[all-commits] [llvm/llvm-project] 0f3524: [mlir][vector] shape_cast(constant) -> constant fo...
James Newling via All-commits
all-commits at lists.llvm.org
Thu Jul 31 12:13:15 PDT 2025
Branch: refs/heads/main
Home: https://github.com/llvm/llvm-project
Commit: 0f3524481680ee31ec38d79be90207898213e217
https://github.com/llvm/llvm-project/commit/0f3524481680ee31ec38d79be90207898213e217
Author: James Newling <james.newling at gmail.com>
Date: 2025-07-31 (Thu, 31 Jul 2025)
Changed paths:
M mlir/lib/Dialect/Vector/IR/VectorOps.cpp
M mlir/test/Dialect/Vector/canonicalize.mlir
Log Message:
-----------
[mlir][vector] shape_cast(constant) -> constant fold for non-splats (#145539)
The folder `shape_cast(splat constant) -> splat constant` was first
introduced
[here](https://github.com/llvm/llvm-project/commit/36480657d8ce97836f76bf5fa8c36677b9cdc19a#diff-484cea976e0c96459027c951733bf2d22d34c5a0c0de6f577069870ef4588983R2600)
(Nov 2020). In that commit there is a comment to _Only handle splat for
now_. Based on that I assume the intention was to, at a later time,
support a general `shape_cast(constant) -> constant` folder. That is
what this PR does
One minor downside: It is possible with this folder end up with, instead
of 1 large constant and 1 shape_cast, 2 large constants:
```mlir
func.func @foo() -> (vector<4xi32>, vector<2x2xi32>) {
%cst = arith.constant dense<[1, 2, 3, 4]> : vector<4xi32> # 'large' constant 1
%0 = vector.shape_cast %cst : vector<4xi32> to vector<2x2xi32>
return %cst, %0 : vector<4xi32>, vector<2x2xi32>
}
```
gets folded with this new folder to
```mlir
func.func @foo() -> (vector<4xi32>, vector<2x2xi32>) {
%cst = arith.constant dense<[1, 2, 3, 4]> : vector<4xi32> # 'large' constant 1
%cst_0 = arith.constant dense<[[1, 2], [3, 4]]> : vector<2x2xi32> # 'large' constant 2
return %cst, %cst_0 : vector<4xi32>, vector<2x2xi32>
}
```
Notes on the above case:
1) This only effects the textual IR, the actual values share the same
context storage (I've verified this by checking pointer values in the
`DenseIntOrFPElementsAttrStorage`
[constructor](https://github.com/llvm/llvm-project/blob/da5c442550a3823fff05c14300c1664d0fbf68c8/mlir/lib/IR/AttributeDetail.h#L59))
so no compile-time memory overhead to this folding. At the LLVM
IR level the constant is shared, too.
2) This only happens when the pre-folded constant cannot be dead code
eliminated (i.e. when it has 2+ uses) which I don't think is common.
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