[Embedded] Prefer linkonce_odr to weak_odr for nonunique definitions

This allows the implementation to drop definitions that it does need.

Author: DougGregor

include/swift/SIL/SILFunction.h

@@ -930,6 +930,10 @@ class SILFunction
   /// function, such as swift_retain.
   bool isSwiftRuntimeFunction() const;
 
+  /// Helper method that determines whether a function with the given name and
+  /// parent module is a Swift runtime function such as swift_retain.
+  static bool isSwiftRuntimeFunction(StringRef name, const ModuleDecl *module);
+
   /// Helper method which returns true if the linkage of the SILFunction
   /// indicates that the object's definition might be required outside the
   /// current SILModule.

lib/IRGen/GenDecl.cpp

@@ -2329,8 +2329,13 @@ getIRLinkage(StringRef name, const UniversalLinkageInfo &info,
   case SILLinkage::Package: {
     auto linkage = llvm::GlobalValue::ExternalLinkage;
 
-    if (hasNonUniqueDefinition)
-      linkage = llvm::GlobalValue::WeakODRLinkage;
+    if (hasNonUniqueDefinition) {
+      // Keep Swift runtime functions around so IRGen can reference them.
+      if (SILFunction::isSwiftRuntimeFunction(name, nullptr))
+        linkage = llvm::GlobalValue::WeakODRLinkage;
+      else
+        linkage = llvm::GlobalValue::LinkOnceODRLinkage;
+    }
 
     return {linkage, PublicDefinitionVisibility,
             info.Internalize ? llvm::GlobalValue::DefaultStorageClass
@@ -2348,7 +2353,7 @@ getIRLinkage(StringRef name, const UniversalLinkageInfo &info,
 
   case SILLinkage::Hidden:
     if (hasNonUniqueDefinition)
-      return RESULT(WeakODR, Hidden, Default);
+      return RESULT(LinkOnceODR, Hidden, Default);
 
     return RESULT(External, Hidden, Default);
 

lib/SIL/IR/SILFunction.cpp

@@ -1038,17 +1038,20 @@ bool SILFunction::hasValidLinkageForFragileRef(SerializedKind_t callerSerialized
   return hasPublicOrPackageVisibility(getLinkage(), /*includePackage*/ true);
 }
 
-bool SILFunction::isSwiftRuntimeFunction() const {
-  if (!getName().starts_with("swift_") &&
-      !getName().starts_with("_swift_"))
+bool SILFunction::isSwiftRuntimeFunction(
+    StringRef name, const ModuleDecl *module) {
+  if (!name.starts_with("swift_") && !name.starts_with("_swift_"))
     return false;
 
-  auto module = getParentModule();
   return !module ||
       module->getName().str() == "Swift" ||
       module->getName().str() == "_Concurrency";
 }
 
+bool SILFunction::isSwiftRuntimeFunction() const {
+  return isSwiftRuntimeFunction(getName(), getParentModule());
+}
+
 bool
 SILFunction::isPossiblyUsedExternally() const {
   auto linkage = getLinkage();

test/embedded/linkage/leaf_application.swift

@@ -21,13 +21,13 @@
 
 //--- Library.swift
 
-// LIBRARY-IR: @"$e7Library10PointClassCN" = weak_odr {{.*}}global
+// LIBRARY-IR: @"$e7Library10PointClassCN" = linkonce_odr {{.*}}global
 
 // Never referenced.
-// LIBRARY-IR-NOT: @"$es23_swiftEmptyArrayStorageSi_S3itvp" = weak_odr {{(protected |dllexport )?}}global
+// LIBRARY-IR-NOT: @"$es23_swiftEmptyArrayStorageSi_S3itvp" = linkonce_odr {{(protected |dllexport )?}}global
 
 // Note: referenced by swift_allocEmptyBox.
-// LIBRARY-IR: @"$es16_emptyBoxStorageSi_Sitvp" = weak_odr {{(protected |dllexport )?}}global
+// LIBRARY-IR: @"$es16_emptyBoxStorageSi_Sitvp" = linkonce_odr {{(protected |dllexport )?}}global
 
 // LIBRARY-IR-NOT: define {{.*}}@"$e7Library5helloSaySiGyF"()
 public func hello() -> [Int] {