[PATCH 34/59] x86: Fix THREAD_GSCOPE_RESET_FLAG build on clang

[Uros Bizjak]

On 10/17/25 21:13, Adhemerval Zanella wrote:
> clang does not support __seg_fs in asm constraint.
> ---
>   sysdeps/x86_64/nptl/tls.h | 21 +++++++++++++++++----
>   1 file changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/sysdeps/x86_64/nptl/tls.h b/sysdeps/x86_64/nptl/tls.h
> index abfeb88054..2c4b9aaa80 100644
> --- a/sysdeps/x86_64/nptl/tls.h
> +++ b/sysdeps/x86_64/nptl/tls.h
> @@ -198,13 +198,26 @@ _Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x80,
>   # define THREAD_GSCOPE_FLAG_UNUSED 0
>   # define THREAD_GSCOPE_FLAG_USED   1
>   # define THREAD_GSCOPE_FLAG_WAIT   2
> +
> +/* clang does not support __seg_fs in asm constraint.  */
> +# ifdef __clang__
> +#  define XCHGL_GSCOPE(__r)						      \
> +   asm volatile ("xchgl %0, %%fs:%P1"					      \
> +                 : "=r" (__r)						      \
> +                 : "i" (offsetof (struct pthread, header.gscope_flag)),	      \
> +		   "0" (THREAD_GSCOPE_FLAG_UNUSED))

This needs a "memory" clobber, because this asm hides memory access
from the compiler. Please see the "memory" special clobber argument
section in [1].

Also, please use the "c" operand modifier instead of "P". Clang allows
"c" with "i" constraint and also checks that operand fits "c".

Clang accepts __seg_fs prefixed addresses, it just "forgets" to emit 
%fs: prefix in the generated code. While this could be considered a bug 
with clang, the simplest solution is to just conditionally put "%%fs:" 
prefix in the asm template for clang.

[1] 
https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Clobbers-and-Scratch-Registers

Uros.