http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61159
Bug ID: 61159 Summary: __builtin_constant_p gives incorrect results with aliases Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: ipa Assignee: unassigned at gcc dot gnu.org Reporter: bugdal at aerifal dot cx This issue is closely related to #61144, but seems to have appeared earlier. Consider the following test case: static int dummy = 0; extern int foo __attribute__((__weak__, __alias__("dummy"))); typedef char check[2*!__builtin_constant_p(dummy)-1]; typedef char check[2*!__builtin_constant_p(foo)-1]; While the static dummy could be considered constant if its address did not leak outside the translation unit, its address DOES leak via the weak alias foo. As long as there is no strong definition for foo in another translation unit, dummy may be externally modified via the alias foo. As for foo, it is external and thus may never be considered constant. Thus, __builtin_constant_p(dummy) and __builtin_constant_p(foo) should both yield zero and the above translation unit should compile successfully. The results observed vary highly with gcc version. A number of versions were observed giving __builtin_constant_p(dummy)==1, including 4.8.1, but __builtin_constant_p(foo) was more rare, showing up only in 4.9.0 and some really old versions like 4.4.7 (tested via gcc.godbolt.org since I don't have that many gcc versions lying around to test locally).