[llvm] [AArch64][GlobalISel] Match G_DUP with undef elements (PR #195237)

[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/lib/Target/AArch64/GISel/AArch64PostLegalizerLowering.cpp llvm/test/CodeGen/AArch64/arm64-vhadd.ll llvm/test/CodeGen/AArch64/cttz.ll llvm/test/CodeGen/AArch64/dup.ll llvm/test/CodeGen/AArch64/fadd-combines.ll llvm/test/CodeGen/AArch64/fcmp.ll llvm/test/CodeGen/AArch64/fcopysign.ll llvm/test/CodeGen/AArch64/fcvt_combine.ll llvm/test/CodeGen/AArch64/fixed-vector-interleave.ll llvm/test/CodeGen/AArch64/fsh.ll llvm/test/CodeGen/AArch64/hadd-combine.ll llvm/test/CodeGen/AArch64/hoist-and-by-const-from-lshr-in-eqcmp-zero.ll llvm/test/CodeGen/AArch64/hoist-and-by-const-from-shl-in-eqcmp-zero.ll llvm/test/CodeGen/AArch64/icmp.ll llvm/test/CodeGen/AArch64/rem-by-const.ll llvm/test/CodeGen/AArch64/select_cc.ll llvm/test/CodeGen/AArch64/shift.ll llvm/test/CodeGen/AArch64/vec-combine-compare-to-bitmask.ll
``````````

</details>


The following files introduce new uses of undef:
 - llvm/test/CodeGen/AArch64/dup.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/195237