Bit scanning operations in emscripten.

[ovostrikov]

I've been trying to port some code that relies on vectorclass library to emscripten. While patching in proper includes and removing dynamic dispatching logic fixed most of compilation errors, there is one blocker remaining: bit scan forward/reverse functions.
Here's link to corresponding code.
It uses built-in functions from <intrin.h> for msvc/intel and assembly snippet for gcc/clang. Neither works in emscripten for obvious reasons.

So are there any built-in analogues in emscripten compiler or should I fall back to less efficient C++ implementation?

[tlively]

There's no single instruction that has the same behavior, but you can use __builtin_ctz and __builtin_popcount to emulate it and those should lower to a single Wasm instruction each. Emscripten is a thin wrapper around clang at compile time, so it supports everything normal clang supports.