[wasm][IRGen] Stop attaching COMDATs to dead stub methods

include/swift/IRGen/Linking.h

@@ -1886,7 +1886,7 @@ class ApplyIRLinkage {
   IRLinkage IRL;
 public:
   ApplyIRLinkage(IRLinkage IRL) : IRL(IRL) {}
-  void to(llvm::GlobalValue *GV, bool definition = true) const {
+  void to(llvm::GlobalValue *GV, bool nonAliasedDefinition = true) const {
     llvm::Module *M = GV->getParent();
     const llvm::Triple Triple(M->getTargetTriple());
 
@@ -1899,9 +1899,16 @@ class ApplyIRLinkage {
     if (Triple.isOSBinFormatELF())
       return;
 
-    // COMDATs cannot be applied to declarations.  If we have a definition,
-    // apply the COMDAT.
-    if (definition)
+    // COMDATs cannot be applied to declarations. Also, definitions that are
+    // exported through aliases should not have COMDATs, because the alias
+    // itself might represent an externally visible symbol but such symbols
+    // are discarded from the symtab when other object files have a COMDAT
+    // group with the same signature.
+    //
+    // If we have a non-aliased definition with ODR-based linkage, attach it
+    // to a COMDAT group so that duplicate definitions across object files
+    // can be merged by the linker.
+    if (nonAliasedDefinition)
       if (IRL.Linkage == llvm::GlobalValue::LinkOnceODRLinkage ||
           IRL.Linkage == llvm::GlobalValue::WeakODRLinkage)
         if (Triple.supportsCOMDAT())

lib/IRGen/GenDecl.cpp

@@ -2201,7 +2201,7 @@ void IRGenModule::emitVTableStubs() {
                                     &Module);
       ApplyIRLinkage(canLinkOnce ? IRLinkage::InternalLinkOnceODR
                                  : IRLinkage::Internal)
-          .to(stub);
+          .to(stub, /* nonAliasedDefinition */ false);
       stub->setAttributes(constructInitialAttributes());
       stub->setCallingConv(DefaultCC);
       auto *entry = llvm::BasicBlock::Create(getLLVMContext(), "entry", stub);

test/IRGen/dead-stub-method-dedup.swift

@@ -0,0 +1,43 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file --leading-lines %s %t
+
+// Note: Windows uses internal linkage for dead stub methods, which doesn't
+// use linkonce_odr or COMDAT groups.
+// UNSUPPORTED: OS=windows-msvc
+
+// Ensure that swift_dead_method_stub is emitted without comdat linkage
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -parse-as-library -O -module-name A -c -primary-file %t/A.swift %t/B.swift -emit-ir -o - | %FileCheck %s -check-prefix CHECK
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -parse-as-library -O -module-name A -c %t/A.swift -primary-file %t/B.swift -emit-ir -o - | %FileCheck %s -check-prefix CHECK
+
+// CHECK-LABEL: define {{(linkonce_odr )?}}hidden void @_swift_dead_method_stub(
+// CHECK-NOT: comdat
+// CHECK: {
+
+
+// Ensure that link-time deduplication for swift_dead_method_stub works correctly
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -parse-as-library -O -module-name A -c -primary-file %t/A.swift %t/B.swift -o %t/A.swift.o
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -parse-as-library -O -module-name A -c %t/A.swift -primary-file %t/B.swift -o %t/B.swift.o
+// RUN: %target-build-swift -target %target-swift-5.1-abi-triple %t/B.swift.o %t/A.swift.o -o %t/a.out
+
+//--- A.swift
+
+// Define an open class with a dead vtable entry
+class C1 {
+  private func dead() {}
+}
+
+//--- B.swift
+
+class C2: C1 {
+  // Define another dead vtable entry to ensure that this object file
+  // also should have dead vtable stub definition
+  private func beef() {}
+}
+
+@main
+struct Entry {
+    static func main() {
+        _ = C2()
+    }
+}
+