Re: vectorization in icc

"Kral Stefan" <>
1 Dec 2002 22:35:47 -0500

          From comp.compilers

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From: "Kral Stefan" <>
Newsgroups: comp.compilers
Date: 1 Dec 2002 22:35:47 -0500
Organization: Vienna University of Technology, Austria
References: 02-11-173
Keywords: parallel, performance
Posted-Date: 01 Dec 2002 22:35:46 EST wrote:

: I've been experimenting with the Intel C/C++ compiler for Linux, and in
: particular, with the automatic vectorization.

: I have the following piece of code (all the arrays are of type char):
: [...]

: Both work just fine, but the vectorized code is significantly slower!
: I certainly expected the vectorized code to be much faster.

Don't worry! I experienced something similar when porting FFTW-GEL (a
special purpose compiler for FFTs) to the Intel Pentium 4. See for details.

: What's going on? Are the sse2 instructions really so slow compared to
: the standard integer instructions? If so, what's the point of the
: vectorization anyways?

I would like to note two things:
    (1) Usually, Intel does not properly support some ISA extension
            until some time passes (or until AMD does it first ;-)

    (2) Vectorized code is very sensitive to alignment issues. Make sure
            that the character arrays you are using are 128-bit aligned.

Best Regards,
Stefan Kral

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