Re: [PATCH, rs6000] don't use unaligned vsx for memset of less than 32 bytes

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Mon, Jun 25, 2018 at 10:41:32AM -0500, Aaron Sawdey wrote:
> In gcc 8 I added support for unaligned vsx in the builtin expansion of
> memset(x,0,y). Turns out that for memset of less than 32 bytes, this
> doesn't really help much, and it also runs into an egregious load-hit-
> store case in CPU2006 components gcc and hmmer.
> 
> This patch reverts to the previous (gcc 7) behavior for memset of 16-31 
> bytes, which is to use vsx stores only if the target is 16 byte
> aligned. For 32 bytes or more, unaligned vsx stores will still be used.
>   Performance testing of the memset expansion shows that not much is
> given up by using scalar stores for 16-31 bytes, and CPU2006 runs show
> the performance regression is fixed.
> 
> Regstrap passes on powerpc64le, ok for trunk and backport to 8?

Yes, okay for both.  Thanks!


Segher


> 2018-06-25  Aaron Sawdey  <acsawdey@linux.ibm.com>
> 
> 	* config/rs6000/rs6000-string.c (expand_block_clear): Don't use
> 	unaligned vsx for 16B memset.