[PATCH] nptl: x86_64: Use stackinfo.h definition for CURRENT_STACK_FRAME

[Noah Goldstein]

On Mon, Aug 29, 2022 at 12:00 PM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> It avoids the possible warning of uninitialized 'frame' variable when
> building with clang:
>
>   ../sysdeps/nptl/jmp-unwind.c:27:42: error: variable 'frame' is
>   uninitialized when used here [-Werror,-Wuninitialized]
>     __pthread_cleanup_upto (env->__jmpbuf, CURRENT_STACK_FRAME);
>
> It increases the generated code by one instruction:
>
> --- master
> +++ patch
> @@ -25068,11 +25068,11 @@ Disassembly of section .text:
>     3db1b:      00 00
>     3db1d:      48 89 44 24 08          mov    %rax,0x8(%rsp)
>     3db22:      31 c0                   xor    %eax,%eax
> -   3db24:      48 8b 44 24 08          mov    0x8(%rsp),%rax
> -   3db29:      64 48 2b 04 25 28 00    sub    %fs:0x28,%rax
> -   3db30:      00 00
> -   3db32:      75 0c                   jne    3db40
>     <_longjmp_unwind+0x30>
> -   3db34:      48 89 e6                mov    %rsp,%rsi
> +   3db24:      48 89 e6                mov    %rsp,%rsi
> +   3db27:      48 8b 44 24 08          mov    0x8(%rsp),%rax
> +   3db2c:      64 48 2b 04 25 28 00    sub    %fs:0x28,%rax
> +   3db33:      00 00
> +   3db35:      75 09                   jne    3db40
>     <_longjmp_unwind+0x30>
>     3db37:      48 83 c4 18             add    $0x18,%rsp
>     3db3b:      e9 60 31 05 00          jmp    90ca0
> <__GI___pthread_cleanup_upto>
>     3db40:      e8 4b ec 0e 00          call   12c790 <__stack_chk_fail>
>
> Checked on x86_64-linux-gnu.
> ---
>  sysdeps/x86/nptl/pthreaddef.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/sysdeps/x86/nptl/pthreaddef.h b/sysdeps/x86/nptl/pthreaddef.h
> index 63fdbcb27c..3ff95efee6 100644
> --- a/sysdeps/x86/nptl/pthreaddef.h
> +++ b/sysdeps/x86/nptl/pthreaddef.h
> @@ -42,7 +42,7 @@
>  #ifdef __x86_64__
>  /* The frame pointer is not usable.  */
>  # define CURRENT_STACK_FRAME \
> -  ({ register char *frame __asm__("rsp"); frame; })
> +  ({ void *p__; asm volatile ("mov %%" RSP_REG ", %0" : "=r" (p__)); p__; })

Is the mov instruction needed at all?

Would:
```
({ register void * p__ __asm__("rsp"); asm volatile("" : "=r"(p__)); p__; });
```
work?
>  #else
>  # define CURRENT_STACK_FRAME   __builtin_frame_address (0)
>  #endif
> --
> 2.34.1
>