https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101175
--- Comment #2 from Mikael Pettersson <mikpelinux at gmail dot com> --- (In reply to Iru Cai from comment #0) > Built with '-march=x86-64-v3 -O1', the following code generates a bsr > instruction, which has undefined behavior when the source operand is zero, > thus gives wrong result The documentation for __builtin_clz(x) states "If x is 0, the result is undefined".
