On Fri, 3 Nov 2023, Richard Biener wrote: > The following tries to clarify the __builtin_constant_p documentation, > stating that the argument expression is not evaluated and side-effects > are discarded. I'm struggling to find the correct terms matching > what the C language standard would call things so I'd appreciate > some help here. > > OK for trunk?
OK. > Shall we diagnose arguments with side-effects? It seems to me > such use is usually unintended? I think rather than dropping The traditional use is definitely in macros to choose between code for constant arguments (evaluating them more than once) and maybe-out-of-line code for non-constant arguments (evaluating them exactly once), in which case having a side effect is definitely OK. > side-effects as a side-effect of folding the frontend should > discard them at parsing time instead, no? I suppose the original expression needs to remain around in some form until the latest point at which optimizers might decide to evaluate __builtin_constant_p to true. Although cases with possible side effects might well be optimized to false earlier; the interesting cases for deciding later are e.g. __builtin_constant_p called on an argument to an inline function (no side effects for __builtin_constant_p to discard, whether or not there are side effects in the caller from evaluating the expression passed to the function). -- Joseph S. Myers jos...@codesourcery.com
