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
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