo.size.isNull()) {
          alreadyHasRequestedShape = false;
        }
      }
    }
  }
 
  // Upper bound the sizes to obtain a static bounding box.
  for (int64_t i = 0, e = shape.size(); i < e; ++i) {
    LLVM_DEBUG(DBGS() << "--computing un-padded size for dim " << i << "\n");
    // Skip dimensions that do not require padding.
    if (!dimToInfo.contains(i)) {
      LLVM_DEBUG(DBGS() << "----dim does not require padding, SKIP\n");
      continue;
    }
    PaddingInfo &info = dimToInfo[i];
    if (info.size) {
      LLVM_DEBUG(DBGS() << "----the user provided the size: " << info.size
                        << "\n");
      continue;
    }
    // Otherwise, try to compute a constant upper bound for the size value.
    FailureOr<int64_t> upperBound =
        ValueBoundsConstraintSet::computeConstantBound(
            presburger::BoundType::UB,
            {opOperand->get(),
             /*dim=*/i},
            /*stopCondition=*/nullptr, /*closedUB=*/true);
    if (failed(upperBound)) {
      LLVM_DEBUG(
          DBGS() << "----could not compute a bounding box for padding\n");
      return failure();
    }
    info.size =
        IntegerAttr::get(IndexType::get(opToPad.getContext()), *upperBound);
    LLVM_DEBUG(DBGS() << "----new un-padded size: " << info.size << "\n");
  }
  return alreadyHasRequestedShape;
}
 
void PaddedShape::computePadding(OpBuilder &builder, Value operand) {
  Location loc = operand.getLoc();
  AffineExpr sizeSym = builder.getAffineSymbolExpr(0);
 
  // Compute the padding for each dimension.
  for (auto &&[i, dim] : llvm::enumerate(shape)) {
    LLVM_DEBUG(DBGS() << "--computing padded size for dim " << i << "\n");
 
    // Get the padding info or default info for the shape dimension.
    PaddingInfo paddingInfo = dimToInfo.lookup(i);
 
    // Skip dimensions that do not require padding.
    if (paddingInfo.size.isNull()) {
      LLVM_DEBUG(DBGS() << "----dim does not require padding, SKIP\n");
 
      // We still need to push the size as `makeComposedPadHighOp` expects a
      // range with all the dynamic sizes, whether they're being padded or not.
      if (ShapedType::isDynamic(dim)) {
        dynDims.push_back(
            cast<Value>(tensor::getMixedSize(builder, loc, operand, i)));
      }
      continue;
    }
 
    // Compute the padded size to be a multiple of `padToMultipleOf`.
    AffineExpr szExpr = (sizeSym).ceilDiv(paddingInfo.padToMultipleOf) *
                        paddingInfo.padToMultipleOf;
    OpFoldResult paddedSize = affine::makeComposedFoldedAffineApply(
        builder, loc, szExpr, paddingInfo.size);
    assert(paddedSize && "invalid arguments to affine apply");
 
    if (auto cstSzAttr = dyn_cast<Attribute>(paddedSize)) {
      // Update the shape as the size is static.
      dim = cast<IntegerAttr>(cstSzAttr).getValue().getZExtValue();
    } else {
      // Add a dynamic dimension.
      dim = ShapedType::kDynamic;
      dynDims.push_back(cast<Value>(paddedSize));
    }
    LLVM_DEBUG(DBGS() << "----new dim size: " << paddedSize << "\n");
  }
}
 
/// Pad the `opOperand` in the "paddingDimensions" using the padding value and
/// the nofold flag found in "paddingValues" and "nofoldFlags", respectively.
///
/// Exit early and return the `opOperand` value if it already has the requested
/// shape. i.e.:
/// - static shape
/// - nofold is not set
/// - dim sizes are multiples of "padToMultipleOf"
///
/// Otherwise, try to pad the shape dimensions that match the iterator
/// dimensions "paddingDimensions" and return the tensor::PadOp result if
/// padding succeeds or failure otherwise.
static FailureOr<Value> padOperandToSmallestStaticBoundingBox(
    RewriterBase &rewriter, linalg::LinalgOp opToPad, OpOperand *opOperand,
    const LinalgPaddingOptions &options) {
  assert(
      (!options.padToMultipleOf.has_value() ||
       options.padToMultipleOf->size() == options.paddingDimensions.size()) &&
      "invalid number of elements in padToMultipleOf");
 
  // Initialize the padded shape and get whether it requires padding.
  PaddedShape shape;
  FailureOr<bool> alreadyHasRequestedShape =
      shape.initialize(opToPad, opOperand, options);
  if (failed(alreadyHasRequestedShape)) {
    return rewriter.notifyMatchFailure(opToPad,
                                       "--failed to compute padded shape");
  }
 
  // Return the un-padded operand if padding to a static shape is not needed and
  // if the nofold flag is not set.
  bool nofold = opOperand->getOperandNumber() < options.nofoldFlags.size()
                    ? bool(options.nofoldFlags[opOperand->getOperandNumber()])
                    : false;
  if (!nofold && *alreadyHasRequestedShape)
    return opOperand->get();
 
  // Fail if `paddingValues` specifies no padding value.
  if (opOperand->getOperandNumber() >= options.paddingValues.size()) {
    return rewriter.notifyMatchFailure(opToPad, "--no padding value specified");
  }
  Attribute paddingAttr = options.paddingValues[opOperand->getOperandNumber()];
 
  Value paddingValue;
  if (auto complexTy = dyn_cast<ComplexType>(
          getElementTypeOrSelf(opOperand->get().getType()))) {
    auto complexAttr = cast<ArrayAttr>(paddingAttr);
    paddingValue = rewriter.create<complex::ConstantOp>(opToPad.getLoc(),
                                                        complexTy, complexAttr);
  } else {
    paddingValue = rewriter.create<arith::ConstantOp>(
        opToPad.getLoc(), cast<TypedAttr>(paddingAttr));
  }
 
  // Computes the padded shape.
  if (!*alreadyHasRequestedShape)
    shape.computePadding(rewriter, opOperand->get());
 
  // Pad the operand to the bounding box defined by `paddedShape`.
  RankedTensorType paddedTensorType =
      shape.getType(getElementTypeOrSelf(opOperand->get()));
  LLVM_DEBUG(DBGS() << "--SUCCESS, makeComposedPadHighOp with type: "
                    << paddedTensorType);
  return makeComposedPadHighOp(rewriter, opToPad->getLoc(), paddedTensorType,
                               opOperand->get(), paddingValue, nofold,
                               shape.dynDims);
}
 
LogicalResult
linalg::rewriteAsPaddedOp(RewriterBase &rewriter, LinalgOp opToPad,
                          const LinalgPaddingOptions &constOptions,
                          LinalgOp &paddedOp, SmallVector<Value> &replacements,
                          SmallVector<tensor::PadOp> &padOps) {
  LLVM_DEBUG(DBGS() << "Start rewriteAsPaddedOp : " << opToPad << "\n");
  Location loc = opToPad->getLoc();
 
  LinalgPaddingOptions options(constOptions);
  // Allow inference of pad values if they are not explicitly specified.
  // TODO: be mindful about the value depending on the actual operation.
  if (options.paddingValues.empty()) {
    SmallVector<Type> types(opToPad->getOperandTypes());
    llvm::append_range(types, opToPad->getResultTypes());
    for (Type t : types) {
      options.paddingValues.push_back(
          rewriter.getZeroAttr(getElementTypeOrSelf(t)));
    }
  }
 
  // TODO: there are cases where we may still want to pad to larger sizes.
  if (!opToPad.hasPureTensorSemantics())
    return rewriter.notifyMatchFailure(opToPad,
                                       "expected operation on tensors");
 
  OpBuilder::InsertionGuard g(rewriter);
  // Set IP after op because we also take the dims of the original output.
  rewriter.setInsertionPointAfter(opToPad);
 
  // Make a copy of the shaped operands and update it.
  SmallVector<Value> newOperands;
  newOperands.reserve(opToPad->getNumOperands());
  for (OpOperand &opOperand : opToPad->getOpOperands()) {
    FailureOr<Value> paddedOperand = padOperandToSmallestStaticBoundingBox(
        rewriter, opToPad, &opOperand, options);
    // Exit if `paddingDimensions` cannot be bounded statically.
    if (failed(paddedOperand)) {
      LLVM_DEBUG(DBGS() << "--operand cannot be bound statically : "
                        << opOperand.get() << " -> FAIL\n");
      return rewriter.notifyMatchFailure(opToPad,
                                         "operand cannot be bound statically");
    }
    newOperands.push_back(*paddedOperand);
    if (auto padOp = paddedOperand->getDefiningOp<tensor::PadOp>())
      padOps.push_back(padOp);
  }
 
  ReifiedRankedShapedTypeDims reifiedResultShapes;
  if (failed(reifyResultShapes(rewriter, opToPad, reifiedResultShapes))) {
    LLVM_DEBUG(DBGS() << "--failed to reify result shapes -> FAIL\n");
    return rewriter.notifyMatchFailure(opToPad,
                                       "failed to reify result shapes");
  }
  assert(reifiedResultShapes.size() == opToPad->getNumResults() &&
         "expected same number of results");
 
  // Clone `opToPad` to operate on the statically padded shapes.
  auto resultTensorTypes =
      ValueRange(newOperands).take_back(opToPad.getNumDpsInits()).getTypes();
  // clone **should** properly notify the rewriter.
  paddedOp = clone(rewriter, opToPad, resultTensorTypes, newOperands);
  LLVM_DEBUG(DBGS() << "--cloned padded op: " << paddedOp << "\n");
 
  // Recover the slice out of the new static results. This keeps the original
  // linalg op around because it uses the dims of the original results.
  SmallVector<Value> paddedSubtensorResults;
  paddedSubtensorResults.reserve(opToPad->getNumResults());
  for (const auto &en : llvm::enumerate(paddedOp->getResults())) {
    Value paddedResult = en.value();
    int64_t resultNumber = en.index();
    int64_t rank = cast<RankedTensorType>(paddedResult.getType()).getRank();
    SmallVector<OpFoldResult> offsets(rank, rewriter.getIndexAttr(0));
    SmallVector<OpFoldResult> strides(rank, rewriter.getIndexAttr(1));
    paddedSubtensorResults.push_back(rewriter.create<tensor::ExtractSliceOp>(
        loc, paddedResult, offsets, reifiedResultShapes[resultNumber],
        strides));
  }
 
  if (options.copyBackOp == LinalgPaddingOptions::CopyBackOp::None) {
    replacements = std::move(paddedSubtensorResults);
    return success();
  }
 
  // Copy back unpadded results to the original destination (i.e., inits of the
  // linalg op), so that the destination buffer of the computation does not
  // change. If the padding folds away, this will materialize as a memcpy
  // between two identical buffers, which will then also fold away.
  assert(static_cast<int64_t>(paddedSubtensorResults.size()) ==
             opToPad.getNumDpsInits() &&
         "expected matching number of results");
  for (auto it :
       llvm::zip(paddedSubtensorResults, opToPad.getDpsInitsMutable())) {
    if (options.copyBackOp == LinalgPaddingOptions::CopyBackOp::LinalgCopy) {
      replacements.push_back(rewriter
                                 .create<linalg::CopyOp>(loc, std::get<0>(it),
                                                         std::get<1>(it).get())
                                 .getResult(0));
    } else if (options.copyBackOp ==
               LinalgPaddingOptions::CopyBackOp::
                   BufferizationMaterializeInDestination) {
      replacements.push_back(
          rewriter
              .create<bufferization::MaterializeInDestinationOp>(
                  loc, std::get<0>(it), std::get<1>(it).get())
              ->getResult(0));
    } else {
      llvm_unreachable("unsupported copy back op");
    }
  }
  return success();
}
 
FailureOr<LinalgOp>
mlir::linalg::padAndHoistLinalgOp(RewriterBase &rewriter, LinalgOp linalgOp,
                                  const LinalgPaddingOptions &options) {
  assert(options.copyBackOp == LinalgPaddingOptions::CopyBackOp::None &&
         "invalid options");
 
  if (!linalgOp.hasPureTensorSemantics())
    return rewriter.notifyMatchFailure(
        linalgOp, "only applies to Linalg ops with tensor semantics");
 
  // Pad the operation.
  LinalgOp paddedOp;
  SmallVector<Value> newResults;
  SmallVector<tensor::PadOp> padOps;
  if (failed(rewriteAsPaddedOp(rewriter, linalgOp, options, paddedOp,
                               newResults, padOps)))
    return rewriter.notifyMatchFailure(linalgOp,
                                       "failed to rewrite as a padded op");
 
  // Hoist the padding.
  for (const auto &en : enumerate(options.hoistPaddings)) {
    if (static_cast<int64_t>(en.index()) >= paddedOp->getNumOperands())
      break;
    OpOperand &opOperand = paddedOp->getOpOperand(en.index());
    auto padOp = opOperand.get().getDefiningOp<tensor::PadOp>();
    if (!padOp || en.value() == 0) {
      (void)rewriter.notifyMatchFailure(linalgOp, "not a tensor.pad -- skip");
      continue;
    }
 
    // Fail hoisting if the operand shape is not fully static.
    if (llvm::any_of(paddedOp.getShape(&opOperand), ShapedType::isDynamic)) {
      (void)rewriter.notifyMatchFailure(linalgOp,
                                        "non static padding shape -- skip");
      continue;
    }
 
    tensor::PadOp hoistedOp;
    SmallVector<TransposeOp> transposeOps;
    SmallVector<int64_t> transposeVector =
        en.index() < options.transposePaddings.size()
            ? options.transposePaddings[en.index()]
            : SmallVector<int64_t>{};
 
    FailureOr<Value> newResult = hoistPaddingOnTensors(
        padOp, en.value(), transposeVector, hoistedOp, transposeOps);
    if (failed(newResult)) {
      (void)rewriter.notifyMatchFailure(linalgOp,
                                        "failed to apply hoistPadding");
      continue;
    }
    rewriter.replaceOp(padOp, *newResult);
  }
 
  // Replace the original operation to pad.
  rewriter.replaceOp(linalgOp, newResults);
 
  return paddedOp;
}