https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78057
--- Comment #2 from Uroš Bizjak <ubizjak at gmail dot com> --- + case IX86_BUILTIN_TZCNT16: + case IX86_BUILTIN_TZCNT32: + case IX86_BUILTIN_TZCNT64: + if (n_args == 1 && TREE_CODE (args[0]) == INTEGER_CST) Maybe: ... case IX86_BUILTIN_TZCNT64: assert (n_args == 1); if (TREE_CODE (args[0]) == INTEGER_CST) These builtins are defined in the compiler and have known prototype.