Re: [LAD] GCC Vector extensions

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>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 25 Jul 2011 12:04:06 +0200
> From: Maurizio De Cecco <jmax@email-addr-hidden>
> Subject: Re: [LAD] GCC Vector extensions
> Cc: linux-audio-dev@email-addr-hidden
> Message-ID: <4E2D3F96.2010206@email-addr-hidden>
> Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"
>
> 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.
>
> I made more tests, isolating a small code example, on Mac OS and Ubuntu,
> and i found out the origin of the problem, even if i do not know what
> exactly happening.
>
> My original test used vectors of float of size 8; the gcc vector
> extension documentation says that if the vector size do not match the
> hardware vector size, the code is synthesized in some way.
>
> With a vector size of 8 i found the above results under Mac OS X, using
> clang and gcc4.2, and under Ubuntu 11.04, using clang and gcc4.5.2.
>
> When i move to a vector size of 4, things go better; clang slow down wrt
> the size of 8 of around 2x, and gcc obtains the same result; the
> interesting point is that gcc obtains essentially the same speed with
> and without vector extensions, meaning probably that the compiler is
> good enough in vectorizing the code, at least in the the test cases i used.
>
> 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.
>
> The code is modeled around the way jmax compute, i.e. one vector
> operation at a time on vectors passed by pointers, and it is not
> designed to be the fastest possible code to implements this computation.
>
> Thanks for the help,
>
> Maurizio

You should really give the produced assembly code in each test case.

Stephane
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Received on Mon Jul 25 20:15:02 2011