[cfe-dev] Floating-point performance question

[Eli Friedman]

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?

-Eli
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