To build sum-primes, just run make. If you want to use a different C++
compiler, or set different flags, set either CXX or CXXFLAGS in your
environment before running make. (Check your shell's manual for information.)
sum-primes will use C++11 if it is available, but it does not require it.
To use sum-primes, run ./sum-primes $MAX replacing $MAX with the number
you want sum-primes to stop at. sum-primes is inclusive, so if you
want the sum of all primes up to and excluding 923869, run ./sum-primes 923868.
sum-primes uses the sieve of Eratosthenes,
so it is rather fast. On a Core 2 Quad @ 2.50 GHz box running Gentoo Linux, it
takes 0.048s with no flags given to g++, and 0.017s with -march=native -O3.
Both times were measured with time (real time) and averaged over five runs.
sum-primes was built leaning heavily toward speed with respect to speed vs.
memory tradeoffs. It would be possible to use std::vector<bool> instead of a
C-style array of bools, but that would make sum-primes run slower while
reducing memory usage (std::vector<bool> frequently, depending on
implementation, packs multiple bools together into one byte whereas C-style
arrays of bools do not).
A script to benchmark different CXXFLAGS is included. Run ./bench-cflags.sh CXXFLAGS
replacing CXXFLAGS with the CXXFLAGS you want to test. The script does
require an implementation of the time command that is not built in to your
shell, which may not be available by default. For example, on Gentoo you need
to install the sys-process/time ebuild. If you see an error such as
which: no time... you are missing the separate time program.
After running the script, you should see five lines of real followed by a
number. That number is the execution time of sum-primes. Note that the
benchmarking script will invoke sum-primes with the argument 50000000
(fifty million) by default, though you can change that by setting the
environment variable PROG_ARGS. This is an example session on the same Core 2
Quad box with Gentoo that was mentioned in the Speed section:
wizard@galio ~/Projects/sum-primes $ ./bench-cflags.sh -march=native -O3 -std=c++0x
g++ -march=native -O3 -std=c++0x -o sum-primes sum-primes.cc
Running `./sum-primes 50000000' five times...
real 1.40
real 1.40
real 1.41
real 1.39
real 1.40