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Date: 16 Sep 2010 08:13:09 -0400 From: "George Spelvin" <linux@...izon.com> To: andi@...stfloor.org, miaox@...fujitsu.com Cc: linux@...izon.com, linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] x86_64/lib: improve the performance of memmove > void *memmove(void *dest, const void *src, size_t count) > { > if (dest < src) { > return memcpy(dest, src, count); > } else { > - char *p = dest + count; > - const char *s = src + count; > - while (count--) > - *--p = *--s; > + return memcpy_backwards(dest, src, count); > } > return dest; > } Er... presumably, the forward-copy case is somewhat better optimized, so should be preferred if the areas don't overlap; that is, dest > src by more than the sount. Assuming that size_t can hold a pointer: if ((size_t)src - (size_t)dest >= count) return memcpy(dest, src, count); else return memcpy_backwards(dest, src, count); Or, using GCC's arithmetic on void * extension, if ((size_t)(src - dest) >= count) ... etc. If src == dest, it doesn't matter which you use. You could skip the copy entirely, but presumably that case doesn't arise often enough to be worth testing for: if ((size_t)(src - dest) >= count) return memcpy(dest, src, count); else if (src - dest != 0) return memcpy_backwards(dest, src, count); else return dest; -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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