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: 2013-11-13 11:48:29

> I have a program that makes considerable use of the gamma distribution 
> from Boost.Math.
>
> Profiling with OProfile has shown that, on Linux, this program spends 
> about 20% of its runtime calling the functions fesetexceptflag() and 
> feclearexcept().  >The code responsible for calling these is in the 
> BOOST_FPU_EXCEPTION_GUARD macro which is defined in 
> boost/math/tools/config.hpp.

Thanks for raising this, it's something that's been niggling in the back of 
my mind for a while.

> I have looked back through the source history to see when and why this 
> macro was added, and it was in changeset 3186 from September 2006 with the 
> comment "More linux fixes: finally got AMD64 machines to behave correctly" 
> (see https://svn.boost.org/trac/boost/changeset/3186).  In Boost 1.54 the 
> conditions under which the guard macro is defined have been refined since 
> it was originally added, but the guard class itself remains the same as in 
> 2006.
>
> Given the performance penalty I am suffering as a result of constantly 
> clearing and reseting the floating point exception flags, I have a couple 
> of questions about this functionality:
> 1) Can anyone provide a specific test case that did not work without the 
> BOOST_FPU_EXCEPTION_GUARD functionality but was fixed by its addition?  > 
> It would be very useful to have such a test case because then I can check 
> if this is still a problem on recent versions of Linux.

Try disabling it and running the test cases in libs/math/test.

> 2) The commit comment for changeset 3186 refers to AMD64.  Does this mean 
> the original problem only occurred on AMD CPUs, or does this refer to the 
>  >architecture, and hence it affected both Intel and AMD CPUs?

As I recall this was a hardware issue on early AMD 64-bit CPU's, possibly a 
GLIBC issue too.  In fact if I recall it was this issue: 
http://sourceware.org/bugzilla/show_bug.cgi?id=2445 which didn't get much 
help from the glibc guys :-(

But basically if any of the FPU exception flags are set prior to a call to 
pow or exp, you risk getting a garbage answer back.  I'll add a link to the 
above in the code.

HTH, John.