PR #2596 introduced regression when configuring with -O3 optimization

[hppritcha]

#2596, which has been merged back in to v2.x and v.2.0.x introduced a major regression. Open MPI can no longer be compiled with gcc (at least) at -O2 or -O3 for x86_64 without immediately segfaulting in MPI_Init. The problem has been observed both for gcc 4.9 and 6.3.

orterun also fails with a similar segfault pattern. If one uses the cpuid instruction directly (avoiding all the exb exchange stuff - which isn't necessary for x86_64), the problem vanishes.

See comments to commit a718743.
Although the problem was originally reported on a system with Opteron processors,

@marksantcroos thanks for catching this.

[hjelmn]

Hmm, I tested this only with gcc on CTS-1. Optimized build (defaults to -O2). Will look at the backtrace and see what is going on.

[hjelmn]

Rather embarrassing. The cpuid code I copied from patcher only saved/restored the lower 32 bits of register bx. PR open to fix.