Re: [LAD] GCC Vector extensions

On Mon, Jul 25, 2011 at 5:04 AM, Maurizio De Cecco <jmax@email-addr-hidden> wrote:
> Short resume of my initial post: i found that using the gcc vector
> extensions induced a 2x slow down using gcc, and a 4x speed up in clang.
[snip]
>
> I include the code, results and scripts to run the tests in a small zip
> if anybody want to make other tests; the test code compute an arbitrary
> vector computation (essentially 100 million multiply add), starting from a
> seed given as argument.

I'm getting SIMD instructions when I compile. However, you have two
things slowing you down:

  - The calculations for the for(;;) loop is slowing you down with
every iteration.
  - You're only using one xmm register, so you're getting some memory slowdowns.

Both of these can be solved by having gcc unroll your loops for you
(recompile with -funroll-loops).

In addition, you're handling 3 buffers at a time. bufc[k] = bufa[k] *
bufb[k]. You might be able to speed it up a little by converting the
code to:

    memcopy(bufc, bufa, N*sizeof(float));
    for(k=0; k<N ; ++k) bufc[k] *= bufb[k];

This way you are only handling 2 buffers at a time (which an x86 CPU
generally does better with). But YMMV on this piece of advice.

-gabriel
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Received on Mon Jul 25 20:15:02 2011