[llvm] [SLP]Reduce number of alternate instruction, where possible (PR #123360)

[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)' 63b0ab84253f29f1f9b9136a02d589552b29c645 b0e4d402159778e38e08b7442607f1c0a9c7c66f llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp llvm/test/Transforms/PhaseOrdering/AArch64/slpordering.ll llvm/test/Transforms/SLPVectorizer/AArch64/gather-with-minbith-user.ll llvm/test/Transforms/SLPVectorizer/AArch64/loadorder.ll llvm/test/Transforms/SLPVectorizer/AArch64/tsc-s116.ll llvm/test/Transforms/SLPVectorizer/RISCV/complex-loads.ll llvm/test/Transforms/SLPVectorizer/RISCV/reductions.ll llvm/test/Transforms/SLPVectorizer/X86/alternate-cast-inseltpoison.ll llvm/test/Transforms/SLPVectorizer/X86/alternate-cast.ll llvm/test/Transforms/SLPVectorizer/X86/alternate-fp-inseltpoison.ll llvm/test/Transforms/SLPVectorizer/X86/alternate-fp.ll llvm/test/Transforms/SLPVectorizer/X86/alternate-int-inseltpoison.ll llvm/test/Transforms/SLPVectorizer/X86/alternate-int.ll llvm/test/Transforms/SLPVectorizer/X86/buildvector-schedule-for-subvector.ll llvm/test/Transforms/SLPVectorizer/X86/long-full-reg-stores.ll llvm/test/Transforms/SLPVectorizer/X86/matched-shuffled-entries.ll llvm/test/Transforms/SLPVectorizer/X86/non-load-reduced-as-part-of-bv.ll llvm/test/Transforms/SLPVectorizer/X86/scatter-vectorize-reused-pointer.ll llvm/test/Transforms/SLPVectorizer/X86/splat-score-adjustment.ll llvm/test/Transforms/SLPVectorizer/addsub.ll llvm/test/Transforms/SLPVectorizer/resized-alt-shuffle-after-minbw.ll
``````````

</details>


The following files introduce new uses of undef:
 - llvm/test/Transforms/SLPVectorizer/AArch64/gather-with-minbith-user.ll
 - llvm/test/Transforms/SLPVectorizer/X86/non-load-reduced-as-part-of-bv.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/123360