O flags

Robert Mijakovic mijakovi@in.tum.de
Sun May 24 18:16:00 GMT 2015


Dear all,

I have done some experiments with optimization flags enabled by 
different O flag levels.
I took optimizations enabled by O[1..3] and tried to enable them all at 
once without calling
O flag levels. I have found out that improvements I got by using all 
flags used by the
particular O flag level are not even close to what I would get by 
explicitly using O flag levels.
What is the reason for not reaching the same level of performance of O 
flag levels?

I did the following tests:
1. Enable optimizations specified in the Optimize Options manual of the 
version of gcc I'm using on my system.
2. Enabled optimizations that were named to be enabled by "-Q -O[1..3] 
--help=optimizers".

In both cases, I have reached much slower performance compared to the 
one reached by calling O[1..3].
In my tests I have reached speedup of below 2%, while equivalent O flag 
would give me 40% speedup.

Best regards,
Robert Mijakovic



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