code with SIMD Extensions runs slower

code with SIMD Extensions runs slower


I've a dual Pentium III (with Streaming SIMD Extensions)and I've tried compiling code with these instructions enabled using the Intel C Compiler v 5.01. The optimizations string is:

/G6 /Gr /MT /W4 /Ob2 /I /QxK

The odd thing, it runs slower (!). Considerably slower than when I leave out the /QxK.

Clearly I'm going wrong somewhere. Any pointers?

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For more complete information about compiler optimizations, see our Optimization Notice.

It has been a while since version 5.0 ...
I dont remember if there were any obvious gotchas.

When you looked at the vectorization report, did you see quite a few loops get vectorized? You might want to check to see if one of these is getting hit.

Also you might want to explicitly try setting Qfp_port-.

Dis that help?

Please open this in when you isolate a testcase.


Addionally you might want to consider trying version 7.0 (trial download at You can install 7.0 in addition to 5.0.

There have been quite a few improvements to the vectorizer.

Take care,

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