[mlir][SCF] Unify tileUsingFor and tileReductionUsingFor implementation (#120115)

This patch unifies the tiling implementation for tileUsingFor and
tileReductionUsingFor. This is done by passing an addition option to
SCFTilingOptions, allowing it to set how reduction dimensions should be
tiled. Currently, there are 3 different options for reduction tiling:
FullReduction (old tileUsingFor), PartialReductionOuterReduction (old
tileReductionUsingFor) and PartialReductionOuterParallel
(linalg::tileReductionUsingForall, this isn't implemented in this
patch).

The patch makes tileReductionUsingFor use the tileUsingFor
implementation with the new reduction tiling options.

There are no test changes because the implementation was doing almost
the exactly same thing. This was also tested in IREE (which uses both
these APIs heavily) and there were no test changes.

Author: Groverkss

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

@@ -85,6 +85,36 @@ struct SCFTilingOptions {
     return *this;
   }
 
+  /// Specify how reduction dimensions should be tiled.
+  ///
+  /// Tiling can be thought of as splitting a dimension into 2 and materializing
+  /// the outer dimension as a loop:
+  ///
+  /// op[original] -> op[original / x, x] -> loop[original] { op[x] }
+  ///
+  /// For parallel dimensions, the split can only happen in one way, with both
+  /// dimensions being parallel. For reduction dimensions however, there is a
+  /// choice in how we split the reduction dimension. This enum exposes this
+  /// choice.
+  enum class ReductionTilingStrategy {
+    // [reduction] -> [reduction1, reduction2]
+    // -> loop[reduction1] { [reduction2] }
+    FullReduction,
+    // [reduction] -> [reduction1, parallel2]
+    // -> loop[reduction1] { [parallel2] }; merge[reduction1]
+    PartialReductionOuterReduction,
+    // [reduction] -> [parallel1, reduction2]
+    // -> loop[parallel1] { [reduction2] }; merge[parallel1]
+    PartialReductionOuterParallel
+  };
+  ReductionTilingStrategy reductionStrategy =
+      ReductionTilingStrategy::FullReduction;
+  SCFTilingOptions &
+  setReductionTilingStrategy(ReductionTilingStrategy strategy) {
+    reductionStrategy = strategy;
+    return *this;
+  }
+
   /// Specify mapping of loops to devices. This is only respected when the loop
   /// constructs support such a mapping (like `scf.forall`). Will be ignored
   /// when using loop constructs that dont support such a mapping (like
@@ -102,11 +132,16 @@ struct SCFTilingResult {
   /// matter except the last op. The replacements are expected to be the results
   /// of the last op.
   SmallVector<Operation *> tiledOps;
+  /// The initial destination values passed to the tiled operations.
+  SmallVector<Value> initialValues;
   /// The `scf.for` operations that iterate over the tiles.
   SmallVector<LoopLikeOpInterface> loops;
-  /// Values to use as replacements for the untiled op. Is the same size as the
-  /// number of results of the untiled op.
-  SmallVector<Value> replacements;
+  /// The result generated by the loop nest in tiling, may hold partial results,
+  /// which need to be merged to match the computation of the untiled operation.
+  /// `mergeResult` contains the operations used to perform this merge from
+  /// partial results and the values that can be used as replacements of
+  /// the untiled operation.
+  MergeResult mergeResult;
   /// Slices generated after tiling that can be used for fusing with the tiled
   /// producer.
   SmallVector<Operation *> generatedSlices;
@@ -300,20 +335,6 @@ tileAndFuseConsumerOfSlice(RewriterBase &rewriter, Operation *candidateSliceOp);
 FailureOr<SmallVector<scf::ForOp>>
 lowerToLoopsUsingSCFForOp(RewriterBase &rewriter, TilingInterface op);
 
-/// Transformation information returned after reduction tiling.
-struct SCFReductionTilingResult {
-  /// The partial reduction tiled op generated.
-  SmallVector<Operation *> parallelTiledOps;
-  /// The final reduction operation merging all the partial reductions.
-  SmallVector<Operation *> mergeOps;
-  /// Initial values used for reduction.
-  SmallVector<Value> initialValues;
-  /// The loop operations that iterate over the tiles.
-  SmallVector<LoopLikeOpInterface> loops;
-  /// The replacements to use for the results of the tiled operation.
-  SmallVector<Value> replacements;
-};
-
 /// Method to tile a reduction and generate a parallel op within a serial loop.
 /// Each of the partial reductions are calculated in parallel. Then after the
 /// loop all the partial reduction are merged into a final reduction.
@@ -338,7 +359,7 @@ struct SCFReductionTilingResult {
 /// %6 = linalg.generic %1 ["parallel", "reduction"]
 ///   : tensor<7x4xf32> -> tensor<7xf32>
 /// ```
-FailureOr<scf::SCFReductionTilingResult>
+FailureOr<scf::SCFTilingResult>
 tileReductionUsingScf(RewriterBase &b, PartialReductionOpInterface op,
                       ArrayRef<OpFoldResult> tileSize);
 

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

@@ -2223,7 +2223,7 @@ transform::ScalarizeOp::applyToOne(transform::TransformRewriter &rewriter,
     return emitDefaultDefiniteFailure(target);
 
   if (target->getNumResults())
-    rewriter.replaceOp(target, maybeTilingResult->replacements);
+    rewriter.replaceOp(target, maybeTilingResult->mergeResult.replacements);
   else
     rewriter.eraseOp(target);
 
@@ -2630,17 +2630,18 @@ DiagnosedSilenceableFailure transform::TileReductionUsingForOp::applyToOne(
     transform::ApplyToEachResultList &results,
     transform::TransformState &state) {
   rewriter.setInsertionPoint(target);
-  FailureOr<scf::SCFReductionTilingResult> result = scf::tileReductionUsingScf(
+  FailureOr<scf::SCFTilingResult> result = scf::tileReductionUsingScf(
       rewriter, cast<PartialReductionOpInterface>(target.getOperation()),
       getAsOpFoldResult(rewriter.getI64ArrayAttr(getTileSizes())));
 
   if (failed(result))
     return emitDefaultSilenceableFailure(target);
+  rewriter.replaceOp(target, result->mergeResult.replacements);
   for (Value initValue : result->initialValues)
     results.push_back(initValue.getDefiningOp());
-  for (auto parallelTiledOp : result->parallelTiledOps)
+  for (auto parallelTiledOp : result->tiledOps)
     results.push_back(parallelTiledOp);
-  for (auto mergeOp : result->mergeOps)
+  for (auto mergeOp : result->mergeResult.mergeOps)
     results.push_back(mergeOp);
   results.push_back(result->loops.front());
   return DiagnosedSilenceableFailure::success();
@@ -3064,7 +3065,7 @@ transform::TileUsingForOp::apply(transform::TransformRewriter &rewriter,
     if (failed(maybeTilingResult))
       return DiagnosedSilenceableFailure::definiteFailure();
 
-    rewriter.replaceOp(op, maybeTilingResult->replacements);
+    rewriter.replaceOp(op, maybeTilingResult->mergeResult.replacements);
 
     tiled.append(maybeTilingResult->tiledOps);
     for (const auto &en2 : llvm::enumerate(maybeTilingResult->loops))
@@ -3303,7 +3304,7 @@ DiagnosedSilenceableFailure transform::tileToForallOpImpl(
   if (failed(maybeTilingResult))
     return transformOp.emitDefaultSilenceableFailure(tileableOp);
 
-  rewriter.replaceOp(tileableOp, maybeTilingResult->replacements);
+  rewriter.replaceOp(tileableOp, maybeTilingResult->mergeResult.replacements);
 
   tilingResult = *maybeTilingResult;