[Mlir-commits] [mlir] [MLIR] Bubble up tensor.extract_slice through tensor.collapse_shape (PR #131982)

[llvmlistbot at llvm.org]

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@@ -428,6 +429,239 @@ struct BubbleUpExpandShapeThroughExtractSlice
   }
 };
 
+/// Converts `tensor.extract_slice(tensor.collapse_shape)` to
+///          `tensor.collapse_shape(tensor.extract_slice)`.
+///
+/// For this transformation to be possible - after bubbling up, the extraction
+/// of the contiguous slice must be representable as a single slice obtained via
+/// tensor.extract_slice within each reassociation group of the src.
+///
+/// In case the size and offset extracted are static then this is possible if
+/// the following conditions are met within each reassociation group:
+/// Let T be a tensor of shape [A0, A1, ..., An] (these are the sizes of the
+/// dimensions in the reassociation group), and let S = [S0, S1, ..., Sn] be the
+/// shape of a desired slice. A slice of shape S can be extracted as a
+/// contiguous span of elements if and only if there exists an index k in {0, 1,
+/// ..., n} such that:
+///      S_i = 1 for all i < k (that is, all leading dimensions are singleton),
+///      1 <= S_k <= A_k (that is, non trivial slicing occurs along exactly
+///                       one dimension),
+///      S_i = A_i for all i > k (that is, all trailing dimensions are preserved
+///      in full).
+/// In other words, the slice shape S must be of the form:
+/// [ 1, 1, ..., 1, Sk, Ak + 1, Ak + 2, ...,An ]
+///
+/// In case the size and/or offset extracted are dynamic then this is possible
+/// only if there is single dimension in the reassociation group that has a size
+/// not equal to 1.
+/// In other words, the tensor shape must be of the form:
+/// [ 1, 1, ..., 1, A, 1, ...,1 ]
+/// Note - it might be possible to enable this pattern for more cases when the
+/// size/offset are dynamic via performing an analysis of the possible values
+/// that could be given to the size/offset.
+///
+/// Example:
+/// The transformation is possible because each reassociation group can be
+/// represented as a contiguous slice (i.e., [8x16->2x16], [1x7->1x?],
+/// [20->10]).
+/// ```
+/// BEFORE:
+/// %collapse = tensor.collapse_shape %src [[0, 1], [2, 3], [4]] ...
+///     tensor<8x16x1x7x20f32> to tensor<128x7x20xf32>
+/// %slice = tensor.extract_slice %slice [0, 0, 0][32, %size, 10][1, 1, 1]
+///     tensor<128x7x20xf32> to tensor<32x?x10xf32>
+///
+/// AFTER:
+/// %slice = tensor.extract_slice %src [0, 0, 0, 0, 0][2, 16, 1, %size, 10]
+//           [1, 1, 1, 1, 1] : tensor<8x16x1x7x20f32> to tensor<2x16x1x?x10xf32>
+/// %collapse = tensor.collapse_shape %slice [[0, 1], [2, 3], [4]] ...
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MaheshRavishankar wrote:

Whew! This is actually correct. Took me a while to work out that it, but this works because for the reassociation the other dimensions are all 1. 

https://github.com/llvm/llvm-project/pull/131982