Messages in this thread | | | Date | Sat, 18 Aug 2007 09:17:58 +0200 | From | Eric Dumazet <> | Subject | Re: [PATCH] x86-64: memset optimization |
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Stephen Hemminger a écrit : > Optimize uses of memset with small constant offsets. > This will generate smaller code, and avoid the slow rep/string instructions. > Code copied from i386 with a little cleanup. >
You obviously didnt test it, did you ?
How can you be sure this is going to speedup things then ?
> Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org> > > --- a/include/asm-x86_64/string.h 2007-08-17 15:14:32.000000000 -0700 > +++ b/include/asm-x86_64/string.h 2007-08-17 15:36:30.000000000 -0700 > @@ -42,9 +42,51 @@ extern void *__memcpy(void *to, const vo > __ret = __builtin_memcpy((dst),(src),__len); \ > __ret; }) > #endif > - > #define __HAVE_ARCH_MEMSET > -void *memset(void *s, int c, size_t n); > +void *__memset(void *s, int c, size_t n); > + > +/* Optimize for cases of trivial memset's > + * Compiler should optimize away all but the case used. > + */ > +static __always_inline void * > +__constant_c_and_count_memset(void *s, int c, size_t count) > +{ > + unsigned long pattern = 0x01010101UL * (unsigned char) c;
Main difference between x86_64 and i386 is sizeof(long) being 8 instead of 4
Why not let gcc do its job about memset() ?
On x86_64 at least, modern gcc are smart enough.
> + > + switch (count) { > + case 0: > + return s; > + case 1: > + *(unsigned char *)s = pattern; > + return s; > + case 2: > + *(unsigned short *)s = pattern; > + return s; > + case 3: > + *(unsigned short *)s = pattern; > + *(2+(unsigned char *)s) = pattern; > + return s; > + case 4: > + *(unsigned long *)s = pattern; > + return s; > + case 6: > + *(unsigned long *)s = pattern; > + *(2+(unsigned short *)s) = pattern; > + return s; > + case 8: > + *(unsigned long *)s = pattern; > + *(1+(unsigned long *)s) = pattern; > + return s; > + default: > + return __memset(s, c, count); > + } > +} > +#define memset(s, c, count) \ > + (__builtin_constant_p(c) \ > + ? __constant_c_and_count_memset((s),(c),(count)) \ > + : __memset((s),(c),(count))) > + > + > > #define __HAVE_ARCH_MEMMOVE > void * memmove(void * dest,const void *src,size_t count); > --- a/arch/x86_64/kernel/x8664_ksyms.c 2007-08-17 15:14:32.000000000 -0700 > +++ b/arch/x86_64/kernel/x8664_ksyms.c 2007-08-17 15:44:58.000000000 -0700 > @@ -48,10 +48,12 @@ EXPORT_SYMBOL(__read_lock_failed); > #undef memmove > > extern void * memset(void *,int,__kernel_size_t); > +extern void * __memset(void *,int,__kernel_size_t); > extern void * memcpy(void *,const void *,__kernel_size_t); > extern void * __memcpy(void *,const void *,__kernel_size_t); > > EXPORT_SYMBOL(memset); > +EXPORT_SYMBOL(__memset); > EXPORT_SYMBOL(memcpy); > EXPORT_SYMBOL(__memcpy); >
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