Add the "thunk" attribute to MS ABI virtual member pointers

This attribute implies indicates that the function musttail calls
another function and returns whatever it returns. The return type of the
thunk is meaningless, as the thunk can dynamically call different
functions with different return types. So long as the callers bitcast
the thunk with the correct type, behavior is well defined.

This attribute was necessary to fix PR20944, where the indirect call
combiner noticed that the thunk returned void and replaced the results
of the indirect call instruction with undef.

Over-the-shoulder reviewed by David Majnemer.

llvm-svn: 226707

2 files changed, 20 insertions, 4 deletions

diff --git a/clang/lib/CodeGen/MicrosoftCXXABI.cpp b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
index 0741e5d7385..40f88f32a99 100644
--- a/clang/lib/CodeGen/MicrosoftCXXABI.cpp
+++ b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
@@ -1540,6 +1540,12 @@ llvm::Function *MicrosoftCXXABI::EmitVirtualMemPtrThunk(
   CGM.SetLLVMFunctionAttributes(MD, FnInfo, ThunkFn);
   CGM.SetLLVMFunctionAttributesForDefinition(MD, ThunkFn);
 
+  // Add the "thunk" attribute so that LLVM knows that the return type is
+  // meaningless. These thunks can be used to call functions with differing
+  // return types, and the caller is required to cast the prototype
+  // appropriately to extract the correct value.
+  ThunkFn->addFnAttr("thunk");
+
   // These thunks can be compared, so they are not unnamed.
   ThunkFn->setUnnamedAddr(false);
 
diff --git a/clang/test/CodeGenCXX/microsoft-abi-virtual-member-pointers.cpp b/clang/test/CodeGenCXX/microsoft-abi-virtual-member-pointers.cpp
index 8462c298037..8f1ff36a1a8 100644
--- a/clang/test/CodeGenCXX/microsoft-abi-virtual-member-pointers.cpp
+++ b/clang/test/CodeGenCXX/microsoft-abi-virtual-member-pointers.cpp
@@ -62,6 +62,7 @@ void f() {
 
 // Thunk for calling the 1st virtual function in C with no parameters.
 // CHECK32-LABEL: define linkonce_odr x86_thiscallcc void @"\01??_9C@@$BA@AE"(%struct.C* %this, ...)
+// CHECK32: #[[ATTR:[0-9]+]]
 // CHECK32-NOT:             unnamed_addr
 // CHECK32:                 comdat
 // CHECK32: [[VPTR:%.*]] = getelementptr inbounds void (%struct.C*, ...)** %{{.*}}, i64 0
@@ -71,6 +72,7 @@ void f() {
 // CHECK32: }
 //
 // CHECK64-LABEL: define linkonce_odr void @"\01??_9C@@$BA@AA"(%struct.C* %this, ...)
+// CHECK64: #[[ATTR:[0-9]+]]
 // CHECK64-NOT:             unnamed_addr
 // CHECK64:                 comdat
 // CHECK64: [[VPTR:%.*]] = getelementptr inbounds void (%struct.C*, ...)** %{{.*}}, i64 0
@@ -80,14 +82,16 @@ void f() {
 // CHECK64: }
 
 // Thunk for calling the 2nd virtual function in C, taking int and double as parameters, returning int.
-// CHECK32-LABEL: define linkonce_odr x86_thiscallcc void @"\01??_9C@@$B3AE"(%struct.C* %this, ...) {{.*}} comdat
+// CHECK32-LABEL: define linkonce_odr x86_thiscallcc void @"\01??_9C@@$B3AE"(%struct.C* %this, ...)
+// CHECK32: #[[ATTR]] comdat
 // CHECK32: [[VPTR:%.*]] = getelementptr inbounds void (%struct.C*, ...)** %{{.*}}, i64 1
 // CHECK32: [[CALLEE:%.*]] = load void (%struct.C*, ...)** [[VPTR]]
 // CHECK32: musttail call x86_thiscallcc void (%struct.C*, ...)* [[CALLEE]](%struct.C* %{{.*}}, ...)
 // CHECK32-NEXT: ret void
 // CHECK32: }
 //
-// CHECK64-LABEL: define linkonce_odr void @"\01??_9C@@$B7AA"(%struct.C* %this, ...) {{.*}} comdat
+// CHECK64-LABEL: define linkonce_odr void @"\01??_9C@@$B7AA"(%struct.C* %this, ...)
+// CHECK64: #[[ATTR]] comdat
 // CHECK64: [[VPTR:%.*]] = getelementptr inbounds void (%struct.C*, ...)** %{{.*}}, i64 1
 // CHECK64: [[CALLEE:%.*]] = load void (%struct.C*, ...)** [[VPTR]]
 // CHECK64: musttail call void (%struct.C*, ...)* [[CALLEE]](%struct.C* %{{.*}}, ...)
@@ -95,14 +99,16 @@ void f() {
 // CHECK64: }
 
 // Thunk for calling the 3rd virtual function in C, taking an int parameter, returning a struct.
-// CHECK32-LABEL: define linkonce_odr x86_thiscallcc void @"\01??_9C@@$B7AE"(%struct.C* %this, ...) {{.*}} comdat
+// CHECK32-LABEL: define linkonce_odr x86_thiscallcc void @"\01??_9C@@$B7AE"(%struct.C* %this, ...)
+// CHECK32: #[[ATTR]] comdat
 // CHECK32: [[VPTR:%.*]] = getelementptr inbounds void (%struct.C*, ...)** %{{.*}}, i64 2
 // CHECK32: [[CALLEE:%.*]] = load void (%struct.C*, ...)** [[VPTR]]
 // CHECK32: musttail call x86_thiscallcc void (%struct.C*, ...)* [[CALLEE]](%struct.C* %{{.*}}, ...)
 // CHECK32-NEXT: ret void
 // CHECK32: }
 //
-// CHECK64-LABEL: define linkonce_odr void @"\01??_9C@@$BBA@AA"(%struct.C* %this, ...) {{.*}} comdat
+// CHECK64-LABEL: define linkonce_odr void @"\01??_9C@@$BBA@AA"(%struct.C* %this, ...)
+// CHECK64: #[[ATTR]] comdat
 // CHECK64: [[VPTR:%.*]] = getelementptr inbounds void (%struct.C*, ...)** %{{.*}}, i64 2
 // CHECK64: [[CALLEE:%.*]] = load void (%struct.C*, ...)** [[VPTR]]
 // CHECK64: musttail call void (%struct.C*, ...)* [[CALLEE]](%struct.C* %{{.*}}, ...)
@@ -111,6 +117,7 @@ void f() {
 
 // Thunk for calling the virtual function in internal class D.
 // CHECK32-LABEL: define internal x86_thiscallcc void @"\01??_9D@?A@@$BA@AE"(%"struct.(anonymous namespace)::D"* %this, ...)
+// CHECK32: #[[ATTR]]
 // CHECK32: [[VPTR:%.*]] = getelementptr inbounds void (%"struct.(anonymous namespace)::D"*, ...)** %{{.*}}, i64 0
 // CHECK32: [[CALLEE:%.*]] = load void (%"struct.(anonymous namespace)::D"*, ...)** [[VPTR]]
 // CHECK32: musttail call x86_thiscallcc void (%"struct.(anonymous namespace)::D"*, ...)* [[CALLEE]](%"struct.(anonymous namespace)::D"* %{{.*}}, ...)
@@ -118,6 +125,7 @@ void f() {
 // CHECK32: }
 //
 // CHECK64-LABEL: define internal void @"\01??_9D@?A@@$BA@AA"(%"struct.(anonymous namespace)::D"* %this, ...)
+// CHECK64: #[[ATTR]]
 // CHECK64: [[VPTR:%.*]] = getelementptr inbounds void (%"struct.(anonymous namespace)::D"*, ...)** %{{.*}}, i64 0
 // CHECK64: [[CALLEE:%.*]] = load void (%"struct.(anonymous namespace)::D"*, ...)** [[VPTR]]
 // CHECK64: musttail call void (%"struct.(anonymous namespace)::D"*, ...)* [[CALLEE]](%"struct.(anonymous namespace)::D"* %{{.*}}, ...)
@@ -139,3 +147,5 @@ void f() {
 // CHECK64: musttail call void (%struct.C*, ...)* [[CALLEE]](%struct.C* %{{.*}}, ...)
 // CHECK64: ret void
 // CHECK64: }
+
+// CHECK32: #[[ATTR]] = {{{.*}}"thunk"{{.*}}}