[all-commits] [llvm/llvm-project] f28f0b: [X86][APX] Fix EFLAGS reuse across cycles (#223613)

[Feng Zou via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: f28f0baf480125fd1a0b70c39c316778e3794b13
      https://github.com/llvm/llvm-project/commit/f28f0baf480125fd1a0b70c39c316778e3794b13
  Author: Feng Zou <feng.zou at intel.com>
  Date:   2026-09-20 (Sun, 20 Sep 2026)

  Changed paths:
    M llvm/lib/Target/X86/X86InstrInfo.cpp
    M llvm/test/CodeGen/X86/apx/optimize-compare-multipred.mir

  Log Message:
  -----------
  [X86][APX] Fix EFLAGS reuse across cycles (#223613)

optimizeCompareInstr's multi-predecessor NF reuse (added in
ab39c491430e) did not scan the below-CmpInstr region of CmpMBB when
CmpMBB was on a CFG cycle: that region is on the back-edge path but is
scanned by neither the caller nor the helper's predecessor walk. A
non-NF-convertible EFLAGS clobber there (e.g. an unrelated CMP feeding a
JCC, as in the reproducer below with VUCOMISS/CTEST/COPY-\$eflags) still
let the helper delete the redundant compare and mark \$eflags live-in on
CmpMBB. That broke the reused EFLAGS on iteration 2+ and tripped
X86FlagsCopyLowering's SuccMBB == TestMBB self-loop check with a fatal
report_fatal_error when a preexisting COPY \$eflags save/restore sat in
the cycle.

Reproducer: https://godbolt.org/z/s58j5zxqd

Detect CmpMBB on a cycle via a forward-reachability BFS on its
successors and, when so, scan from just after CmpInstr to the block's
end: bail on any non-NF-convertible EFLAGS clobber, stage NF-convertible
ones. The rest of the helper is unchanged.

Tests exercise the bail paths (selfloop_shadow_below,
twoblock_cycle_shadow_below, threeblock_cycle_shadow_below,
selfloop_nfconvertible_below_cap), the positive stage-and-succeed path
(selfloop_nfconvertible_below), and the CmpMBB-not-on-cycle skip path
(multipred_above_cmpmbb).

Fixes: https://github.com/llvm/llvm-project/issues/223282

Assisted by Claude Code.



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