[Bug target/57309] New: Spill code degrades vectorized loop for 437.leslie3d on PPC64

[Bug target/57309] New: Spill code degrades vectorized loop for 437.leslie3d on PPC64

[wschmidt at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57309

            Bug ID: 57309
           Summary: Spill code degrades vectorized loop for 437.leslie3d
                    on PPC64
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wschmidt at gcc dot gnu.org
                CC: bergner at vnet dot ibm.com
              Host: powerpc*-*-*
            Target: powerpc*-*-*
             Build: powerpc*-*-*

Note: This bug does NOT occur on current trunk.

To reproduce, it's necessary to patch config/rs6000/rs6000.h so that
MALLOC_ABI_ALIGNMENT is defined as:

#define MALLOC_ABI_ALIGNMENT (TARGET_64BIT ? 128 : 64)

This allows more vectorization opportunities for loops that access malloc'd
arrays that can be vectorized with 128-bit vectors.

I observed that making this change introduces a degradation of SPEC CPU2006
437.leslie3d, built for 64-bit PowerPC Linux.  There are a number of degraded
loops in the code, which seem to all be pretty similar.  In all cases the loops
are vectorized with and without the patch, but with the patch there is no need
for prolog code to align the data.  Unfortunately, with the patch, the loops
also contain a great deal of spill code (ld, addi, lxvd2x, stxvd2x) which
reloads not only vector registers, but also GPRs used for address computation
of vector loads and stores.  Without the spill code, the main loop body would
be vectorized identically with and without the patch.

One of the worst degraded loops is in function fluxk.  I have oprofile data
available to identify the loop as well as some dumps showing how the loop is
transformed in various phases, available by request.

[Bug target/57309] Spill code degrades vectorized loop for 437.leslie3d on PPC64

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57309

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Can you isolate a testcase for the worst loop?

[Bug target/57309] Spill code degrades vectorized loop for 437.leslie3d on PPC64

[wschmidt at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57309

--- Comment #2 from Bill Schmidt <wschmidt at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> Can you isolate a testcase for the worst loop?

Not yet.  It's one of these horrible gargantuan functions (leslie3d is one big
file and fluxi, fluxj, fluxk are all quite large functions) and the issue
appears to involve global register allocation, so it will be sensitive to
control flow changes.  Not an easy thing to attack with delta, either.

Peter is planning to look at this at some point, and I have a bunch of dump
files saved away to help with the analysis.  Not sure what else to do for now.