[mlir][vector] Clarify the semantics of BroadcastOp (#101928)

Clarifies the semantics of `vector.broadcast` in the context of scalable
vectors. In particular, broadcasting a unit scalable dim, `[1]`, is not
valid unless there's a match between the output and the input dims.
See the examples below for an illustration:

```mlir
// VALID
 %0 = vector.broadcast %arg0 : vector<[1]xf32> to vector<4x[1]xf32>
// INVALID
 %0 = vector.broadcast %arg0 : vector<[1]xf32> to vector<[4]xf32>
// VALID FIXED-WIDTH EQUIVALENT
 %0 = vector.broadcast %arg0 : vector<1xf32> to vector<4xf32>
```

Documentation, the Op verifier and tests are updated accordingly.

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

@@ -68,9 +68,14 @@ enum class BroadcastableToResult {
   DimensionMismatch = 2,
   SourceTypeNotAVector = 3
 };
+
+struct VectorDim {
+  int64_t dim;
+  bool isScalable;
+};
 BroadcastableToResult
 isBroadcastableTo(Type srcType, VectorType dstVectorType,
-                  std::pair<int, int> *mismatchingDims = nullptr);
+                  std::pair<VectorDim, VectorDim> *mismatchingDims = nullptr);
 
 /// Collect a set of vector-to-vector canonicalization patterns.
 void populateVectorToVectorCanonicalizationPatterns(RewritePatternSet &patterns,

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

@@ -367,6 +367,8 @@ def Vector_BroadcastOp :
                                s_1     x .. x s_j x .. x s_k
                <duplication>         <potential stretch>
        ```
+       * in addition, any scalable unit dimension, `[1]`, must match exactly.
+
     The source operand is duplicated over all the missing leading dimensions
     and stretched over the trailing dimensions where the source has a non-equal
     dimension of 1. These rules imply that any scalar broadcast (k=0) to any

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

@@ -2371,9 +2371,9 @@ Value BroadcastOp::createOrFoldBroadcastOp(
   return res;
 }
 
-BroadcastableToResult
-mlir::vector::isBroadcastableTo(Type srcType, VectorType dstVectorType,
-                                std::pair<int, int> *mismatchingDims) {
+BroadcastableToResult mlir::vector::isBroadcastableTo(
+    Type srcType, VectorType dstVectorType,
+    std::pair<VectorDim, VectorDim> *mismatchingDims) {
   // Broadcast scalar to vector of the same element type.
   if (srcType.isIntOrIndexOrFloat() && dstVectorType &&
       getElementTypeOrSelf(srcType) == getElementTypeOrSelf(dstVectorType))
@@ -2390,13 +2390,31 @@ mlir::vector::isBroadcastableTo(Type srcType, VectorType dstVectorType,
   // Source has an exact match or singleton value for all trailing dimensions
   // (all leading dimensions are simply duplicated).
   int64_t lead = dstRank - srcRank;
-  for (int64_t r = 0; r < srcRank; ++r) {
-    int64_t srcDim = srcVectorType.getDimSize(r);
-    int64_t dstDim = dstVectorType.getDimSize(lead + r);
-    if (srcDim != 1 && srcDim != dstDim) {
-      if (mismatchingDims) {
-        mismatchingDims->first = srcDim;
-        mismatchingDims->second = dstDim;
+  for (int64_t dimIdx = 0; dimIdx < srcRank; ++dimIdx) {
+    // Have mismatching dims (in the sense of vector.broadcast semantics) been
+    // encountered?
+    bool foundMismatchingDims = false;
+
+    // Check fixed-width dims.
+    int64_t srcDim = srcVectorType.getDimSize(dimIdx);
+    int64_t dstDim = dstVectorType.getDimSize(lead + dimIdx);
+    if (srcDim != 1 && srcDim != dstDim)
+      foundMismatchingDims = true;
+
+    // Check scalable flags.
+    bool srcDimScalableFlag = srcVectorType.getScalableDims()[dimIdx];
+    bool dstDimScalableFlag = dstVectorType.getScalableDims()[lead + dimIdx];
+    if ((srcDim == 1 && srcDimScalableFlag && dstDim != 1) ||
+        (srcDimScalableFlag != dstDimScalableFlag))
+      foundMismatchingDims = true;
+
+    if (foundMismatchingDims) {
+      if (mismatchingDims != nullptr) {
+        mismatchingDims->first.dim = srcDim;
+        mismatchingDims->first.isScalable = srcDimScalableFlag;
+
+        mismatchingDims->second.dim = dstDim;
+        mismatchingDims->second.isScalable = dstDimScalableFlag;
       }
       return BroadcastableToResult::DimensionMismatch;
     }
@@ -2406,16 +2424,22 @@ mlir::vector::isBroadcastableTo(Type srcType, VectorType dstVectorType,
 }
 
 LogicalResult BroadcastOp::verify() {
-  std::pair<int, int> mismatchingDims;
+  std::pair<VectorDim, VectorDim> mismatchingDims;
   BroadcastableToResult res = isBroadcastableTo(
       getSourceType(), getResultVectorType(), &mismatchingDims);
   if (res == BroadcastableToResult::Success)
     return success();
   if (res == BroadcastableToResult::SourceRankHigher)
     return emitOpError("source rank higher than destination rank");
-  if (res == BroadcastableToResult::DimensionMismatch)
+  if (res == BroadcastableToResult::DimensionMismatch) {
     return emitOpError("dimension mismatch (")
-           << mismatchingDims.first << " vs. " << mismatchingDims.second << ")";
+           << (mismatchingDims.first.isScalable ? "[" : "")
+           << mismatchingDims.first.dim
+           << (mismatchingDims.first.isScalable ? "]" : "") << " vs. "
+           << (mismatchingDims.second.isScalable ? "[" : "")
+           << mismatchingDims.second.dim
+           << (mismatchingDims.second.isScalable ? "]" : "") << ")";
+  }
   if (res == BroadcastableToResult::SourceTypeNotAVector)
     return emitOpError("source type is not a vector");
   llvm_unreachable("unexpected vector.broadcast op error");

mlir/test/Dialect/Vector/invalid.mlir

@@ -35,6 +35,27 @@ func.func @broadcast_dim2_mismatch(%arg0: vector<4x8xf32>) {
 
 // -----
 
+func.func @broadcast_scalable_unit_dim(%arg0: vector<[1]xf32>) {
+  // expected-error@+1 {{'vector.broadcast' op dimension mismatch ([1] vs. [4])}}
+  %0 = vector.broadcast %arg0 : vector<[1]xf32> to vector<[4]xf32>
+}
+
+// -----
+
+func.func @broadcast_fixed_to_scalable(%arg0: vector<2xf32>) {
+  // expected-error@+1 {{'vector.broadcast' op dimension mismatch (2 vs. [2])}}
+  %0 = vector.broadcast %arg0 : vector<2xf32> to vector<[2]xf32>
+}
+
+// -----
+
+func.func @broadcast_scalable_to_fixed(%arg0: vector<[1]xf32>) {
+  // expected-error@+1 {{'vector.broadcast' op dimension mismatch ([1] vs. 1)}}
+  %0 = vector.broadcast %arg0 : vector<[1]xf32> to vector<4x1xf32>
+}
+
+// -----
+
 func.func @broadcast_unknown(%arg0: memref<4x8xf32>) {
   // expected-error@+1 {{'vector.broadcast' op source type is not a vector}}
   %1 = vector.broadcast %arg0 : memref<4x8xf32> to vector<1x8xf32>