[PATCH][rs6000] better use of unaligned vsx in memset() expansion

[Aaron Sawdey]

The first version of this had a big bug and cleared past the requested bytes.
This version passes regstrap on ppc64le(power7/8/9), ppc64be(power6/7/8),
and ppc32(power8).

OK for trunk (and 8 backport after a week)?

Thanks!
   Aaron

Index: gcc/config/rs6000/rs6000-string.c
===================================================================
--- gcc/config/rs6000/rs6000-string.c	(revision 266524)
+++ gcc/config/rs6000/rs6000-string.c	(working copy)
@@ -85,6 +85,8 @@
   if (! optimize_size && bytes > 8 * clear_step)
     return 0;

+  bool unaligned_vsx_ok = (bytes >= 32 && TARGET_EFFICIENT_UNALIGNED_VSX);
+
   for (offset = 0; bytes > 0; offset += clear_bytes, bytes -= clear_bytes)
     {
       machine_mode mode = BLKmode;
@@ -91,8 +93,7 @@
       rtx dest;

       if (TARGET_ALTIVEC
-	  && ((bytes >= 16 && align >= 128)
-	      || (bytes >= 32 && TARGET_EFFICIENT_UNALIGNED_VSX)))
+	  && (bytes >= 16 && ( align >= 128 || unaligned_vsx_ok)))
 	{
 	  clear_bytes = 16;
 	  mode = V4SImode;


On 11/26/18 4:29 PM, Segher Boessenkool wrote:
> On Mon, Nov 26, 2018 at 03:08:32PM -0600, Aaron Sawdey wrote:
>> When I previously added the use of unaligned vsx stores to inline expansion
>> of memset, I didn't do a good job of managing boundary conditions. The intention
>> was to only use unaligned vsx if the block being cleared was more than 32 bytes.
>> What it actually did was to prevent the use of unaligned vsx for the last 32
>> bytes of any block being cleared. So this change puts the test up front so it
>> is not affected by the decrement of bytes.
> 
> Oh wow.  Yes, that isn't so great.  Okay for trunk (and whatever backports).
> Thanks,
> 
> 
> Segher
> 
> 
>> 2018-11-26  Aaron Sawdey  <acsawdey@linux.ibm.com>
>>
>> 	* config/rs6000/rs6000-string.c (expand_block_clear): Change how
>> 	we determine if unaligned vsx is ok.
> 

-- 
Aaron Sawdey, Ph.D.  acsawdey@linux.vnet.ibm.com
050-2/C113  (507) 253-7520 home: 507/263-0782
IBM Linux Technology Center - PPC Toolchain