Generating available_externally vtables for outline virtual functions

Generating available_externally vtables for optimizations purposes.
Unfortunatelly ItaniumABI doesn't guarantee that we will be able to
refer to virtual inline method by name.
But when we don't have any inline virtual methods, and key function is
not defined in this TU, we can generate that there will be vtable and
mark it as available_externally.

This is patch will help devirtualize better.
Differential Revision: http://reviews.llvm.org/D11441

llvm-svn: 243090

1 files changed, 30 insertions, 12 deletions

diff --git a/clang/lib/CodeGen/CGVTables.cpp b/clang/lib/CodeGen/CGVTables.cpp
index 7321dbf..c6d7e06 100644
--- a/clang/lib/CodeGen/CGVTables.cpp
+++ b/clang/lib/CodeGen/CGVTables.cpp
@@ -679,6 +679,12 @@ CodeGenVTables::GenerateConstructionVTable(const CXXRecordDecl *RD,
   return VTable;
 }
 
+static bool shouldEmitAvailableExternallyVTable(const CodeGenModule &CGM,
+                                                const CXXRecordDecl *RD) {
+  return CGM.getCodeGenOpts().OptimizationLevel > 0 &&
+            CGM.getCXXABI().canEmitAvailableExternallyVTable(RD);
+}
+
 /// Compute the required linkage of the v-table for the given class.
 ///
 /// Note that we only call this at the end of the translation unit.
@@ -700,7 +706,12 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
     switch (keyFunction->getTemplateSpecializationKind()) {
       case TSK_Undeclared:
       case TSK_ExplicitSpecialization:
-        assert(def && "Should not have been asked to emit this");
+        assert((def || CodeGenOpts.OptimizationLevel > 0) &&
+               "Shouldn't query vtable linkage without key function or "
+               "optimizations");
+        if (!def && CodeGenOpts.OptimizationLevel > 0)
+          return llvm::GlobalVariable::AvailableExternallyLinkage;
+
         if (keyFunction->isInlined())
           return !Context.getLangOpts().AppleKext ?
                    llvm::GlobalVariable::LinkOnceODRLinkage :
@@ -742,16 +753,18 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
   }
 
   switch (RD->getTemplateSpecializationKind()) {
-  case TSK_Undeclared:
-  case TSK_ExplicitSpecialization:
-  case TSK_ImplicitInstantiation:
-    return DiscardableODRLinkage;
-
-  case TSK_ExplicitInstantiationDeclaration:
-    return llvm::GlobalVariable::ExternalLinkage;
-
-  case TSK_ExplicitInstantiationDefinition:
-    return NonDiscardableODRLinkage;
+    case TSK_Undeclared:
+    case TSK_ExplicitSpecialization:
+    case TSK_ImplicitInstantiation:
+      return DiscardableODRLinkage;
+
+    case TSK_ExplicitInstantiationDeclaration:
+      return shouldEmitAvailableExternallyVTable(*this, RD)
+                 ? llvm::GlobalVariable::AvailableExternallyLinkage
+                 : llvm::GlobalVariable::ExternalLinkage;
+
+    case TSK_ExplicitInstantiationDefinition:
+      return NonDiscardableODRLinkage;
   }
 
   llvm_unreachable("Invalid TemplateSpecializationKind!");
@@ -819,7 +832,12 @@ bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
 /// we define that v-table?
 static bool shouldEmitVTableAtEndOfTranslationUnit(CodeGenModule &CGM,
                                                    const CXXRecordDecl *RD) {
-  return !CGM.getVTables().isVTableExternal(RD);
+  // If vtable is internal then it has to be done
+  if (!CGM.getVTables().isVTableExternal(RD))
+    return true;
+
+  // If it's external then maybe we will need it as available_externally
+  return shouldEmitAvailableExternallyVTable(CGM, RD);
 }
 
 /// Given that at some point we emitted a reference to one or more