linux-ext4 - Re: [PATCH] x86_64/lib: improve the performance of memmove

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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;
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