[4.4] Strange performance regression?

Tim Prince n8tm@aol.com
Wed Oct 14 14:28:00 GMT 2009


Joern Rennecke wrote:
> Quoting Mark Tall <mtall.qld@gmail.com>:
> 
>> Joern Rennecke wrote:
>>> But at any rate, the subject does not agree with
>>> the content of the original post.  When we talk
>>> about a 'regression' in a particular gcc version,
>>> we generally mean that this version is in some
>>> way worse than a previous version of gcc.
>>
>> Didn't the original poster indicate that gcc 4.3 was faster than 4.4 ?
>>  In my book that is a regression.
> 
> He also said that it was a different machine, Core 2 Q6600 vs
> some kind of Xeon Core 2 system with a total of eight cores.
> As different memory subsystems are likely to affect the code, it
> is not an established regression till he can reproduce a performance
> drop going from an older to a current compiler on the same or
> sufficiently similar machines, under comparable load conditions -
> which generally means that the machine must be idle apart from the
> benchmark.
Ian's judgment in diverting to gcc-help was born out when it developed 
that -funroll-loops was wanted.  This appeared to confirm his suggestion 
that it might have had to do with loop alignments.
As long as everyone is editorializing, I'll venture say this case raises 
the suspicion that gcc might benefit from better default loop 
alignments, at least for that particular CPU.  However, I've played a 
lot of games on Core i7 with varying unrolling etc. I find the behavior 
of current gcc entirely satisfactory, aside from the verbosity of the 
options required.



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