[Bug c++/78636] New: PPC Optimization Bug in Libelemental unit test

[me at ryanlewis dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78636

            Bug ID: 78636
           Summary: PPC Optimization Bug in Libelemental unit test
           Product: gcc
           Version: 6.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: me at ryanlewis dot net
  Target Milestone: ---

Created attachment 40218
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40218&action=edit
example of failing code (which isn't failing)

I am working on packaging libelemental for Fedora. I have found that a few of
our unit tests fail on PPC with in Release mode and pass in Debug mode, only on
this architecture. I suspect this to be a compiler bug, as the code is quite
vanilla. 


Here is the associated build log demonstrating how the software located here:
https://github.com/elemental/Elemental.git

was compiled and ran: 
https://copr-be.cloud.fedoraproject.org/results/rhl/elemental/fedora-24-ppc64le/00482253-elemental/build.log.gz

here are other examples of it failing (it looks like a timeout issue here):

https://kojipkgs.fedoraproject.org//work/tasks/5356/16555356/build.log


Here is an example of a failure in the debugger:

Program received signal SIGSEGV, Segmentation fault.
__memcpy_power7 () at ../sysdeps/powerpc/powerpc64/power7/memcpy.S:387
387             stvx    6,0,dst
Missing separate debuginfos, use: dnf debuginfo-install
mpich-3.2-5.fc24.ppc64le
(gdb) bt
#0  __memcpy_power7 () at ../sysdeps/powerpc/powerpc64/power7/memcpy.S:387
#1  0x00003fffb59da5f4 in std::char_traits<char>::copy
(__n=13839561654909534208, __s2=<optimized out>, __s1=<optimized out>)
    at
/usr/src/debug/gcc-6.2.1-20160916/obj-ppc64le-redhat-linux/ppc64le-redhat-linux/libstdc++-v3/include/bits/char_traits.h:290
#2  std::basic_streambuf<char, std::char_traits<char> >::xsputn
(this=0x3fffffffe568, __s=<optimized out>, __n=11)
    at
/usr/src/debug/gcc-6.2.1-20160916/obj-ppc64le-redhat-linux/ppc64le-redhat-linux/libstdc++-v3/include/bits/streambuf.tcc:90
#3  0x00003fffb59c5cd4 in std::basic_streambuf<char, std::char_traits<char>
>::sputn (__n=11, __s=0x2000e0a8 "  runtime: ", this=<optimized out>)
    at
/usr/src/debug/gcc-6.2.1-20160916/obj-ppc64le-redhat-linux/ppc64le-redhat-linux/libstdc++-v3/include/streambuf:451
#4  std::__ostream_write<char, std::char_traits<char> > (__n=11, __s=0x2000e0a8
"  runtime: ", __out=...)
    at
/usr/src/debug/gcc-6.2.1-20160916/obj-ppc64le-redhat-linux/ppc64le-redhat-linux/libstdc++-v3/include/bits/ostream_insert.h:50
#5  std::__ostream_insert<char, std::char_traits<char> > (__out=...,
__s=0x2000e0a8 "  runtime: ", __n=11)
    at
/usr/src/debug/gcc-6.2.1-20160916/obj-ppc64le-redhat-linux/ppc64le-redhat-linux/libstdc++-v3/include/bits/ostream_insert.h:101
#6  0x000000002000402c in std::operator<< <std::char_traits<char> >
(__s=0x2000e0a8 "  runtime: ", __out=...) at /usr/include/c++/6.2.1/ostream:561
#7  El::BuildStream<char [12], double, char [9]> (item=..., os=...) at
/home/fedora/rpmbuild/BUILD/Elemental-master/include/El/core/environment/impl.hpp:167
#8  El::Output<char [12], double, char [9]> () at
/home/fedora/rpmbuild/BUILD/Elemental-master/include/El/core/environment/impl.hpp:227
#9  main (argc=<optimized out>, argv=<optimized out>) at
/home/fedora/rpmbuild/BUILD/Elemental-master/examples/number_theory/ZDependenceSearch.cpp:86

I isolated this bit of code to produce an elemental free minimal test case,
but, it appears that it does not fail when freed from this example.

I ran the code through valgrind I have attached the output, and I also removed
the function El::Output() (in favor of std::cout) and the failure disappears. I
then reran valgrind again. I will attach the valgrind output for both runs.