https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90838
--- Comment #21 from Wilco <wilco at gcc dot gnu.org> --- (In reply to Gabriel Ravier from comment #19) > If the original code being branchless makes it faster, wouldn't that imply > that we should use the table-based implementation when generating code for > `__builtin_ctz` ? __builtin_ctz is 3-4 times faster than the table implementation, so this optimization is always worth it. This is why I believe the current situation is not ideal since various targets still set CTZ_DEFINED_VALUE_AT_ZERO to 0 or 1. One option would be to always allow it in Gimple (perhaps add an extra argument for the value to return for a zero input), and at expand time check whether the backend supports the requested value. It it doesn't, emit branches.
