[all-commits] [llvm/llvm-project] f8de1a: [X86] Prevent legalization cycles in lane-permute ...

[azwolski via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: f8de1a4a91af108860545b696cce9b2ef8d8fa09
      https://github.com/llvm/llvm-project/commit/f8de1a4a91af108860545b696cce9b2ef8d8fa09
  Author: azwolski <antoni.zwolski at intel.com>
  Date:   2026-09-19 (Sat, 19 Sep 2026)

  Changed paths:
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    A llvm/test/CodeGen/X86/shuffle-lane-permute-cycle.ll

  Log Message:
  -----------
  [X86] Prevent legalization cycles in lane-permute shuffle lowering (#224265)

When a cross-lane shuffle reproduces the original mask, trying a finer
sublane size can produce a decomposition that legalization transforms
back into the original shuffle. This causes legalization to cycle
indefinitely.

Stop trying finer sublane sizes when this condition is detected, and add
a regression test for the shuffle that previously caused `llc` to hang.

Fixes #224266



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