[all-commits] [llvm/llvm-project] abe92a: [DAGCombine] Fix an incorrect folding of extract_s...

[Min-Yih Hsu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: abe92a50005a2a4998e4a006417fdfe6ff36eb6b
      https://github.com/llvm/llvm-project/commit/abe92a50005a2a4998e4a006417fdfe6ff36eb6b
  Author: Min-Yih Hsu <min.hsu at sifive.com>
  Date:   2025-08-14 (Thu, 14 Aug 2025)

  Changed paths:
    M llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    A llvm/test/CodeGen/RISCV/rvv/incorrect-extract-subvector-combine.ll

  Log Message:
  -----------
  [DAGCombine] Fix an incorrect folding of extract_subvector (#153709)

Reported from
https://github.com/llvm/llvm-project/pull/153393#issuecomment-3189898813

During DAGCombine, an intermediate extract_subvector sequence was
generated:
```
  t8: v9i16 = extract_subvector t3, Constant:i64<9>
t24: v8i16 = extract_subvector t8, Constant:i64<0>
```
And one of the DAGCombine rule which turns `(extract_subvector
(extract_subvector X, C), 0)` into `(extract_subvector X, C)` kicked in
and turn that into `v8i16 = extract_subvector t3, Constant:i64<9>`. But
it forgot to check if the extracted index is a multiple of the minimum
vector length of the result type, hence the crash.

This patch fixes this by adding an additional check.



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