r212233 - Fix linkage bug that miscompiled variable templates instantiated from similarly named local types. In essence, this bug ensures that the x<Foo> specialization in function foo() defined as follows differs between two distinct translation units.

[David Blaikie]

Test coverage?

On Wed, Jul 2, 2014 at 4:08 PM, Larisse Voufo <lvoufo at google.com> wrote:
> Author: lvoufo
> Date: Wed Jul  2 18:08:34 2014
> New Revision: 212233
>
> URL: http://llvm.org/viewvc/llvm-project?rev=212233&view=rev
> Log:
> Fix linkage bug that miscompiled variable templates instantiated from similarly named local types. In essence, this bug ensures that the x<Foo> specialization in function foo() defined as follows differs between two distinct translation units.
>     static int &foo() {
>        struct Foo { };
>        return x<Foo>;
>     }
>
> Modified:
>     cfe/trunk/lib/AST/Decl.cpp
>
> Modified: cfe/trunk/lib/AST/Decl.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/Decl.cpp?rev=212233&r1=212232&r2=212233&view=diff
> ==============================================================================
> --- cfe/trunk/lib/AST/Decl.cpp (original)
> +++ cfe/trunk/lib/AST/Decl.cpp Wed Jul  2 18:08:34 2014
> @@ -471,6 +471,58 @@ static void mergeTemplateLV(LinkageInfo
>    LV.mergeExternalVisibility(argsLV);
>  }
>
> +/// Should we consider visibility associated with the template
> +/// arguments and parameters of the given variable template
> +/// specialization? As usual, follow class template specialization
> +/// logic up to initialization.
> +static bool shouldConsiderTemplateVisibility(
> +                                 const VarTemplateSpecializationDecl *spec,
> +                                 LVComputationKind computation) {
> +  // Include visibility from the template parameters and arguments
> +  // only if this is not an explicit instantiation or specialization
> +  // with direct explicit visibility (and note that implicit
> +  // instantiations won't have a direct attribute).
> +  if (!spec->isExplicitInstantiationOrSpecialization())
> +    return true;
> +
> +  // An explicit variable specialization is an independent, top-level
> +  // declaration.  As such, if it has an explicit visibility attribute,
> +  // that must directly express the user's intent, and we should honor
> +  // it.
> +  if (spec->isExplicitSpecialization() &&
> +      hasExplicitVisibilityAlready(computation))
> +    return false;
> +
> +  return !hasDirectVisibilityAttribute(spec, computation);
> +}
> +
> +/// Merge in template-related linkage and visibility for the given
> +/// variable template specialization. As usual, follow class template
> +/// specialization logic up to initialization.
> +static void mergeTemplateLV(LinkageInfo &LV,
> +                            const VarTemplateSpecializationDecl *spec,
> +                            LVComputationKind computation) {
> +  bool considerVisibility = shouldConsiderTemplateVisibility(spec, computation);
> +
> +  // Merge information from the template parameters, but ignore
> +  // visibility if we're only considering template arguments.
> +
> +  VarTemplateDecl *temp = spec->getSpecializedTemplate();
> +  LinkageInfo tempLV =
> +    getLVForTemplateParameterList(temp->getTemplateParameters(), computation);
> +  LV.mergeMaybeWithVisibility(tempLV,
> +           considerVisibility && !hasExplicitVisibilityAlready(computation));
> +
> +  // Merge information from the template arguments.  We ignore
> +  // template-argument visibility if we've got an explicit
> +  // instantiation with a visibility attribute.
> +  const TemplateArgumentList &templateArgs = spec->getTemplateArgs();
> +  LinkageInfo argsLV = getLVForTemplateArgumentList(templateArgs, computation);
> +  if (considerVisibility)
> +    LV.mergeVisibility(argsLV);
> +  LV.mergeExternalVisibility(argsLV);
> +}
> +
>  static bool useInlineVisibilityHidden(const NamedDecl *D) {
>    // FIXME: we should warn if -fvisibility-inlines-hidden is used with c.
>    const LangOptions &Opts = D->getASTContext().getLangOpts();
> @@ -662,6 +714,14 @@ static LinkageInfo getLVForNamespaceScop
>      // C99 6.2.2p4 and propagating the visibility attribute, so we don't have
>      // to do it here.
>
> +    // As per function and class template specializations (below),
> +    // consider LV for the template and template arguments.  We're at file
> +    // scope, so we do not need to worry about nested specializations.
> +    if (const VarTemplateSpecializationDecl *spec
> +              = dyn_cast<VarTemplateSpecializationDecl>(Var)) {
> +      mergeTemplateLV(LV, spec, computation);
> +    }
> +
>    //     - a function, unless it has internal linkage; or
>    } else if (const FunctionDecl *Function = dyn_cast<FunctionDecl>(D)) {
>      // In theory, we can modify the function's LV by the LV of its
> @@ -869,6 +929,10 @@ static LinkageInfo getLVForClassMember(c
>
>    // Static data members.
>    } else if (const VarDecl *VD = dyn_cast<VarDecl>(D)) {
> +    if (const VarTemplateSpecializationDecl *spec
> +        = dyn_cast<VarTemplateSpecializationDecl>(VD))
> +      mergeTemplateLV(LV, spec, computation);
> +
>      // Modify the variable's linkage by its type, but ignore the
>      // type's visibility unless it's a definition.
>      LinkageInfo typeLV = getLVForType(*VD->getType(), computation);
>
>
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