[Bug target/124740] New: x86 (32bit) could handle musttail with -fPIC

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124740

            Bug ID: 124740
           Summary: x86 (32bit) could handle musttail with -fPIC
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: rejects-valid, tail-call
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: i?86-*-* x86_64-*-*

Created attachment 64111
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=64111&action=edit
testcase

With this testcase at -O2 -fPIC -m32; GCC rejects the attached because of:
```
  /* If we are generating position-independent code, we cannot sibcall
     optimize direct calls to global functions, as the PLT requires
     %ebx be live. (Darwin does not have a PLT.)  */
  if (!TARGET_MACHO
      && !TARGET_64BIT
      && flag_pic
      && flag_plt
      && bind_global)
    return false;

```

But with must-tail you could do an indirect tail call instead. For must-tail it
is better to do an indirect tail call than rejecting it.

This was noticed while adding musttail to clang:
https://github.com/llvm/llvm-project/pull/188419#issuecomment-4148497506