error with gcc 5 for varargs function: suffix or o

Question

When trying to install OpenMPI the .configure filed failed in the section checking for ISO C99 ability of the selected compiler.

Puzzled as gcc of course has both the std=gnu99 and std=c99 option, I pruning the 20,000 lines configure file to isolate the offending section. In the end I traced it back to gcc 5 not compiling variable argument functions.

The below code generates the "suffix or operands invalid for `movq'" error, which is the root cause of the .configure file failing.

(The step-by-step installation for OpenMPI I followed is here https://wiki.helsinki.fi/display/HUGG/Installing+Open+MPI+on+Mac+OS+X)

Anyone has seen this and knows how to fix it?

//-----------------------------------------------------------------------------
// This fails to compile with gcc 5.3
//
// Command: gcc test4.c
//
// Error is: 
//  /var/folders/4s/gkkpz000gn/T//ccAraq59.s:47:suffix or operands invalid for `movq'
//  /var/folders/4s/gkg0r000gn/T//ccAraq59.s:52:suffix or operands invalid for `movq'
//
// gcc version is 5.3.0:
//
// $ gcc --version
// gcc (GCC) 5.3.0
// Copyright (C) 2015 Free Software Foundation, Inc.
// This is free software; see the source for copying conditions.  There is NO
// warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
//
//-----------------------------------------------------------------------------


// Check varargs
static void
test_varargs (const char *format, ...)
{
}


//--------------------------------------------------------------
//                         MAIN {}   
//--------------------------------------------------------------

 int
 main ()
 {

    //====================================================
    // Check varargs.
    // test_varargs ("s", "string");            //WORKS
    // test_varargs ("d' ", 65);                //WORKS
    // test_varargs ("s, d' ", "string", 65);   //WORKS

    test_varargs ("f .", 34.234);                       // FAILS!!!
    test_varargs ("s, d' f .", "string", 65, 34.234);   //ALSO FAILS
    //====================================================


   return 0;
 }

Answer 1

TLDR: conflict between brew and XCode gcc installs.

I ended up uninstalling all gcc versions installed, deleting g++, deleting all dangling links left over. Next I upgraded from XCode 6 to 7 and used the gcc version coming with it. This fixed the above problem. Deleting all dangling symlinks and the unziped tar-ball of OpenMPI and then re-unzipping allowed the OpenMPI to be installed without a hitch. cd ./examples, make all, mpirun -np 4 hello_c worked and we have a happy MPI installation.