[Mlir-commits] [mlir] [mlir][vector] transpose(broadcast) -> broadcast canonicalization (PR #135096)

[Andrzej Warzyński]

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@@ -6155,12 +6155,115 @@ class FoldTransposeCreateMask final : public OpRewritePattern<TransposeOp> {
   }
 };
 
+/// Folds transpose(broadcast(x)) to broadcast(x) if the transpose is
+/// 'order preserving', where 'order preserving' means the flattened
+/// inputs and outputs of the transpose have identical (numerical) values.
+///
+/// Example:
+/// ```
+///  %0 = vector.broadcast %input : vector<1x1xi32> to vector<1x8xi32>
+///  %1 = vector.transpose %0, [1, 0] : vector<1x8xi32>
+///                                                 to vector<8x1xi32>
+/// ```
+/// can be rewritten as the equivalent
+/// ```
+///  %0 = vector.broadcast %input : vector<1x1xi32> to vector<8x1xi32>.
+/// ```
+/// The algorithm works by partitioning dimensions into groups that can be
+/// locally permuted while preserving order, and checks that the transpose
+/// only permutes within these groups.
+class FoldTransposeBroadcast : public OpRewritePattern<vector::TransposeOp> {
+public:
+  using OpRewritePattern::OpRewritePattern;
+  FoldTransposeBroadcast(MLIRContext *context, PatternBenefit benefit = 1)
+      : OpRewritePattern<vector::TransposeOp>(context, benefit) {}
+
+  static bool canFoldIntoPrecedingBroadcast(vector::TransposeOp transpose) {
+
+    vector::BroadcastOp broadcast =
+        transpose.getVector().getDefiningOp<vector::BroadcastOp>();
+    if (!broadcast)
+      return false;
+
+    auto inputType = dyn_cast<VectorType>(broadcast.getSourceType());
+    bool inputIsScalar = !inputType;
+    ArrayRef<int64_t> inputShape = inputType.getShape();
+    int64_t inputRank = inputType.getRank();
+    int64_t outputRank = transpose.getType().getRank();
+    int64_t deltaRank = outputRank - inputRank;
+
+    // transpose(broadcast(scalar)) -> broadcast(scalar) is always valid
+    if (inputIsScalar)
+      return true;
+
+    // Return true if all permutation destinations for indices in [low, high)
+    // are in [low, high), so the permutation is local to the group.
+    auto isGroupBound = [&](int low, int high) {
+      ArrayRef<int64_t> permutation = transpose.getPermutation();
+      for (int j = low; j < high; ++j) {
+        if (permutation[j] < low || permutation[j] >= high) {
+          return false;
+        }
+      }
+      return true;
+    };
+
+    // Groups are either contiguous sequences  of 1s and non-1s (1-element
+    // groups). Consider broadcasting 4x1x1x7 to 2x3x4x5x6x7. This is equivalent
+    // to broadcasting from 1x1x4x1x1x7.
+    //                      ^^^ ^ ^^^ ^
+    //               groups: 0  1  2  3
+    // Order preserving permutations for this example are ones that only permute
+    // within the groups [0,1] and [3,4], like (1 0 2 4 3 5 6).
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banach-space wrote:

To me this part is key and I would move it to the top comment for this pattern. Btw, very clearly explained, thanks!

https://github.com/llvm/llvm-project/pull/135096