[Mlir-commits] [mlir] [mlir][vector] Folder: shape_cast(extract) -> extract (PR #146368)

[Andrzej Warzyński]

================
@@ -1707,59 +1707,98 @@ static bool hasZeroDimVectors(Operation *op) {
          llvm::any_of(op->getResultTypes(), hasZeroDimVectorType);
 }
 
-/// Fold extractOp with scalar result coming from BroadcastOp or SplatOp.
+/// All BroadcastOps and SplatOps, as well as ShapeCastOps that only prepend
+/// 1s, are considered to be 'broadcastlike'.
+static bool isBroadcastLike(Operation *op) {
+  if (isa<BroadcastOp, SplatOp>(op))
+    return true;
+
+  auto shapeCast = dyn_cast<ShapeCastOp>(op);
+  if (!shapeCast)
+    return false;
+
+  // Check that shape_cast **only** prepends 1s, like (2,3) -> (1,1,2,3).
+  // Checking that the destination shape has a prefix of 1s is not sufficient,
+  // for example (2,3) -> (1,3,2) is not broadcastlike. A sufficient condition
+  // is that the source shape is a suffix of the destination shape.
+  VectorType srcType = shapeCast.getSourceVectorType();
+  ArrayRef<int64_t> srcShape = srcType.getShape();
+  uint64_t srcRank = srcType.getRank();
+  ArrayRef<int64_t> dstShape = shapeCast.getType().getShape();
+  return dstShape.size() >= srcRank && dstShape.take_back(srcRank) == srcShape;
+}
+
+/// Fold extract(broadcast(X)) to either extract(X) or just X.
+///
+/// Example:
+///
+///        broadcast             extract [1][2]
+/// (3, 4) --------> (2, 3, 4) ----------------> (4)
+///
+/// becomes
+///                  extract [1]
+/// (3,4) -------------------------------------> (4)
+///
+///
+/// The variable names used in this implementation correspond to the above
+/// shapes as,
+///
+/// - (3, 4) is `input` shape.
+/// - (2, 3, 4) is `broadcast` shape.
+/// - (4) is `extract` shape.
+///
+/// This folding is possible when the suffix of `input` shape is the same as
+/// `extract` shape.
 static Value foldExtractFromBroadcast(ExtractOp extractOp) {
+
   Operation *defOp = extractOp.getVector().getDefiningOp();
-  if (!defOp || !isa<vector::BroadcastOp, SplatOp>(defOp))
+  if (!defOp || !isBroadcastLike(defOp))
     return Value();
 
-  Value source = defOp->getOperand(0);
-  if (extractOp.getType() == source.getType())
-    return source;
-  auto getRank = [](Type type) {
-    return llvm::isa<VectorType>(type) ? llvm::cast<VectorType>(type).getRank()
-                                       : 0;
-  };
+  Value input = defOp->getOperand(0);
 
-  // If splat or broadcast from a scalar, just return the source scalar.
-  unsigned broadcastSrcRank = getRank(source.getType());
-  if (broadcastSrcRank == 0 && source.getType() == extractOp.getType())
-    return source;
+  // Replace extract(broadcast(X)) with X
+  if (extractOp.getType() == input.getType())
+    return input;
 
-  unsigned extractResultRank = getRank(extractOp.getType());
-  if (extractResultRank > broadcastSrcRank)
-    return Value();
-  // Check that the dimension of the result haven't been broadcasted.
-  auto extractVecType = llvm::dyn_cast<VectorType>(extractOp.getType());
-  auto broadcastVecType = llvm::dyn_cast<VectorType>(source.getType());
-  if (extractVecType && broadcastVecType &&
-      extractVecType.getShape() !=
-          broadcastVecType.getShape().take_back(extractResultRank))
+  // Get required types and ranks in the chain
+  //    input -> broadcast -> extract
----------------
banach-space wrote:

```suggestion
  //    input -> broadcast -> extract
  // (scalars are treated as rank-0).
```

https://github.com/llvm/llvm-project/pull/146368