Re: [PATCH] ARM: make memzero optimization smarter

[Nicolas Pitre]

On Tue, 16 Jan 2018, Arnd Bergmann wrote:

> While testing with a gcc-8.0.0 snapshot, I ran into a harmless
> build warning:
> 
> In file included from include/linux/string.h:18:0,
> 		 ...
>                  from drivers/net/ethernet/hisilicon/hns/hns_ethtool.c:10:
> drivers/net/ethernet/hisilicon/hns/hns_ethtool.c: In function '__lb_other_process':
> arch/arm/include/asm/string.h:50:5: error: 'memset' specified size 4294967295 exceeds maximum object size 2147483647 [-Werror=stringop-overflow=]
>      memset((__p),(v),(__n));  \
>      ^~~~~~~~~~~~~~~~~~~~~~~
> drivers/net/ethernet/hisilicon/hns/hns_ethtool.c:394:3: note: in expansion of macro 'memset'
>    memset(&skb->data[frame_size / 2], 0xAA, frame_size / 2 - 1);
>    ^~~~~~
> 
> I think the warning is unintentional here, and gcc should not actually
> warn, so I reported this in the gcc bugzilla as pr82103. From the
> discussion there, it seems unlikely that this gets addressed in
> the compiler.
> 
> The problem here is that testing the 'frame_size' variable for non-zero
> in the first memset() macro invocation leads to a code path in which
> gcc thinks it may be zero, and that code path would lead to an overly
> large length for the following memset that is now "(u32)-1". We know
> this won't happen as the skb len is already guaranteed to be nonzero
> when we get here (it has just been allocated with a nonzero size).
> 
> However, we can avoid this class of bogus warnings for the memset() macro
> by only doing the micro-optimization for zero-length arguments when the
> length is a compile-time constant. This should also reduce code size by
> a few bytes, and avoid an extra branch for the cases that a variable-length
> argument is always nonzero, which is probably the common case anyway.
> 
> I have made sure that the __memzero implementation can safely handle
> a zero length argument.

Why not simply drop the test on (__n) != 0 then? I fail to see what the 
advantage is in that case.


> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82103
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> Originally sent in September 2017, but then forgot about it
> as nobody replied.
> 
> I slightly updated the change text now to reflect that the gcc
> developers treat it as an invalid bug.
> ---
>  arch/arm/include/asm/string.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/include/asm/string.h b/arch/arm/include/asm/string.h
> index f54a3136aac6..a8b90e33cc87 100644
> --- a/arch/arm/include/asm/string.h
> +++ b/arch/arm/include/asm/string.h
> @@ -44,7 +44,7 @@ extern void __memzero(void *ptr, __kernel_size_t n);
>  #define memset(p,v,n)							\
>  	({								\
>  	 	void *__p = (p); size_t __n = n;			\
> -		if ((__n) != 0) {					\
> +		if (!__builtin_constant_p(__n) || (__n) != 0) {		\
>  			if (__builtin_constant_p((v)) && (v) == 0)	\
>  				__memzero((__p),(__n));			\
>  			else						\
> -- 
> 2.9.0
> 
>