Re: [RFC] Don't schedule prefetch INSNs

[Andreas Krebbel <krebbel at linux dot vnet dot ibm dot com>]

> Isn't that a big hammer as it will also prevent unrelated accesses from
> being scheduled?

I'm trying to optimize memcpy loops with prefetches. Therefore I 
prefetch the cache line for the next iteration. So the prefetching 
instructions issued are not related to the memory access in the current 
iteration.

loop over cache lines:
   read prefetch x + cache_line_size
   write prefetch y + cache_line_size
   copy cache line x to y

Here I want the prefetching instructions to stay before the copy.

Another point is that the change also seems to prevent DCE to throw IFs 
around prefetches away. As in:

loop over cache lines:
   if (cache_lines_to_be_copied > 0)
     {
       read prefetch x + cache_line_size
       write prefetch y + cache_line_size
     }
   copy cache line x to y

Bye,

-Andreas-