On Thu, 16 May 2024 at 14:52, Bart Van Assche <bvanassche@xxxxxxx> wrote: > > It seems like sparse verifies the types of all expressions in a > _Generic() argument list instead of only the expression for which the > type matches. Yes. > Could this indicate a bug in sparse? On > https://en.cppreference.com/w/c/language/generic I found the > following (I'm not sure whether that website is a good reference): > > "The controlling-expression and the expressions of the selections that > are not chosen are never evaluated." Not really a bug, because "never evaluated" in the above context means that they don't generate code. The expressions are still obviously parsed for syntax and validity. It definitely might be seen as a misfeature, though - the "degrades to integer" warning is done before code reachability has been determined. So it's done even for code that is never executed. So you'd get it even if you had something like if (0) .. some bad bitwise expression ... Sadly, that's fairly deeply ingrained in how sparse deals with the bitwise types: they degrade to their regular base type as part of the type evaluation, which happens fairly early on the syntax tree, long before it has been converted to SSA form and reachability analysis. It's *fixable* - instead of warning when evaluating the types of the expression, sparse could leave in a "warning node" into the tree, linearize it to a "warning instruction" in the SSA form, and only actually output a warning if that instruction still exists after dead code elimination etc. But that kind of fix would be a pretty big change, we don't have that kind of thing right now at all. So "fixable in theory" is probably not "practical with the current lack of sparse development". What would be much easier is probably to hack together a couple of builtins for type checking: a "__builtin_signed_p()" should not be hard. Linus
