[mlir][SCF] Allow tiling by specifying maximum number of tiles.

Author: MaheshRavishankar

mlir/include/mlir/Dialect/Linalg/TransformOps/LinalgTransformOps.h

@@ -30,6 +30,10 @@ class GenericOp;
 class LinalgOp;
 } // namespace linalg
 
+namespace scf {
+struct SCFTilingResult;
+} // namespace scf
+
 namespace tensor {
 class InsertSliceOp;
 class PackOp;
@@ -60,7 +64,7 @@ tileToForallOpImpl(RewriterBase &rewriter, transform::TransformState &state,
                    ArrayRef<OpFoldResult> mixedNumThreads,
                    ArrayRef<OpFoldResult> mixedTileSizes,
                    std::optional<ArrayAttr> mapping,
-                   linalg::ForallTilingResult &tilingResult);
+                   scf::SCFTilingResult &tilingResult);
 
 } // namespace transform
 } // namespace mlir

mlir/include/mlir/Dialect/Linalg/Transforms/Transforms.h

@@ -866,31 +866,8 @@ FailureOr<ContinuousTileSizeSpecification>
 computeContinuousTileSizes(OpBuilder &builder, TilingInterface op,
                            unsigned dimension, OpFoldResult targetSize,
                            bool emitAssertions);
-/// Rewrite a TilingInterface `op` to a tiled `scf.forall`, applying
-/// tiling by `numThreads`.
-/// If non-empty, the `mapping` is added as an attribute to the
-/// resulting `scf.forall`.
-/// Zero tile sizes indicate that the dimension is not tiled, and can be
-/// thought of as tiling by the full size of data. It is the user's
-/// responsibility to ensure that `numThreads` is a valid tiling specification
-/// (i.e. that only tiles parallel dimensions, e.g. in the Linalg case).
-struct ForallTilingResult {
-  Operation *tileOp;
-  Operation *tiledOp;
-};
-FailureOr<ForallTilingResult> tileToForallOp(RewriterBase &builder,
-                                             TilingInterface op,
-                                             ArrayRef<OpFoldResult> numThreads,
-                                             std::optional<ArrayAttr> mapping);
-
-/// Same as `tileToForallOp`, but calculate the number of threads
-/// required using the given tileSizes.
-FailureOr<ForallTilingResult>
-tileToForallOpUsingTileSizes(RewriterBase &builder, TilingInterface op,
-                             ArrayRef<OpFoldResult> tileSizes,
-                             std::optional<ArrayAttr> mapping);
-
-/// Transformation information returned after reduction tiling.
+
+  /// Transformation information returned after reduction tiling.
 struct ForallReductionTilingResult {
   /// The partial reduction tiled op generated.
   SmallVector<Operation *> parallelTiledOps;

mlir/include/mlir/Dialect/SCF/Transforms/TileUsingInterface.h

@@ -32,9 +32,13 @@ using SCFTileSizeComputationFunction =
 
 /// Options to use to control tiling.
 struct SCFTilingOptions {
-  /// Computation function that returns the tile sizes for each operation.
-  /// Delayed construction of constant tile sizes should occur to interoperate
-  /// with folding.
+  /// Computation function that returns the tile sizes to use for each loop.
+  /// Returning a tile size of zero implies no tiling for that loop. If the
+  /// size of the returned vector is smaller than the number of loops, the inner
+  /// loops are not tiled. If the size of the returned vector is larger, then
+  /// the vector is truncated to number of loops.  Only one of
+  /// `tileSizeComputationFunction` or `maxNumTilesComputationFunction` should
+  /// be used.
   SCFTileSizeComputationFunction tileSizeComputationFunction = nullptr;
 
   SCFTilingOptions &
@@ -45,7 +49,25 @@ struct SCFTilingOptions {
   /// Convenience function to set the `tileSizeComputationFunction` to a
   /// function that computes tile sizes at the point they are needed. Allows
   /// proper interaction with folding.
-  SCFTilingOptions &setTileSizes(ArrayRef<OpFoldResult> ts);
+  SCFTilingOptions &setTileSizes(ArrayRef<OpFoldResult> tileSizes);
+
+  /// Computation function that returns the maximum number of tile to use for
+  /// each loop. Returning a tile size of zero implies no tiling for that loop.
+  /// If the size of the returned vector is smaller than the number of loops,
+  /// the inner loops are not tiled. If the size of the returned vector is
+  /// larger, then the vector is truncated to number of loops. Only one of
+  /// `tileSizeComputationFunction` or `maxNumTilesComputationFunction` should
+  /// be used.
+  SCFTileSizeComputationFunction maxNumTilesComputationFunction = nullptr;
+
+  SCFTilingOptions &
+  setMaxNumTilesComputationFunction(SCFTileSizeComputationFunction fun) {
+    maxNumTilesComputationFunction = std::move(fun);
+    return *this;
+  }
+  /// Convenience function to set the `tileSizeComputationFunction` to a
+  /// function that computes tile sizes at the point they are needed.
+  SCFTilingOptions &setMaxNumTiles(ArrayRef<OpFoldResult> numTiles);
 
   /// The interchange vector to reorder the tiled loops.
   SmallVector<int64_t> interchangeVector = {};
@@ -67,9 +89,8 @@ struct SCFTilingOptions {
   /// when using loop constructs that dont support such a mapping (like
   /// `scf.for`)
   SmallVector<Attribute> mappingVector = {};
-  SCFTilingOptions &setMapping(ArrayRef<DeviceMappingAttrInterface> mapping) {
-    mappingVector = llvm::map_to_vector(
-        mapping, [](auto attr) -> Attribute { return attr; });
+  SCFTilingOptions &setMapping(ArrayRef<Attribute> mapping) {
+    mappingVector = llvm::to_vector(mapping);
     return *this;
   }
 };

mlir/lib/Dialect/Linalg/TransformOps/LinalgTransformOps.cpp

@@ -3156,7 +3156,7 @@ DiagnosedSilenceableFailure transform::tileToForallOpImpl(
     TransformOpInterface transformOp, Operation *target,
     ArrayRef<OpFoldResult> mixedNumThreads,
     ArrayRef<OpFoldResult> mixedTileSizes, std::optional<ArrayAttr> mapping,
-    linalg::ForallTilingResult &tilingResult) {
+    scf::SCFTilingResult &tilingResult) {
   // Transform all targets one by one.
   auto tileableOp = dyn_cast<TilingInterface>(target);
   if (!tileableOp) {
@@ -3167,18 +3167,38 @@ DiagnosedSilenceableFailure transform::tileToForallOpImpl(
     return diag;
   }
   rewriter.setInsertionPoint(tileableOp);
-  FailureOr<linalg::ForallTilingResult> maybeTilingResult = failure();
+  scf::SCFTilingOptions options;
+  options.setLoopType(scf::SCFTilingOptions::LoopType::ForallOp);
   if (!mixedNumThreads.empty()) {
-    maybeTilingResult =
-        linalg::tileToForallOp(rewriter, tileableOp, mixedNumThreads, mapping);
+    options.setMaxNumTiles(mixedNumThreads);
   } else {
-    maybeTilingResult = linalg::tileToForallOpUsingTileSizes(
-        rewriter, tileableOp, mixedTileSizes, mapping);
+    SmallVector<Range> loopRanges = tileableOp.getIterationDomain(rewriter);
+    unsigned nLoops = loopRanges.size();
+    SmallVector<OpFoldResult> numThreads;
+    numThreads.reserve(nLoops);
+    AffineExpr s0, s1;
+    bindSymbols(rewriter.getContext(), s0, s1);
+    AffineExpr divExpr = s0.ceilDiv(s1);
+    for (int i = 0, e = std::min(mixedTileSizes.size(), loopRanges.size());
+         i < e; ++i) {
+      OpFoldResult numTiles = mixedTileSizes[i];
+      if (!isConstantIntValue(numTiles, 0))
+        numTiles = affine::makeComposedFoldedAffineApply(
+            rewriter, tileableOp.getLoc(), divExpr,
+            {loopRanges[i].size, numTiles});
+      numThreads.push_back(numTiles);
+    }
+    options.setMaxNumTiles(numThreads);
+  }
+  if (mapping) {
+    options.setMapping(mapping.value().getValue());
   }
+  FailureOr<scf::SCFTilingResult> maybeTilingResult =
+      scf::tileUsingSCF(rewriter, tileableOp, options);
 
   if (failed(maybeTilingResult))
     return transformOp.emitDefaultSilenceableFailure(tileableOp);
-  rewriter.replaceOp(tileableOp, maybeTilingResult->tileOp->getResults());
+  rewriter.replaceOp(tileableOp, maybeTilingResult->replacements);
 
   tilingResult = *maybeTilingResult;
   return DiagnosedSilenceableFailure::success();
@@ -3214,14 +3234,14 @@ DiagnosedSilenceableFailure transform::TileUsingForallOp::apply(
     return status;
 
   for (Operation *target : state.getPayloadOps(getTarget())) {
-    linalg::ForallTilingResult tilingResult;
+    scf::SCFTilingResult tilingResult;
     DiagnosedSilenceableFailure diag = tileToForallOpImpl(
         rewriter, state, transformOp, target, mixedNumThreads, mixedTileSizes,
         getMapping(), tilingResult);
     if (!diag.succeeded())
       return diag;
-    tileOps.push_back(tilingResult.tileOp);
-    tiledOps.push_back(tilingResult.tiledOp);
+    tileOps.push_back(tilingResult.loops.front());
+    tiledOps.append(tilingResult.tiledOps);
   }
 
   transformResults.set(cast<OpResult>(getForallOp()), tileOps);
@@ -3699,7 +3719,7 @@ DiagnosedSilenceableFailure transform::MapCopyToThreadsOp::applyToOne(
 
   // OpBuilder only used to compute attributes.
   OpBuilder b(getContext());
-  linalg::ForallTilingResult tilingResult;
+  scf::SCFTilingResult tilingResult;
   DiagnosedSilenceableFailure diag = tileToForallOpImpl(
       /*rewriter=*/rewriter,
       /*state=*/state,
@@ -3712,8 +3732,9 @@ DiagnosedSilenceableFailure transform::MapCopyToThreadsOp::applyToOne(
   if (!diag.succeeded())
     return diag;
 
-  results.push_back(tilingResult.tileOp);
-  results.push_back(tilingResult.tiledOp);
+  results.push_back(tilingResult.loops.front());
+  for (auto op : tilingResult.tiledOps)
+    results.push_back(op);
   return DiagnosedSilenceableFailure::success();
 }
 

mlir/lib/Dialect/Linalg/Transforms/Tiling.cpp

@@ -435,188 +435,6 @@ static void calculateTileOffsetsAndSizes(
   }
 }
 
-/// Returns a vector of bools representing if, for each axis, `op` can be tiled
-/// without incurring in a race condition and thus it is thread-safe to do the
-/// tiling. This is checked by iterating over numThreads and ensuring that the
-/// corresponding iterator type is "parallel". If it is not, then we know that
-/// such dimension is unsafe to tile.
-SmallVector<bool> safeToTileToForall(mlir::MLIRContext *ctx, LinalgOp linalgOp,
-                                     ArrayRef<OpFoldResult> numThreads) {
-  auto iterators = linalgOp.getIteratorTypesArray();
-  SmallVector<bool> safeToTile(numThreads.size(), true);
-
-  for (unsigned i = 0, e = numThreads.size(); i != e; i++) {
-    if (auto attr = llvm::dyn_cast_if_present<Attribute>(numThreads[i])) {
-      if (cast<IntegerAttr>(attr).getValue().getSExtValue() > 1) {
-        safeToTile[i] = iterators[i] == utils::IteratorType::parallel;
-      }
-    } else {
-      safeToTile[i] = iterators[i] == utils::IteratorType::parallel;
-    }
-  }
-  return safeToTile;
-}
-
-/// Rewrite a TilingInterface `op` to a tiled `scf.forall`. The
-/// tiling is specified by the number of tiles/threads `numThreads` and the
-/// optional nominal tile size `nominalTileSizes`. If `nominalTilSizes` is
-/// not specified, then  it is derived from `numThreads` as `ceilDiv(dimSize[i],
-/// numThreads[i])`. If non-empty, the `mapping` is added as an
-/// attribute to the resulting `scf.forall`. A zero tile sizes indicate
-/// that the dimension is not tiled, and can be thought of as tiling by the full
-/// size of data.
-/// It is the user's responsibility to ensure that `numThreads` is a valid
-/// tiling specification (i.e. that only tiles parallel dimensions, e.g. in the
-/// Linalg case). If the dimension is not parallelizable, a warning is issued to
-/// notify the user that the generated code is not safe to parallelize. If
-/// `omitTileOffsetBoundsCheck` is true, then the function will assume that
-/// `tileSize[i] * (numThread[i] -1) <= dimSize[i]` holds.
-static FailureOr<ForallTilingResult> tileToForallOpImpl(
-    RewriterBase &b, TilingInterface op, ArrayRef<OpFoldResult> numThreads,
-    std::optional<ArrayRef<OpFoldResult>> nominalTileSizes,
-    std::optional<ArrayAttr> mapping, bool omitTileOffsetBoundsCheck) {
-  Location loc = op->getLoc();
-  OpBuilder::InsertionGuard g(b);
-
-  SmallVector<Range> loopRanges = op.getIterationDomain(b);
-  if (loopRanges.empty())
-    return op->emitOpError("expected non-empty loop ranges");
-  auto hasStrideOne = [](Range r) { return !isConstantIntValue(r.stride, 1); };
-  if (llvm::any_of(loopRanges, hasStrideOne))
-    return op->emitOpError("only stride-1 supported atm");
-
-  // Gather destination tensors.
-  SmallVector<Value> dest;
-  if (failed(tensor::getOrCreateDestinations(b, loc, op, dest)))
-    return op->emitOpError("failed to get destination tensors");
-
-  SmallVector<OpFoldResult> nonZeroNumThreads =
-      llvm::to_vector(llvm::make_filter_range(numThreads, [](OpFoldResult ofr) {
-        return !isConstantIntValue(ofr, 0);
-      }));
-  SmallVector<Value> materializedNonZeroNumThreads =
-      llvm::to_vector(llvm::map_range(nonZeroNumThreads, [&](OpFoldResult ofr) {
-        return getValueOrCreateConstantIndexOp(b, loc, ofr);
-      }));
-
-  LinalgOp linalgOp = dyn_cast<LinalgOp>(op.getOperation());
-  if (linalgOp) {
-    // Check if tiling is thread safe and print a warning if not.
-    SmallVector<bool> tilingSafety =
-        safeToTileToForall(b.getContext(), linalgOp, numThreads);
-    for (size_t i = 0; i < tilingSafety.size(); i++)
-      if (!tilingSafety[i])
-        op.emitWarning() << "tiling is not thread safe at axis #" << i;
-  }
-
-  // 1. Create the ForallOp. We don't use the lambda body-builder
-  // version because we require the use of RewriterBase in the body, so we
-  // manually move the insertion point to the body below.
-  scf::ForallOp forallOp = b.create<scf::ForallOp>(
-      loc, getAsOpFoldResult((materializedNonZeroNumThreads)), dest, mapping);
-
-  // 2. Fill out the ForallOp body.
-  SmallVector<OpFoldResult> tiledOffsets, tiledSizes;
-  calculateTileOffsetsAndSizes(b, loc, forallOp, numThreads, loopRanges,
-                               omitTileOffsetBoundsCheck, nominalTileSizes,
-                               tiledOffsets, tiledSizes);
-
-  // 3. Clone the tileable op and update its destination operands to use the
-  // output bbArgs of the ForallOp.
-  ArrayRef<BlockArgument> destBbArgs = forallOp.getRegionIterArgs();
-  Operation *tiledOp = nullptr;
-  SmallVector<Value> tiledValues;
-  {
-    // 3.a. RAII guard, inserting within forallOp, before terminator.
-    OpBuilder::InsertionGuard g(b);
-    b.setInsertionPoint(forallOp.getTerminator());
-    Operation *clonedOp = b.clone(*op.getOperation());
-    auto destinationStyleOp = dyn_cast<DestinationStyleOpInterface>(clonedOp);
-    if (destinationStyleOp) {
-      for (OpOperand &outOperand : destinationStyleOp.getDpsInitsMutable()) {
-        // Swap tensor inits with the corresponding block argument of the
-        // scf.forall op. Memref inits remain as is.
-        if (isa<TensorType>(outOperand.get().getType())) {
-          auto *it = llvm::find(dest, outOperand.get());
-          assert(it != dest.end() && "could not find destination tensor");
-          unsigned destNum = std::distance(dest.begin(), it);
-          outOperand.set(destBbArgs[destNum]);
-        }
-      }
-    }
-
-    // 4. Tile the cloned op and delete the clone.
-    FailureOr<TilingResult> tilingResult =
-        cast<TilingInterface>(clonedOp).getTiledImplementation(b, tiledOffsets,
-                                                               tiledSizes);
-    if (failed(tilingResult))
-      return clonedOp->emitError("Failed to tile op: ");
-    if (tilingResult->tiledOps.size() != 1) {
-      return clonedOp->emitError("expected a single produced tiled op, got ")
-             << tilingResult->tiledOps.size();
-    }
-
-    b.eraseOp(clonedOp);
-    tiledOp = tilingResult->tiledOps.front();
-    tiledValues = tilingResult->tiledValues;
-  }
-
-  // 5. Parallel insert back into the result tensor.
-  for (auto it : llvm::zip(llvm::seq(unsigned(0), unsigned(dest.size())),
-                           tiledValues, destBbArgs)) {
-    // 5.a. Partial subset information is inserted just before the terminator.
-    OpBuilder::InsertionGuard g(b);
-    b.setInsertionPoint(forallOp.getTerminator());
-
-    SmallVector<OpFoldResult> resultOffsets, resultSizes;
-    if (failed(op.getResultTilePosition(b, std::get<0>(it), tiledOffsets,
-                                        tiledSizes, resultOffsets,
-                                        resultSizes)))
-      return op->emitOpError("output offsets couldn't be calculated");
-    SmallVector<OpFoldResult> strides(resultSizes.size(), b.getIndexAttr(1));
-
-    // 5.b. Parallel insertions are inserted at the end of the combining
-    // terminator.
-    b.setInsertionPointToEnd(forallOp.getTerminator().getBody());
-    b.create<tensor::ParallelInsertSliceOp>(loc, std::get<1>(it),
-                                            std::get<2>(it), resultOffsets,
-                                            resultSizes, strides);
-  }
-  return ForallTilingResult{forallOp, tiledOp};
-}
-
-FailureOr<ForallTilingResult>
-linalg::tileToForallOp(RewriterBase &b, TilingInterface op,
-                       ArrayRef<OpFoldResult> numThreads,
-                       std::optional<ArrayAttr> mapping) {
-  return tileToForallOpImpl(b, op, numThreads,
-                            /*nominalTileSizes=*/std::nullopt, mapping,
-                            /*omitTileOffsetBoundsCheck=*/false);
-}
-
-FailureOr<ForallTilingResult>
-linalg::tileToForallOpUsingTileSizes(RewriterBase &b, TilingInterface op,
-                                     ArrayRef<OpFoldResult> tileSizes,
-                                     std::optional<ArrayAttr> mapping) {
-  SmallVector<Range> loopRanges = op.getIterationDomain(b);
-  unsigned nLoops = loopRanges.size();
-  SmallVector<OpFoldResult> numThreads;
-  numThreads.reserve(nLoops);
-  AffineExpr s0, s1;
-  bindSymbols(b.getContext(), s0, s1);
-  AffineExpr divExpr = s0.ceilDiv(s1);
-  for (const auto &it : llvm::zip(tileSizes, loopRanges)) {
-    OpFoldResult numTiles = std::get<0>(it);
-    if (!isConstantIntValue(numTiles, 0))
-      numTiles = makeComposedFoldedAffineApply(
-          b, op.getLoc(), divExpr, {std::get<1>(it).size, std::get<0>(it)});
-    numThreads.push_back(numTiles);
-  }
-  return tileToForallOpImpl(b, op, numThreads,
-                            /*nominalTileSizes=*/tileSizes, mapping,
-                            /*omitTileOffsetBoundsCheck=*/true);
-}
-
 template <typename LoopTy>
 static FailureOr<TiledLinalgOp>
 tileLinalgOpImpl(RewriterBase &b, LinalgOp op, ArrayRef<OpFoldResult> tileSizes,