[llvm] [SLP]Improved reduction cost/codegen (PR #118293)

[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)' 2a922903bf5d5b0012c1f8f2a5396d44cfff4630 d526472ca1f526c9e0017a52f344d523c8ff76f5 llvm/include/llvm/Analysis/TargetTransformInfo.h llvm/include/llvm/Analysis/TargetTransformInfoImpl.h llvm/include/llvm/CodeGen/BasicTTIImpl.h llvm/lib/Analysis/TargetTransformInfo.cpp llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp llvm/test/Transforms/SLPVectorizer/AArch64/reduce-fadd.ll llvm/test/Transforms/SLPVectorizer/AMDGPU/reduction.ll llvm/test/Transforms/SLPVectorizer/RISCV/horizontal-list.ll llvm/test/Transforms/SLPVectorizer/RISCV/partial-vec-invalid-cost.ll llvm/test/Transforms/SLPVectorizer/RISCV/reductions.ll llvm/test/Transforms/SLPVectorizer/X86/bool-mask.ll llvm/test/Transforms/SLPVectorizer/X86/external-used-across-reductions.ll llvm/test/Transforms/SLPVectorizer/X86/extract-scalar-from-undef.ll llvm/test/Transforms/SLPVectorizer/X86/horizontal-list.ll llvm/test/Transforms/SLPVectorizer/X86/horizontal-minmax.ll llvm/test/Transforms/SLPVectorizer/X86/malformed_phis.ll llvm/test/Transforms/SLPVectorizer/X86/multi-tracked-reduced-value.ll llvm/test/Transforms/SLPVectorizer/X86/phi-node-reshuffled-part.ll llvm/test/Transforms/SLPVectorizer/X86/reduced-val-extracted-and-externally-used.ll llvm/test/Transforms/SLPVectorizer/X86/reduction-extracted-value.ll llvm/test/Transforms/SLPVectorizer/X86/reduction-logical.ll llvm/test/Transforms/SLPVectorizer/X86/reduction-transpose.ll llvm/test/Transforms/SLPVectorizer/X86/revectorized_rdx_crash.ll llvm/test/Transforms/SLPVectorizer/X86/undef_vect.ll llvm/test/Transforms/SLPVectorizer/partial-register-extract.ll llvm/test/Transforms/SLPVectorizer/reduction-modified-values.ll llvm/test/Transforms/SLPVectorizer/scalarization-overhead.ll
``````````

</details>


The following files introduce new uses of undef:
 - llvm/test/Transforms/SLPVectorizer/X86/revectorized_rdx_crash.ll
 - llvm/test/Transforms/SLPVectorizer/X86/undef_vect.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/118293