rsmith added a comment.
Are we at a point where we can test this now? Perhaps by adding an assert in
codegen that we always have an evaluated value for any `constexpr` variable
that we emit?
================
Comment at: clang/lib/Sema/SemaDecl.cpp:11883-11885
ExprResult Result =
ActOnFinishFullExpr(Init, VDecl->getLocation(),
/*DiscardedValue*/ false, VDecl->isConstexpr());
----------------
This may create additional AST nodes outside the `ConstantExpr`, which
`VarDecl::evaluateValue` is not expecting to see (in particular, if we have
cleanups for the initializer). Should the `ConstantExpr` go outside those nodes
rather than inside?
================
Comment at: clang/lib/Serialization/ASTReader.cpp:9635
+ if (IsExpr) {
+ Base = APValue::LValueBase(ReadExpr(F), CallIndex, Version);
+ ElemTy = Base.get<const Expr *>()->getType();
----------------
Tyker wrote:
> Tyker wrote:
> > rsmith wrote:
> > > This is problematic.
> > >
> > > `ReadExpr` will read a new copy of the expression, creating a distinct
> > > object. But in the case where we reach this when deserializing (for a
> > > `MaterializeTemporaryExpr`), we need to refer to the existing
> > > `MaterializeTemporaryExpr` in the initializer of its lifetime-extending
> > > declaration. We will also need to serialize the `ASTContext`'s
> > > `MaterializedTemporaryValues` collection so that the temporaries
> > > lifetime-extended in a constant initializer get properly handled.
> > >
> > > That all sounds very messy, so I think we should reconsider the model
> > > that we use for lifetime-extended materialized temporaries. As a
> > > half-baked idea:
> > >
> > > * When we lifetime-extend a temporary, create a
> > > `MaterializedTemporaryDecl` to hold its value, and modify
> > > `MaterializeTemporaryExpr` to refer to the `MaterializedTemporaryDecl`
> > > rather than to just hold the subexpression for the temporary.
> > > * Change the `LValueBase` representation to denote the declaration
> > > rather than the expression.
> > > * Store the constant evaluated value for a materialized temporary on the
> > > `MaterializedTemporaryDecl` rather than on a side-table in the
> > > `ASTContext`.
> > >
> > > With that done, we should verify that all remaining `Expr*`s used as
> > > `LValueBase`s are either only transiently used during evaluation or don't
> > > have these kinds of identity problems.
> > Would it be possible to adapt serialization/deserialization so that they
> > make sure that `MaterializeTemporaryExpr` are unique.
> > by:
> >
> > - When serializing `MaterializeTemporaryExpr` serialize a key obtained
> > from the pointer to the expression as it is unique.
> > - When deserializing `MaterializeTemporaryExpr` deserializing the key,
> > and than have a cache for previously deserialized expression that need to
> > be unique.
> >
> > This would make easier adding new `Expr` that require uniqueness and seem
> > less complicated.
> > What do you think ?
> i added a review that does the refactoring https://reviews.llvm.org/D69360.
What are the cases for which we still encounter expressions as lvalue bases
during serialization? I think all the other ones should be OK, but maybe
there's another interesting one we've overlooked.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D63640/new/
https://reviews.llvm.org/D63640
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