[petsc-dev] de-global variablelizing PETSc

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:
>    I assume this depends on what configure finds?

Yeah, we can test for a bunch of them, but the interfaces are not
equivalent.  The typical inline assembly approach has (memory
barrier-free) atomic operations and memory barriers while C11 has
acquire/release semantics.  There can be a (minor) loss of efficiency
when implementing one in terms of the other, depending on the memory
model of the hardware.  OpenMP's memory model has changed over the years
(e.g., http://www.nic.uoregon.edu/iwomp2005/Papers/f29.pdf has a
nonsensical interpretation of the volatile keyword, which was later
removed) and omp flush is a blunt instrument.

If all we need are simple locks, then this doesn't matter, but
an asynchronous threadcomm needs sharper tools.
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