Fast or not? Collective wisdom on what is faster than other things.
This is a collection of recipes/dirty tricks for how to write C++11 code which will run fast. Some will be examples of how not to do it. Some will show that it does not matter what you do.
The aim is to provide a collection of examples that can be consulted for enlightenment. Things to ponder when investigating speed ups to your C++.
Links to other places that collect information on this topic:
- Andrei Alexandrescu talking about Writing Quick Code in C++, Quickly and Three Optimization Tips for C++
- Herb Sutter's Modern C++: What You Need to Know
- Agner Fog's excellent guide to fast C++ http://www.agner.org/optimize/
searching.cpp compares three different way of finding the first element larger than X in a vector. Compile it with:
for d in FIND STD SELF; do
clang++ -std=c++11 -stdlib=libc++ -O3 searching.cpp -D$d -o search_$d.x;
done;
This creates three executables one for each method. The three methods are a home made binary search, std::lower_bound and std::find_if.
timing.cpp is an example of how to measure the passage of time. Benchmarks all require some kind of time measurement. The more precise the clock the better. Though there are a lot of pitfalls when it comes to building a good benchmark (are you testing what you think you are?), especially when it is a micro-benchmark.
clang++ -std=c++11 -stdlib=libc++ -O3 timing.cpp -o timing.x;
The timers which are part of the C++11 standard library are pretty good!
Do you know how big your various caches are? This example measures the
throughput by reading from an array about 100MB in size. Each read
uses a different step size, making it increasingly harder for the
computer to prefetch elements of the array.
clang++ -std=c++11 -stdlib=libc++ -O3 cache_sizes.cpp -o cache.x;
It prints the step size as well as MB/s read for that step size. The script
cache_plot.gp is a gnuplot script that will plot the results.