builtinpopcount

Builtinpopcount refers to a set of compiler intrinsic functions that count the number of set bits (1s) in the binary representation of an unsigned integer. The operation is widely used in bit-twiddling, bitboard algorithms, and algorithms that rely on population counts for performance or simplicity. In practice, these intrinsics usually map to a single hardware instruction on modern CPUs, when such an instruction is available.

In GCC and Clang the common names are __builtin_popcount for unsigned int, __builtin_popcountl for unsigned long, and __builtin_popcountll for unsigned long long. The return type is int, and the result is the number of 1 bits in the input, ranging from 0 to the number of bits in the input type. The argument is treated as unsigned; if a signed type is supplied, it is converted to unsigned before counting.

Other compilers provide similar functionality under different names. For example, Microsoft compilers expose a family of intrinsics such as __popcnt in their headers. In addition, the C++20 standard library offers a portable alternative: std::popcount in the <bit> header, which performs the same operation in a standard, type-safe way.

Usage considerations include type width and portability. If you are counting bits in wider types, use the corresponding built-in variant or cast to the appropriate unsigned type. When the argument is a constant, many compilers can evaluate the result at compile time. While built-ins offer high performance on supported hardware, portable code should rely on standard library facilities where possible.