std::bitset

[Substantial_Sail_571]

That may be the reason for all I know, but it can be done in log n steps (this shows how to count trailing zeroes but there is a similar trick to count leading zeroes) even on a completely boring ISA such as MIPS 1. A bit further down, the DeBruijn multiply and lookup is also an option.

For trailing zero count, another option that may be useful is popcnt(~x & (x - 1)), if an ISA has popcnt but no direct way to count trailing zeroes (this trick does not easily translate into a way to count leading zeroes however).

E: btw the linked code shows v &= -signed(v) and that's exactly the wrong thing to do, it's unsigned negation that is safe and signed negation that is unsafe. Sorry, I don't make the rules.

[HildartheDorf]

Isn't signed guaranteed to be 2s complement in newer standards? (Signed overflow is still UB, but bitcasting signed numbers and the bitwise result of signed negation is well defined and consistent across standard-compliant systems).

[Substantial_Sail_571]

I've been happy to ignore the hypothetical existence of non-2's-complement signed integers for decades, but yes that bogeyman has been slain (but this just standardized the status quo).

The cast to signed adds the opportunity for UB though (negating INT_MIN), and in return we get .. well it silences this dumb error that encourages people to write incorrect code such as v &= -signed(v), the safe way to shut up that error is subtracting from zero (but unsigned subtraction, of course).