[clang] [llvm] [InstCombine] remove dead loads, such as memcpy from undef (PR #143958)

[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/test/Transforms/InstCombine/dead-alloc-elim.ll clang/test/Misc/loop-opt-setup.c llvm/lib/Transforms/InstCombine/InstructionCombining.cpp llvm/test/Transforms/InstCombine/and-or-icmps.ll llvm/test/Transforms/InstCombine/apint-shift.ll llvm/test/Transforms/InstCombine/call-cast-target.ll llvm/test/Transforms/InstCombine/fp-ret-bitcast.ll llvm/test/Transforms/InstCombine/getelementptr.ll llvm/test/Transforms/InstCombine/malloc-free.ll llvm/test/Transforms/InstCombine/multiple-uses-load-bitcast-select.ll llvm/test/Transforms/InstCombine/objsize.ll llvm/test/Transforms/InstCombine/select-load.ll llvm/test/Transforms/InstCombine/shift-amount-reassociation.ll llvm/test/Transforms/InstCombine/vscale_gep.ll
``````````

</details>


The following files introduce new uses of undef:
 - llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
 - llvm/test/Transforms/InstCombine/call-cast-target.ll
 - llvm/test/Transforms/InstCombine/dead-alloc-elim.ll
 - llvm/test/Transforms/InstCombine/fp-ret-bitcast.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/143958