Re: [gentoo-user] openmp flag

[Peter Weilbacher]

On 2012-09-26 20:25, Florian Philipp wrote:
> Am 25.09.2012 17:01, schrieb Michael Mol:
>> On Tue, Sep 25, 2012 at 10:42 AM, James <wireless@×××××××××××.com>
>> wrote:
>>> OH, anyone is encouraged to "chime in" about openmp
>>> and your thoughts as to it's viability and usefulness.
>>> Do you believe it will become a core technology,
>>> embedded into GCC? Used widely?

I didn't understand this statement. It is a core "technology" and has 
been
part of GCC since 4.2 or so. I certainly have used it since several 
years
in some of my projects. But it certainly needs some little 
modifications
to the code to work.

>> If you can use it, use it. OpenMP is little more than a set of
>> extensions to C (and C++) which allows the normally-scalar language
>> to
>> do some things in a parallel fashion without resorting to the costs
>> of
>> multithreading. This is good, because vector instructions have been
>> available in x86 since MMX came out, and improvements to the vector
>> instructions available to x86 still goes on.
>
> I guess this is just poorly phrased but to clarify: OpenMP *does* use
> multithreading and nothing else. It does not, in any way, make more
> use
> of vector instructions than GCC without -fopenmp. I guess what you
> mean
> is avoiding the costs of *manual* multithreading using POSIX threads
> and
> the like.

To get GCC to try and use vectorization pass -ftree-vectorize.
(You can see what loops it optimized using vectorization with
-ftree-vectorizer-verbose=1).

Cheers,
    Peter.