Optimizing the source code

Optimizing the source code

In general, normal C and C++ code will compile and run at good speeds. However, there are some fundamental differences between JavaScript and native code, that make certain types of source code much faster in one or the other. Typical C and C++ code has been optimized for native code obviously, so some tweaking might help for compiling to JavaScript.

• #define ABS(x) (x > 0 ? x : -x) and similar things are common macros in C and C++ code. The assumption is that using math.h's abs/fabs etc. will not be faster. However, when compiling to JavaScript, the condition in this macro will be much slower: Doing define ABS(x) abs(x), that is, using math.h, would be faster, since then it will be compiled by Emscripten into a single operation, a call to Math.abs. Math.abs has been optimized by JS engines, and will run faster than any JavaScript code that emulates the same behavior, whereas in native code, the conditional macro might be just as fast, or faster, and simpler in that math.h is not needed - so what makes sense in native code makes less sense for JavaScript. In this case, the solution is simple, just switch the macro to use abs().