[llvm] SystemZ: Mark unread CC defs dead at build time (PR #226434)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-systemz

Author: Matt Arsenault (arsenm)

<details>
<summary>Changes</summary>

The custom inserters for the subword atomic pseudos and for probed
allocas, plus the inline stack probe emitted by frame lowering, build
instructions whose CC def is never read. Set the dead flags.

Marking the RISBG32 in the subword cmpxchg loop dead lets MachineSink
move it out of the compare block and into the store block, so it is
skipped when the comparison fails.

Co-Authored-By: Claude Opus 5 <noreply@<!-- -->anthropic.com>

---
Full diff: https://github.com/llvm/llvm-project/pull/226434.diff


6 Files Affected:

- (modified) llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp (+6-3) 
- (modified) llvm/lib/Target/SystemZ/SystemZISelLowering.cpp (+53-21) 
- (modified) llvm/test/CodeGen/SystemZ/atomicrmw-fadd-04.ll (+3-3) 
- (modified) llvm/test/CodeGen/SystemZ/cmpxchg-01.ll (+10-10) 
- (modified) llvm/test/CodeGen/SystemZ/cmpxchg-02.ll (+12-12) 
- (added) llvm/test/CodeGen/SystemZ/dead-cc-defs.ll (+75) 


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</details>


https://github.com/llvm/llvm-project/pull/226434