[PATCH] D76096: [clang] allow const structs to be constant expressions in initializer lists

[Eli Friedman via Phabricator via cfe-commits]

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.


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https://reviews.llvm.org/D76096