r183283 - Model temporary lifetime-extension explicitly in the AST. Use this model to

[Eli Friedman]

On Mon, Jul 1, 2013 at 7:05 PM, Richard Smith <richard at metafoo.co.uk> wrote:

> On Mon, Jul 1, 2013 at 3:53 PM, Eli Friedman <eli.friedman at gmail.com>
> wrote:
> > On Mon, Jul 1, 2013 at 3:42 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> >>
> >> On Sun, Jun 30, 2013 at 6:39 PM, Eli Friedman <eli.friedman at gmail.com>
> >> wrote:
> >> > On Tue, Jun 4, 2013 at 5:46 PM, Richard Smith
> >> > <richard-llvm at metafoo.co.uk>
> >> > wrote:
> >> >>
> >> >> Author: rsmith
> >> >> Date: Tue Jun  4 19:46:14 2013
> >> >> New Revision: 183283
> >> >>
> >> >> URL: http://llvm.org/viewvc/llvm-project?rev=183283&view=rev
> >> >> Log:
> >> >> Model temporary lifetime-extension explicitly in the AST. Use this
> >> >> model
> >> >> to
> >> >> handle temporaries which have been lifetime-extended to static
> storage
> >> >> duration
> >> >> within constant expressions. This correctly handles nested lifetime
> >> >> extension
> >> >> (through reference members of aggregates in aggregate initializers)
> but
> >> >> non-constant-expression emission hasn't yet been updated to do the
> >> >> same.
> >> >>
> >> >> Modified:
> >> >>     cfe/trunk/include/clang/AST/ASTContext.h
> >> >>     cfe/trunk/include/clang/AST/Decl.h
> >> >>     cfe/trunk/include/clang/AST/ExprCXX.h
> >> >>     cfe/trunk/include/clang/AST/Stmt.h
> >> >>     cfe/trunk/include/clang/Basic/DiagnosticASTKinds.td
> >> >>     cfe/trunk/include/clang/Basic/Specifiers.h
> >> >>     cfe/trunk/include/clang/Sema/Initialization.h
> >> >>     cfe/trunk/lib/AST/ASTContext.cpp
> >> >>     cfe/trunk/lib/AST/ASTDumper.cpp
> >> >>     cfe/trunk/lib/AST/ExprConstant.cpp
> >> >>     cfe/trunk/lib/CodeGen/CGExprConstant.cpp
> >> >>     cfe/trunk/lib/CodeGen/CodeGenModule.cpp
> >> >>     cfe/trunk/lib/CodeGen/CodeGenModule.h
> >> >>     cfe/trunk/lib/Sema/SemaExpr.cpp
> >> >>     cfe/trunk/lib/Sema/SemaInit.cpp
> >> >>     cfe/trunk/lib/Serialization/ASTReaderStmt.cpp
> >> >>     cfe/trunk/lib/Serialization/ASTWriterStmt.cpp
> >> >>     cfe/trunk/test/CodeGenCXX/const-init-cxx11.cpp
> >> >>     cfe/trunk/test/CodeGenCXX/const-init-cxx1y.cpp
> >> >>
> cfe/trunk/test/CodeGenCXX/cxx0x-initializer-stdinitializerlist.cpp
> >> >>     cfe/trunk/test/SemaCXX/constant-expression-cxx11.cpp
> >> >>     cfe/trunk/test/SemaCXX/constant-expression-cxx1y.cpp
> >> >>
> >> >> Modified: cfe/trunk/include/clang/AST/ASTContext.h
> >> >> URL:
> >> >>
> >> >>
> http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/AST/ASTContext.h?rev=183283&r1=183282&r2=183283&view=diff
> >> >>
> >> >>
> >> >>
> ==============================================================================
> >> >> --- cfe/trunk/include/clang/AST/ASTContext.h (original)
> >> >> +++ cfe/trunk/include/clang/AST/ASTContext.h Tue Jun  4 19:46:14 2013
> >> >> @@ -55,6 +55,7 @@ namespace clang {
> >> >>    class ExternalASTSource;
> >> >>    class ASTMutationListener;
> >> >>    class IdentifierTable;
> >> >> +  class MaterializeTemporaryExpr;
> >> >>    class SelectorTable;
> >> >>    class TargetInfo;
> >> >>    class CXXABI;
> >> >> @@ -163,6 +164,11 @@ class ASTContext : public RefCountedBase
> >> >>    llvm::DenseMap<const FunctionDecl*, FunctionDecl*>
> >> >>      ClassScopeSpecializationPattern;
> >> >>
> >> >> +  /// \brief Mapping from materialized temporaries with static
> storage
> >> >> duration
> >> >> +  /// that appear in constant initializers to their evaluated
> values.
> >> >> +  llvm::DenseMap<const MaterializeTemporaryExpr*, APValue>
> >> >> +    MaterializedTemporaryValues;
> >> >> +
> >> >>    /// \brief Representation of a "canonical" template template
> >> >> parameter
> >> >> that
> >> >>    /// is used in canonical template names.
> >> >>    class CanonicalTemplateTemplateParm : public llvm::FoldingSetNode
> {
> >> >> @@ -2117,7 +2123,12 @@ public:
> >> >>    /// \brief Used by ParmVarDecl to retrieve on the side the
> >> >>    /// index of the parameter when it exceeds the size of the normal
> >> >> bitfield.
> >> >>    unsigned getParameterIndex(const ParmVarDecl *D) const;
> >> >> -
> >> >> +
> >> >> +  /// \brief Get the storage for the constant value of a
> materialized
> >> >> temporary
> >> >> +  /// of static storage duration.
> >> >> +  APValue *getMaterializedTemporaryValue(const
> >> >> MaterializeTemporaryExpr
> >> >> *E,
> >> >> +                                         bool MayCreate);
> >> >>
> >> >
> >> > Having a global table of temporary values seems really fragile given
> the
> >> > ability to modify the values of the temporaries in C++1y.
> >> > (const-init-cxx1y.cpp happens to work given the precise order we call
> >> > into
> >> > the constant evaluator, but could easily break if we change how we do
> >> > CodeGen.)  Have you given any thought to this?
> >>
> >> Yes; I'm also not entirely happy with the current state of affairs.
> >>
> >> The best approach I've thought of to improve the situation is to
> >> ensure the ASTContext storage is only used when evaluating the
> >> initializer for the variable that lifetime-extends the temporary (that
> >> is, refuse to evaluate the temporary otherwise, and clear out all the
> >> storage for the variable's extended temporaries if the overall
> >> evaluation fails). It's not possible for us to emit a reference to a
> >> MaterializeTemporaryExpr from outside the variable's initializer if
> >> we've not succeeded in evaluating its initializer, so we shouldn't
> >> emit it with the "wrong" initializer.
> >>
> >> In addition, we should probably move the code that emits the constant
> >> in the ASTContext storage from GetAddrOfGlobalTemporary into
> >> CGExprConstant, so that we can more aggressively assert that we get
> >> this right.
> >>
> >> This would still be a little untidy. Do you have a better suggestion?
> >
> >
> > This might be a bit crazy, but have you considered extending APValue?
> > Specifically, we could add a new kind of LValue whose base is a temporary
> > with the given value.  That way, when CGExprConstant is doing evaluation,
> > the value is right there, instead of messing with a global table.
>
> We'd need all the LValues pointing at the same temporary to share the
> same value; that risks misbehavior if we evaluate the initializer for
> a variable multiple times for whatever reason,


I'm not sure why this is an issue; as long as the LValue in question
represents a reference temporary with the same mangled name and the same
value, it doesn't really matter how many instances of it exist.


> but I think it can be
> made to work. The semantic model feels a bit strange; the initial
> value for a global temporary really is a global thing, and shouldn't
> depend on how you constructed the LValue referring to it.
>

I think that's just the nature of the C++1y model.

-Eli
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