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Re: Macbook pro performance issue
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Re: Macbook pro performance issue




On May 4, 2006, at 12:29 PM, Paul Messmer wrote:


I have stumbled upon a serious performance anomaly with the Macbook pro and wondered if anyone had any brilliant suggestions on how to track down this mystery.

I'm always up for wild suggestions. How about... your problem is about the performance penalty in dealing with denormals, which on x86 you incur for both scalar and SSE code by default.


Perhaps you're fetching the floating point data from some place other than you think you are (uninitialized, or from some other data structure, or big endian FP data on a little endian machine), and the FP data you are processing ends up being denormals and is really slow.

As the size of tmp[] goes between 1800 and 2600, the stack frame changes, causing window of memory where you fetch this bad data from to slide between fetching nearly all denormals, to fetching some, to fetching none.

You could test this theory by making the call (can't remember off the top of my head) to disable denormal handling, run the code, and see if it makes a difference.

#include <fenv.h>

#pragma STDC FENV_ACCESS ON

#if defined( FE_DFL_DISABLE_SSE_DENORMS_ENV )
//Disables denormal handling and resets FP masks & flags to default values
fesetenv( FE_DFL_DISABLE_SSE_DENORMS_ENV );
#endif
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