Re: [Boost-users] [Math] BOOST_FPU_EXCEPTION_GUARD on Linux

Subject: Re: [Boost-users] [Math] BOOST_FPU_EXCEPTION_GUARD on Linux
From: John Maddock (john_at_[hidden])
Date: 2013-11-15 12:47:57

> That Bugzilla link was extremely helpful - thanks!
>
> I've done a bit more research and I'm pretty sure this WAS a bug in glibc. 
> It's fixed on recent versions of Linux though.
>
> Taking your test program from 
> http://sourceware.org/bugzilla/show_bug.cgi?id=2445 and trying it on some 
> different Linux versions and CPUs resulted in this:

Great research!

Thanks for that, I'll patch the source and add some comments, plus a link to 
this thread.

> I'm not suggesting that you put this hack into the official Boost 
> codeline, but just thought it might be useful to anyone in my situation 
> who finds this post via a search engine.

One thing you could try: if you only care about double precision, then you 
could set:

#define BOOST_MATH_PROMOTE_DOUBLE_POLICY false
#define BOOST_NO_FENV_H

Then this whole exception flag code would be disabled, and evaluation would 
proceed via the SSE2 double precision registers with the x87 long double 
code unused.  You'd lose a touch of precision (no guard digits in the 
evaluation) but it should be a bit quicker.

HTH, John.