3 Replies Latest reply on Apr 1, 2013 2:11 AM by saravanan_ekanathan

    AVX instruction in runtime library

    dmantione

      Hello,

       

      I am trying to compile OpenMPI with Open64 4.5.2.1. I want to support pre-Buldozer processors. The configure fails with an "Illegal instruction" error.

       

      After analysis, the configure script tries to compile this program:

       

      program f90findintkind

          open(8, file="conftestval")

          write(8, fmt="(I5)") selected_int_kind(19)

          close(8)

      end program

       

      And indeed:

       

      [root@benchmark openmpi-1.6.4]# openf90 -march=anyx86 -o conftestf conftestf.f90

      [root@benchmark openmpi-1.6.4]# ./conftestf

      Illegal instruction (core dumped)

      [root@benchmark openmpi-1.6.4]#

       

      Analysis shows the error to occur in _s2ui, which is part of libffio:

       

      (gdb) info stack

      #0  _s2ui () at /local/home/qa/nightly_build_avx/sandbox/open64/osprey/libu/numconv/mpp/s2uboiz.c:440

      #1  0x00002aaaaab0bf03 in _wrfmt () at /local/home/qa/nightly_build_avx/sandbox/open64/osprey/libf/fio/wrfmt.c:606

      #2  0x00002aaaaaad7017 in _xfer_iolist () at /local/home/qa/nightly_build_avx/sandbox/open64/osprey/libf/fio/dopexfer.c:264

      #3  0x00002aaaaab0a243 in _FWF () at /local/home/qa/nightly_build_avx/sandbox/open64/osprey/libf/fio/wf90.c:442

      #4  0x000000000040099e in MAIN__ () at /usr/src/openmpi-1.6.4/conftestf.f90:3

       

      (gdb) disass _s2ui

      Dump of assembler code for function _s2ui:

      => 0x00002aaaab349e80 <+0>:     vzeroupper

         0x00002aaaab349e83 <+3>:     add    $0xfffffffffffffed8,%rsp

         0x00002aaaab349e8a <+10>:    mov    (%rdx),%r10

         0x00002aaaab349e8d <+13>:    test   $0x10,%r10

       

      vzeroupper is an AVX instruction!

       

      Why does the Open64 runtime library use AVX instructions in default code paths?