Re: [PATCH] x86-64: Use testl to check __x86_string_control

[Carlos O'Donell <carlos@redhat.com>]

On 8/28/21 9:15 AM, H.J. Lu via Libc-alpha wrote:
> Use testl, instead of andl, to check __x86_string_control to avoid
> updating __x86_string_control.

The __x86_string_control is a global variable that is hidden from external
linkage and used internally by various routines. Today the value is RW,
but in the future it could become RO (and probably should after a call
to init_cacheinfo()). We don't want to do an idempotent update (we have
only one bit constant for now), but instead just want to check for the bit.
This code will break when we get another bit, or when it becomes RO.

LGTM.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> ---
>  sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
> index 9f02624375..abde8438d4 100644
> --- a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
> +++ b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
> @@ -325,7 +325,7 @@ L(movsb):
>  	/* Avoid slow backward REP MOVSB.  */
>  	jb	L(more_8x_vec_backward)
>  # if AVOID_SHORT_DISTANCE_REP_MOVSB
> -	andl	$X86_STRING_CONTROL_AVOID_SHORT_DISTANCE_REP_MOVSB, __x86_string_control(%rip)
> +	testl	$X86_STRING_CONTROL_AVOID_SHORT_DISTANCE_REP_MOVSB, __x86_string_control(%rip)
>  	jz	3f
>  	movq	%rdi, %rcx
>  	subq	%rsi, %rcx
> @@ -333,7 +333,7 @@ L(movsb):
>  # endif
>  1:
>  # if AVOID_SHORT_DISTANCE_REP_MOVSB
> -	andl	$X86_STRING_CONTROL_AVOID_SHORT_DISTANCE_REP_MOVSB, __x86_string_control(%rip)
> +	testl	$X86_STRING_CONTROL_AVOID_SHORT_DISTANCE_REP_MOVSB, __x86_string_control(%rip)
>  	jz	3f
>  	movq	%rsi, %rcx
>  	subq	%rdi, %rcx
> 


-- 
Cheers,
Carlos.