[IRGen] Be smarter about adding the FORCE_LINK symbols for overlays

lib/IRGen/IRGenModule.cpp

@@ -910,6 +910,23 @@ static bool replaceModuleFlagsEntry(llvm::LLVMContext &Ctx,
   llvm_unreachable("Could not replace old linker options entry?");
 }
 
+/// Returns true if the object file generated by \p IGM will be the "first"
+/// object file in the module. This lets us determine where to put a symbol
+/// that must be unique.
+static bool isFirstObjectFileInModule(IRGenModule &IGM) {
+  if (IGM.getSILModule().isWholeModule())
+    return IGM.IRGen.getPrimaryIGM() == &IGM;
+
+  const DeclContext *DC = IGM.getSILModule().getAssociatedContext();
+  if (!DC)
+    return false;
+
+  assert(!isa<ModuleDecl>(DC) && "that would be a whole module build");
+  assert(isa<FileUnit>(DC) && "compiling something smaller than a file?");
+  ModuleDecl *containingModule = cast<FileUnit>(DC)->getParentModule();
+  return containingModule->getFiles().front() == DC;
+}
+
 void IRGenModule::emitAutolinkInfo() {
   // Collect the linker options already in the module (from ClangCodeGen).
   // FIXME: This constant should be vended by LLVM somewhere.
@@ -964,12 +981,13 @@ void IRGenModule::emitAutolinkInfo() {
     addUsedGlobal(var);
   }
 
-  if (!IRGen.Opts.ForceLoadSymbolName.empty()) {
+  if (!IRGen.Opts.ForceLoadSymbolName.empty() &&
+      isFirstObjectFileInModule(*this)) {
     llvm::SmallString<64> buf;
     encodeForceLoadSymbolName(buf, IRGen.Opts.ForceLoadSymbolName);
     auto ForceImportThunk =
         llvm::Function::Create(llvm::FunctionType::get(VoidTy, false),
-                               llvm::GlobalValue::WeakODRLinkage, buf,
+                               llvm::GlobalValue::ExternalLinkage, buf,
                                &Module);
     if (useDllStorage())
       ForceImportThunk

test/IRGen/autolink-force-link.swift

@@ -0,0 +1,29 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %swift -target x86_64-apple-macosx10.9 -parse-stdlib -autolink-force-load -module-name TEST -module-link-name TEST -emit-ir %s %S/../Inputs/empty.swift | %FileCheck -check-prefix=CHECK-WMO %s
+
+// CHECK-WMO: source_filename = "-"
+// CHECK-WMO: define void @"_swift_FORCE_LOAD_$_TEST"()
+// CHECK-WMO-NOT: source_filename
+
+
+// RUN: %swift -target x86_64-apple-macosx10.9 -parse-stdlib -autolink-force-load -module-name TEST -module-link-name TEST -emit-ir -num-threads 1 %s %S/../Inputs/empty.swift | %FileCheck -check-prefix=CHECK-WMO-THREADED %s
+
+// CHECK-WMO-THREADED: source_filename = "-"
+// CHECK-WMO-THREADED: define void @"_swift_FORCE_LOAD_$_TEST"()
+// CHECK-WMO-THREADED: source_filename = "-"
+// CHECK-WMO-THREADED-NOT: _swift_FORCE_LOAD_$_TEST
+// CHECK-WMO-THREADED-NOT: source_filename
+
+
+// RUN: %swift -target x86_64-apple-macosx10.9 -parse-stdlib -autolink-force-load -module-name TEST -module-link-name TEST -emit-ir -primary-file %s %S/../Inputs/empty.swift | %FileCheck -check-prefix=CHECK-SINGLE-FILE-FIRST %s
+// RUN: %swift -target x86_64-apple-macosx10.9 -parse-stdlib -autolink-force-load -module-name TEST -module-link-name TEST -emit-ir %S/../Inputs/empty.swift -primary-file %s | %FileCheck -check-prefix=CHECK-SINGLE-FILE-SECOND %s
+
+// CHECK-SINGLE-FILE-FIRST: source_filename = "-"
+// CHECK-SINGLE-FILE-FIRST: define void @"_swift_FORCE_LOAD_$_TEST"()
+// CHECK-SINGLE-FILE-FIRST-NOT: source_filename
+
+// CHECK-SINGLE-FILE-SECOND: source_filename = "-"
+// CHECK-SINGLE-FILE-SECOND-NOT: _swift_FORCE_LOAD_$_TEST
+// CHECK-SINGLE-FILE-SECOND-NOT: source_filename
+

test/Serialization/autolinking.swift

@@ -38,10 +38,10 @@ import someModule
 // FRAMEWORK-DAG: !{{[0-9]+}} = !{!"-framework", !"someModule"}
 
 // NO-FORCE-LOAD-NOT: FORCE_LOAD
-// FORCE-LOAD: define weak_odr void @"_swift_FORCE_LOAD_$_module"() {
+// FORCE-LOAD: define void @"_swift_FORCE_LOAD_$_module"() {
 // FORCE-LOAD:   ret void
 // FORCE-LOAD: }
-// FORCE-LOAD-HEX: define weak_odr void @"_swift_FORCE_LOAD_$306d6f64756c65"() {
+// FORCE-LOAD-HEX: define void @"_swift_FORCE_LOAD_$306d6f64756c65"() {
 // FORCE-LOAD-HEX:   ret void
 // FORCE-LOAD-HEX: }