<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/98859>98859</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [PowerPC] [LLD][Compiler] support tail call across modules for ELFv2
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            lld
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          chenzheng1030
      </td>
    </tr>
</table>

<pre>
    This is from https://github.com/llvm/llvm-project/issues/63214#issuecomment-2128605743 and https://github.com/llvm/llvm-project/issues/63214#issuecomment-2128669372

For ELFv2, tail call optimization should be allowed even across modules. The idea is:
(Function A and function B share the TOC; B calls function C which does not share TOC with B.)

1: Changes in the compiler side:
- For the call between A and B, use the `st_other === 1` feature to let linker know that when B is called, the caller A needs to restore the TOC although they share the TOC.
  - From ABI: for `st_other == 1`: The local and global entry points are the same, and r2 should be treated as caller-saved for local and global callers."
  - inside the compiler, for the call between A and B, we should not use `PPCISD::CALL`, instead `PPCISD::CALL_NOP` should be used to allow the tail call.
  - B should set `st_other` to 1
- For the tail call between A and B, use the tail call opcode

2: Changes in the lld linker side:
- no need for the `nop` after `b` even for the b cross the modules. For now, linker will report an error `call to bar lacks nop, can't restore toc`, see https://github.com/llvm/llvm-project/issues/63214#issuecomment-2217053717 and https://github.com/llvm/llvm-project/issues/63214#issuecomment-2219264350
</pre>
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