[swift-vfe] Disable for @objc classes

Swift methods can be called from Objective-C without a vtable
load, and more importantly without emitting an `llvm.type.checked.load`
instruction. These are effectively virtual callsites that are not
visible to the VFE logic in GlobalDCE.

To to safely enable swift-vfe we must prevent vtables whose functions
are callable from Objective-C from being eliminated.

To do this we ensure such vtables have public vcall_visibility, which
indirectly prevents GlobalDCE from stripping them.

Author: Nuri Amari

lib/IRGen/GenMeta.cpp

@@ -382,7 +382,15 @@ void IRGenModule::addVTableTypeMetadata(
   using VCallVisibility = llvm::GlobalObject::VCallVisibility;
   VCallVisibility vis = VCallVisibility::VCallVisibilityPublic;
   auto AS = decl->getFormalAccessScope();
-  if (AS.isFileScope()) {
+  if (decl->isObjC()) {
+    // Swift methods are called from Objective-C via objc_MsgSend
+    // and thus such call sites are not taken into consideration
+    // by VFE in GlobalDCE. We cannot for the timebeing at least
+    // safely eliminate a virtual function that might be called from
+    // Objective-C. Setting vcall_visibility to public ensures this is
+    // prevented.
+    vis = VCallVisibility::VCallVisibilityPublic;
+  } else if (AS.isFileScope()) {
     vis = VCallVisibility::VCallVisibilityTranslationUnit;
   } else if (AS.isPrivate() || AS.isInternal()) {
     vis = VCallVisibility::VCallVisibilityLinkageUnit;

test/IRGen/virtual-function-elimination-objc-interop-opt-out.swift

@@ -0,0 +1,64 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -enable-llvm-vfe -internalize-at-link -emit-ir %s -o %t/ir.ll
+// RUN: %FileCheck --input-file %t/ir.ll %s --check-prefix=OBJC-BASE
+// RUN: %FileCheck --input-file %t/ir.ll %s --check-prefix=OBJC-DERIVED
+// RUN: %FileCheck --input-file %t/ir.ll %s --check-prefix=BASE
+// RUN: %FileCheck --input-file %t/ir.ll %s --check-prefix=DERIVED
+// RUN: echo '' > %t/used-symbols
+// RUN: %target-build-swift -emit-library -lto=llvm-full %lto_flags -Xfrontend -enable-llvm-vfe -Xfrontend -internalize-at-link -Xlinker -exported_symbols_list -Xlinker %t/used-symbols %s -o %t/main
+// RUN: %llvm-nm --defined-only %t/main | %FileCheck %s --check-prefix=NM
+
+// REQUIRES: objc_interop
+// REQUIRES: no_asan
+// UNSUPPORTED: remote_run
+
+// Tests that under -enable-llvm-vfe @objc virtual functions are not eligible for elimination
+
+import Foundation
+
+// Objective-C exposed types should have public vcall_visibility even under
+// -internalize-at-link
+
+@objc public class ObjCBase : NSObject {
+    @objc public func foo() { print("ObjCBase.foo") }
+    @objc public func bar() { print("ObjCBase.bar") }
+}
+
+// OBJC-BASE: @"$s2ir8ObjCBaseCMn" = {{.*}}!vcall_visibility ![[VCALL_VISIBILITY_METADATA_NODE:[0-9]+]]
+// OBJC-BASE: ![[VCALL_VISIBILITY_METADATA_NODE]] = !{i64 0
+
+@objc public class ObjCDerived : ObjCBase {
+    @objc override public func foo() { print("ObjCDerived.foo") }
+    @objc override public func bar() { print("ObjCDerived.bar") }
+}
+
+// OBJC-DERIVED: @"$s2ir11ObjCDerivedCMn" = {{.*}}!vcall_visibility ![[VCALL_VISIBILITY_METADATA_NODE:[0-9]+]]
+// OBJC-DERIVED: ![[VCALL_VISIBILITY_METADATA_NODE]] = !{i64 0
+
+// Regular Swift types should have linkage unit vcall_visibility
+
+public class Base {
+    public func foo() { print("Base.foo") }
+    public func bar() { print("Base.bar") }
+}
+
+// BASE: @"$s2ir4BaseCMn" = {{.*}}!vcall_visibility ![[VCALL_VISIBILITY_METADATA_NODE:[0-9]+]]
+// BASE: ![[VCALL_VISIBILITY_METADATA_NODE]] = !{i64 1
+
+public class Derived : Base {
+    override public func foo() { print("Derived.foo") }
+    override public func bar() { print("Derived.bar") }
+}
+
+// DERIVED: @"$s2ir7DerivedCMn" = {{.*}}!vcall_visibility ![[VCALL_VISIBILITY_METADATA_NODE:[0-9]+]]
+// DERIVED: ![[VCALL_VISIBILITY_METADATA_NODE]] = !{i64 1
+
+// NM:     $s4main11ObjCDerivedC3baryyF{{$}}
+// NM:     $s4main11ObjCDerivedC3fooyyF{{$}}
+// NM-NOT: $s4main4BaseC3baryyF{{$}}
+// NM-NOT: $s4main4BaseC3fooyyF{{$}}
+// NM-NOT: $s4main7DerivedC3baryyF{{$}}
+// NM-NOT: $s4main7DerivedC3fooyyF{{$}}
+// NM:     $s4main8ObjCBaseC3baryyF{{$}}
+// NM:     $s4main8ObjCBaseC3fooyyF{{$}}