GCC 14: abseil-src/absl/numeric/internal/bits.h:309:24: error: '__builtin_ctzs' needs isa option -mbmi about abseil-cpp

[derekmauro]

It looks like we can use __builtin_clzg here.

[ChrisDodd]

Why not just use __builtin_ctz here? Per gcc docs __builtin_ctzT returns an unspecified value for a 0 input, so ctz might return a well-defined value where ctzs would return unspecified (when the bottom 16 bits are all 0). I guess that it would be desirable to use the more efficient 16-bit instruction from the bmi instruction set when that is available, but using the 32 bit instruction should be fine when it is not. Overall, this really seems like a gcc bug -- __builtin_X should use whatever sequence of instructions is appropriate for any target. Saying it is not supported when it is perfectly supportable seems wrong.

[derekmauro]

There is no reason to guess when you have benchmarks.