[PATCH] arm: Enable USE_ATOMIC_COMPILER_BUILTINS (BZ #24774)

[Aurelien Jarno]

On 2021-09-29 16:14, Adhemerval Zanella via Libc-alpha wrote:
> As per other architectures.  I have checked on a armv8 hardware with
> the following configurations:
> 
>   arm-linux-gnueabihf (gcc built with --with-float=hard --with-cpu=arm926ej-s)
>   armv5-linux-gnueabihf (-march=armv5te -mfpu=vfpv3)
>   armv7-linux-gnueabihf (-march=armv7-a -mfpu=vfpv3)
>   armv7-thumb-linux-gnueabihf (-march=armv7-a -mfpu=vfpv3 -mthumb)
>   armv7-neon-linux-gnueabihf (-march=armv7-a -mfpu=neon)
>   armv7-neonhard-linux-gnueabihf (-march=armv7-a -mfpu=neon -mfloat-abi=hard)
> 
> Without any regression.
> 
> I haven't dig into the code, but since Linux atomic-machine.h handle
> pre-ARMv6 and ARMv6 I expect the compiler might have some small room
> to optimize.
> 
> The code size also improves is most of the configurations:
> 
> * master
> 
>    text    data     bss     dec     hex filename
> 1727801    9720   37928 1775449  1b1759	 arm-linux-gnueabihf/libc.so
> 1691729    9720   37928 1739377  1a8a71	 arm-linux-gnueabihf-armv7-disable-multi-arch/libc.so
> 1725509    9720   37928 1773157  1b0e65	 armv5-linux-gnueabihf/libc.so
> 1700757    9720   37928 1748405  1aadb5	 armv6-linux-gnueabihf/libc.so
> 1698973    9720   37928 1746621  1aa6bd	 armv6t2-linux-gnueabihf/libc.so
> 1695481    9752   37928 1743161  1a9939	 armv7-linux-gnueabihf/libc.so
> 1692917    9744   37928 1740589  1a8f2d	 armv7-neonhard-linux-gnueabihf/libc.so
> 1692917    9744   37928 1740589  1a8f2d	 armv7-neon-linux-gnueabihf/libc.so
> 1225353    9752   37928 1273033  136cc9	 armv7-thumb-linux-gnueabihf/libc.so
> 
> * patched
> 
>    text    data     bss     dec     hex filename
> 1726805    9720   37928 1774453  1b1375 arm-linux-gnueabihf/libc.so
> 1689321    9720   37928 1736969  1a8109	arm-linux-gnueabihf-armv7-disable-multi-arch/libc.so
> 1724433    9720   37928 1772081  1b0a31 armv5-linux-gnueabihf/libc.so
> 1698301    9720   37928 1745949  1aa41d armv6-linux-gnueabihf/libc.so
> 1696525    9720   37928 1744173  1a9d2d armv6t2-linux-gnueabihf/libc.so
> 1693009    9752   37928 1740689  1a8f91 armv7-linux-gnueabihf/libc.so
> 1690493    9744   37928 1738165  1a85b5 armv7-neonhard-linux-gnueabihf/libc.so
> 1690493    9744   37928 1738165  1a85b5 armv7-neon-linux-gnueabihf/libc.so
> 1223837    9752   37928 1271517  1366dd armv7-thumb-linux-gnueabihf/libc.so
> 
> The idea is eventually move all architectures to use compiler builtins.
> ---
>  sysdeps/arm/atomic-machine.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sysdeps/arm/atomic-machine.h b/sysdeps/arm/atomic-machine.h
> index 7928ff29d8..0275fcd2f7 100644
> --- a/sysdeps/arm/atomic-machine.h
> +++ b/sysdeps/arm/atomic-machine.h
> @@ -34,7 +34,7 @@ typedef intmax_t atomic_max_t;
>  typedef uintmax_t uatomic_max_t;
>  
>  #define __HAVE_64B_ATOMICS 0
> -#define USE_ATOMIC_COMPILER_BUILTINS 0
> +#define USE_ATOMIC_COMPILER_BUILTINS 1
>  #define ATOMIC_EXCHANGE_USES_CAS 1
>  
>  void __arm_link_error (void);
> -- 

I have just found your patch after I stumbled on BZ #24774. I confirm it
fixes this bug and also the flaky nptl/tst-rwlock9 test on arm. I did
not find any regression on the testsuite on Debian armel
(armv5-linux-gnueabi) and Debian armhf (armv7-linux-gnueabihf), using
GCC 11.

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Tested-by: Aurelien Jarno <aurelien@aurel32.net>

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net