Don't emit an available_externally vtable pointing to linkonce_odr funcs.

This fixes pr13124.

From the discussion at
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2012-June/022606.html
we know that we cannot make funcions in a weak_odr vtable also weak_odr. They
should remain linkonce_odr.

The side effect is that we cannot emit a available_externally vtable unless we
also emit a copy of the function. This also has an issue: If codegen is going
to output a function, sema has to mark it used. Given llvm.org/pr9114, it looks
like sema cannot be more aggressive at marking functions used because
of vtables.

This leaves us with a few unpleasant options:

* Marking functions in vtables used if possible. This sounds a bit sloppy, so
  we should avoid it.
* Producing available_externally vtables only when all the functions in it are
  already used or weak_odr. This would cover cases like

--------------------
struct foo {
  virtual ~foo();
};
struct bar : public foo {
  virtual void zed();
};
void f() {
  foo *x(new bar);
  delete x;
}
void g(bar *x) {
  x->~bar(); // force the destructor to be used
}
--------------------------

and

----------------------------------
template<typename T>
struct bar {
  virtual ~bar();
};
template<typename T>
bar<T>::~bar() {
}

// make the destructor weak_odr instead of linkonce_odr
extern template class bar<int>;

void f() {
  bar<int> *x(new bar<int>);
  delete x;
}
----------------------------

These look like corner cases, so it is unclear if it is worth it.

* And finally: Just nuke this optimization. That is what this patch implements.

llvm-svn: 189852

1 files changed, 3 insertions, 20 deletions

diff --git a/clang/lib/CodeGen/CGVTables.cpp b/clang/lib/CodeGen/CGVTables.cpp
index 26ebf913a9b..3585c42dc9f 100644
--- a/clang/lib/CodeGen/CGVTables.cpp
+++ b/clang/lib/CodeGen/CGVTables.cpp
@@ -753,12 +753,7 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
     switch (keyFunction->getTemplateSpecializationKind()) {
       case TSK_Undeclared:
       case TSK_ExplicitSpecialization:
-        // When compiling with optimizations turned on, we emit all vtables,
-        // even if the key function is not defined in the current translation
-        // unit. If this is the case, use available_externally linkage.
-        if (!def && CodeGenOpts.OptimizationLevel)
-          return llvm::GlobalVariable::AvailableExternallyLinkage;
-
+        assert(def && "Should not have been asked to emit this");
         if (keyFunction->isInlined())
           return !Context.getLangOpts().AppleKext ?
                    llvm::GlobalVariable::LinkOnceODRLinkage :
@@ -777,9 +772,7 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
                  llvm::Function::InternalLinkage;
   
       case TSK_ExplicitInstantiationDeclaration:
-        return !Context.getLangOpts().AppleKext ?
-                 llvm::GlobalVariable::AvailableExternallyLinkage :
-                 llvm::Function::InternalLinkage;
+        llvm_unreachable("Should not have been asked to emit this");
     }
   }
 
@@ -795,7 +788,7 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
     return llvm::GlobalVariable::LinkOnceODRLinkage;
 
   case TSK_ExplicitInstantiationDeclaration:
-    return llvm::GlobalVariable::AvailableExternallyLinkage;
+    llvm_unreachable("Should not have been asked to emit this");
 
   case TSK_ExplicitInstantiationDefinition:
       return llvm::GlobalVariable::WeakODRLinkage;
@@ -897,16 +890,6 @@ bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
 /// we define that v-table?
 static bool shouldEmitVTableAtEndOfTranslationUnit(CodeGenModule &CGM,
                                                    const CXXRecordDecl *RD) {
-  // If we're building with optimization, we always emit v-tables
-  // since that allows for virtual function calls to be devirtualized.
-  // If the v-table is defined strongly elsewhere, this definition
-  // will be emitted available_externally.
-  //
-  // However, we don't want to do this in -fapple-kext mode, because
-  // kext mode does not permit devirtualization.
-  if (CGM.getCodeGenOpts().OptimizationLevel && !CGM.getLangOpts().AppleKext)
-    return true;
-
   return !CGM.getVTables().isVTableExternal(RD);
 }