Thanks you just helped tons of my code go faster. Turns out false output dependency is a much bigger issue for bsr and bsf with 32-bit operands since it impacts all models and GCC builtins like __builtin_clz(x)^31 won't xor the output unless you use -march=native. So once again I find myself replacing compiler apis with stallman notation. For what it's worth:

    asm("popcnt\t%0,%0" : "=r"(res) : "0"(pop) : "cc");

Can generate the mov statement automatically.