[mlir][vector] vector.splat and vector.broadcast folding/canonicalizing parity (#150284)

This PR ensures parity in folding/canonicalizing of vector.broadcast
(from a scalar) and vector.splat. This means that by using
vector.broadcast instead of vector.splat (which is currently
deprecated), there is no loss in optimizations performed. All tests
which were previously checking folding/canonicalizing of vector.splat
are now done for vector.broadcast. The vector.splat canonicalization
tests are now in a separate file, ready for removal when, in the future,
we remove vector.splat completely.

This PR also adds a canonicalizer to vector.splat to always convert it
to vector.broadcast. This is to reduce the 'traffic' through
vector.splat.

There is a chance that this PR will break downstream users who create/expect 
for vector.splat. Changing all such logic to work just vector.broadcast instead
should fix.

Author: newling

mlir/include/mlir/Dialect/Vector/IR/VectorOps.td

@@ -2780,6 +2780,10 @@ def Vector_SplatOp : Vector_Op<"splat", [
   let assemblyFormat = "$input attr-dict `:` type($aggregate)";
 
   let hasFolder = 1;
+
+  // vector.splat is deprecated, and vector.broadcast should be used instead.
+  // Canonicalize vector.splat to vector.broadcast.
+  let hasCanonicalizer = 1;
 }
 
 //===----------------------------------------------------------------------===//

mlir/lib/Dialect/Vector/IR/VectorOps.cpp

@@ -2476,17 +2476,19 @@ OpFoldResult FromElementsOp::fold(FoldAdaptor adaptor) {
   return {};
 }
 
-/// Rewrite a vector.from_elements into a vector.splat if all elements are the
-/// same SSA value. E.g.:
-///
-/// %0 = vector.from_elements %a, %a, %a : vector<3xf32>
-/// ==> rewrite to vector.splat %a : vector<3xf32>
-static LogicalResult rewriteFromElementsAsSplat(FromElementsOp fromElementsOp,
-                                                PatternRewriter &rewriter) {
+/// Rewrite vector.from_elements as vector.broadcast if the elements are the
+/// same. Example:
+///    %0 = vector.from_elements %a, %a, %a : vector<3xf32>
+/// =>
+///    %0 = vector.broadcast %a : f32 to vector<3xf32>
+static LogicalResult
+rewriteFromElementsAsBroadcast(FromElementsOp fromElementsOp,
+                               PatternRewriter &rewriter) {
   if (!llvm::all_equal(fromElementsOp.getElements()))
     return failure();
-  rewriter.replaceOpWithNewOp<SplatOp>(fromElementsOp, fromElementsOp.getType(),
-                                       fromElementsOp.getElements().front());
+  rewriter.replaceOpWithNewOp<BroadcastOp>(
+      fromElementsOp, fromElementsOp.getType(),
+      fromElementsOp.getElements().front());
   return success();
 }
 
@@ -2517,7 +2519,7 @@ class FromElementsToShapeCast : public OpRewritePattern<FromElementsOp> {
   LogicalResult matchAndRewrite(FromElementsOp fromElements,
                                 PatternRewriter &rewriter) const override {
 
-    // Handled by `rewriteFromElementsAsSplat`
+    // Handled by `rewriteFromElementsAsBroadcast`.
     if (fromElements.getType().getNumElements() == 1)
       return failure();
 
@@ -2610,7 +2612,7 @@ class FromElementsToShapeCast : public OpRewritePattern<FromElementsOp> {
 
 void FromElementsOp::getCanonicalizationPatterns(RewritePatternSet &results,
                                                  MLIRContext *context) {
-  results.add(rewriteFromElementsAsSplat);
+  results.add(rewriteFromElementsAsBroadcast);
   results.add<FromElementsToShapeCast>(context);
 }
 
@@ -3058,23 +3060,50 @@ struct Canonicalize0DShuffleOp : public OpRewritePattern<ShuffleOp> {
   }
 };
 
-/// Pattern to rewrite a ShuffleOp(SplatOp, SplatOp) to SplatOp.
+/// Consider the defining operation `defOp` of `value`. If `defOp` is a
+/// vector.splat or a vector.broadcast with a scalar operand, return the scalar
+/// value that is splatted. Otherwise return null.
+///
+/// Examples:
+///
+/// scalar_source --> vector.splat --> value     - return scalar_source
+/// scalar_source --> vector.broadcast --> value - return scalar_source
+static Value getScalarSplatSource(Value value) {
+  // Block argument:
+  Operation *defOp = value.getDefiningOp();
+  if (!defOp)
+    return {};
+
+  // Splat:
+  if (auto splat = dyn_cast<vector::SplatOp>(defOp))
+    return splat.getInput();
+
+  auto broadcast = dyn_cast<vector::BroadcastOp>(defOp);
+
+  // Not broadcast (and not splat):
+  if (!broadcast)
+    return {};
+
+  // Broadcast of a vector:
+  if (isa<VectorType>(broadcast.getSourceType()))
+    return {};
+
+  // Broadcast of a scalar:
+  return broadcast.getSource();
+}
+
+/// Pattern to rewrite shuffle(splat-like(v), splat-like(v)) as broadcast(v).
 class ShuffleSplat final : public OpRewritePattern<ShuffleOp> {
 public:
   using OpRewritePattern::OpRewritePattern;
 
   LogicalResult matchAndRewrite(ShuffleOp op,
                                 PatternRewriter &rewriter) const override {
-    auto v1Splat = op.getV1().getDefiningOp<SplatOp>();
-    auto v2Splat = op.getV2().getDefiningOp<SplatOp>();
-
-    if (!v1Splat || !v2Splat)
+    Value splat = getScalarSplatSource(op.getV1());
+    if (!splat || getScalarSplatSource(op.getV2()) != splat)
       return failure();
 
-    if (v1Splat.getInput() != v2Splat.getInput())
-      return failure();
-
-    rewriter.replaceOpWithNewOp<SplatOp>(op, op.getType(), v1Splat.getInput());
+    rewriter.replaceOpWithNewOp<BroadcastOp>(op, op.getType(), splat);
     return success();
   }
 };
@@ -3230,23 +3259,19 @@ class InsertToBroadcast final : public OpRewritePattern<InsertOp> {
   }
 };
 
-/// Pattern to rewrite a InsertOp(SplatOp, SplatOp) to SplatOp.
+/// Pattern to rewrite a insert(splat-like(v), splat-like(v)) as broadcast(v).
 class InsertSplatToSplat final : public OpRewritePattern<InsertOp> {
 public:
   using OpRewritePattern::OpRewritePattern;
 
   LogicalResult matchAndRewrite(InsertOp op,
                                 PatternRewriter &rewriter) const override {
-    auto srcSplat = op.getValueToStore().getDefiningOp<SplatOp>();
-    auto dstSplat = op.getDest().getDefiningOp<SplatOp>();
-
-    if (!srcSplat || !dstSplat)
-      return failure();
 
-    if (srcSplat.getInput() != dstSplat.getInput())
+    Value splat = getScalarSplatSource(op.getValueToStore());
+    if (!splat || getScalarSplatSource(op.getDest()) != splat)
       return failure();
 
-    rewriter.replaceOpWithNewOp<SplatOp>(op, op.getType(), srcSplat.getInput());
+    rewriter.replaceOpWithNewOp<BroadcastOp>(op, op.getType(), splat);
     return success();
   }
 };
@@ -3514,27 +3539,21 @@ LogicalResult InsertStridedSliceOp::verify() {
 }
 
 namespace {
-/// Pattern to rewrite an InsertStridedSliceOp(SplatOp(X):src_type,
-/// SplatOp(X):dst_type) to SplatOp(X):dst_type.
+/// Rewrite insert_strided_slice(splat-like(v), splat-like(v)) as v.
 class FoldInsertStridedSliceSplat final
     : public OpRewritePattern<InsertStridedSliceOp> {
 public:
   using OpRewritePattern::OpRewritePattern;
 
   LogicalResult matchAndRewrite(InsertStridedSliceOp insertStridedSliceOp,
                                 PatternRewriter &rewriter) const override {
-    auto srcSplatOp =
-        insertStridedSliceOp.getValueToStore().getDefiningOp<vector::SplatOp>();
-    auto destSplatOp =
-        insertStridedSliceOp.getDest().getDefiningOp<vector::SplatOp>();
 
-    if (!srcSplatOp || !destSplatOp)
+    auto dst = insertStridedSliceOp.getDest();
+    auto splat = getScalarSplatSource(insertStridedSliceOp.getValueToStore());
+    if (!splat || getScalarSplatSource(dst) != splat)
       return failure();
 
-    if (srcSplatOp.getInput() != destSplatOp.getInput())
-      return failure();
-
-    rewriter.replaceOp(insertStridedSliceOp, insertStridedSliceOp.getDest());
+    rewriter.replaceOp(insertStridedSliceOp, dst);
     return success();
   }
 };
@@ -4189,17 +4208,18 @@ class StridedSliceBroadcast final
   }
 };
 
-/// Pattern to rewrite an ExtractStridedSliceOp(SplatOp) to SplatOp.
+/// Rewrite extract_strided_slice(splat-like(v)) with broadcast(v).
 class StridedSliceSplat final : public OpRewritePattern<ExtractStridedSliceOp> {
 public:
   using OpRewritePattern::OpRewritePattern;
 
   LogicalResult matchAndRewrite(ExtractStridedSliceOp op,
                                 PatternRewriter &rewriter) const override {
-    auto splat = op.getVector().getDefiningOp<SplatOp>();
+
+    Value splat = getScalarSplatSource(op.getVector());
     if (!splat)
       return failure();
-    rewriter.replaceOpWithNewOp<SplatOp>(op, op.getType(), splat.getInput());
+    rewriter.replaceOpWithNewOp<BroadcastOp>(op, op.getType(), splat);
     return success();
   }
 };
@@ -6354,19 +6374,19 @@ class TransposeFolder final : public OpRewritePattern<vector::TransposeOp> {
   }
 };
 
-// Folds transpose(splat x : src_type) : res_type into splat x : res_type.
+/// Replace transpose(splat-like(v)) with broadcast(v)
 class FoldTransposeSplat final : public OpRewritePattern<TransposeOp> {
 public:
   using OpRewritePattern::OpRewritePattern;
 
   LogicalResult matchAndRewrite(TransposeOp transposeOp,
                                 PatternRewriter &rewriter) const override {
-    auto splatOp = transposeOp.getVector().getDefiningOp<vector::SplatOp>();
-    if (!splatOp)
+    Value splat = getScalarSplatSource(transposeOp.getVector());
+    if (!splat)
       return failure();
 
-    rewriter.replaceOpWithNewOp<vector::SplatOp>(
-        transposeOp, transposeOp.getResultVectorType(), splatOp.getInput());
+    rewriter.replaceOpWithNewOp<vector::BroadcastOp>(
+        transposeOp, transposeOp.getResultVectorType(), splat);
     return success();
   }
 };
@@ -7117,6 +7137,23 @@ OpFoldResult SplatOp::fold(FoldAdaptor adaptor) {
   return SplatElementsAttr::get(getType(), {constOperand});
 }
 
+// Canonicalizer for vector.splat. It always gets canonicalized to a
+// vector.broadcast.
+class SplatToBroadcastPattern final : public OpRewritePattern<SplatOp> {
+public:
+  using OpRewritePattern<SplatOp>::OpRewritePattern;
+  LogicalResult matchAndRewrite(SplatOp splatOp,
+                                PatternRewriter &rewriter) const override {
+    rewriter.replaceOpWithNewOp<vector::BroadcastOp>(splatOp, splatOp.getType(),
+                                                     splatOp.getOperand());
+    return success();
+  }
+};
+void SplatOp::getCanonicalizationPatterns(RewritePatternSet &results,
+                                          MLIRContext *context) {
+  results.add<SplatToBroadcastPattern>(context);
+}
+
 void SplatOp::inferResultRanges(ArrayRef<ConstantIntRanges> argRanges,
                                 SetIntRangeFn setResultRanges) {
   setResultRanges(getResult(), argRanges.front());