[Bug c/107285] Incorrect code generation when we use __builtin_constant_p built-in function.

["alex.curiou at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

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.