How undefined are __builtin_ctz(0) or __builtin_clz(0)?

Question

Background

For a long time, gcc has been providing a number of builtin bit-twiddling functions, in particular the number of trailing and leading 0-bits (also for <

Answer 1

Unfortunately, even x86-64 implementations can differ - from Intel's instruction set reference,BSF and BSR, with a source operand value of (0), leaves the destination undefined, and sets the ZF (zero flag). So the behaviour may not be consistent between micro-architectures or, say, AMD and Intel. (I believe AMD leaves the destination unmodified.)

The newer LZCNT and TZCNT instructions are not ubiquitous. Both are present only as of the Haswell architecture (for Intel).