[libc] register exhaustion with gcc-14

[nickdesaulniers]

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.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

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.

[pinskia]

Do you have a preprocessed source for this? I might take a look to understand the inline-asm to see if something is wrong but it is easier with the preprocessed source.

[michaelrj-google]

The code for thread.cpp is at libc/src/__support/threads/linux/thread.cpp and the inline asm is for syscalls and is at libc/src/__support/OSUtil/linux/x86_64/syscall.h