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From: "meissner at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/58160] New: Power8 fusion support has a bug that shows up in running spec 2006
Date: Wed, 14 Aug 2013 20:24:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58160

            Bug ID: 58160
           Summary: Power8 fusion support has a bug that shows up in
                    running spec 2006
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: meissner at gcc dot gnu.org

In running the spec 2006 testsuite with several options, we discovered that
-mtune=power8 (which turns on the -mpower8-fusion option) has a segmentation
fault in two benchmarks:

403.gcc when built with -O2 -mcpu=power7 -mtune=power8 -m32
435.gromacs when built with -O3 -funroll-loops -mcpu=power7 -mtune=power8 -m32

In the gcc case, I tracked it down to the fusion op in the function
strength_reduce in the file loop.c.  It was wanting to use the result of the
addis instruction after the loop.

It wanted to optimize:
        lis 10,loop_dump_stream@ha
        lwz 6,loop_dump_stream@l(6)
        stw 10,24(1)

to:
        lis 6,loop_dump_stream@ha
        lwz 6,loop_dump_stream@l(6)
        stw 10,24(1)

The problem is fusion was implemented using define_peephole, and the register
live notes are not correct when peepholes are done.