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

On Fri, Jan 20, 2012 at 3:43 AM, <> wrote:
> On 1/19/2012 6:29 AM, Richard Guenther wrote:
>> On Thu, Jan 19, 2012 at 3:27 PM,<> ?wrote:
>>> On 1/19/2012 2:59 AM, Richard Guenther wrote:
>>>> On Thu, Jan 19, 2012 at 7:37 AM, Marc Glisse<>
>>>> ?wrote:
>>>>> On Wed, 18 Jan 2012, 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:
>>>>> 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?
>> Yes. ?Intel merely comes with its own (optimized) math library while GCC
>> has to rely on the operating system one.
> Wouldn't it be possible to in-line the standard C math functions in math.h,
> though, if the correct compiler flags were set? ?I realize this could be a
> big task and would potentially have a lot of dependencies on the CPU flag
> settings, but is it at least conceivable? ? Or is it highly undesirable for
> some reason? ?(I almost don't see how Intel could be so fast on some of
> those functions without in-lining.)

The functions are too large for inlining and the function call overhead is
never the problem.  The Intel routines are faster because they are highly
optimized, which neither Windows nor glibcs are.

> Alternately, is anybody developing an open-source, x86-based or x64-based
> fast math library (just for standard C math functions--I don't need
> vector/array/BLAS/etc.) that auto-detects and takes advantage of modern CPU
> capabilities?

AMD libm is open source AFAIK and has optimizations for several AMD CPUs.
Other than that, don't hold your breath - developing math library routines is
not an easy task.


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