On 07/26/2009 04:45 PM, Aurelien Jarno wrote:
> Knowing that $31 could be used for prefetch, I have modified the
> assembly code from memchr.S to use it. It passes all the testsuite.
>

This isn't intended to be a prefetch instruction, it's
meant to be fetching the data for the next word.  I.e.
we've unrolled the loop and there's at least 8 bytes
left in the search.

Note the

         # At least two quads remain to be accessed.

comment.  At that point we're supposed to be more
than 16 bytes away from the end of the input buffer.

Actually, the confusion I see is farther upthread:

> > >>>>> The problem is that the memchr() function on alpha uses 
> prefetch, which
> > >>>>> can cause a page boundary to be crossed, while the standards 
> (POSIX and
> > >>>>> C99) says it should stop when a match is found.

I didn't realize this when I wrote the function.

The entire function should be rewritten, since there's little
point in using a prefetch instruction that close to the load.
Prefetch instructions should be used to move data into the L1
cache, not hide the 3 cycle load delay.  Thus a prefetch, if
used, should be several cache lines ahead, not just a single word.

Perhaps a better solution would be to read words until we get
cacheline aligned, then read the entire line into 8 registers,
prefetch the next line, then process each register one by one.

Try this.


r~