Re: C Compiler benchmark: gcc 4.6.3 vs. Intel v11 and others

[Tim Prince <n8tm at aol dot com>]

On 1/19/2012 9:27 AM, willus.com wrote:
> On 1/19/2012 2:59 AM, Richard Guenther wrote:
> > On Thu, Jan 19, 2012 at 7:37 AM, Marc Glisse<marc.glisse@inria.fr> wrote:
> > > On Wed, 18 Jan 2012, willus.com wrote:
> > >
> > > > For those who might be interested, I've recently benchmarked gcc 4.6.3
> > > > (and 3.4.2) vs. Intel v11 and Microsoft (in Windows 7) here:
> > > >
> > > > http://willus.com/ccomp_benchmark2.shtml
> > >
> > > http://en.wikipedia.org/wiki/Microsoft_Windows_SDK#64-bit_development
> > >
> > > For the math functions, this is normally more a libc feature, so you
> > > might
> > > get very different results on different OS. Then again, by using
> > > -ffast-math, you allow the math functions to return any random value,
> > > so I
> > > can think of ways to make it even faster ;-)
> > Also for math functions you can simply substitute the Intel compilers one
> > (GCC uses the Microsoft ones) by linking against libimf. You can also
> > make
> > use of their vectorized variants from GCC by specifying -mveclibabi=svml
> > and link against libimf (the GCC autovectorizer will then use the
> > routines
> > from the Intel compiler math library). That makes a huge difference for
> > code using functions from math.h.
> >
> > Richard.
> >
> > > --
> > > Marc Glisse
> Thank you both for the tips. Are you certain that with the flags I used
> Intel doesn't completely in-line the math2.h functions at the compile
> stage? gcc? I take it to use libimf.a (legally) I would have to purchase
> the Intel compiler?
In-line math functions, beyond what gcc does automatically (sqrt...) are 
possible only with x87 code; those aren't vectorizable nor remarkably 
fast, although quality can be made good (with care).

As Richard said, the icc svml library is the one supporting the fast 
vector math functions.  There is also an arch-consistency version of 
svml (different internal function names) which is not as fast but may 
give more accurate results or avoid platform-dependent bugs.

Yes, the Intel library license makes restrictions on usage:
http://software.intel.com/en-us/articles/faq-intel-parallel-composer-redistributable-package/?wapkw=%28redistributable+license%29

You might use it for personal purposes under terms of this linux license:
http://software.intel.com/en-us/articles/Non-Commercial-license/?wapkw=%28non-commercial+license%29

It isn't supported in the gcc context.  Needless to say, I don't speak 
for my employer.....


--
Tim Prince