Coding Efficiency

[Steve Williams]

I saw this in an otherwise worthless thread on Slashdot about OOP:

   Another reason ... (Score:5, Informative)
           by taniwha on Tuesday January 09, @12:40PM EST (#54)
           (User #70410 Info) http://www.taniwha.com/nospam.jpg
           (for the record I first wrote smalltalk code in the 70's, I
regularly code in C++ ...)

           I'm a sometimes chip designer, sometimes programmer ... a
while back while
           working on an unnamed CPU project I did some low level
performance analysis on
           a number of well known programs (maybe even the browser
you're using now)
           basicly we were taking very long instruction/data traces and
then modelling them
           against various potential CPU pipeline/tlb/cache
architectures - we were looking for
           things that would help guide us to a better architecture for
our CPU.

           I found that quite early on I could figure out which language
something was coded
           in from the cooked numbers pretty easily - OO (ie C++) coded
stuff always had a
           really sucky CPI (clocks per instruction - a measure of
architectural efficiency that
           includes pipe breaks, stalls and cache misses) - I spent some
time looking at this
           (since it seemed that C++ code would probably become more
common in our CPU's
           lifetime) - basicly C++ code sucked because it took more
icache hits (because the
           coding style encourages lots of subroutine calls which tend
to spread over the
           cache more filling the I$ quickly) and it took more pipe
breaks (also due to the
           subrotine calls and returns - it turned out that some code
generators did stuff that
           broke CPU's return stack caches causing many more
mispredicts) and finally virtual
           method dispatches (basicly load a pointer, save the pc on the
stack and jump to the
           new pointer) tended to cause double pipe stalls that couldn't
be predicted well at all
           even though these weren't done much they were a real killer
(if you've one a bit of
           modern CPU architecture you learn that with long pipes you
live or die on your
           branch predictor's hit rate - these were very bad news)

           In short C++ and more genrally OO result in code and coding
styles that tend to
           make code that makes modern CPU's run less efficiently.

           Anyway - you often hear about 'efficiency of programmers' etc
etc for OO - I
           thought I'd add a data point from the other end of the
spectrum.
           [