<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/82387>82387</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Missed tail call optimization on undefined return value for all architectures
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          ekliptik
      </td>
    </tr>
</table>

<pre>
    See [compiler explorer](https://godbolt.org/z/7T7P3foed) to confirm it to affect x64, arm, aarch64, RISC-V clang, but not gcc, at `-O3`.

## LLVM

`TaillRecursionElimination.cpp` does kick in and marks the `call` with `tail`, but sdag lowers it to `PsuedoCALL`. Interestingly, adding `-fglobal-isel -mllvm -global-isel-abort=1` to AArch64 shows that with gisel, the IRTranslator turns `tail call` directly to `BL`, so it's not sdag-specific.

## GCC

I believe gcc's TCO in this case to be correct even just based on the C language spec since when a function doesn't `return` a value, the function call expression's value is undefined

## Reproducer

Build with `-O3` with any gcc, any clang:

```c
int i;
int bar(void);
int foo (void)
{
  if (i)
  {
    bar();
  }
}
```

Code has been reduced from `Proc_1` calling `Proc_7` in Dhrystone, where functions with prefix `Proc_` for "procedure" have unspecified return value types, and therefore default to `int` (since C89) despite the original intention clearly being `void`. Nevertheless this stinky use case is probably rather common and worth addressing
</pre>
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