Code Generation Modes

Code Generation Modes

Memory Compression (a.k.a QUANTUM_SIZE == 1)

By default, Emscripten sets QUANTUM_SIZE to 4, which means that a 'normal' element - like an int or a pointer - takes 4 'bytes'. That is the normal behavior on 32-bit systems, and allows Emscripten to properly compile a great deal of C/C++ code. However, the downside is that each int or pointer takes 4 places in the HEAP array, which is how Emscripten models the normal C/C++ memory space. This makes HEAP much bigger than it needs to be, which takes more memory, and also slows things down (for example, memset ends up clearing larger areas of memory).

By setting QUANTUM_SIZE to 1, memory is 'compressed' - it is not longer isomorphic to normal C/C++ memory. So for example a structure with an int, a double, and a pointer would have a size of 3 (one for each variable), instead of the normal 16 (4 for the int and pointer, 8 for the double). This leads to significantly more optimized code, both in terms of speed and memory use.

However, this is risky, since with QUANTUM_SIZE of 1, Emscripten rewrites the .ll, trying to fix all the hardcoded places where it assumes the normal memory space. So consider this experimental for now.

Notes:

• Clang works much better than llvm-gcc (llvm-gcc hardcodes more numbers).
• With QUANTUM_SIZE == 1, Emscripten will warn you about ptrtoint and inttoptr operations (in the generated .js file). You should make sure there are no problems in each of those cases, and perhaps modify the original C/C++ to avoid generating that kind of code (the changes are often simple).

The second version of the Bullet demo in Emscripten uses QUANTUM_SIZE == 1. It appears to give a speedup of around 25%.