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.
