[PATCH v12 18/31] arm: Add string-fza.h

[Szabolcs Nagy]

The 02/20/2023 11:01, Adhemerval Zanella Netto wrote:
> 
> 
> On 20/02/23 10:45, Szabolcs Nagy wrote:
> > The 02/20/2023 13:24, Szabolcs Nagy via Libc-alpha wrote:
> >> The 02/02/2023 15:11, Adhemerval Zanella via Libc-alpha wrote:
> >>> +static __always_inline find_t
> >>> +find_zero_all (op_t x)
> >>> +{
> >>> +  /* Use unsigned saturated subtraction from 1 in each byte.
> >>> +     That leaves 1 for every byte that was zero.  */
> >>> +  op_t ones = repeat_bytes (0x01);
> >>> +  return __builtin_arm_uqsub8 (ones, x);
> >>> +}
> >>
> >> __builtin_arm_uqsub8 is only available >=gcc-10
> >>
> >> so now the build fails with gcc-9
> >>
> >> ../sysdeps/arm/armv6t2/string-fza.h:36:10: error: implicit declaration of function ‘__builtin_arm_uqsub8’; did you mean ‘__builtin_arm_stc’? [-Werror=implicit-function-declaration]
> >>    36 |   return __builtin_arm_uqsub8 (ones, x);
> >>
> >> so this code should be conditional on gcc version.
> >>
> > 
> > i think
> > 
> >  asm ("uqsub8  %0, %0, %1" : "+r" (ones) : "r" (x));
> > 
> > should be a good fallback (untested).
> 
> This is what we have on v7:
> 
> static __always_inline op_t
> find_zero_all (op_t x)
> {
>   /* Use unsigned saturated subtraction from 1 in each byte.
>      That leaves 1 for every byte that was zero.  */
>   op_t ret, ones = repeat_bytes (0x01);
>   asm ("uqsub8 %0,%1,%2" : "=r"(ret) : "r"(ones), "r"(x));
>   return ret;
> }
> 
> Maybe extend with:
> 
> static __always_inline op_t
> find_zero_all (op_t x)
> {
>   op_t ones = repeat_bytes (0x01);
> #if __GNUC_PREREQ (10, 0)
>   return __builtin_arm_uqsub8 (ones, x);
> #else
>   op_t ret;
>   asm ("uqsub8 %0,%1,%2" : "=r"(ret) : "r"(ones), "r"(x));
>   return ret;
> #endif
> }

yes this looks good to me.