c++cbit-manipulationundefined-behaviorbranchless

Generating prefix bitmasks

I am looking for a portable way to generate prefix bitmasks which have the first n bits set for 0 <= n <= 32 (or 64 or an arbitrary integer type bit width).

Examples:

prefix_bitmask(0)  = 0b00000000000000000000000000000000u
prefix_bitmask(4)  = 0b00000000000000000000000000001111u
prefix_bitmask(32) = 0b11111111111111111111111111111111u

There are two ways this can already work if we ignore the cases n == 0 or n == 32:

// "constructive": set only the required bits
uint32_t prefix_mask1(int i) { return (uint32_t(1) << i) - 1; }
// "destructive": shift unneeded bits out
uint32_t prefix_mask2(int i) { return ~uint32_t(0) >> (32 - i); }

prefix_mask1 fails for 32 and prefix_mask2 fails for 0, both because shifts larger than the integer type are undefined behavior (because CPUs are allowed to only use the lowest 5 bits of the shift size).

Is there a "canonical" way to solve this without branching?

Solution

  • ((uint32_t) 1 << i/2 << i-i/2) - 1.

    The above works where uint32_t may be replaced with any unsigned type, and no other changes are needed. Other options that require knowing the number of bits b in the type and a mask m = 2b−1 include:

    ((uint32_t) 1 << (i & m)) - 1 - (i >> b) (from supercat)

    and:

    ((uint32_t) i >> b) ^ 1) << (i & m)) - 1 (derived from a suggestion by Matt Timmermans).