https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445
--- Comment #41 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It is documented to be undefined:
-- Built-in Function: int __builtin_clz (unsigned int x)
Returns the number of leading 0-bits in X, starting at the most
significant bit position. If X is 0, the result is undefined.
Especially GCC 11 (but e.g. clang too) will e.g. during value range propagation
assume that e.g. the builtin return value will be only 0 to 31, not to 32, etc.
The portable way how to write this is x ? __builtin_clz (x) :
whatever_value_you_want_for_clz_0;
and the compiler should recognize that and if the instruction is well defined
for 0 and matches your choice, use optimal sequence.
