[llvm] StructurizeCFG: Use poison instead of undef (PR #130459)

[via llvm-commits]

github-actions[bot] wrote:

<!--LLVM CODE FORMAT COMMENT: {undef deprecator}-->


:warning: undef deprecator found issues in your code. :warning:

<details>
<summary>
You can test this locally with the following command:
</summary>

``````````bash
git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 0715c6cc6d11814b8a31bc0368ca080b72810177 c2f7c83683e432cb7c1850de36e25614a3efddbb llvm/lib/Transforms/Scalar/StructurizeCFG.cpp llvm/test/CodeGen/AMDGPU/GlobalISel/divergent-control-flow.ll llvm/test/CodeGen/AMDGPU/agpr-copy-no-free-registers.ll llvm/test/CodeGen/AMDGPU/blender-no-live-segment-at-def-implicit-def.ll llvm/test/CodeGen/AMDGPU/branch-folding-implicit-def-subreg.ll llvm/test/CodeGen/AMDGPU/itofp.i128.bf.ll llvm/test/CodeGen/AMDGPU/itofp.i128.ll llvm/test/CodeGen/AMDGPU/loop_break.ll llvm/test/CodeGen/AMDGPU/multi-divergent-exit-region.ll llvm/test/CodeGen/AMDGPU/multilevel-break.ll llvm/test/CodeGen/AMDGPU/nested-loop-conditions.ll llvm/test/CodeGen/AMDGPU/optimize-negated-cond.ll llvm/test/CodeGen/AMDGPU/scheduler-rp-calc-one-successor-two-predecessors-bug.ll llvm/test/CodeGen/AMDGPU/si-annotate-dbg-info.ll llvm/test/CodeGen/AMDGPU/tuple-allocation-failure.ll llvm/test/CodeGen/AMDGPU/vgpr-liverange-ir.ll llvm/test/Transforms/StructurizeCFG/AMDGPU/loop-subregion-misordered.ll llvm/test/Transforms/StructurizeCFG/AMDGPU/uniform-regions.ll llvm/test/Transforms/StructurizeCFG/interleaved-loop-order.ll llvm/test/Transforms/StructurizeCFG/loop-break-phi.ll llvm/test/Transforms/StructurizeCFG/loop-continue-phi.ll llvm/test/Transforms/StructurizeCFG/one-loop-multiple-backedges.ll llvm/test/Transforms/StructurizeCFG/structurizer-keep-perf-md.ll llvm/test/Transforms/StructurizeCFG/workarounds/needs-fix-reducible.ll llvm/test/Transforms/StructurizeCFG/workarounds/needs-fr-ule.ll llvm/test/Transforms/StructurizeCFG/workarounds/needs-unified-loop-exits.ll
``````````

</details>


The following files introduce new uses of undef:
 - llvm/test/CodeGen/AMDGPU/loop_break.ll
 - llvm/test/CodeGen/AMDGPU/vgpr-liverange-ir.ll

[Undef](https://llvm.org/docs/LangRef.html#undefined-values) is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields `undef`. You should use `poison` values for placeholders instead.

In tests, avoid using `undef` and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:
```llvm
define void @fn() {
  ...
  br i1 undef, ...
}
```

Please use the following instead:
```llvm
define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}
```

Please refer to the [Undefined Behavior Manual](https://llvm.org/docs/UndefinedBehavior.html) for more information.



https://github.com/llvm/llvm-project/pull/130459