<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/124852>124852</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc] register exhaustion with gcc-14
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            build-problem,
            libc
      </td>
    </tr>

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

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

<pre>
    llvm-project/libc/src/__support/threads/linux/thread.cpp fails to compile for me with gcc-14:
```
In file included from /android0/llvm-project/libc/src/__support/CPP/type_traits/integral_constant.h:11,
                 from /android0/llvm-project/libc/src/__support/CPP/type_traits/has_unique_object_representations.h:11,
                 from /android0/llvm-project/libc/src/__support/CPP/atomic.h:12,
                 from /android0/llvm-project/libc/src/__support/threads/thread.h:12,
                 from /android0/llvm-project/libc/src/__support/threads/linux/thread.cpp:9:
In function ‘long int __llvm_libc_20_0_0_git::syscall_impl(long int, long int, long int)’,
    inlined from ‘R __llvm_libc_20_0_0_git::syscall_impl(long int, Ts ...) [with R = long int; Ts = {void*, long unsigned int}]’ at /android0/llvm-project/libc/src/__support/OSUtil/linux/syscall.h:34:49,
    inlined from ‘void __llvm_libc_20_0_0_git::free_stack(void*, size_t, size_t)’ at /android0/llvm-project/libc/src/__support/threads/linux/thread.cpp:128:37,
    inlined from ‘void __llvm_libc_20_0_0_git::cleanup_thread_resources(ThreadAttributes*)’ at /android0/llvm-project/libc/src/__support/threads/linux/thread.cpp:153:15,
    inlined from ‘int __llvm_libc_20_0_0_git::Thread::run(__llvm_libc_20_0_0_git::ThreadStyle, __llvm_libc_20_0_0_git::ThreadRunner, void*, void*, size_t, size_t, bool)’ at /android0/llvm-project/libc/src/__support/threads/linux/thread.cpp:335:29:
/android0/llvm-project/libc/src/__support/macros/attributes.h:28:25: error: ‘asm’ operand has impossible constraints or there are not enough registers
   28 | #define LIBC_INLINE_ASM __asm__ __volatile__
      |                         ^~~~~~~
/android0/llvm-project/libc/src/__support/OSUtil/linux/x86_64/syscall.h:39:3: note: in expansion of macro ‘LIBC_INLINE_ASM’
   39 |   LIBC_INLINE_ASM("syscall"
      |   ^~~~~~~~~~~~~~~
```
If I change the arity-2 syscall to use just `asm` rather than `LIBC_INLINE_ASM` (removing the `volatile` qualifier), we do not observe register exhaustion.  It's not immediately clear why that would fail; only with gcc and only with newer gcc, but I've been of the opinion that `LIBC_INLINE_ASM` needs to die.  I'd rather put [[nodiscard]] on our syscall wrappers.  Found while trying to test #124036.
</pre>
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