[cfe-dev] Floating-point performance question

[Halfdan Ingvarsson]

glibc's expf() function changes the FP rounding mode on every call -- 
which are the fe* calls you're seeing -- resulting in a dreadful 
performance (IIRC there's a pipeline stall when rounding mode changes).

Have a look at sysdeps/ieee754/flt-32/e_expf.c in the glibc sources to 
verify. This is true as of glibc 2.14, at least.

We had to roll our own to work around it.

  - ½

On 13-09-05 03:33 PM, Stephen Canon wrote:
> On Sep 5, 2013, at 12:20 PM, Eli Friedman <eli.friedman at gmail.com 
> <mailto:eli.friedman at gmail.com>> wrote:
>
>> On Thu, Sep 5, 2013 at 12:15 PM, Richard Hadsell 
>> <hadsell at blueskystudios.com <mailto: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 <http://libm-2.13.so/>   feholdexcept
>>     262358    8.7032 libm-2.13.so <http://libm-2.13.so/>   fesetenv
>>     258225    8.5661  studio++       cgi...
>>     207915    6.8971 libm-2.13.so <http://libm-2.13.so/>   fesetround
>>     193316    6.4129  studio++       dcopy2
>>     126933 4.2107 <tel:126933%20%20%20%204.2107> libm-2.13.so
>>     <http://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 <http://libm-2.13.so/>   __ieee754_pow
>>     126938 5.7928 <tel:126938%20%20%20%205.7928> 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
>
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20130905/20a20bd6/attachment.html>