Re: How to get GCC on par with ICC?

[Jeff Law <law@redhat.com>]

On 06/15/2018 05:39 AM, Wilco Dijkstra wrote:
> Martin wrote:
> 
>> Keep in mind that when discussing FP benchmarks, the used math library
>> can be (almost) as important as the compiler.  In the case of 481.wrf,
>> we found that the GCC 8 + glibc 2.26 (so the "out-of-the box" GNU)
>> performance is about 70% of ICC's.  When we just linked against AMD's
>> libm, we got to 83%. When we instructed GCC to generate calls to Intel's
>> SVML library and linked against it, we got to 91%.  Using both SVML and
>> AMD's libm, we achieved 93%.
>>
>> That means that there likely still is 7% to be gained from more clever
>> optimizations in GCC but the real problem is in GNU libm.  And 481.wrf
>> is perhaps the most extreme example but definitely not the only one.
> 
> You really should retry with GLIBC 2.27 since several key math functions were
> rewritten from scratch by Szabolcs Nagy (all in generic C code), resulting in huge
> performance gains on all targets (eg. wrf improved over 50%).
> 
> I fixed several double precision functions in current GLIBC to avoid extremely bad
> performance which had been complained about for years. There are more math
> functions on the way, so the GNU libm will not only catch up, but become the fastest
> math library available.
And resolution on -fno-math-errno as the default.  Setting errno can be
more expensive than people realize.

Jeff