Running GMP on Ryzen

[Torbjörn Granlund]

The GMP project got a Ryzen system last spring, and we have since
implemented special optimisations for Ryzen, resulting in stellar
Ryzen performance.

However, it turns out that current Ryzen hardware does not build or run
GMP without issues.  Our initial assumption was that our non-ECC RAM or
our motherboard were to blame, but now it is clear the problem is
related to the CPU itself.

The problem is not unique to GMP, but happpens when loading the systems
with e.g. parallel compiles.

The symptom is that one gets segfaults which are very hard too
reproduce.  Most segfaults happen during compilation, but running GMP
applications will also give some spurious segfaults.

AMD has confirmed there is an issue, but we're still waiting for a clear
statement from them.  Their marketing department has chosen to call it a
"performance marginality problem", which is untrue.

-- 
Torbjörn
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