efriedma added a comment. I think the code that disables constant evaluation for C is just https://github.com/llvm/llvm-project/blob/dcaf13a4048df3dad55f1a28cde7cefc99ccc057/clang/lib/AST/ExprConstant.cpp#L13918 and https://github.com/llvm/llvm-project/blob/dcaf13a4048df3dad55f1a28cde7cefc99ccc057/clang/lib/AST/ExprConstant.cpp#L13744 . The performance implications of deleting those lines is the complicated part.
================ Comment at: clang/lib/AST/Expr.cpp:3164 + const QualType &QT = cast<DeclRefExpr>(this)->getDecl()->getType(); + if (QT->isStructureType() && QT.isConstQualified()) + return true; ---------------- efriedma wrote: > nickdesaulniers wrote: > > efriedma wrote: > > > nickdesaulniers wrote: > > > > nickdesaulniers wrote: > > > > > Interesting, playing with this more in godbolt, it looks like the > > > > > struct doesn't even have to be const qualified. > > > > Or, rather, behaves differently between C and C++ mode; > > > > > > > > C -> const required > > > > C++ -> const not required > > > In C++, global variable initializers don't have to be constant > > > expressions at all. > > > > > > Do we really need to check GNUMode here? We try to avoid it except for > > > cases where we would otherwise reject valid code. > > > > > > Do we need to worry about arrays here? > > > In C++, global variable initializers don't have to be constant > > > expressions at all. > > > > It looks like my test cases are supported already in Clang today, in C++ > > mode only and not C. Maybe there's some alternative code path that I > > should be looking to reuse? > > > > > Do we really need to check GNUMode here? > > > > Maybe a `-Wpedantic` diag would be more appropriate otherwise? (GCC does > > not warn for these cases with `-Wpedantic`. If the answer to your question > > is `no`, then that means supporting these regardless of language mode. > > (I'm ok with that, was just being maybe overly cautious with `GNUMode`, but > > maybe folks with better knowledge of the language standards have better > > thoughts?) > > > > > Do we need to worry about arrays here? > > > > I don't think arrays are supported: https://godbolt.org/z/RiZPpM > Also, do we need to check that we actually have a definition for the variable? The C++ standard is substantially different from C. C++ global initializers can be evaluated at runtime. So we don't call this code at all in C++. Independent of that, we do have pretty complete support for constant evaluation of structs in C++ to support constexpr, and we should be able to leverage that. ---- For arrays, I was thinking of something like this: ``` const int foo[3] = { 0, 1, 2 }; int bar = foo[0]; ``` ---- We generally don't generate pedantic warnings unless the user uses an extension that's disallowed by the C standard. (The idea is that clang with -pedantic should generate a diagnostic every place the C standard requires a diagnostic. It's not a catch-all for extensions.) We could separately generate some sort of portability warning, but not sure anyone would care to enable it. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D76096/new/ https://reviews.llvm.org/D76096 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits