MPICH 3.2 + Clang 8.0 + OSX Sierra + MPI_Alltoallv = MPID_Request_create segfault

[poulson]

My lldb session is showing a segfault within an MPI_Alltoallv call of the form

Process 67961 stopped
* thread #1: tid = 0x55d6af, 0x00000001040b70e3 libpmpi.12.dylib`MPID_Request_create + 275, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
    frame #0: 0x00000001040b70e3 libpmpi.12.dylib`MPID_Request_create + 275
libpmpi.12.dylib`MPID_Request_create:
->  0x1040b70e3 <+275>: movaps %xmm0, 0x230(%rax)
    0x1040b70ea <+282>: movq   $0x0, 0x240(%rax)
    0x1040b70f5 <+293>: movl   $0x2c000000, 0x210(%rax)  ; imm = 0x2C000000 
    0x1040b70ff <+303>: popq   %rbp

This toolchain issue appears to be related to http://stackoverflow.com/questions/38155812/segmentation-fault-while-using-mpi-barrier-in-libpmpi-12-dylib

[jeffhammond]

See http://lists.mpich.org/pipermail/discuss/2016-May/004764.html and http://lists.mpich.org/pipermail/discuss/2016-August/004861.html for root cause analysis.

[poulson]

It seems that OS X alignment issues strike again.

[poulson]

I have confirmed that building Elemental using the git version of MPICH (compiling using clang) resolves the issue.