Re: [PATCH, rs6000, testsuite] Fix PR65484

[Bill Schmidt <wschmidt at linux dot vnet dot ibm dot com>]

> On Jan 26, 2017, at 4:29 PM, Segher Boessenkool <segher@kernel.crashing.org> wrote:
> 
> On Thu, Jan 26, 2017 at 03:14:47PM -0600, Bill Schmidt wrote:
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65484 identifies an
>> issue whether g++.dg/vect/pr36648.cc fails on older POWER hardware.
>> This is due to a decision made in November 2010 to include the
>> flag -mno-allow-movmisalign in check_vect_support_and_set_flags,
>> which governs the vectorizer tests in that directory.  This flag
>> sometimes inhibits vectorization when to vectorize the code
>> requires that misaligned loads and stores be used.  This flag is
>> not added to the command line for POWER8 hardware and later.
>> 
>> pr36648.cc is an example of the kind of vectorization that
>> requires misaligned memory accesses, so it is vectorized on 
>> POWER8 and later hardware, but not on POWER7 or earlier with
>> the default testsuite flags.  This patch modifies the dg-final
>> checks in pr36648.cc to be consistent with this behavior.  I've
>> added commentary to explain what might otherwise seem to be a
>> somewhat arcane choice of tests.
>> 
>> Tested on trunk and GCC 6 for POWER8 LE and POWER7 BE systems.
>> Is this ok for trunk?
> 
>> 2017-01-26  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>
>> 
>> 	PR target/65484
>> 	* g++.dg/vect/pr36648.cc: Modify to reflect that the loop is not
>> 	vectorized on POWER unless hardware misaligned loads are
>> 	available.
> 
>> --- gcc/testsuite/g++.dg/vect/pr36648.cc	(revision 244811)
>> +++ gcc/testsuite/g++.dg/vect/pr36648.cc	(working copy)
>> @@ -19,7 +19,12 @@ Foo foo;
>> 
>> int main() { }
>> 
>> -/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect" { target { !  vect_no_align } } } } */
>> -/* { dg-final { scan-tree-dump-times "vectorizing stmts using SLP" 1 "vect" { target { ! vect_no_align } } } } */
>> +/* On older powerpc hardware (POWER7 and earlier), the default flag
>> +   -mno-allow-movmisalign prevents vectorization.  On POWER8 and later,
>> +   when vect_hw_misalign is true, vectorization occurs.  For other
>> +   targets, ! vect_no_align is a sufficient test.  */
>> 
>> +/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect" { target { { { !  vect_no_align } && { ! powerpc*-*-* } } || { powerpc*-*-* && vect_hw_misalign } } } } } */
>> +/* { dg-final { scan-tree-dump-times "vectorizing stmts using SLP" 1 "vect" { target { { { ! vect_no_align } && { ! powerpc*-*-* } } || { powerpc*-*-* && vect_hw_misalign } } } } } */
>> 
> 
> What does this do if no_align and powerpc and vect_hw_misalign?  Or can that
> not happen?
> 
That's the usual case.  Both vect_no_align and vect_hw_misalign are 1 for
POWER8 or later, and 0 otherwise.

If you're asking what happens with !vect_no_align && powerpc*-*-* && 
vect_hw_misalign, the answer is that it can't happen.

I used vect_hw_misalign here because it better documents what's going on.

The first clause maintains the existing condition for non-powerpc targets,
while the second expects vectorization for POWER8 and later, but not
for earlier hardware.

Bill

> 
> Segher
>