[cfe-dev] Floating-point performance question

[Stephen Canon]

On Sep 5, 2013, at 12:20 PM, Eli Friedman <eli.friedman at gmail.com> wrote:

> On Thu, Sep 5, 2013 at 12:15 PM, Richard Hadsell <hadsell at blueskystudios.com> wrote:
> We have been comparing the performance of code generated by Clang++ 3.3 with G++ 4.5.1.  The results have been mixed.
> 
> We ran a profiler to look for what could cause some cases to run slower with Clang++ and found that some floating-point routines were taking a lot of time:
> 
> samples  %        image name     symbol name
> 596677   19.7935  studio++       gcopy2
> 274870    9.1182  libm-2.13.so   feholdexcept
> 262358    8.7032  libm-2.13.so   fesetenv
> 258225    8.5661  studio++       cgi...
> 207915    6.8971  libm-2.13.so   fesetround
> 193316    6.4129  studio++       dcopy2
> 126933 4.2107  libm-2.13.so   __ieee754_exp2
> 122614    4.0675  studio++       fcopy2
> 
> For g++ the top contributors were these:
> 
> samples  %        image name     symbol name
> 466893   21.3064  studio++       gcopy2
> 300240   13.7013  studio++       cgi...
> 176191    8.0404  studio++       dcopy2
> 132491    6.0462  studio++       cgi...
> 129580    5.9133  libm-2.13.so   __ieee754_pow
> 126938 5.7928  studio++       ecopy2
> 119610    5.4583  studio++       fcopy2
> 
> The libm floating-point routines 'fe...' only show up with Clang++, so I suspect they account for the slower performance.
> 
> We are not purposely changing the floating-point precision or rounding mode, so I am looking for a way to avoid code that uses these functions unnecessarily.
> 
> We are compiling with these options:
> 
> -march=core2 -msse4.1 -m64 -std=c++0x -fPIC -pthread -gcc-toolchain /opt/gcc-4.7.2 -Wno-logical-op-parentheses -Wno-shift-op-parentheses -O2
> 
> 
> There isn't any obvious reason why feholdexcept etc. would be called from clang-compiled code, but not gcc-compiled code; clang never generates calls to it implicitly.
> 
> Can you hop into a debugger and get a stack trace from a call to feholdexcept?

Usually the reason these symbols show up on linux is that you’re hitting the errno-versions of the libm entry points (i.e. GCC is likely generating calls to a different set of more streamlined libm entry points, while clang is hitting the default versions).

– Steve
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