Re: How to get GCC on par with ICC?

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, Jun 6, 2018 at 5:52 PM Paul Menzel
<pmenzel+gcc.gnu.org@molgen.mpg.de> wrote:
>
> Dear GCC folks,
>
>
> Some scientists in our organization still want to use the Intel
> compiler, as they say, it produces faster code, which is then executed
> on clusters. Some resources on the Web [1][2] confirm this. (I am aware,
> that it’s heavily dependent on the actual program.)
>
> My question is, is it realistic, that GCC could catch up and that the
> scientists will start to use it over Intel’s compiler? Or will Intel
> developers always have the lead, because they have secret documentation
> and direct contact with the processor designers?

They will of course have an edge in timing when supporting a new architecture
because they have access to NDA material and hardware.  For example the
OSS community doesn't yet have access to any AVX512 capable machine
(speaking of the GNU compile-farm), and those are prohibitly expensive
for a private contributor.

Similar stories apply to the access to proprietary benchmarks or simply
having resources to continuously work with folks in HPC to make sure ICC
works great for their codes.

> If it is realistic, how can we get there? Would first the program be
> written, and then the compiler be optimized for that? Or are just more
> GCC developers needed?

I think a big part of the story is perception and training.  This means that
for example a coherent and up-to-date source for information on how
to use GCC in a HPC environment (optimizing your code, recommended
compiler options, pitfalls to avoid, etc.) is desperately missing.

When we do our own comparisons of GCC vs. ICC on benchmarks
like SPEC CPU 2006/2017 ICC doesn't have a big lead over GCC
(in fact it even trails in some benchmarks) unless you get to
"SPEC tricks" like data structure re-organization optimizations that
probably never apply in practice on real-world code (and people
should fix such things at the source level being pointed at them
via actually profiling their codes).

In my own experience which dates back nearly 15 years now ICC is
buggy (generates wrong-code / simulation results) and cannot compile
a "simple" C++ program ;)  This made me start working on GCC.

Note that the very best strength of GCC is the first-class high-quality
(insert more buzzwords here) support infrastructure if you actually
run into issues with the compiler!  Even when using paid ICC I never
got timely fixes (if at all) for wrong-code issues I reported to them!

I've separately replied to specific points in other posts where ICC has
an edge over GCC.

Richard.

>
> Kind regards,
>
> Paul
>
>
> [1]: https://colfaxresearch.com/compiler-comparison/
> [2]:
> http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.679.1280&rep=rep1&type=pdf
>