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SubjectRe: generic strncpy - off-by-one error


Peter Kjellerstedt wrote:

>
> For loops 2.867568 5.620561 8.128734 28.286289
> Multi byte fill 2.868031 5.670782 6.312027 11.336015
>
> And here are the numbers for my P4:
>
> For loops 3.060262 5.927378 8.796814 30.659818
> Multi byte fill 3.126607 5.898459 7.096685 13.135379
>
> So there is no doubt that the multi byte version is a clear
> winner (which was expected, I suppose).

Cool! Hey, is this just an exercise, or are we actually going to use
this? I would be very happy to have something I contributed to put into
the kernel. :)

>
> Here is the code that I used:
>
> char *strncpy(char *dest, const char *src, size_t count)
> {
> char *tmp = dest;
>
> while (count && *src) {
> *tmp++ = *src++;
> count--;
> }
>
> if (count) {

Good idea... bad to do so many checks if count is zero. On the other
hand, if count is rarely zero, then it's a loss. Maybe benchmark with
and without?

> size_t count2;
>
> while (count & (sizeof(long) - 1)) {
> *tmp++ = '\0';
> count--;
> }
>
> count2 = count / sizeof(long);

I know that a good compiler should migrate code to help the CPU
pipeline, but how about moving this "count2 = " line up to before the
first fill loop. See if that helps any. Always good to precompute well
in advance.

> while (count2) {
> *((long *)tmp)++ = '\0';
> count2--;
> }
>
> count &= (sizeof(long) - 1);

And move this to before the middle fill loop.

> while (count) {
> *tmp++ = '\0';
> count--;
> }
> }
>
> return dest;
> }
>
> //Peter
>
>


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