https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80265
--- Comment #17 from Marc Glisse <glisse at gcc dot gnu.org> --- (In reply to Jason Merrill from comment #16) > (In reply to Marc Glisse from comment #13) > > it seems better than abusing __builtin_constant_p, which is getting > > contradictory requirements from its various uses: > > - constexpr (forces very early lowering) > > I'm not sure what you mean here; constexpr specifically delays lowering > within a constexpr function until we're actually trying to evaluate to a > constant value. Evaluating a constexpr function forces the front-end to evaluate __builtin_constant_p. That's very early compared to usual __builtin_constant_p lowering during gimple optimizations. However, now that I think about it, I can't remember why I thought this would be an issue. constexpr evaluation only happens when required, not as an optimization, and when it happens the whole function gets evaluated at compile-time, so I can't think of when we would miss an optimization this way...
