diff --git a/stdlib/public/SwiftShims/CoreFoundationShims.h b/stdlib/public/SwiftShims/CoreFoundationShims.h
index ee8841476aa..44f517156a2 100644
--- a/stdlib/public/SwiftShims/CoreFoundationShims.h
+++ b/stdlib/public/SwiftShims/CoreFoundationShims.h
@@ -20,6 +20,7 @@
 #define SWIFT_STDLIB_SHIMS_COREFOUNDATIONSHIMS_H
 
 #include "SwiftStdint.h"
+#include "Visibility.h"
 
 #ifdef __cplusplus
 namespace swift { extern "C" {
@@ -54,37 +55,46 @@ typedef unsigned char _swift_shims_Boolean;
 typedef __swift_uint16_t _swift_shims_UniChar;
 
 // Buffer is nullable in case the string is zero-length.
+SWIFT_RUNTIME_STDLIB_INTERFACE
 void _swift_stdlib_CFStringGetCharacters(_swift_shims_CFStringRef __nonnull theString,
                            _swift_shims_CFRange range,
                            _swift_shims_UniChar *__nullable buffer);
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 const _swift_shims_UniChar *__nullable _swift_stdlib_CFStringGetCharactersPtr(
     _swift_shims_CFStringRef __nonnull theString);
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 _swift_shims_CFIndex
     _swift_stdlib_CFStringGetLength(_swift_shims_CFStringRef __nonnull theString);
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 _swift_shims_CFStringRef __nonnull _swift_stdlib_CFStringCreateWithSubstring(
     _swift_shims_CFAllocatorRef __nullable alloc,
     _swift_shims_CFStringRef __nonnull str, _swift_shims_CFRange range);
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 _swift_shims_UniChar
 _swift_stdlib_CFStringGetCharacterAtIndex(_swift_shims_CFStringRef __nonnull theString,
                             _swift_shims_CFIndex idx);
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 _swift_shims_CFStringRef __nonnull _swift_stdlib_CFStringCreateCopy(
     _swift_shims_CFAllocatorRef __nullable alloc,
     _swift_shims_CFStringRef __nonnull theString);
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 const char *__nullable _swift_stdlib_CFStringGetCStringPtr(
     _swift_shims_CFStringRef __nonnull theString,
     _swift_shims_CFStringEncoding encoding);
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 _swift_shims_CFComparisonResult
 _swift_stdlib_CFStringCompare(_swift_shims_CFStringRef __nonnull theString1,
                 _swift_shims_CFStringRef __nonnull theString2,
                 _swift_shims_CFStringCompareFlags compareOptions);
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 _swift_shims_Boolean
 _swift_stdlib_CFStringFindWithOptions(_swift_shims_CFStringRef __nonnull theString,
                         _swift_shims_CFStringRef __nonnull stringToFind,
@@ -92,6 +102,7 @@ _swift_stdlib_CFStringFindWithOptions(_swift_shims_CFStringRef __nonnull theStri
                         _swift_shims_CFStringCompareFlags searchOptions,
                         _swift_shims_CFRange *__nullable result);
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 _swift_shims_CFStringRef __nonnull _swift_stdlib_objcDebugDescription(id __nonnull nsObject);
 #endif // __OBJC2__
 
diff --git a/stdlib/public/SwiftShims/FoundationShims.h b/stdlib/public/SwiftShims/FoundationShims.h
index 7212935d8cf..3800fa01bdf 100644
--- a/stdlib/public/SwiftShims/FoundationShims.h
+++ b/stdlib/public/SwiftShims/FoundationShims.h
@@ -29,6 +29,10 @@
 
 #include "SwiftStdint.h"
 
+#ifdef __cplusplus
+namespace swift { extern "C" {
+#endif
+
 typedef struct {
   __swift_intptr_t location;
   __swift_intptr_t length;
@@ -50,7 +54,12 @@ typedef struct {
   __swift_intptr_t patchVersion;
 } _SwiftNSOperatingSystemVersion;
 
+SWIFT_RUNTIME_STDLIB_INTERFACE
 _SwiftNSOperatingSystemVersion _swift_stdlib_operatingSystemVersion();
 
+#ifdef __cplusplus
+}} // extern "C", namespace swift
+#endif
+
 #endif // SWIFT_STDLIB_SHIMS_FOUNDATIONSHIMS_H
 
diff --git a/stdlib/public/SwiftShims/HeapObject.h b/stdlib/public/SwiftShims/HeapObject.h
index 98c95eeb83b..585095f2492 100644
--- a/stdlib/public/SwiftShims/HeapObject.h
+++ b/stdlib/public/SwiftShims/HeapObject.h
@@ -60,7 +60,7 @@ static_assert(swift::IsTriviallyConstructible<HeapObject>::value,
 static_assert(std::is_trivially_destructible<HeapObject>::value,
               "HeapObject must be trivially destructible");
 
-}
-#endif 
+} // end namespace swift
+#endif
 
 #endif
diff --git a/stdlib/public/stubs/Availability.mm b/stdlib/public/stubs/Availability.mm
index a3d98d1a437..537058d3163 100644
--- a/stdlib/public/stubs/Availability.mm
+++ b/stdlib/public/stubs/Availability.mm
@@ -17,6 +17,7 @@
 #include "swift/Runtime/Debug.h"
 #import <Foundation/Foundation.h>
 #include <TargetConditionals.h>
+#include "../SwiftShims/FoundationShims.h"
 
 using namespace swift;
 
@@ -74,8 +75,7 @@ static NSOperatingSystemVersion operatingSystemVersionFromPlist() {
 
 /// Return the version of the operating system currently running for use in
 /// API availability queries.
-SWIFT_RUNTIME_STDLIB_INTERFACE
-extern "C" NSOperatingSystemVersion _swift_stdlib_operatingSystemVersion() {
+_SwiftNSOperatingSystemVersion swift::_swift_stdlib_operatingSystemVersion() {
   static NSOperatingSystemVersion version = ([]{
     // Use -[NSProcessInfo.operatingSystemVersion] when present
     // (on iOS 8 and OS X 10.10 and above).
@@ -88,5 +88,5 @@ extern "C" NSOperatingSystemVersion _swift_stdlib_operatingSystemVersion() {
     }
   })();
 
-  return version;
+  return { version.majorVersion, version.minorVersion, version.patchVersion };
 }
diff --git a/stdlib/public/stubs/GlobalObjects.cpp b/stdlib/public/stubs/GlobalObjects.cpp
index f059d18ebfa..11a5d7ee6d2 100644
--- a/stdlib/public/stubs/GlobalObjects.cpp
+++ b/stdlib/public/stubs/GlobalObjects.cpp
@@ -15,17 +15,18 @@
 //  defined here.
 //
 //===----------------------------------------------------------------------===//
+
 #include "../SwiftShims/GlobalObjects.h"
 #include "swift/Runtime/Metadata.h"
 
 namespace swift {
-
+// FIXME(ABI): does this declaration need SWIFT_RUNTIME_STDLIB_INTERFACE?
 // _direct type metadata for Swift._EmptyArrayStorage
 SWIFT_RUNTIME_STDLIB_INTERFACE
 extern "C" ClassMetadata _TMCs18_EmptyArrayStorage;
+}
 
-SWIFT_RUNTIME_STDLIB_INTERFACE
-extern "C" _SwiftEmptyArrayStorage _swiftEmptyArrayStorage = {
+swift::_SwiftEmptyArrayStorage swift::_swiftEmptyArrayStorage = {
   // HeapObject header;
   {
     &_TMCs18_EmptyArrayStorage, // isa pointer
@@ -38,16 +39,10 @@ extern "C" _SwiftEmptyArrayStorage _swiftEmptyArrayStorage = {
   }
 };
 
-SWIFT_RUNTIME_STDLIB_INTERFACE
-extern "C"
-__swift_uint64_t _swift_stdlib_HashingDetail_fixedSeedOverride = 0;
+__swift_uint64_t swift::_swift_stdlib_HashingDetail_fixedSeedOverride = 0;
 
 /// Backing storage for Swift.Process.arguments.
-SWIFT_RUNTIME_STDLIB_INTERFACE
-extern "C"
-void *_swift_stdlib_ProcessArguments = nullptr;
-
-}
+void *swift::_swift_stdlib_ProcessArguments = nullptr;
 
 namespace llvm { namespace hashing { namespace detail {
   // An extern variable expected by LLVM's hashing templates. We don't link any
diff --git a/test/1_stdlib/RuntimeObjC.swift b/test/1_stdlib/RuntimeObjC.swift
index 7097ed4f244..947d9215e34 100644
--- a/test/1_stdlib/RuntimeObjC.swift
+++ b/test/1_stdlib/RuntimeObjC.swift
@@ -682,11 +682,6 @@ RuntimeFoundationWrappers.test("_stdlib_NSStringUppercaseString/NoLeak") {
   expectEqual(0, nsStringCanaryCount)
 }
 
-// FIXME: Temporarily commented out due to linker failure in finding
-// __swift_stdlib_CFStringCreateCopy, __swift_stdlib_CFStringGetCharactersPtr,
-// and  __swift_stdlib_CFStringGetLength during optimized builds.
-// rdar://problem/26239602
-/*
 RuntimeFoundationWrappers.test("_stdlib_CFStringCreateCopy/NoLeak") {
   nsStringCanaryCount = 0
   autoreleasepool {
@@ -716,7 +711,6 @@ RuntimeFoundationWrappers.test("_stdlib_CFStringGetCharactersPtr/NoLeak") {
   }
   expectEqual(0, nsStringCanaryCount)
 }
-*/
 
 RuntimeFoundationWrappers.test("bridgedNSArray") {
   var c = [NSObject]()