[all-commits] [llvm/llvm-project] c8c6c0: [flang] Drop nuw on XArrayCoor for non-positive sl...

[Matsu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c8c6c0ff7461a5a4c19c110d830945967bc97329
      https://github.com/llvm/llvm-project/commit/c8c6c0ff7461a5a4c19c110d830945967bc97329
  Author: Matsu <47756807+khaki3 at users.noreply.github.com>
  Date:   2026-07-29 (Wed, 29 Jul 2026)

  Changed paths:
    M flang/lib/Optimizer/CodeGen/CodeGen.cpp
    M flang/test/Fir/array_coor_nuw_nusw.fir
    M flang/test/Fir/convert-to-llvm.fir

  Log Message:
  -----------
  [flang] Drop nuw on XArrayCoor for non-positive slice steps (#212639)

Example:
```fortran
z(:, 3:2:-1) = z(:, 1:2)
```

In this code, reverse-section indexing lowers `idx*step` / `diff+adj`
with `nuw`. A negative step can make the product negative, so `nuw` is
invalid and LLVM `-O2` can drop the stores.

Fix: keep `nsw|nuw` only for known positive steps; otherwise keep `nsw`
and drop `nuw` (negative, zero, or unknown).



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