diff --git a/CMakeLists.txt b/CMakeLists.txt
index 31808174c5804..bfc677f49479f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -235,8 +235,8 @@ option(SWIFT_RUNTIME_CRASH_REPORTER_CLIENT
     FALSE)
 
 option(SWIFT_DARWIN_ENABLE_STABLE_ABI_BIT
-    "Enable the Swift stable ABI's class marker bit"
-    FALSE)
+    "Enable the Swift stable ABI's class marker bit for new deployment targets"
+    TRUE)
 
 set(SWIFT_DARWIN_XCRUN_TOOLCHAIN "XcodeDefault" CACHE STRING
     "The name of the toolchain to pass to 'xcrun'")
diff --git a/include/swift/ABI/Metadata.h b/include/swift/ABI/Metadata.h
index a453603dacd0e..a24735720b8fd 100644
--- a/include/swift/ABI/Metadata.h
+++ b/include/swift/ABI/Metadata.h
@@ -1192,6 +1192,31 @@ struct TargetClassMetadata : public TargetAnyClassMetadata<Runtime> {
     return bounds;
   }
 
+  /// Given a statically-emitted metadata template, this sets the correct
+  /// "is Swift" bit for the current runtime. Depending on the deployment
+  /// target a binary was compiled for, statically emitted metadata templates
+  /// may have a different bit set from the one that this runtime canonically
+  /// considers the "is Swift" bit.
+  void setAsTypeMetadata() {
+    // If the wrong "is Swift" bit is set, set the correct one.
+    //
+    // Note that the only time we should see the "new" bit set while
+    // expecting the "old" one is when running a binary built for a
+    // new OS on an old OS, which is not supported, however we do
+    // have tests that exercise this scenario.
+    auto otherSwiftBit = (3ULL - SWIFT_CLASS_IS_SWIFT_MASK);
+    assert(otherSwiftBit == 1ULL || otherSwiftBit == 2ULL);
+
+    if ((this->Data & 3) == otherSwiftBit) {
+      this->Data ^= 3;
+    }
+
+    // Otherwise there should be nothing to do, since only the old "is
+    // Swift" bit is used for backward-deployed runtimes.
+    
+    assert(isTypeMetadata());
+  }
+
   static bool classof(const TargetMetadata<Runtime> *metadata) {
     return metadata->getKind() == MetadataKind::Class;
   }
diff --git a/include/swift/Runtime/BackDeployment.h b/include/swift/Runtime/BackDeployment.h
new file mode 100644
index 0000000000000..1e6737f182d39
--- /dev/null
+++ b/include/swift/Runtime/BackDeployment.h
@@ -0,0 +1,45 @@
+//===--- BackDeployment.h - Support for running on older OS versions. -----===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_STDLIB_BACKDEPLOYMENT_H
+#define SWIFT_STDLIB_BACKDEPLOYMENT_H
+
+#if defined(__APPLE__) && defined(__MACH__)
+
+#include "swift/Runtime/Config.h"
+#include "../../../stdlib/public/SwiftShims/Visibility.h"
+
+#ifdef __cplusplus
+namespace swift { extern "C" {
+#endif
+
+#if SWIFT_CLASS_IS_SWIFT_MASK_GLOBAL_VARIABLE
+# ifndef __cplusplus
+// This file gets included from some C/ObjC files and
+// SWIFT_RUNTIME_STDLIB_SPI doesn't imply extern in C.
+extern
+# endif
+SWIFT_RUNTIME_STDLIB_SPI unsigned long long _swift_classIsSwiftMask;
+#endif
+
+/// Returns true if the current OS version, at runtime, is a back-deployment
+/// version.
+SWIFT_RUNTIME_STDLIB_INTERNAL
+int _swift_isBackDeploying();
+
+#ifdef __cplusplus
+}} // extern "C", namespace swift
+#endif
+
+#endif // defined(__APPLE__) && defined(__MACH__)
+
+#endif // SWIFT_STDLIB_BACKDEPLOYMENT_H
diff --git a/include/swift/Runtime/CMakeConfig.h.in b/include/swift/Runtime/CMakeConfig.h.in
index 5a19acb3adaa7..5bf783b453e05 100644
--- a/include/swift/Runtime/CMakeConfig.h.in
+++ b/include/swift/Runtime/CMakeConfig.h.in
@@ -5,5 +5,7 @@
 #define SWIFT_RUNTIME_CMAKECONFIG_H
 
 #cmakedefine01 SWIFT_DARWIN_ENABLE_STABLE_ABI_BIT
+#cmakedefine01 SWIFT_BNI_OS_BUILD
+#cmakedefine01 SWIFT_BNI_XCODE_BUILD
 
 #endif
diff --git a/include/swift/Runtime/CMakeLists.txt b/include/swift/Runtime/CMakeLists.txt
index 4b0687145867b..7dea0d2c2a39f 100644
--- a/include/swift/Runtime/CMakeLists.txt
+++ b/include/swift/Runtime/CMakeLists.txt
@@ -1,2 +1,13 @@
+# Detect B&I builds.
+set(SWIFT_BNI_OS_BUILD FALSE)
+set(SWIFT_BNI_XCODE_BUILD FALSE)
+if(DEFINED ENV{RC_XBS})
+  if(NOT DEFINED ENV{RC_XCODE} OR NOT "$ENV{RC_XCODE}")
+    set(SWIFT_BNI_OS_BUILD TRUE)
+  else()
+    set(SWIFT_BNI_XCODE_BUILD TRUE)
+  endif()
+endif()
+
 configure_file(CMakeConfig.h.in ${CMAKE_CURRENT_BINARY_DIR}/CMakeConfig.h
                ESCAPE_QUOTES @ONLY)
diff --git a/include/swift/Runtime/Config.h b/include/swift/Runtime/Config.h
index 94eace943b2c7..7377828924de6 100644
--- a/include/swift/Runtime/Config.h
+++ b/include/swift/Runtime/Config.h
@@ -141,10 +141,27 @@
 /// Which bits in the class metadata are used to distinguish Swift classes
 /// from ObjC classes?
 #ifndef SWIFT_CLASS_IS_SWIFT_MASK
-# if defined(__APPLE__) && SWIFT_OBJC_INTEROP && SWIFT_DARWIN_ENABLE_STABLE_ABI_BIT
+
+// Non-Apple platforms always use 1.
+# if !defined(__APPLE__)
+#  define SWIFT_CLASS_IS_SWIFT_MASK 1ULL
+
+// Builds for Swift-in-the-OS always use 2.
+# elif SWIFT_BNI_OS_BUILD
 #  define SWIFT_CLASS_IS_SWIFT_MASK 2ULL
-# else
+
+// Builds for Xcode always use 1.
+# elif SWIFT_BNI_XCODE_BUILD
 #  define SWIFT_CLASS_IS_SWIFT_MASK 1ULL
+
+// Other builds (such as local builds on developers' computers)
+// dynamically choose the bit at runtime based on the current OS
+// version.
+# else
+#  define SWIFT_CLASS_IS_SWIFT_MASK _swift_classIsSwiftMask
+#  define SWIFT_CLASS_IS_SWIFT_MASK_GLOBAL_VARIABLE 1
+#  include "BackDeployment.h"
+
 # endif
 #endif
 
diff --git a/include/swift/SwiftRemoteMirror/SwiftRemoteMirror.h b/include/swift/SwiftRemoteMirror/SwiftRemoteMirror.h
index 16d3661471144..01c12cabf0317 100644
--- a/include/swift/SwiftRemoteMirror/SwiftRemoteMirror.h
+++ b/include/swift/SwiftRemoteMirror/SwiftRemoteMirror.h
@@ -37,6 +37,8 @@
 extern "C" {
 #endif
 
+extern unsigned long long swift_reflection_classIsSwiftMask;
+
 /// Get the metadata version supported by the Remote Mirror library.
 SWIFT_REMOTE_MIRROR_LINKAGE
 uint16_t swift_reflection_getSupportedMetadataVersion(void);
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 48486e5079e49..35e2041ca3166 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -429,7 +429,11 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableSILOpaqueValues |= Args.hasArg(OPT_enable_sil_opaque_values);
 
 #if SWIFT_DARWIN_ENABLE_STABLE_ABI_BIT
-  Opts.UseDarwinPreStableABIBit = false;
+  Opts.UseDarwinPreStableABIBit =
+    (Target.isMacOSX() && Target.isMacOSXVersionLT(10, 14, 4)) ||
+    (Target.isiOS() && Target.isOSVersionLT(12, 2)) ||
+    (Target.isTvOS() && Target.isOSVersionLT(12, 2)) ||
+    (Target.isWatchOS() && Target.isOSVersionLT(5, 2));
 #else
   Opts.UseDarwinPreStableABIBit = true;
 #endif
diff --git a/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp b/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp
index 2491668b7a1d9..b6536d0baff49 100644
--- a/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp
+++ b/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp
@@ -10,6 +10,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+#define SWIFT_CLASS_IS_SWIFT_MASK swift_reflection_classIsSwiftMask
+extern "C" unsigned long long swift_reflection_classIsSwiftMask = 2;
+
 #include "swift/Reflection/ReflectionContext.h"
 #include "swift/Reflection/TypeLowering.h"
 #include "swift/Remote/CMemoryReader.h"
diff --git a/stdlib/public/runtime/BackDeployment.cpp b/stdlib/public/runtime/BackDeployment.cpp
new file mode 100644
index 0000000000000..c92c2db104e75
--- /dev/null
+++ b/stdlib/public/runtime/BackDeployment.cpp
@@ -0,0 +1,64 @@
+//===--- BackDeployment.cpp - Support for running on older OS versions. ---===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/Runtime/BackDeployment.h"
+#include "swift/Runtime/Config.h"
+#include "../SwiftShims/FoundationShims.h"
+#include <stdlib.h>
+
+#if defined(__APPLE__) && defined(__MACH__)
+
+#if SWIFT_CLASS_IS_SWIFT_MASK_GLOBAL_VARIABLE
+static unsigned long long computeIsSwiftMask() {
+  if (swift::_swift_isBackDeploying())
+    return 1ULL;
+  return 2ULL;
+}
+
+SWIFT_ALLOWED_RUNTIME_GLOBAL_CTOR_BEGIN
+extern "C" unsigned long long
+_swift_classIsSwiftMask = computeIsSwiftMask();
+SWIFT_ALLOWED_RUNTIME_GLOBAL_CTOR_END
+#endif // SWIFT_CLASS_IS_SWIFT_MASK_GLOBAL_VARIABLE
+
+static swift::_SwiftNSOperatingSystemVersion swiftInOSVersion = {
+#if __MAC_OS_X_VERSION_MIN_REQUIRED
+  10, 14, 4
+// WatchOS also pretends to be iOS, so check it first.
+#elif __WATCH_OS_VERSION_MIN_REQUIRED
+   5,  2, 0
+#elif __IPHONE_OS_VERSION_MIN_REQUIRED || __TV_OS_VERSION_MIN_REQUIRED
+  12,  2, 0
+#else
+  9999, 0, 0
+#endif
+};
+
+static bool versionLessThan(swift::_SwiftNSOperatingSystemVersion lhs,
+                            swift::_SwiftNSOperatingSystemVersion rhs) {
+  if (lhs.majorVersion < rhs.majorVersion) return true;
+  if (lhs.majorVersion > rhs.majorVersion) return false;
+  
+  if (lhs.minorVersion < rhs.minorVersion) return true;
+  if (lhs.minorVersion > rhs.minorVersion) return false;
+  
+  if (lhs.patchVersion < rhs.patchVersion) return true;
+  
+  return false;
+}
+
+SWIFT_RUNTIME_STDLIB_INTERNAL
+int _swift_isBackDeploying() {
+  auto version = swift::_swift_stdlib_operatingSystemVersion();
+  return versionLessThan(version, swiftInOSVersion);
+}
+#endif
diff --git a/stdlib/public/runtime/CMakeLists.txt b/stdlib/public/runtime/CMakeLists.txt
index db0f67de5e4e3..8f5d79fb6be49 100644
--- a/stdlib/public/runtime/CMakeLists.txt
+++ b/stdlib/public/runtime/CMakeLists.txt
@@ -31,6 +31,7 @@ set(swift_runtime_objc_sources
 set(swift_runtime_sources
     AnyHashableSupport.cpp
     Array.cpp
+    BackDeployment.cpp
     Casting.cpp
     CompatibilityOverride.cpp
     CygwinPort.cpp
diff --git a/stdlib/public/runtime/Metadata.cpp b/stdlib/public/runtime/Metadata.cpp
index 7d06adf85c8f3..d08fb47ba14df 100644
--- a/stdlib/public/runtime/Metadata.cpp
+++ b/stdlib/public/runtime/Metadata.cpp
@@ -96,7 +96,15 @@ Metadata *TargetSingletonMetadataInitialization<InProcess>::allocate(
   // If this is a class, we have to initialize the value witness table early
   // so that two-phase initialization can proceed as if this metadata is
   // complete for layout purposes when it appears as part of an aggregate type.
-  if (auto *classMetadata = dyn_cast<ClassMetadata>(metadata)) {
+  //
+  // Note that we can't use (dyn_)cast<ClassMetadata> here because the static
+  // template may have the "wrong" isSwift bit set in its Data pointer, if the
+  // binary was built to deploy back to pre-stable-Swift Objective-C runtimes.
+  // Such a template will fail the `isTypeMetadata` test and we'll think that it
+  // isn't Swift metadata but a plain old ObjC class instead.
+  if (metadata->getKind() == MetadataKind::Class) {
+    auto *classMetadata = static_cast<ClassMetadata*>(metadata);
+    classMetadata->setAsTypeMetadata();
     auto *fullMetadata = asFullMetadata(metadata);
 
     // Begin by initializing the value witness table; everything else is
diff --git a/stdlib/tools/swift-reflection-test/swift-reflection-test.c b/stdlib/tools/swift-reflection-test/swift-reflection-test.c
index 98980f8f4f1f7..0f8d5b34af4a7 100644
--- a/stdlib/tools/swift-reflection-test/swift-reflection-test.c
+++ b/stdlib/tools/swift-reflection-test/swift-reflection-test.c
@@ -551,6 +551,20 @@ int doDumpHeapInstance(const char *BinaryFilename) {
   return EXIT_SUCCESS;
 }
 
+#if defined(__APPLE__) && defined(__MACH__)
+#include <dlfcn.h>
+static unsigned long long computeClassIsSwiftMask(void) {
+  uintptr_t *objc_debug_swift_stable_abi_bit_ptr =
+    (uintptr_t *)dlsym(RTLD_DEFAULT, "objc_debug_swift_stable_abi_bit");
+  return objc_debug_swift_stable_abi_bit_ptr ?
+           *objc_debug_swift_stable_abi_bit_ptr : 1;
+}
+#else
+static unsigned long long computeClassIsSwiftMask(void) {
+  return 1;
+}
+#endif
+
 void printUsageAndExit() {
   fprintf(stderr, "swift-reflection-test <binary filename>\n");
   exit(EXIT_FAILURE);
@@ -561,6 +575,8 @@ int main(int argc, char *argv[]) {
     printUsageAndExit();
 
   const char *BinaryFilename = argv[1];
+  
+  swift_reflection_classIsSwiftMask = computeClassIsSwiftMask();
 
   uint16_t Version = swift_reflection_getSupportedMetadataVersion();
   printf("Metadata version: %u\n", Version);
diff --git a/test/IRGen/class.sil b/test/IRGen/class.sil
index 3dde15cbe5162..57b0b62c6f400 100644
--- a/test/IRGen/class.sil
+++ b/test/IRGen/class.sil
@@ -41,7 +41,7 @@ sil_vtable C {}
 // CHECK-INDIRECT-SAME:   %swift.type* null,
 // CHECK-SAME:   [[OPAQUE]]* @_objc_empty_cache,
 // CHECK-SAME:   [[OPAQUE]]* null,
-// CHECK-SAME:   i64 add (i64 ptrtoint ({{.*}}* @_DATA__TtC5class1C to i64), i64 1)
+// CHECK-SAME:   i64 add (i64 ptrtoint ({{.*}}* @_DATA__TtC5class1C to i64), i64 [[IS_SWIFT_BIT:1|2]])
 // CHECK-SAME: }>
 
 // Destroying destructor
diff --git a/test/IRGen/class_resilience.swift b/test/IRGen/class_resilience.swift
index 4066f8ffb1947..e7d1f03dd20d0 100644
--- a/test/IRGen/class_resilience.swift
+++ b/test/IRGen/class_resilience.swift
@@ -92,7 +92,7 @@
 // --       ivar destroyer:
 // CHECK-SAME:   i32 0,
 // --       flags:
-// CHECK-SAME:   i32 3,
+// CHECK-SAME:   i32 2,
 // --       RO data:
 // CHECK-objc-SAME: @_DATA__TtC16class_resilience14ResilientChild
 // CHECK-native-SAME: i32 0,
diff --git a/test/IRGen/subclass.swift b/test/IRGen/subclass.swift
index c611f8b841006..447317e74f567 100644
--- a/test/IRGen/subclass.swift
+++ b/test/IRGen/subclass.swift
@@ -20,7 +20,7 @@
 // CHECK-INDIRECT-SAME:   [[TYPE]]* null,
 // CHECK-SAME:   [[OPAQUE]]* @_objc_empty_cache,
 // CHECK-SAME:   [[OPAQUE]]* null,
-// CHECK-SAME:   i64 add (i64 ptrtoint ({ {{.*}} }* @_DATA__TtC8subclass1A to i64), i64 1),
+// CHECK-SAME:   i64 add (i64 ptrtoint ({ {{.*}} }* @_DATA__TtC8subclass1A to i64), i64 [[IS_SWIFT_BIT:1|2]]),
 // CHECK-SAME:   i64 ([[A]]*)* @"$s8subclass1AC1fSiyF",
 // CHECK-SAME:   [[A]]* ([[TYPE]]*)* @"$s8subclass1AC1gACyFZ"
 // CHECK-SAME: }>
@@ -34,7 +34,7 @@
 // CHECK-INDIRECT-SAME:   [[TYPE]]* null,
 // CHECK-SAME:   [[OPAQUE]]* @_objc_empty_cache,
 // CHECK-SAME:   [[OPAQUE]]* null,
-// CHECK-SAME:   i64 add (i64 ptrtoint ({ {{.*}} }* @_DATA__TtC8subclass1B to i64), i64 1),
+// CHECK-SAME:   i64 add (i64 ptrtoint ({ {{.*}} }* @_DATA__TtC8subclass1B to i64), i64 [[IS_SWIFT_BIT]]),
 // CHECK-SAME:   i64 ([[B]]*)* @"$s8subclass1BC1fSiyF",
 // CHECK-SAME:   [[A]]* ([[TYPE]]*)* @"$s8subclass1AC1gACyFZ"
 // CHECK-SAME: }>
diff --git a/test/Runtime/stable-bit-backward-deployment.swift b/test/Runtime/stable-bit-backward-deployment.swift
new file mode 100644
index 0000000000000..aa0e7b33dea43
--- /dev/null
+++ b/test/Runtime/stable-bit-backward-deployment.swift
@@ -0,0 +1,29 @@
+// RUN: %empty-directory(%t)
+// -- Deployment target is set to pre-10.14.4 so that we use the "old"
+//    Swift runtime bit in compiler-emitted classes
+// RUN: %target-build-swift -target x86_64-apple-macosx10.9 %s -module-name main -o %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s
+
+// REQUIRES: executable_test
+// REQUIRES: OS=macosx
+
+import Foundation
+
+// A fixed-layout class should be considered Swift metadata by the OS runtime.
+class FixedLayout { }
+
+debugPrint(FixedLayout.self) // CHECK: main.FixedLayout
+
+// A generic class
+class GenericBase<T> { }
+debugPrint(GenericBase<Int>.self) // CHECK-NEXT: main.GenericBase<Swift.Int>
+
+// A singleton-initialized class
+class SingletonInit: GenericBase<Int> { }
+debugPrint(SingletonInit.self) // CHECK-NEXT: main.SingletonInit
+
+// A resilient-heritage class
+class ResilientSubInit: JSONEncoder {}
+debugPrint(ResilientSubInit.self) // CHECK-NEXT: main.ResilientSubInit
+
+print("nailed it") // CHECK-NEXT: nailed it
diff --git a/tools/swift-remoteast-test/swift-remoteast-test.cpp b/tools/swift-remoteast-test/swift-remoteast-test.cpp
index 6d6f207fefb70..391c7b3edb98e 100644
--- a/tools/swift-remoteast-test/swift-remoteast-test.cpp
+++ b/tools/swift-remoteast-test/swift-remoteast-test.cpp
@@ -38,6 +38,23 @@ using namespace swift;
 using namespace swift::remote;
 using namespace swift::remoteAST;
 
+#if defined(__APPLE__) && defined(__MACH__)
+#include <dlfcn.h>
+static unsigned long long computeClassIsSwiftMask(void) {
+  uintptr_t *objc_debug_swift_stable_abi_bit_ptr =
+    (uintptr_t *)dlsym(RTLD_DEFAULT, "objc_debug_swift_stable_abi_bit");
+  return objc_debug_swift_stable_abi_bit_ptr ?
+           *objc_debug_swift_stable_abi_bit_ptr : 1;
+}
+#else
+static unsigned long long computeClassIsSwiftMask(void) {
+  return 1;
+}
+#endif
+
+extern "C" unsigned long long _swift_classIsSwiftMask =
+  computeClassIsSwiftMask();
+
 /// The context for the code we're running.  Set by the observer.
 static ASTContext *context = nullptr;