[Boost-users] C++ performance - one reference

From: Mike Marchywka (marchywka_at_[hidden])
Date: 2007-12-05 17:45:19

Performance optimization has come up recently but  I had avoided posting anything
to the list as it is not exactly a boost specific topic. However, John has suggested
that gcc produces slower boost results due to lack of inlining. In a quick test on
some of my own code, I couldn't find any evidence of that. Further, most of the discussion
here seems to center on things like instruction count.  As someone who learned to
program on an 8080, I have a lot of sympathy but I was shocked to learn in this
century  (quite literally true for myself) that there are a lot of additional factors
that effect performance on today's architectures. 

Interested parties may wish to look at things like this:

ftp://download.intel.com/design/Pentium4/papers/24943801.pdf

In particular, after skimming the memory hierachy, consider also the specific FPU
you are using. I know in one case, a functioning audio codec was having noticeable
speed problems visible on task manager. Further investigation found perceptibly
irrelevant things like near-zero values or NAN/ overlfow problems that didn't effect
the final results. My point here is that even a few denormals or exceptional cases can have
a big impact on performance- you could have these even if your output looks right.

Certainly vectorized code would help but if the memory access patterns are wrong these
will limit performance- it can be hard to keep one execution unit busy with slow memory. 

Just some things to think about if you care.

Mike Marchywka
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