https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107285
--- Comment #3 from Alexey <alex.curiou at gmail dot com> --- (In reply to Jakub Jelinek from comment #1) > That is just bogus expectation. > __builtin_constant_p works that it has a cut-off after certain optimizations > (for -O0 already very early), if optimizations before it manage to optimize > the argument into a constant, it is true. After the cut-off the remaining > occurrences are folded into false. > So, if bar is inlined or say IPA constant propagated, the argument is > constant, while if it isn't inlined nor IPA constant propagated (etc.), as > seems to be the case for -Og, it is not constant, thus you get what you > asked for. > __builtin_constant_p better should be used as a way to optimize code without > changing the observable behavior... For -O2: the generated code and the result should be consistent. Otherwise the usage of the __builtin_constant_p function is just unpredictable. But now the code of bar func will calculate 60 and the real return value is 13. Or just change the explanation for the __builtin_constant_p (https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html) and write that some times the branching on the result of this function will have nothing to do with the final result provided by the compiled code.
