https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80378
--- Comment #8 from Andi Kleen <andi-gcc at firstfloor dot org> --- __builtin_constant_p does not cover variable range information, which is what we're looking for here to prevent security bugs. Also in my experience these explicit expressions tend to be somewhat fragile and is not well specified. It has to assume that the optimizer does specific operations which are nowhere guaranteed. An explicit builtin could be much tighter defined.
