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Old 04-28-2004, 03:07 AM   #1
duckie
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Registered: Apr 2004
Location: Switzerland
Distribution: SuSE
Posts: 2

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gcc on Opteron: 64bit-mode slower than 32bit-mode!!!?


I recently tried a 'numerical hello world' on our new Opteron-system but got somewhat confused when it turned out that the 32-bit mode ran twice as fast as the 64-bit mode!

The test-code was essentially:
for (i=0; i<i_stop; i++) {
for (j=0; j<j_stop; j++)
sum+=sin(double(i))+cos(double(i));
}

compile 64-bit
g++ test.cpp -O2

compile 32-bit
g++ test.cpp -O2 -m32

It does not matter if I add more optimization switches, the 32-bit version still runs on half the time...

The system:
Dual Opteron 244, 8GB Ram
SuSE 9.0-64bit for AMD64, kernel 2.4.21-209-smp, gcc-3.3.1-23

Do anyone have a clue where the problem is?

Hopefully

Anders

Last edited by duckie; 04-28-2004 at 03:11 AM.
 
Old 05-20-2004, 09:00 AM   #2
bluie
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Registered: May 2004
Location: Europe
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Hi duckie!
A pragmatic answer: use the -ffast-math compiler-option for better results.
Maybe the sin-function is not optimized by default for 64-bit.

Hope this helps some, bluie
 
Old 05-20-2004, 10:51 AM   #3
duckie
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Registered: Apr 2004
Location: Switzerland
Distribution: SuSE
Posts: 2

Original Poster
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Thank you,

now I got the exspected performance

Anders
 
  


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