[llvm-branch-commits] [mlir] [mlir][linalg] Restrict linalg.pack to not have artificial padding. (PR #149624)

[Adam Siemieniuk via llvm-branch-commits]

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@@ -150,9 +150,15 @@ def Linalg_PackOp : Linalg_RelayoutOp<"pack", [
 
     `padding_value` specifies a padding value at the boundary on non-perfectly
     divisible dimensions. Padding is optional:
-    - If absent, it is UB if the tile does not perfectly divide the dimension.
+    - If absent, it is assumed that for all inner tiles,
+      `shape(source)[inner_dims_pos[i]] % inner_tiles[i] == 0`, i.e. all inner
+      tiles divide perfectly the corresponding outer dimension in the result
+      tensor.
     - If present, it will pad along high dimensions (high-padding) to make the
-      tile complete.
+      tile complete. Note that it is not allowed to have artificial padding that
+      is not strictly required by linalg.pack (i.e., padding past what is needed
+      to complete the last tile along each packed dimension). It is UB if extra
+      padding is requested.
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adam-smnk wrote:

> Shouldn't that be verification error?

It's not possible to enforce that with dynamic source.
`UB` is more of "catch all" here and allows `linalg::lowerPack` to remain as is.

> restore UB for the previous point

It could remain there to reinforce the message. But no strong preference here.

https://github.com/llvm/llvm-project/pull/149624