diff --git a/cmake/modules/AddSwift.cmake b/cmake/modules/AddSwift.cmake
index 64902335dd37e..fd6db69b0ddc0 100644
--- a/cmake/modules/AddSwift.cmake
+++ b/cmake/modules/AddSwift.cmake
@@ -886,7 +886,7 @@ function(_add_swift_library_single target name)
   elseif("${SWIFTLIB_SINGLE_SDK}" STREQUAL "CYGWIN")
     set_target_properties("${target}"
       PROPERTIES
-      INSTALL_RPATH "$ORIGIN:/usr/lib/swift/windows")
+      INSTALL_RPATH "$ORIGIN:/usr/lib/swift/cygwin")
   endif()
 
   set_target_properties("${target}" PROPERTIES BUILD_WITH_INSTALL_RPATH YES)
@@ -1256,6 +1256,9 @@ endfunction()
 # SWIFT_MODULE_DEPENDS_LINUX
 #   Swift modules this library depends on when built for Linux.
 #
+# SWIFT_MODULE_DEPENDS_CYGWIN
+#   Swift modules this library depends on when built for Cygwin.
+#
 # FRAMEWORK_DEPENDS
 #   System frameworks this library depends on.
 #
@@ -1329,7 +1332,7 @@ function(add_swift_library name)
   cmake_parse_arguments(SWIFTLIB
     "${SWIFTLIB_options}"
     "INSTALL_IN_COMPONENT;DEPLOYMENT_VERSION_OSX;DEPLOYMENT_VERSION_IOS;DEPLOYMENT_VERSION_TVOS;DEPLOYMENT_VERSION_WATCHOS"
-    "DEPENDS;LINK_LIBRARIES;SWIFT_MODULE_DEPENDS;SWIFT_MODULE_DEPENDS_OSX;SWIFT_MODULE_DEPENDS_IOS;SWIFT_MODULE_DEPENDS_TVOS;SWIFT_MODULE_DEPENDS_WATCHOS;SWIFT_MODULE_DEPENDS_FREEBSD;SWIFT_MODULE_DEPENDS_LINUX;FRAMEWORK_DEPENDS;FRAMEWORK_DEPENDS_WEAK;FRAMEWORK_DEPENDS_OSX;FRAMEWORK_DEPENDS_IOS_TVOS;LLVM_COMPONENT_DEPENDS;FILE_DEPENDS;TARGET_SDKS;C_COMPILE_FLAGS;SWIFT_COMPILE_FLAGS;SWIFT_COMPILE_FLAGS_OSX;SWIFT_COMPILE_FLAGS_IOS;SWIFT_COMPILE_FLAGS_TVOS;SWIFT_COMPILE_FLAGS_WATCHOS;LINK_FLAGS;PRIVATE_LINK_LIBRARIES;INTERFACE_LINK_LIBRARIES;INCORPORATE_OBJECT_LIBRARIES;INCORPORATE_OBJECT_LIBRARIES_SHARED_ONLY"
+    "DEPENDS;LINK_LIBRARIES;SWIFT_MODULE_DEPENDS;SWIFT_MODULE_DEPENDS_OSX;SWIFT_MODULE_DEPENDS_IOS;SWIFT_MODULE_DEPENDS_TVOS;SWIFT_MODULE_DEPENDS_WATCHOS;SWIFT_MODULE_DEPENDS_FREEBSD;SWIFT_MODULE_DEPENDS_LINUX;SWIFT_MODULE_DEPENDS_CYGWIN;FRAMEWORK_DEPENDS;FRAMEWORK_DEPENDS_WEAK;FRAMEWORK_DEPENDS_OSX;FRAMEWORK_DEPENDS_IOS_TVOS;LLVM_COMPONENT_DEPENDS;FILE_DEPENDS;TARGET_SDKS;C_COMPILE_FLAGS;SWIFT_COMPILE_FLAGS;SWIFT_COMPILE_FLAGS_OSX;SWIFT_COMPILE_FLAGS_IOS;SWIFT_COMPILE_FLAGS_TVOS;SWIFT_COMPILE_FLAGS_WATCHOS;LINK_FLAGS;PRIVATE_LINK_LIBRARIES;INTERFACE_LINK_LIBRARIES;INCORPORATE_OBJECT_LIBRARIES;INCORPORATE_OBJECT_LIBRARIES_SHARED_ONLY"
     ${ARGN})
   set(SWIFTLIB_SOURCES ${SWIFTLIB_UNPARSED_ARGUMENTS})
 
@@ -1462,6 +1465,9 @@ function(add_swift_library name)
         elseif("${sdk}" STREQUAL "LINUX" OR "${sdk}" STREQUAL "ANDROID")
           list(APPEND swiftlib_module_depends_flattened
               ${SWIFTLIB_SWIFT_MODULE_DEPENDS_LINUX})
+        elseif("${sdk}" STREQUAL "CYGWIN")
+          list(APPEND swiftlib_module_depends_flattened
+               ${SWIFTLIB_SWIFT_MODULE_DEPENDS_CYGWIN})
         endif()
 
         # Swift compiles depend on swift modules, while links depend on
diff --git a/stdlib/private/StdlibUnittest/CMakeLists.txt b/stdlib/private/StdlibUnittest/CMakeLists.txt
index e18d39fd8c330..fb9d19bb37883 100644
--- a/stdlib/private/StdlibUnittest/CMakeLists.txt
+++ b/stdlib/private/StdlibUnittest/CMakeLists.txt
@@ -38,6 +38,7 @@ add_swift_library(swiftStdlibUnittest ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STD
   SWIFT_MODULE_DEPENDS_WATCHOS Darwin Foundation
   SWIFT_MODULE_DEPENDS_LINUX Glibc
   SWIFT_MODULE_DEPENDS_FREEBSD Glibc
+  SWIFT_MODULE_DEPENDS_CYGWIN Glibc
   SWIFT_COMPILE_FLAGS ${swift_stdlib_unittest_compile_flags}
   TARGET_SDKS ALL_POSIX_PLATFORMS
   INSTALL_IN_COMPONENT stdlib-experimental)
diff --git a/stdlib/private/StdlibUnittest/RaceTest.swift b/stdlib/private/StdlibUnittest/RaceTest.swift
index ea07016f3c6d2..e53d7762c7161 100644
--- a/stdlib/private/StdlibUnittest/RaceTest.swift
+++ b/stdlib/private/StdlibUnittest/RaceTest.swift
@@ -41,7 +41,7 @@ import SwiftPrivateLibcExtras
 import SwiftPrivatePthreadExtras
 #if os(OSX) || os(iOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
 import Glibc
 #endif
 
diff --git a/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift b/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
index 784f075d2af4c..7307e81e9d724 100644
--- a/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
+++ b/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
@@ -14,7 +14,7 @@ import SwiftPrivate
 import SwiftPrivateLibcExtras
 #if os(OSX) || os(iOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
 import Glibc
 #endif
 
diff --git a/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb b/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb
index 46e61fb3de171..fa6c8cae31f37 100644
--- a/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb
+++ b/stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb
@@ -20,7 +20,7 @@ import SwiftPrivateLibcExtras
 
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
 import Glibc
 #endif
 
@@ -1429,6 +1429,8 @@ public enum OSVersion : CustomStringConvertible {
   case freeBSD
   case android
   case ps4
+  case windowsCygnus
+  case windows
 
   public var description: String {
     switch self {
@@ -1454,6 +1456,10 @@ public enum OSVersion : CustomStringConvertible {
       return "PS4"
     case .android:
       return "Android"
+    case .windowsCygnus:
+      return "Cygwin"
+    case .windows:
+      return "Windows"
     }
   }
 }
@@ -1492,6 +1498,10 @@ func _getOSVersion() -> OSVersion {
   return .ps4
 #elseif os(Android)
   return .android
+#elseif os(Windows) && CYGWIN
+  return .windowsCygnus
+#elseif os(Windows)
+  return .windows
 #else
   let productVersion = _stdlib_getSystemVersionPlistProperty("ProductVersion")!
   let (major, minor, bugFix) = _parseDottedVersionTriple(productVersion)
@@ -1570,6 +1580,10 @@ public enum TestRunPredicate : CustomStringConvertible {
 
   case androidAny(reason: String)
 
+  case windowsAny(reason: String)
+
+  case windowsCygnusAny(reason: String)
+
   case objCRuntime(/*reason:*/ String)
   case nativeRuntime(/*reason:*/ String)
 
@@ -1656,6 +1670,12 @@ public enum TestRunPredicate : CustomStringConvertible {
     case .ps4Any(reason: let reason):
       return "ps4Any(*, reason: \(reason))"
 
+    case .windowsAny(reason: let reason):
+      return "windowsAny(*, reason: \(reason))"
+
+    case .windowsCygnusAny(reason: let reason):
+      return "windowsCygnusAny(*, reason: \(reason))"
+
     case .objCRuntime(let reason):
       return "Objective-C runtime, reason: \(reason))"
     case .nativeRuntime(let reason):
@@ -1921,6 +1941,24 @@ public enum TestRunPredicate : CustomStringConvertible {
         return false
       }
 
+    case .windowsAny:
+      switch _getRunningOSVersion() {
+      case .windowsCygnus:
+        return true
+      case .windows:
+        return true
+      default:
+        return false
+      }
+
+    case .windowsCygnusAny:
+      switch _getRunningOSVersion() {
+      case .windowsCygnus:
+        return true
+      default:
+        return false
+      }
+
     case .objCRuntime:
 #if _runtime(_ObjC)
       return true
diff --git a/stdlib/private/SwiftPrivateLibcExtras/CMakeLists.txt b/stdlib/private/SwiftPrivateLibcExtras/CMakeLists.txt
index 842a2348db0a2..5381c5aa80641 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/CMakeLists.txt
+++ b/stdlib/private/SwiftPrivateLibcExtras/CMakeLists.txt
@@ -13,4 +13,5 @@ add_swift_library(swiftSwiftPrivateLibcExtras ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES
   SWIFT_MODULE_DEPENDS_WATCHOS Darwin
   SWIFT_MODULE_DEPENDS_LINUX Glibc
   SWIFT_MODULE_DEPENDS_FREEBSD Glibc
+  SWIFT_MODULE_DEPENDS_CYGWIN Glibc
   INSTALL_IN_COMPONENT stdlib-experimental)
diff --git a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
index 1b60d3ed49293..2e240ed400d9b 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
@@ -13,7 +13,7 @@
 import SwiftPrivate
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
 import Glibc
 #endif
 
@@ -264,9 +264,19 @@ public enum ProcessTerminationStatus : CustomStringConvertible {
 
 public func posixWaitpid(_ pid: pid_t) -> ProcessTerminationStatus {
   var status: CInt = 0
+#if CYGWIN
+  withUnsafeMutablePointer(to: &status) {
+    statusPtr in
+    let statusPtrWrapper = __wait_status_ptr_t(__int_ptr: statusPtr)
+    if waitpid(pid, statusPtrWrapper, 0) < 0 {
+      preconditionFailure("waitpid() failed")
+    }
+  }
+#else
   if waitpid(pid, &status, 0) < 0 {
     preconditionFailure("waitpid() failed")
   }
+#endif
   if WIFEXITED(status) {
     return .exit(Int(WEXITSTATUS(status)))
   }
@@ -290,6 +300,8 @@ internal func _getEnviron() -> UnsafeMutablePointer<UnsafeMutablePointer<CChar>?
   return environ
 #elseif os(Android)
   return environ
+#elseif CYGWIN
+  return environ
 #else
   return __environ
 #endif
diff --git a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
index 870305d3618c9..2850883eff664 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
@@ -99,6 +99,9 @@ public func _stdlib_select(
         let readAddr = readfds.baseAddress
         let writeAddr = writefds.baseAddress
         let errorAddr = errorfds.baseAddress
+#if CYGWIN
+        typealias fd_set = _types_fd_set
+#endif
         func asFdSetPtr(
           _ p: UnsafeMutablePointer<UInt>?
         ) -> UnsafeMutablePointer<fd_set>? {
diff --git a/stdlib/private/SwiftPrivatePthreadExtras/CMakeLists.txt b/stdlib/private/SwiftPrivatePthreadExtras/CMakeLists.txt
index 92ed8064db968..cf867d60287a3 100644
--- a/stdlib/private/SwiftPrivatePthreadExtras/CMakeLists.txt
+++ b/stdlib/private/SwiftPrivatePthreadExtras/CMakeLists.txt
@@ -10,6 +10,7 @@ add_swift_library(swiftSwiftPrivatePthreadExtras ${SWIFT_STDLIB_LIBRARY_BUILD_TY
   SWIFT_MODULE_DEPENDS_WATCHOS Darwin
   SWIFT_MODULE_DEPENDS_LINUX Glibc
   SWIFT_MODULE_DEPENDS_FREEBSD Glibc
+  SWIFT_MODULE_DEPENDS_CYGWIN Glibc
   SWIFT_COMPILE_FLAGS ${STDLIB_SIL_SERIALIZE_ALL}
   TARGET_SDKS ALL_POSIX_PLATFORMS
   INSTALL_IN_COMPONENT stdlib-experimental)
diff --git a/stdlib/private/SwiftPrivatePthreadExtras/PthreadBarriers.swift b/stdlib/private/SwiftPrivatePthreadExtras/PthreadBarriers.swift
index 987ade1013715..2ebe4b3accaf7 100644
--- a/stdlib/private/SwiftPrivatePthreadExtras/PthreadBarriers.swift
+++ b/stdlib/private/SwiftPrivatePthreadExtras/PthreadBarriers.swift
@@ -12,7 +12,7 @@
 
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
 import Glibc
 #endif
 
diff --git a/stdlib/private/SwiftPrivatePthreadExtras/SwiftPrivatePthreadExtras.swift b/stdlib/private/SwiftPrivatePthreadExtras/SwiftPrivatePthreadExtras.swift
index 62b7cc8e22f21..fe0c4f06586d7 100644
--- a/stdlib/private/SwiftPrivatePthreadExtras/SwiftPrivatePthreadExtras.swift
+++ b/stdlib/private/SwiftPrivatePthreadExtras/SwiftPrivatePthreadExtras.swift
@@ -17,7 +17,7 @@
 
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
 import Glibc
 #endif
 
diff --git a/stdlib/public/Platform/Platform.swift b/stdlib/public/Platform/Platform.swift
index 63e82afeb3883..1512aa04bb1f0 100644
--- a/stdlib/public/Platform/Platform.swift
+++ b/stdlib/public/Platform/Platform.swift
@@ -382,7 +382,7 @@ public var SIG_HOLD: sighandler_t {
 }
 #elseif os(Windows)
 #if CYGWIN
-public typealias sighandler_t = __sighandler_t
+public typealias sighandler_t = _sig_func_ptr
 
 public var SIG_DFL: sighandler_t? { return nil }
 public var SIG_IGN: sighandler_t {
@@ -417,16 +417,9 @@ public var SEM_FAILED: UnsafeMutablePointer<sem_t>? {
 #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
   // The value is ABI.  Value verified to be correct for OS X, iOS, watchOS, tvOS.
   return UnsafeMutablePointer<sem_t>(bitPattern: -1)
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
   // The value is ABI.  Value verified to be correct on Glibc.
   return UnsafeMutablePointer<sem_t>(bitPattern: 0)
-#elseif os(Windows)
-#if CYGWIN
-  // The value is ABI.  Value verified to be correct on Glibc.
-  return UnsafeMutablePointer<sem_t>(bitPattern: 0)
-#else
-  _UnsupportedPlatformError()
-#endif
 #else
   _UnsupportedPlatformError()
 #endif
diff --git a/stdlib/public/Platform/tgmath.swift.gyb b/stdlib/public/Platform/tgmath.swift.gyb
index ae688e01a878f..55be7b3863a1f 100644
--- a/stdlib/public/Platform/tgmath.swift.gyb
+++ b/stdlib/public/Platform/tgmath.swift.gyb
@@ -215,22 +215,14 @@ public func fpclassify(_ value: ${T}) -> Int {
 %if T == 'Double':
 #if os(Linux)
   return Int(__fpclassify(CDouble(value)))
-#elseif os(Windows)
-#if CYGWIN
-  return Int(__fpclassify(CDouble(value)))
-#else
+#elseif os(Windows) && !CYGWIN
   return Int(_dclass(CDouble(value)))
-#endif
 #else
   return Int(__fpclassifyd(CDouble(value)))
 #endif
 %else:
-#if os(Windows)
-#if CYGWIN
-  return Int(__fpclassify${f}(${CT}(value)))
-#else
+#if os(Windows) && !CYGWIN
   return Int(_${f}dclass(${CT}(value)))
-#endif
 #else
   return Int(__fpclassify${f}(${CT}(value)))
 #endif
@@ -305,7 +297,7 @@ public func scalbn(_ x: ${T}, _ n: Int) -> ${T} {
 
 % # This is AllFloatTypes not OverlayFloatTypes because of the tuple return.
 % for T, CT, f in AllFloatTypes():
-#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || CYGWIN
 @_transparent
 public func lgamma(_ x: ${T}) -> (${T}, Int) {
   var sign = Int32(0)
diff --git a/stdlib/public/core/CTypes.swift b/stdlib/public/core/CTypes.swift
index fc1351b280667..8ae4b6110c0b1 100644
--- a/stdlib/public/core/CTypes.swift
+++ b/stdlib/public/core/CTypes.swift
@@ -43,10 +43,18 @@ public typealias CShort = Int16
 public typealias CInt = Int32
 
 /// The C 'long' type.
+#if os(Windows) && !CYGWIN && arch(x86_64)
+public typealias CLong = Int32
+#else
 public typealias CLong = Int
+#endif
 
 /// The C 'long long' type.
+#if os(Windows) && !CYGWIN && arch(x86_64)
+public typealias CLongLong = Int
+#else
 public typealias CLongLong = Int64
+#endif
 
 /// The C 'float' type.
 public typealias CFloat = Float
diff --git a/stdlib/public/core/FloatingPoint.swift.gyb b/stdlib/public/core/FloatingPoint.swift.gyb
index 446b457fb229e..2e617190d242c 100644
--- a/stdlib/public/core/FloatingPoint.swift.gyb
+++ b/stdlib/public/core/FloatingPoint.swift.gyb
@@ -1577,7 +1577,7 @@ public protocol BinaryFloatingPoint: FloatingPoint, ExpressibleByFloatLiteral {
   /// - Parameter value: A floating-point value.
   init(_ value: Double)
 
-#if !os(Windows) && (arch(i386) || arch(x86_64))
+#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
   /// Creates a new instance from the given value, rounded to the closest
   /// possible representation.
   ///
diff --git a/stdlib/public/core/FloatingPointParsing.swift.gyb b/stdlib/public/core/FloatingPointParsing.swift.gyb
index b64710810b277..1fe02dd4f9317 100644
--- a/stdlib/public/core/FloatingPointParsing.swift.gyb
+++ b/stdlib/public/core/FloatingPointParsing.swift.gyb
@@ -38,7 +38,7 @@ internal func _isspace_clocale(_ u: UTF16.CodeUnit) -> Bool {
 %   Self = floatName(bits)
 
 % if bits == 80:
-#if !os(Windows) && (arch(i386) || arch(x86_64))
+#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
 % end
 
 //===--- Parsing ----------------------------------------------------------===//
diff --git a/stdlib/public/core/FloatingPointTypes.swift.gyb b/stdlib/public/core/FloatingPointTypes.swift.gyb
index 395e58e44b8d2..bb226614ceb78 100644
--- a/stdlib/public/core/FloatingPointTypes.swift.gyb
+++ b/stdlib/public/core/FloatingPointTypes.swift.gyb
@@ -66,7 +66,7 @@ else:
 }%
 
 % if bits == 80:
-#if !os(Windows) && (arch(i386) || arch(x86_64))
+#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
 % end
 
 ${SelfDocComment}
@@ -706,7 +706,7 @@ extension ${Self} : _ExpressibleByBuiltinIntegerLiteral, ExpressibleByIntegerLit
   }
 }
 
-#if !os(Windows) && (arch(i386) || arch(x86_64))
+#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
 
 % builtinFloatLiteralBits = 80
 extension ${Self} : _ExpressibleByBuiltinFloatLiteral {
@@ -851,7 +851,7 @@ extension ${Self} {
 %   That = src_type.stdlib_name
 
 %   if srcBits == 80:
-#if !os(Windows) && (arch(i386) || arch(x86_64))
+#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
 %   end
 
 %   if srcBits == bits:
diff --git a/stdlib/public/core/Policy.swift b/stdlib/public/core/Policy.swift
index cdf9a896ec53b..992de1d2545f8 100644
--- a/stdlib/public/core/Policy.swift
+++ b/stdlib/public/core/Policy.swift
@@ -104,7 +104,7 @@ public typealias StringLiteralType = String
 // IEEE Binary64, and we need 1 bit to represent the sign.  Instead of using
 // 1025, we use the next round number -- 2048.
 public typealias _MaxBuiltinIntegerType = Builtin.Int2048
-#if !os(Windows) && (arch(i386) || arch(x86_64))
+#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
 public typealias _MaxBuiltinFloatType = Builtin.FPIEEE80
 #else
 public typealias _MaxBuiltinFloatType = Builtin.FPIEEE64
diff --git a/stdlib/public/core/Runtime.swift.gyb b/stdlib/public/core/Runtime.swift.gyb
index fb01bdecec553..1d11ee489987c 100644
--- a/stdlib/public/core/Runtime.swift.gyb
+++ b/stdlib/public/core/Runtime.swift.gyb
@@ -348,7 +348,7 @@ internal struct _Buffer72 {
 % for bits in [ 32, 64, 80 ]:
 
 % if bits == 80:
-#if !os(Windows) && (arch(i386) || arch(x86_64))
+#if (!os(Windows) || CYGWIN) && (arch(i386) || arch(x86_64))
 % end
 
 @_silgen_name("swift_float${bits}ToString")
diff --git a/stdlib/public/runtime/MutexWin32.cpp b/stdlib/public/runtime/MutexWin32.cpp
index 6f195dfc57be4..0bcc5005fd27d 100644
--- a/stdlib/public/runtime/MutexWin32.cpp
+++ b/stdlib/public/runtime/MutexWin32.cpp
@@ -15,7 +15,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if defined(_WIN32) || defined(__CYGWIN__)
+#if defined(_WIN32)
 #include "swift/Runtime/Mutex.h"
 #include "swift/Runtime/Debug.h"
 
diff --git a/validation-test/StdlibUnittest/ChildProcessShutdown/FailIfChildCrashesDuringShutdown.swift b/validation-test/StdlibUnittest/ChildProcessShutdown/FailIfChildCrashesDuringShutdown.swift
index 7d9ba05a633aa..1589952c84102 100644
--- a/validation-test/StdlibUnittest/ChildProcessShutdown/FailIfChildCrashesDuringShutdown.swift
+++ b/validation-test/StdlibUnittest/ChildProcessShutdown/FailIfChildCrashesDuringShutdown.swift
@@ -2,7 +2,7 @@
 // REQUIRES: executable_test
 
 import StdlibUnittest
-#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Windows)
 import Glibc
 #else
 import Darwin
diff --git a/validation-test/StdlibUnittest/ChildProcessShutdown/FailIfChildExitsDuringShutdown.swift b/validation-test/StdlibUnittest/ChildProcessShutdown/FailIfChildExitsDuringShutdown.swift
index 25334f2a39b31..77339259b14b1 100644
--- a/validation-test/StdlibUnittest/ChildProcessShutdown/FailIfChildExitsDuringShutdown.swift
+++ b/validation-test/StdlibUnittest/ChildProcessShutdown/FailIfChildExitsDuringShutdown.swift
@@ -2,7 +2,7 @@
 // REQUIRES: executable_test
 
 import StdlibUnittest
-#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Windows)
 import Glibc
 #else
 import Darwin
diff --git a/validation-test/StdlibUnittest/ChildProcessShutdown/PassIfChildCrashedDuringTestExecution.swift b/validation-test/StdlibUnittest/ChildProcessShutdown/PassIfChildCrashedDuringTestExecution.swift
index 5f719e6a2063c..567a38b5df9d6 100644
--- a/validation-test/StdlibUnittest/ChildProcessShutdown/PassIfChildCrashedDuringTestExecution.swift
+++ b/validation-test/StdlibUnittest/ChildProcessShutdown/PassIfChildCrashedDuringTestExecution.swift
@@ -2,7 +2,7 @@
 // REQUIRES: executable_test
 
 import StdlibUnittest
-#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Windows)
 import Glibc
 #else
 import Darwin
diff --git a/validation-test/StdlibUnittest/Cygwin.swift b/validation-test/StdlibUnittest/Cygwin.swift
new file mode 100644
index 0000000000000..74786f0c195ab
--- /dev/null
+++ b/validation-test/StdlibUnittest/Cygwin.swift
@@ -0,0 +1,36 @@
+// RUN: %target-run-stdlib-swift | %FileCheck %s
+// REQUIRES: executable_test
+
+import Swift
+import StdlibUnittest
+
+_setOverrideOSVersion(.windowsCygnus)
+_setTestSuiteFailedCallback() { print("abort()") }
+
+var XFailsWindows = TestSuite("XFailsCygwin")
+
+// CHECK: [   UXPASS ] XFailsCygwin.xfail Cygwin passes{{$}}
+XFailsWindows.test("xfail Cygwin passes").xfail(.windowsCygnusAny(reason: "")).code {
+  expectEqual(1, 1)
+}
+
+// CHECK: [    XFAIL ] XFailsCygwin.xfail Cygwin fails{{$}}
+XFailsWindows.test("xfail Cygwin fails").xfail(.windowsCygnusAny(reason: "")).code {
+  expectEqual(1, 2)
+}
+
+// CHECK: [   UXPASS ] XFailsCygwin.xfail Windows passes{{$}}
+XFailsWindows.test("xfail Windows passes").xfail(.windowsAny(reason: "")).code {
+  expectEqual(1, 1)
+}
+
+// CHECK: [    XFAIL ] XFailsCygwin.xfail Windows fails{{$}}
+XFailsWindows.test("xfail Windows fails").xfail(.windowsAny(reason: "")).code {
+  expectEqual(1, 2)
+}
+
+// CHECK: XFailsCygwin: Some tests failed, aborting
+// CHECK: abort()
+
+runAllTests()
+
diff --git a/validation-test/StdlibUnittest/Stdin.swift b/validation-test/StdlibUnittest/Stdin.swift
index 253f1bb14b77d..e0c282edb7cf1 100644
--- a/validation-test/StdlibUnittest/Stdin.swift
+++ b/validation-test/StdlibUnittest/Stdin.swift
@@ -6,7 +6,7 @@ import StdlibUnittest
 
 #if os(OSX) || os(iOS) || os(tvOS) || os(watchOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Windows)
 import Glibc
 #endif