Add an IR-level testcase to exercise LLVM IR VFE using thunks (cross-module vcalls) (swiftlang#39526)

Author: kubamracek

test/IRGen/virtual-function-elimination-ir-thunks.swift

@@ -0,0 +1,43 @@
+// Tests that under -enable-llvm-vfe, virtual function calls to classes defined
+// by other modules are using thunk calls (instead of direct vtable loads).
+
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -Xfrontend -disable-objc-interop -Xfrontend -enable-llvm-vfe -parse-as-library %s -DLIBRARY -module-name Library -emit-module -o %t/Library.swiftmodule
+// RUN: %target-build-swift -Xfrontend -disable-objc-interop -Xfrontend -enable-llvm-vfe -parse-as-library %s -DCLIENT -module-name Main -I%t -emit-ir -o - | %FileCheck %s
+
+#if LIBRARY
+
+public class MyLibraryClass {
+  public func foo() { }
+}
+
+#endif
+
+#if CLIENT
+
+import Library
+
+public class MyLocalClass {
+  public func bar() { }
+}
+
+func func1(o: MyLocalClass) {
+  // CHECK: define hidden swiftcc void @"$s4Main5func11oyAA12MyLocalClassC_tF"(%T4Main12MyLocalClassC* %0)
+  o.bar()
+  // CHECK:  [[SLOT:%.*]] = getelementptr inbounds void (%T4Main12MyLocalClassC*)*, void (%T4Main12MyLocalClassC*)** {{.*}}, {{i64|i32}} {{.*}}
+  // CHECK:  [[SLOTASPTR:%.*]] = bitcast void (%T4Main12MyLocalClassC*)** [[SLOT]] to i8*
+  // CHECK:  call { i8*, i1 } @llvm.type.checked.load(i8* [[SLOTASPTR]], i32 0, metadata !"$s4Main12MyLocalClassC3baryyFTq")
+
+  // CHECK: ret void
+}
+
+func func2(o: MyLibraryClass) {
+  // CHECK: define hidden swiftcc void @"$s4Main5func21oy7Library02MyC5ClassC_tF"(%T7Library02MyA5ClassC* %0)
+  o.foo()
+  // CHECK: call swiftcc void @"$s7Library02MyA5ClassC3fooyyFTj"
+
+  // CHECK-NOT: @llvm.type.checked.load
+  // CHECK: ret void
+}
+
+#endif