<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/107730>107730</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            MIPS tail calls are disabled by default at all optimization levels
        </td>
    </tr>

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

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

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

<pre>
    Hello, I noticed that tail call optimization is disabled on MIPS at all optimization levels by default.   This is causing issues with the `musttail` attribute in C and C++ programs, where tail calls that otherwise would be eligible are not being treated as such.

The relevant flag is defined here:

https://github.com/llvm/llvm-project/blob/98563b19c2d13bac7fb5ee2717ccae6279699e4c/llvm/lib/Target/Mips/MipsSEISelLowering.cpp#L54-L56

When building with `-mllvm -mips-tail-calls=1`, tail calls appear to be optimized accordingly (in the few tests I've done).  I noticed that there are test cases for this flag throughout the backend (just search for `mips-tail-calls=1`).

Here are a few issues that are affected by this behaviour:
https://github.com/llvm/llvm-project/issues/47971
https://github.com/llvm/llvm-project/issues/51709
https://github.com/llvm/llvm-project/issues/57795

Is it possible we could make the MIPS backend behave like the others in this respect?  It seems to me that if a call site is marked `musttail`, the backend should make every effort towards performing the tail call regardless of the current optimization level.  I would be willing to submit a PR for this change if there's interest.
</pre>
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