[llvm] [SLP]Vectorize single-user instructions as the last-attempt seeds (PR #212579)

[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)' 'HEAD~1' HEAD llvm/include/llvm/Transforms/Vectorize/SLPVectorizer.h llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp llvm/lib/Transforms/Vectorize/SLPVectorizer/SLPUtils.cpp llvm/lib/Transforms/Vectorize/SLPVectorizer/SLPUtils.h llvm/test/Transforms/PhaseOrdering/X86/avg.ll llvm/test/Transforms/SLPVectorizer/AArch64/extractelements-to-shuffle.ll llvm/test/Transforms/SLPVectorizer/AArch64/unsigned-after-sext-node.ll llvm/test/Transforms/SLPVectorizer/AArch64/vectorize-free-extracts-inserts.ll llvm/test/Transforms/SLPVectorizer/RISCV/revec.ll llvm/test/Transforms/SLPVectorizer/RISCV/runtime-strided-stores.ll llvm/test/Transforms/SLPVectorizer/X86/alternate-calls-inseltpoison.ll llvm/test/Transforms/SLPVectorizer/X86/alternate-calls.ll llvm/test/Transforms/SLPVectorizer/X86/alternate-int-inseltpoison.ll llvm/test/Transforms/SLPVectorizer/X86/alternate-int.ll llvm/test/Transforms/SLPVectorizer/X86/arith-fp-inseltpoison.ll llvm/test/Transforms/SLPVectorizer/X86/arith-fp.ll llvm/test/Transforms/SLPVectorizer/X86/broadcast.ll llvm/test/Transforms/SLPVectorizer/X86/c-ray.ll llvm/test/Transforms/SLPVectorizer/X86/copyable-operands-reordering.ll llvm/test/Transforms/SLPVectorizer/X86/crash_getpointersdiff-nullopt.ll llvm/test/Transforms/SLPVectorizer/X86/debug-info-salvage.ll llvm/test/Transforms/SLPVectorizer/X86/deleted-inst-reduction-attempt.ll llvm/test/Transforms/SLPVectorizer/X86/extractelement-multi-register-use.ll llvm/test/Transforms/SLPVectorizer/X86/horizontal-list.ll llvm/test/Transforms/SLPVectorizer/X86/intrinsic.ll llvm/test/Transforms/SLPVectorizer/X86/lookahead.ll llvm/test/Transforms/SLPVectorizer/X86/non-vectorizable-inst-operand.ll llvm/test/Transforms/SLPVectorizer/X86/parent-node-split-non-schedulable.ll llvm/test/Transforms/SLPVectorizer/X86/phi-comparator-fix-vec-ops-compare.ll llvm/test/Transforms/SLPVectorizer/X86/pr42022-inseltpoison.ll llvm/test/Transforms/SLPVectorizer/X86/pr42022.ll llvm/test/Transforms/SLPVectorizer/X86/pr52275.ll llvm/test/Transforms/SLPVectorizer/X86/reorder_diamond_match.ll llvm/test/Transforms/SLPVectorizer/X86/reused-mask-with-poison-index.ll llvm/test/Transforms/SLPVectorizer/X86/scalarize-ctlz.ll llvm/test/Transforms/SLPVectorizer/alternate-non-profitable.ll llvm/test/Transforms/SLPVectorizer/insert-element-build-vector-inseltpoison.ll llvm/test/Transforms/SLPVectorizer/insert-element-build-vector.ll llvm/test/Transforms/SLPVectorizer/minbitwidth-multiuse-with-insertelement.ll
``````````

</details>


The following files introduce new uses of undef:
 - llvm/test/Transforms/SLPVectorizer/X86/reused-mask-with-poison-index.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/212579