library_build.md

Accelerating Jngen build

Jngen is distributed as a single header. As the header is sufficiently large, compilation lasts fairly long. To speed it up you may use JNGEN_DECLARE_ONLY macro.

Many functions in the library look like this:

#ifdef JNGEN_DECLARE_ONLY
void doSomething();
#else
void doSomething() {
    // crunching numbers
}
#endif

If JNGEN_DECLARE_ONLY is defined, the compiler expects to find the definitions in some other translation unit, otherwise the header is used standalone. When working with Jngen locally, you may create a static library which includes jngen.h and does nothing else, compile it with g++ lib.cpp -c, and then link your main.cpp with generated lib.o. If you add #define JNGEN_DECLARE_ONLY to the top of your main.cpp or specify -DJNGEN_DECLARE_ONLY flag in compiler options, function definitions will be taken from the static library and thus will be not recompiled every time.

$ echo '#include "jngen.h"' > lib.cpp
$ g++ -O2 -std=c++11 -Wall lib.cpp -c
$ g++ -O2 -std=c++11 -Wall -DJNGEN_DECLARE_ONLY main.cpp lib.o

On the author's laptop this trick reduces compilation time by approximately 2.5 times.

Note that if you use some other Jngen defines, like JNGEN_EXTRA_WEIGHT_TYPES, the library and your program must be compiled with the same set of defines.