[U-Boot] [PATCH V2 1/3] memcpy: copy one word at a time if possible

[Chris Moore]

Wolfgang Denk a écrit :
> I think we should change this if-else into a plain if, something like
> that:
>
> void * memcpy(void *dest, const void *src, size_t count)
> {
> 	char *tmp = (char *) dest, *s = (char *) src;
> 	char *d8 = (char *)dest, *s8 = (char *)src;
> 	unsigned long *dl = (unsigned long *)dest, *sl = (unsigned long *)src;
>
> 	/* while all data is aligned (common case), copy a word at a time */
> 	if ( (((int)dest | (int)src | count) & (sizeof(long) - 1)) == 0) {
> 		while (count) {
> 			*dl++ = *sl++;
> 			count -= sizeof(unsigned long);
> 		}
> 	}
> 	while (count--)
> 		*d8++ = *s8++;
>
> 	return dest;
> }
>
> This way we can have both - the "long" copy of a potential aligne
> dfirst part, and the byte copy of any trailing (or unaligned) part.
>
>   

I agree wholeheartedly with the idea but shouldn't it be more like this 
(untested) code :

void * memcpy(void *dest, const void *src, size_t count)

{
	char *d8, *s8;
	unsigned long *dl = dest, *sl = src;

	/* while all data is aligned (common case), copy multiple bytes at a time */
	if ( (((int)(long)dest | (int)(long)src) & (sizeof(*dl) - 1)) == 0) {
		while (count >= sizeof(*dl)) {
			*dl++ = *sl++;
			count -= sizeof(*dl);
		}
	}

	d8 = (char *)dl;
	s8 = (char *)sl;

	/* copy any remaining data byte by byte */
	while (count--)
		*d8++ = *s8++;

	return dest;
}


Remarks :
1) My curious (int) (long) pointer casts are intended to avoid compiler 
warnings while avoiding unnecessary calculations in long.
On some architectures long calculations are less efficient than int ones.
In fact I wonder whether, on such architectures, it might not also be 
better to perform the copy with int size chunks.
2) Personally I prefer sizeof(*dl) to sizeof(unsigned long) as there is 
less risk of error if the type of the chunks is changed.
3) In C (but not in C++) I think the casts from void * to unsigned long 
* are unnecessary.

But as I said all this is completely untested :(

Cheers,
Chris