CodeGen: Assign an appropriate comdat to thunks.

[pcc]

Summary

Previously we were not assigning a comdat to thunks in the Microsoft
ABI, which
would have required us to emit these functions outside of a comdat.
(Due to an inconsistency in how we were emitting objects, we
were getting this right most of the time, but only when compiling
without function sections.) This code generator change causes
us to create a comdat for each thunk.

[majnemer]

Can't this be fixed by moving the call of CGM.getCXXABI().setThunkLinkage in CodeGenVTables::emitThunk to CodeGenFunction::GenerateThunk right after the call to CGM.setFunctionLinkage?

[pcc]

Would that do the right thing with varargs thunks?

It seems that moving the call to setComdat to emitThunk would do the right thing.

[pcc]

New implementation

[majnemer]

LGTM

cfe/trunk/lib/CodeGen/CGVTables.cpp

void CodeGenFunction::GenerateThunk(llvm::Function *Fn,
   llvm::Value *Callee = CGM.GetAddrOfFunction(GD, Ty, /*ForVTable=*/true);
 
   // Make the call and return the result.
   EmitCallAndReturnForThunk(Callee, &Thunk);
 
   // Set the right linkage.
   CGM.setFunctionLinkage(GD, Fn);
 
-  if (CGM.supportsCOMDAT() && Fn->isWeakForLinker())
-    Fn->setComdat(CGM.getModule().getOrInsertComdat(Fn->getName()));
-
   // Set the right visibility.
   const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
   setThunkVisibility(CGM, MD, Thunk, Fn);
 }
 
 void CodeGenVTables::emitThunk(GlobalDecl GD, const ThunkInfo &Thunk,
                                bool ForVTable) {
   const CGFunctionInfo &FnInfo = CGM.getTypes().arrangeGlobalDeclaration(GD);
void CodeGenVTables::emitThunk(GlobalDecl GD, const ThunkInfo &Thunk,
   CGM.SetLLVMFunctionAttributesForDefinition(GD.getDecl(), ThunkFn);
 
   if (ThunkFn->isVarArg()) {
     // Varargs thunks are special; we can't just generate a call because
     // we can't copy the varargs.  Our implementation is rather
     // expensive/sucky at the moment, so don't generate the thunk unless
     // we have to.
     // FIXME: Do something better here; GenerateVarArgsThunk is extremely ugly.
-    if (!UseAvailableExternallyLinkage) {
-      CodeGenFunction(CGM).GenerateVarArgsThunk(ThunkFn, FnInfo, GD, Thunk);
-      CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable, GD,
-                                      !Thunk.Return.isEmpty());
-    }
+    if (UseAvailableExternallyLinkage)
+      return;
   } else {
     // Normal thunk body generation.
     CodeGenFunction(CGM).GenerateThunk(ThunkFn, FnInfo, GD, Thunk);
-    CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable, GD,
-                                    !Thunk.Return.isEmpty());
-  }
+  }
+
+  if (CGM.supportsCOMDAT() && ThunkFn->isWeakForLinker())
+    ThunkFn->setComdat(CGM.getModule().getOrInsertComdat(ThunkFn->getName()));
 }
 
 void CodeGenVTables::maybeEmitThunkForVTable(GlobalDecl GD,
                                              const ThunkInfo &Thunk) {
   // If the ABI has key functions, only the TU with the key function should emit
   // the thunk. However, we can allow inlining of thunks if we emit them with
   // available_externally linkage together with vtables when optimizations are
   // enabled.

cfe/trunk/test/CodeGenCXX/microsoft-abi-thunks.cpp

struct E : D {
   // MANGLING-DAG: @"\01?goo@E@@UAEPAUC@@XZ"
   // MANGLING-DAG: @"\01?goo@E@@QAEPAUB@@XZ"
   // MANGLING-X64-DAG: @"\01?goo@E@@UEAAPEAUC@@XZ"
   // MANGLING-X64-DAG: @"\01?goo@E@@QEAAPEAUB@@XZ"
 };
 
 E::E() {}  // Emits vftable and forces thunk generation.
 
-// CODEGEN-LABEL: define weak_odr x86_thiscallcc %struct.C* @"\01?goo@E@@QAEPAUB@@XZ"
+// CODEGEN-LABEL: define weak_odr x86_thiscallcc %struct.C* @"\01?goo@E@@QAEPAUB@@XZ"{{.*}} comdat
 // CODEGEN:   call x86_thiscallcc %struct.C* @"\01?goo@E@@UAEPAUC@@XZ"
 // CODEGEN:   getelementptr inbounds i8, i8* {{.*}}, i32 4
 // CODEGEN: ret
 
 struct F : virtual A, virtual B {
   virtual void own_method();
   virtual ~F();
 };

 
 struct I : D {
   I();
   virtual F* goo();
 };
 
 I::I() {}  // Emits vftable and forces thunk generation.
 
-// CODEGEN-LABEL: define weak_odr x86_thiscallcc %struct.{{[BF]}}* @"\01?goo@I@@QAEPAUB@@XZ"
+// CODEGEN-LABEL: define weak_odr x86_thiscallcc %struct.{{[BF]}}* @"\01?goo@I@@QAEPAUB@@XZ"{{.*}} comdat
 // CODEGEN: %[[ORIG_RET:.*]] = call x86_thiscallcc %struct.F* @"\01?goo@I@@UAEPAUF@@XZ"
 // CODEGEN: %[[ORIG_RET_i8:.*]] = bitcast %struct.F* %[[ORIG_RET]] to i8*
 // CODEGEN: %[[VBPTR_i8:.*]] = getelementptr inbounds i8, i8* %[[ORIG_RET_i8]], i32 4
 // CODEGEN: %[[VBPTR:.*]] = bitcast i8* %[[VBPTR_i8]] to i32**
 // CODEGEN: %[[VBTABLE:.*]] = load i32*, i32** %[[VBPTR]]
 // CODEGEN: %[[VBASE_OFFSET_PTR:.*]] = getelementptr inbounds i32, i32* %[[VBTABLE]], i32 2
 // CODEGEN: %[[VBASE_OFFSET:.*]] = load i32, i32* %[[VBASE_OFFSET_PTR]]
 // CODEGEN: %[[RES_i8:.*]] = getelementptr inbounds i8, i8* %[[VBPTR_i8]], i32 %[[VBASE_OFFSET]]