Please check with array initialized from file or other dynamic source. With maximal inlining, there's a risk of constant propagation resulting in a lot of computation happening at compile time rather than run time.
As a quick and dirty hack that took about half hour to implement, I decided to just have the program as a template string and the parameters be substituted in by a python script that called GCC. To the amazement of a very sceptical friend, the result was a 5x speedup.
A lot of this speedup was consistent with replacing the integer divisions with multiplicative inverses, but also merging constants in a way that would have made things quite unreadable. I was quite impressed with how far GCC had gone.
Considering each instance of this simulation ran for tens of minutes, it was well worth compilation time of a few seconds. It kept the code really simple, too.
edit: oh nm, reread GP...
I think if I had to do something similar again today, any script would output a shell script that used command line macro definitions. That didn't really occur to me at the time.
The simulation was run about 100 times with different parameters. Instead of taking about an hour each they instead took about 12 minutes each, with the only difference being baking in all the parameters as constants and letting GCC do the rest.
If you're going to figure this out, please output a warning and ask me to call the optimized intrinsic or something.
These optimizations really mean that as software writers we can declare our intent and have the compiler do safe optimizations.
It seems like these optimizations of Clang are also quite insensitive to the many different ways of writing the same thing that C++ has earned notoriety for.
Huh? It's not explicit, the purpose is implicit. You are writing down an algorithm that implements a popcount algorithm. The compiler than has to infer the fact that you actually wanted popcount (maybe you just wanted to waste a bit of time in a timing loop?) and then substitute something based on that inference.
I am all for it being explicit, i.e. some library function that has a name that somewhere mentions "popcount" or some such. Then have the compiler generate whatever code it needs for that function.
Without even reading TFA, this was my first thought, remembering back to implementing Fibonnaci's sequence in template metaprogramming from "Thinking in C++, Volume 2": http://web.mit.edu/merolish/ticpp/TicV2.html#_Toc53985733
It's cool and useful to be sure, but as costrouc pointed out, LAPACK has been optimized for a very long time by some very smart people. Most people who have not studied the field for a bit are unlikely to come across a more optimal method.