Re: [patch] Improve prefetch heuristics

[Christian Borntraeger <borntraeger at de dot ibm dot com>]

> Patch1: 0001-Do-not-insert-prefetches-if-they-would-hit-the-same-.patch
> This patch modify the prefetch generation logic. We don't issue a prefetch
> if it would fall on the same cache line with an existing memory reference or
> prefetch.  This patch improves the following benchmarks: 416.gamess (~7%),
> 434.zeusmp (~4%), 454.calculix (~2%) and 445.gobmk (~2%).

>+      /* Don't issue a prefetch if its address falls on the same cache line
>+         with a previous memory reference (prefetch/load/store).  */
>+      if (abs (delta - start_offset) < L1_CACHE_LINE_SIZE)
>+        /* Drop the prefetch.  */

This might need a debug log entry about a dropped prefetch.

>+        continue;
>+      else

I looked a bit further into these patches.
This patch causes a regression with lbm on s390.

This is causes by the fact that ahead is 1 step is 160 and with
cache-line-size=256 this patch drops all prefetches on s390 in 
lbms hotloop.

Thinking more about the whole logic, wouldnt it make sense to check for 
abs (delta - start_offset) < L1_CACHE_LINE_SIZE / 2
                                                ^^^
because we cannot assume that we were starting at the beginning of a cache line?

Or maybe have something like
abs (delta - start_offset) < L1_CACHE_LINE_SIZE / CACHE_LINE_AGAIN_FACTOR

which can be defined by the architecture.

Christian