diff --git a/CMakeLists.txt b/CMakeLists.txt
index 783af079c767b..a291359b9c517 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -439,6 +439,8 @@ else()
     set(SWIFT_HOST_VARIANT_ARCH_default "powerpc64")
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64le")
     set(SWIFT_HOST_VARIANT_ARCH_default "powerpc64le")
+  elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "s390x")
+    set(SWIFT_HOST_VARIANT_ARCH_default "s390x")
   # FIXME: Only matches v6l/v7l - by far the most common variants
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "armv6l")
     set(SWIFT_HOST_VARIANT_ARCH_default "armv6")
@@ -507,6 +509,8 @@ if("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "LINUX")
       set(SWIFT_HOST_TRIPLE "aarch64-unknown-linux-gnu")
     elseif("${SWIFT_HOST_VARIANT_ARCH}" MATCHES "(powerpc64|powerpc64le)")
       set(SWIFT_HOST_TRIPLE "${SWIFT_HOST_VARIANT_ARCH}-unknown-linux-gnu")
+    elseif("${SWIFT_HOST_VARIANT_ARCH}" MATCHES "s390x")
+      set(SWIFT_HOST_TRIPLE "s390x-unknown-linux-gnu")
     elseif("${SWIFT_HOST_VARIANT_ARCH}" MATCHES "(armv6|armv7)")
       set(SWIFT_HOST_TRIPLE "${SWIFT_HOST_VARIANT_ARCH}-unknown-linux-gnueabihf")
     else()
diff --git a/cmake/modules/SwiftSetIfArchBitness.cmake b/cmake/modules/SwiftSetIfArchBitness.cmake
index 5dc03cc89a6cc..1849ee0d884f4 100644
--- a/cmake/modules/SwiftSetIfArchBitness.cmake
+++ b/cmake/modules/SwiftSetIfArchBitness.cmake
@@ -16,7 +16,8 @@ function(set_if_arch_bitness var_name)
          "${SIA_ARCH}" STREQUAL "arm64" OR
          "${SIA_ARCH}" STREQUAL "aarch64" OR
          "${SIA_ARCH}" STREQUAL "powerpc64" OR
-         "${SIA_ARCH}" STREQUAL "powerpc64le")
+         "${SIA_ARCH}" STREQUAL "powerpc64le" OR
+         "${SIA_ARCH}" STREQUAL "s390x")
     set("${var_name}" "${SIA_CASE_64_BIT}" PARENT_SCOPE)
   else()
     message(FATAL_ERROR "Unknown architecture: ${SIA_ARCH}")
diff --git a/include/swift/ABI/System.h b/include/swift/ABI/System.h
index f681b96b331bd..0753a0cbcd453 100644
--- a/include/swift/ABI/System.h
+++ b/include/swift/ABI/System.h
@@ -97,4 +97,9 @@
 // Heap objects are pointer-aligned, so the low three bits are unused.
 #define SWIFT_ABI_POWERPC64_SWIFT_SPARE_BITS_MASK 0x0000000000000007ULL
 
+/*********************************** s390x ************************************/
+
+// Top byte of pointers is unused, and heap objects are eight-byte aligned.
+#define SWIFT_ABI_S390X_SWIFT_SPARE_BITS_MASK 0x0000000000000007ULL
+
 #endif /* SWIFT_ABI_SYSTEM_H */
diff --git a/include/swift/Basic/LangOptions.h b/include/swift/Basic/LangOptions.h
index e06b89143bd03..b027f675240eb 100644
--- a/include/swift/Basic/LangOptions.h
+++ b/include/swift/Basic/LangOptions.h
@@ -233,6 +233,10 @@ namespace swift {
     /// a supported target architecture.
     static bool isPlatformConditionArchSupported(StringRef ArchName);
 
+    /// Returns true if the 'endian' platform condition argument represents
+    /// a supported target endianness.
+    static bool isPlatformConditionEndiannessSupported(StringRef endianness);
+
   private:
     llvm::SmallVector<std::pair<std::string, std::string>, 3>
         PlatformConditionValues;
diff --git a/include/swift/Runtime/Config.h b/include/swift/Runtime/Config.h
index 3c22ecad8b4e2..187c1e5717175 100644
--- a/include/swift/Runtime/Config.h
+++ b/include/swift/Runtime/Config.h
@@ -33,8 +33,12 @@
 /// Does the current Swift platform use LLVM's intrinsic "swiftcall"
 /// calling convention for Swift functions?
 #ifndef SWIFT_USE_SWIFTCALL
+#ifdef __s390x__
+#define SWIFT_USE_SWIFTCALL 1
+#else
 #define SWIFT_USE_SWIFTCALL 0
 #endif
+#endif
 
 /// Does the current Swift platform allow information other than the
 /// class pointer to be stored in the isa field?  If so, when deriving
diff --git a/include/swift/Runtime/HeapObject.h b/include/swift/Runtime/HeapObject.h
index 96b489e326a7a..e052feeccca28 100644
--- a/include/swift/Runtime/HeapObject.h
+++ b/include/swift/Runtime/HeapObject.h
@@ -157,10 +157,12 @@ using BoxPair = TwoWordPair<HeapObject *, OpaqueValue *>;
 /// The heap object has an initial retain count of 1, and its metadata is set
 /// such that destroying the heap object destroys the contained value.
 SWIFT_RUNTIME_EXPORT
-extern "C" BoxPair::Return swift_allocBox(Metadata const *type);
+extern "C" BoxPair::Return swift_allocBox(Metadata const *type)
+           SWIFT_CC(swift);
 
 SWIFT_RUNTIME_EXPORT
-extern "C" BoxPair::Return (*_swift_allocBox)(Metadata const *type);
+extern "C" BoxPair::Return (*_swift_allocBox)(Metadata const *type)
+           SWIFT_CC(swift);
 
 
 // Allocate plain old memory. This is the generalized entry point
diff --git a/include/swift/Runtime/Metadata.h b/include/swift/Runtime/Metadata.h
index 077690563643d..1dd5998d62847 100644
--- a/include/swift/Runtime/Metadata.h
+++ b/include/swift/Runtime/Metadata.h
@@ -1046,6 +1046,17 @@ static const uintptr_t ObjCReservedBitsMask =
 static const unsigned ObjCReservedLowBits =
   SWIFT_ABI_DEFAULT_OBJC_NUM_RESERVED_LOW_BITS;
 
+#elif defined(__s390x__)
+
+static const uintptr_t LeastValidPointerValue =
+  SWIFT_ABI_DEFAULT_LEAST_VALID_POINTER;
+static const uintptr_t SwiftSpareBitsMask =
+  SWIFT_ABI_S390X_SWIFT_SPARE_BITS_MASK;
+static const uintptr_t ObjCReservedBitsMask =
+  SWIFT_ABI_DEFAULT_OBJC_RESERVED_BITS_MASK;
+static const unsigned ObjCReservedLowBits =
+  SWIFT_ABI_DEFAULT_OBJC_NUM_RESERVED_LOW_BITS;
+
 #else
 
 static const uintptr_t LeastValidPointerValue =
diff --git a/lib/Basic/LangOptions.cpp b/lib/Basic/LangOptions.cpp
index 2d157d91a201c..eee99069e40ee 100644
--- a/lib/Basic/LangOptions.cpp
+++ b/lib/Basic/LangOptions.cpp
@@ -40,7 +40,13 @@ static const StringRef SupportedConditionalCompilationArches[] = {
   "i386",
   "x86_64",
   "powerpc64",
-  "powerpc64le"
+  "powerpc64le",
+  "s390x"
+};
+
+static const StringRef SupportedConditionalCompilationEndianness[] = {
+  "little",
+  "big"
 };
 
 bool LangOptions::isPlatformConditionOSSupported(StringRef OSName) {
@@ -58,6 +64,14 @@ LangOptions::isPlatformConditionArchSupported(StringRef ArchName) {
   return foundIt != std::end(SupportedConditionalCompilationArches);
 }
 
+bool
+LangOptions::isPlatformConditionEndiannessSupported(StringRef Endianness) {
+  auto foundIt = std::find(std::begin(SupportedConditionalCompilationEndianness),
+                           std::end(SupportedConditionalCompilationEndianness),
+                           Endianness);
+  return foundIt != std::end(SupportedConditionalCompilationEndianness);
+}
+
 StringRef
 LangOptions::getPlatformConditionValue(StringRef Name) const {
   // Last one wins.
@@ -141,6 +155,9 @@ std::pair<bool, bool> LangOptions::setTarget(llvm::Triple triple) {
   case llvm::Triple::ArchType::x86_64:
     addPlatformConditionValue("arch", "x86_64");
     break;
+  case llvm::Triple::ArchType::systemz:
+    addPlatformConditionValue("arch", "s390x");
+    break;
   default:
     UnsupportedArch = true;
   }
@@ -148,6 +165,34 @@ std::pair<bool, bool> LangOptions::setTarget(llvm::Triple triple) {
   if (UnsupportedOS || UnsupportedArch)
     return { UnsupportedOS, UnsupportedArch };
 
+  // Set the "_endian" platform condition.
+  switch (Target.getArch()) {
+  case llvm::Triple::ArchType::arm:
+  case llvm::Triple::ArchType::thumb:
+    addPlatformConditionValue("_endian", "little");
+    break;
+  case llvm::Triple::ArchType::aarch64:
+    addPlatformConditionValue("_endian", "little");
+    break;
+  case llvm::Triple::ArchType::ppc64:
+    addPlatformConditionValue("_endian", "big");
+    break;
+  case llvm::Triple::ArchType::ppc64le:
+    addPlatformConditionValue("_endian", "little");
+    break;
+  case llvm::Triple::ArchType::x86:
+    addPlatformConditionValue("_endian", "little");
+    break;
+  case llvm::Triple::ArchType::x86_64:
+    addPlatformConditionValue("_endian", "little");
+    break;
+  case llvm::Triple::ArchType::systemz:
+    addPlatformConditionValue("_endian", "big");
+    break;
+  default:
+    llvm_unreachable("undefined architecture endianness");
+  }
+
   // Set the "runtime" platform condition.
   if (EnableObjCInterop)
     addPlatformConditionValue("_runtime", "_ObjC");
diff --git a/lib/IRGen/GenMeta.cpp b/lib/IRGen/GenMeta.cpp
index fc635d3b757bf..f094f2e2af4cc 100644
--- a/lib/IRGen/GenMeta.cpp
+++ b/lib/IRGen/GenMeta.cpp
@@ -4338,16 +4338,20 @@ irgen::emitClassResilientInstanceSizeAndAlignMask(IRGenFunction &IGF,
          && "didn't find size or alignment in metadata?!");
   Address metadataAsBytes(IGF.Builder.CreateBitCast(metadata, IGF.IGM.Int8PtrTy),
                           IGF.IGM.getPointerAlignment());
-  auto loadZExtInt32AtOffset = [&](Size offset) {
-    Address slot = IGF.Builder.CreateConstByteArrayGEP(metadataAsBytes, offset);
-    slot = IGF.Builder.CreateBitCast(slot, IGF.IGM.Int32Ty->getPointerTo());
-    llvm::Value *result = IGF.Builder.CreateLoad(slot);
-    if (IGF.IGM.SizeTy != IGF.IGM.Int32Ty)
-      result = IGF.Builder.CreateZExt(result, IGF.IGM.SizeTy);
-    return result;
-  };
-  llvm::Value *size = loadZExtInt32AtOffset(scanner.InstanceSize);
-  llvm::Value *alignMask = loadZExtInt32AtOffset(scanner.InstanceAlignMask);
+
+  Address slot = IGF.Builder.CreateConstByteArrayGEP(metadataAsBytes,
+                                                     scanner.InstanceSize);
+  slot = IGF.Builder.CreateBitCast(slot, IGF.IGM.Int32Ty->getPointerTo());
+  llvm::Value *size = IGF.Builder.CreateLoad(slot);
+  if (IGF.IGM.SizeTy != IGF.IGM.Int32Ty)
+    size = IGF.Builder.CreateZExt(size, IGF.IGM.SizeTy);
+
+  slot = IGF.Builder.CreateConstByteArrayGEP(metadataAsBytes,
+                                             scanner.InstanceAlignMask);
+  slot = IGF.Builder.CreateBitCast(slot, IGF.IGM.Int16Ty->getPointerTo());
+  llvm::Value *alignMask = IGF.Builder.CreateLoad(slot);
+  alignMask = IGF.Builder.CreateZExt(alignMask, IGF.IGM.SizeTy);
+
   return {size, alignMask};
 }
 
diff --git a/lib/IRGen/SwiftTargetInfo.cpp b/lib/IRGen/SwiftTargetInfo.cpp
index 9e033737db62b..1c587e0af4092 100644
--- a/lib/IRGen/SwiftTargetInfo.cpp
+++ b/lib/IRGen/SwiftTargetInfo.cpp
@@ -103,6 +103,13 @@ static void configurePowerPC64(IRGenModule &IGM, const llvm::Triple &triple,
             SWIFT_ABI_POWERPC64_SWIFT_SPARE_BITS_MASK);
 }
 
+/// Configures target-specific information for SystemZ platforms.
+static void configureSystemZ(IRGenModule &IGM, const llvm::Triple &triple,
+                             SwiftTargetInfo &target) {
+  setToMask(target.PointerSpareBits, 64,
+            SWIFT_ABI_S390X_SWIFT_SPARE_BITS_MASK);
+}
+
 /// Configure a default target.
 SwiftTargetInfo::SwiftTargetInfo(
   llvm::Triple::ObjectFormatType outputObjectFormat,
@@ -154,6 +161,10 @@ SwiftTargetInfo SwiftTargetInfo::get(IRGenModule &IGM) {
     configurePowerPC64(IGM, triple, target);
     break;
 
+  case llvm::Triple::systemz:
+    configureSystemZ(IGM, triple, target);
+    break;
+
   default:
     // FIXME: Complain here? Default target info is unlikely to be correct.
     break;
diff --git a/lib/Parse/ParseStmt.cpp b/lib/Parse/ParseStmt.cpp
index b570d03f5855e..921cefbf82984 100644
--- a/lib/Parse/ParseStmt.cpp
+++ b/lib/Parse/ParseStmt.cpp
@@ -1608,7 +1608,7 @@ ParserResult<Stmt> Parser::parseStmtGuard() {
 //    "_compiler_version"), and whose argument is a named decl ref expression
 ConditionalCompilationExprState
 Parser::evaluateConditionalCompilationExpr(Expr *condition) {
-    // Evaluate a ParenExpr.
+  // Evaluate a ParenExpr.
   if (auto *PE = dyn_cast<ParenExpr>(condition))
     return evaluateConditionalCompilationExpr(PE->getSubExpr());
   
@@ -1709,6 +1709,7 @@ Parser::evaluateConditionalCompilationExpr(Expr *condition) {
     }
 
     if (!fnName.equals("arch") && !fnName.equals("os") &&
+        !fnName.equals("_endian") &&
         !fnName.equals("_runtime") &&
         !fnName.equals("swift") &&
         !fnName.equals("_compiler_version")) {
@@ -1799,6 +1800,11 @@ Parser::evaluateConditionalCompilationExpr(Expr *condition) {
                      "architecture", fnName);
             return ConditionalCompilationExprState::error();
           }
+        } else if (fnName == "_endian") {
+          if (!LangOptions::isPlatformConditionEndiannessSupported(argument)) {
+            diagnose(UDRE->getLoc(), diag::unknown_platform_condition_argument,
+                     "endianness", fnName);
+          }
         }
         auto target = Context.LangOpts.getPlatformConditionValue(fnName);
         return {target == argument, ConditionalCompilationExprKind::DeclRef};
diff --git a/stdlib/private/SwiftPrivate/PRNG.swift b/stdlib/private/SwiftPrivate/PRNG.swift
index 335c9166c6975..f5a2276f25cc8 100644
--- a/stdlib/private/SwiftPrivate/PRNG.swift
+++ b/stdlib/private/SwiftPrivate/PRNG.swift
@@ -29,7 +29,7 @@ public func rand64() -> UInt64 {
 public func randInt() -> Int {
 #if arch(i386) || arch(arm)
   return Int(Int32(bitPattern: rand32()))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   return Int(Int64(bitPattern: rand64()))
 #else
   fatalError("unimplemented")
diff --git a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
index af20d7fbf65b5..52d6eccd2d2b3 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
@@ -38,13 +38,13 @@ public var _stdlib_FD_SETSIZE: CInt {
 }
 
 public struct _stdlib_fd_set {
-  var _data: [UInt32]
+  var _data: [UInt]
   static var _wordBits: Int {
-    return sizeof(UInt32) * 8
+    return sizeof(UInt) * 8
   }
 
   public init() {
-    _data = [UInt32](
+    _data = [UInt](
       repeating: 0,
       count: Int(_stdlib_FD_SETSIZE) / _stdlib_fd_set._wordBits)
   }
@@ -53,20 +53,20 @@ public struct _stdlib_fd_set {
     let fdInt = Int(fd)
     return (
         _data[fdInt / _stdlib_fd_set._wordBits] &
-          UInt32(1 << (fdInt % _stdlib_fd_set._wordBits))
+          UInt(1 << (fdInt % _stdlib_fd_set._wordBits))
       ) != 0
   }
 
   public mutating func set(_ fd: CInt) {
     let fdInt = Int(fd)
     _data[fdInt / _stdlib_fd_set._wordBits] |=
-      UInt32(1 << (fdInt % _stdlib_fd_set._wordBits))
+      UInt(1 << (fdInt % _stdlib_fd_set._wordBits))
   }
 
   public mutating func clear(_ fd: CInt) {
     let fdInt = Int(fd)
     _data[fdInt / _stdlib_fd_set._wordBits] &=
-      ~UInt32(1 << (fdInt % _stdlib_fd_set._wordBits))
+      ~UInt(1 << (fdInt % _stdlib_fd_set._wordBits))
   }
 
   public mutating func zero() {
diff --git a/stdlib/public/core/Builtin.swift b/stdlib/public/core/Builtin.swift
index 000952169d283..2802b6aca2639 100644
--- a/stdlib/public/core/Builtin.swift
+++ b/stdlib/public/core/Builtin.swift
@@ -394,6 +394,19 @@ internal var _objectPointerLowSpareBitShift: UInt {
 internal var _objCTaggedPointerBits: UInt {
     @inline(__always) get { return 0 }
 }
+#elseif arch(s390x)
+internal var _objectPointerSpareBits: UInt {
+  @inline(__always) get { return 0x0000_0000_0000_0007 }
+}
+internal var _objectPointerIsObjCBit: UInt {
+  @inline(__always) get { return 0x0000_0000_0000_0002 }
+}
+internal var _objectPointerLowSpareBitShift: UInt {
+  @inline(__always) get { return 0 }
+}
+internal var _objCTaggedPointerBits: UInt {
+  @inline(__always) get { return 0 }
+}
 #endif
 
 /// Extract the raw bits of `x`.
diff --git a/stdlib/public/core/FixedPoint.swift.gyb b/stdlib/public/core/FixedPoint.swift.gyb
index 7c76ad654ee60..61d5a09f881b0 100644
--- a/stdlib/public/core/FixedPoint.swift.gyb
+++ b/stdlib/public/core/FixedPoint.swift.gyb
@@ -265,12 +265,10 @@ public struct ${Self}
   /// byte order if necessary.
   @_transparent public
   init(bigEndian value: ${Self}) {
-#if arch(i386) || arch(x86_64) || arch(arm) || arch(arm64) || arch(powerpc64le)
-    self = ${Self}(Builtin.int_bswap_${BuiltinName}(value._value) )
-#elseif arch(powerpc64)
+#if _endian(big)
     self = value
 #else
-    _UnsupportedArchitectureError()
+    self = ${Self}(Builtin.int_bswap_${BuiltinName}(value._value) )
 #endif
   }
 
@@ -278,12 +276,10 @@ public struct ${Self}
   /// byte order if necessary.
   @_transparent public
   init(littleEndian value: ${Self}) {
-#if arch(i386) || arch(x86_64) || arch(arm) || arch(arm64) || arch(powerpc64le)
+#if _endian(little)
     self = value
-#elseif arch(powerpc64)
-    self = ${Self}(Builtin.int_bswap_${BuiltinName}(value._value) )
 #else
-    _UnsupportedArchitectureError()
+    self = ${Self}(Builtin.int_bswap_${BuiltinName}(value._value) )
 #endif
   }
 %  end
@@ -303,23 +299,19 @@ public struct ${Self}
   /// Returns the big-endian representation of the integer, changing the
   /// byte order if necessary.
   public var bigEndian: ${Self} {
-#if arch(i386) || arch(x86_64) || arch(arm) || arch(arm64) || arch(powerpc64le)
-    return ${Self}(Builtin.int_bswap_${BuiltinName}(_value))
-#elseif arch(powerpc64)
+#if _endian(big)
     return self
 #else
-    _UnsupportedArchitectureError()
+    return ${Self}(Builtin.int_bswap_${BuiltinName}(_value))
 #endif
   }
   /// Returns the little-endian representation of the integer, changing the
   /// byte order if necessary.
   public var littleEndian: ${Self} {
-#if arch(i386) || arch(x86_64) || arch(arm) || arch(arm64) || arch(powerpc64le)
+#if _endian(little)
     return self
-#elseif arch(powerpc64)
-    return ${Self}(Builtin.int_bswap_${BuiltinName}(_value))
 #else
-    _UnsupportedArchitectureError()
+    return ${Self}(Builtin.int_bswap_${BuiltinName}(_value))
 #endif
   }
 % end
diff --git a/stdlib/public/core/Hashing.swift b/stdlib/public/core/Hashing.swift
index 94f1d3286a28b..c5b63f95bd955 100644
--- a/stdlib/public/core/Hashing.swift
+++ b/stdlib/public/core/Hashing.swift
@@ -111,7 +111,7 @@ public // @testable
 func _mixUInt(_ value: UInt) -> UInt {
 #if arch(i386) || arch(arm)
   return UInt(_mixUInt32(UInt32(value)))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   return UInt(_mixUInt64(UInt64(value)))
 #endif
 }
@@ -121,7 +121,7 @@ public // @testable
 func _mixInt(_ value: Int) -> Int {
 #if arch(i386) || arch(arm)
   return Int(_mixInt32(Int32(value)))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   return Int(_mixInt64(Int64(value)))
 #endif
 }
diff --git a/stdlib/public/core/Runtime.swift.gyb b/stdlib/public/core/Runtime.swift.gyb
index 39ce694988329..1f91d1d3fc589 100644
--- a/stdlib/public/core/Runtime.swift.gyb
+++ b/stdlib/public/core/Runtime.swift.gyb
@@ -190,7 +190,7 @@ func _stdlib_atomicCompareExchangeStrongInt(
     object: UnsafeMutablePointer(target),
     expected: UnsafeMutablePointer(expected),
     desired: UInt32(bitPattern: Int32(desired)))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   return _stdlib_atomicCompareExchangeStrongUInt64(
     object: UnsafeMutablePointer(target),
     expected: UnsafeMutablePointer(expected),
@@ -205,7 +205,7 @@ func _swift_stdlib_atomicStoreInt(
   return _swift_stdlib_atomicStoreUInt32(
     object: UnsafeMutablePointer(target),
     desired: UInt32(bitPattern: Int32(desired)))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   return _swift_stdlib_atomicStoreUInt64(
     object: UnsafeMutablePointer(target),
     desired: UInt64(bitPattern: Int64(desired)))
@@ -219,7 +219,7 @@ public func _swift_stdlib_atomicLoadInt(
   return Int(Int32(bitPattern:
     _swift_stdlib_atomicLoadUInt32(
       object: UnsafeMutablePointer(target))))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   return Int(Int64(bitPattern:
     _swift_stdlib_atomicLoadUInt64(
       object: UnsafeMutablePointer(target))))
@@ -259,7 +259,7 @@ public func _swift_stdlib_atomicFetch${operation}Int(
     _swift_stdlib_atomicFetch${operation}UInt32(
       object: UnsafeMutablePointer(target),
       operand: UInt32(bitPattern: Int32(operand)))))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   return Int(Int64(bitPattern:
     _swift_stdlib_atomicFetch${operation}UInt64(
       object: UnsafeMutablePointer(target),
diff --git a/stdlib/public/core/StringUTF8.swift b/stdlib/public/core/StringUTF8.swift
index e5e3955cbef19..0ffe78e974843 100644
--- a/stdlib/public/core/StringUTF8.swift
+++ b/stdlib/public/core/StringUTF8.swift
@@ -47,8 +47,10 @@ extension _StringCore {
         src: UnsafeMutablePointer(startASCII + i),
         size: numericCast(utf16Count))
 
-      return (i + utf16Count, result)
+      // Convert the _UTF8Chunk into host endianness.
+      return (i + utf16Count, _UTF8Chunk(littleEndian: result))
     } else if _fastPath(_baseAddress != nil) {
+      // Transcoding should return a _UTF8Chunk in host endianness.
       return _encodeSomeContiguousUTF16AsUTF8(from: i)
     } else {
 #if _runtime(_ObjC)
diff --git a/stdlib/public/runtime/Enum.cpp b/stdlib/public/runtime/Enum.cpp
index ca1b08f35e718..dd14e458c2ce7 100644
--- a/stdlib/public/runtime/Enum.cpp
+++ b/stdlib/public/runtime/Enum.cpp
@@ -154,12 +154,16 @@ swift::swift_getEnumCaseSinglePayload(const OpaqueValue *value,
     auto *valueAddr = reinterpret_cast<const uint8_t*>(value);
     auto *extraTagBitAddr = valueAddr + payloadSize;
     unsigned extraTagBits = 0;
-    // FIXME: endianness
     unsigned numBytes = getNumTagBytes(payloadSize,
                                        emptyCases-payloadNumExtraInhabitants,
                                        1 /*payload case*/);
 
+#if defined(__BIG_ENDIAN__)
+    small_memcpy(reinterpret_cast<uint8_t*>(&extraTagBits) + 4 - numBytes,
+                 extraTagBitAddr, numBytes);
+#else
     small_memcpy(&extraTagBits, extraTagBitAddr, numBytes);
+#endif
 
     // If the extra tag bits are zero, we have a valid payload or
     // extra inhabitant (checked below). If nonzero, form the case index from
@@ -170,10 +174,15 @@ swift::swift_getEnumCaseSinglePayload(const OpaqueValue *value,
 
       // In practice we should need no more than four bytes from the payload
       // area.
-      // FIXME: endianness.
       unsigned caseIndexFromValue = 0;
+#if defined(__BIG_ENDIAN__)
+      unsigned numPayloadTagBytes = std::min(size_t(4), payloadSize);
+      memcpy(reinterpret_cast<uint8_t*>(&caseIndexFromValue) + 4 -
+             numPayloadTagBytes, valueAddr, numPayloadTagBytes);
+#else
       memcpy(&caseIndexFromValue, valueAddr,
              std::min(size_t(4), payloadSize));
+#endif
       return (caseIndexFromExtraTagBits | caseIndexFromValue)
         + payloadNumExtraInhabitants;
     }
@@ -247,11 +256,22 @@ swift::swift_storeEnumTagSinglePayload(OpaqueValue *value,
   }
   
   // Store into the value.
-  // FIXME: endianness.
+#if defined(__BIG_ENDIAN__)
+  unsigned numPayloadTagBytes = std::min(size_t(4), payloadSize);
+  memcpy(valueAddr,
+         reinterpret_cast<uint8_t*>(&payloadIndex) + 4 - numPayloadTagBytes,
+         numPayloadTagBytes);
+  if (payloadSize > 4)
+    memset(valueAddr + 4, 0, payloadSize - 4);
+  memcpy(extraTagBitAddr,
+         reinterpret_cast<uint8_t*>(&extraTagIndex) + 4 - numExtraTagBytes,
+         numExtraTagBytes);
+#else
   memcpy(valueAddr, &payloadIndex, std::min(size_t(4), payloadSize));
   if (payloadSize > 4)
     memset(valueAddr + 4, 0, payloadSize - 4);
   memcpy(extraTagBitAddr, &extraTagIndex, numExtraTagBytes);
+#endif
 }
 
 void
@@ -312,17 +332,29 @@ static void storeMultiPayloadTag(OpaqueValue *value,
                                  MultiPayloadLayout layout,
                                  unsigned tag) {
   auto tagBytes = reinterpret_cast<char *>(value) + layout.payloadSize;
+#if defined(__BIG_ENDIAN__)
+  small_memcpy(tagBytes,
+               reinterpret_cast<char *>(&tag) + 4 - layout.numTagBytes,
+               layout.numTagBytes);
+#else
   small_memcpy(tagBytes, &tag, layout.numTagBytes);
+#endif
 }
 
 static void storeMultiPayloadValue(OpaqueValue *value,
                                    MultiPayloadLayout layout,
                                    unsigned payloadValue) {
   auto bytes = reinterpret_cast<char *>(value);
-  
+#if defined(__BIG_ENDIAN__)
+  unsigned numPayloadValueBytes =
+      std::min(layout.payloadSize, sizeof(payloadValue));
+  memcpy(bytes,
+         reinterpret_cast<char *>(&payloadValue) + 4 - numPayloadValueBytes,
+         numPayloadValueBytes);
+#else
   memcpy(bytes, &payloadValue,
          std::min(layout.payloadSize, sizeof(payloadValue)));
-  
+#endif
   // If the payload is larger than the value, zero out the rest.
   if (layout.payloadSize > sizeof(payloadValue))
     memset(bytes + sizeof(payloadValue), 0,
@@ -334,7 +366,12 @@ static unsigned loadMultiPayloadTag(const OpaqueValue *value,
   auto tagBytes = reinterpret_cast<const char *>(value) + layout.payloadSize;
 
   unsigned tag = 0;
+#if defined(__BIG_ENDIAN__)
+  small_memcpy(reinterpret_cast<char *>(&tag) + 4 - layout.numTagBytes,
+               tagBytes, layout.numTagBytes);
+#else
   small_memcpy(&tag, tagBytes, layout.numTagBytes);
+#endif
 
   return tag;
 }
@@ -343,8 +380,15 @@ static unsigned loadMultiPayloadValue(const OpaqueValue *value,
                                       MultiPayloadLayout layout) {
   auto bytes = reinterpret_cast<const char *>(value);
   unsigned payloadValue = 0;
+#if defined(__BIG_ENDIAN__)
+  unsigned numPayloadValueBytes =
+      std::min(layout.payloadSize, sizeof(payloadValue));
+  memcpy(reinterpret_cast<char *>(&payloadValue) + 4 - numPayloadValueBytes,
+         bytes, numPayloadValueBytes);
+#else
   memcpy(&payloadValue, bytes,
          std::min(layout.payloadSize, sizeof(payloadValue)));
+#endif
   return payloadValue;
 }
 
diff --git a/stdlib/public/runtime/ErrorObject.cpp b/stdlib/public/runtime/ErrorObject.cpp
index bb72cdea7edc8..31b721b667b2a 100644
--- a/stdlib/public/runtime/ErrorObject.cpp
+++ b/stdlib/public/runtime/ErrorObject.cpp
@@ -62,6 +62,8 @@ static const FullMetadata<HeapMetadata> ErrorProtocolMetadata{
   Metadata{MetadataKind::ErrorObject},
 };
 
+SWIFT_CC(swift) SWIFT_RUNTIME_EXPORT
+extern "C"
 BoxPair::Return
 swift::swift_allocError(const swift::Metadata *type,
                         const swift::WitnessTable *errorConformance,
diff --git a/stdlib/public/runtime/ErrorObject.h b/stdlib/public/runtime/ErrorObject.h
index a2b4ad00af44d..431e3ba8a182b 100644
--- a/stdlib/public/runtime/ErrorObject.h
+++ b/stdlib/public/runtime/ErrorObject.h
@@ -142,7 +142,7 @@ struct SwiftError : SwiftErrorHeader {
 /// copied (or taken if \c isTake is true) into the newly-allocated error box.
 /// If value is null, the box's contents will be left uninitialized, and
 /// \c isTake should be false.
-SWIFT_RUNTIME_EXPORT
+SWIFT_CC(swift) SWIFT_RUNTIME_EXPORT
 extern "C" BoxPair::Return swift_allocError(const Metadata *type,
                                           const WitnessTable *errorConformance,
                                           OpaqueValue *value, bool isTake);
diff --git a/stdlib/public/runtime/HeapObject.cpp b/stdlib/public/runtime/HeapObject.cpp
index cc09828bc3cee..a241277c37301 100644
--- a/stdlib/public/runtime/HeapObject.cpp
+++ b/stdlib/public/runtime/HeapObject.cpp
@@ -176,13 +176,13 @@ class BoxCacheEntry : public CacheEntry<BoxCacheEntry> {
 
 static Lazy<MetadataCache<BoxCacheEntry>> Boxes;
 
-SWIFT_RUNTIME_EXPORT
+SWIFT_CC(swift) SWIFT_RUNTIME_EXPORT
 BoxPair::Return
 swift::swift_allocBox(const Metadata *type) {
   return SWIFT_RT_ENTRY_REF(swift_allocBox)(type);
 }
 
-SWIFT_RT_ENTRY_IMPL_VISIBILITY
+SWIFT_CC(swift) SWIFT_RT_ENTRY_IMPL_VISIBILITY
 extern "C"
 BoxPair::Return SWIFT_RT_ENTRY_IMPL(swift_allocBox)(const Metadata *type) {
   // Get the heap metadata for the box.
diff --git a/stdlib/public/runtime/SwiftObject.mm b/stdlib/public/runtime/SwiftObject.mm
index adfee3ebe0f8d..b5073ca696fde 100644
--- a/stdlib/public/runtime/SwiftObject.mm
+++ b/stdlib/public/runtime/SwiftObject.mm
@@ -1510,7 +1510,7 @@ static bool usesNativeSwiftReferenceCounting_nonNull(
 
 using ClassExtents = TwoWordPair<size_t, size_t>;
 
-SWIFT_RUNTIME_EXPORT
+SWIFT_CC(swift) SWIFT_RUNTIME_EXPORT
 extern "C"
 ClassExtents::Return
 swift_class_getInstanceExtents(const Metadata *c) {
diff --git a/stdlib/public/stubs/Stubs.cpp b/stdlib/public/stubs/Stubs.cpp
index d22044c027b61..f3119a9fe4eb3 100644
--- a/stdlib/public/stubs/Stubs.cpp
+++ b/stdlib/public/stubs/Stubs.cpp
@@ -283,6 +283,7 @@ extern "C" long double _swift_fmodl(long double lhs, long double rhs) {
 #if (defined(__linux__) && defined(__x86_64__)) || \
     (defined(__linux__) && defined(__aarch64__)) || \
     (defined(__linux__) && defined(__powerpc64__)) || \
+    (defined(__linux__) && defined(__s390x__)) || \
     (defined(__ANDROID__) && defined(__arm64__))
 
 typedef int ti_int __attribute__((__mode__(TI)));
diff --git a/test/1_stdlib/FloatingPointIR.swift b/test/1_stdlib/FloatingPointIR.swift
index 938ae8547a499..17fb44aa57a15 100644
--- a/test/1_stdlib/FloatingPointIR.swift
+++ b/test/1_stdlib/FloatingPointIR.swift
@@ -53,3 +53,6 @@ func testConstantFoldFloatLiterals() {
 
 // powerpc64le: call void @{{.*}}_TF15FloatingPointIR13acceptFloat32FSfT_(float 1.000000e+00)
 // powerpc64le: call void @{{.*}}_TF15FloatingPointIR13acceptFloat64FSdT_(double 1.000000e+00)
+
+// s390x: call void @{{.*}}_TF15FloatingPointIR13acceptFloat32FSfT_(float 1.000000e+00)
+// s390x: call void @{{.*}}_TF15FloatingPointIR13acceptFloat64FSdT_(double 1.000000e+00)
diff --git a/test/1_stdlib/ReflectionHashing.swift b/test/1_stdlib/ReflectionHashing.swift
index 6994e9f2ad190..c6b4235382643 100644
--- a/test/1_stdlib/ReflectionHashing.swift
+++ b/test/1_stdlib/ReflectionHashing.swift
@@ -49,7 +49,7 @@ Reflection.test("Dictionary") {
   expected += "  ▿ (2 elements)\n"
   expected += "    - .0: \"Three\"\n"
   expected += "    - .1: 3\n"
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   var expected = ""
   expected += "▿ 5 key/value pairs\n"
   expected += "  ▿ (2 elements)\n"
@@ -88,7 +88,7 @@ Reflection.test("Set") {
   expected += "  - 5\n"
   expected += "  - 2\n"
   expected += "  - 4\n"
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   var expected = ""
   expected += "▿ 5 members\n"
   expected += "  - 5\n"
diff --git a/test/1_stdlib/Runtime.swift b/test/1_stdlib/Runtime.swift
index 96f0727262d20..6eba1320c8abc 100644
--- a/test/1_stdlib/Runtime.swift
+++ b/test/1_stdlib/Runtime.swift
@@ -1493,7 +1493,7 @@ BitTwiddlingTestSuite.test("_isPowerOf2/Int") {
   expectTrue(_isPowerOf2(asInt(1024)))
 #if arch(i386) || arch(arm)
   // Not applicable to 32-bit architectures.
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   expectTrue(_isPowerOf2(asInt(0x8000_0000)))
 #else
   fatalError("implement")
diff --git a/test/1_stdlib/UnsafePointer.swift.gyb b/test/1_stdlib/UnsafePointer.swift.gyb
index e8fa3d0e7c96e..9a2f5cd772554 100644
--- a/test/1_stdlib/UnsafePointer.swift.gyb
+++ b/test/1_stdlib/UnsafePointer.swift.gyb
@@ -331,7 +331,7 @@ UnsafePointerTestSuite.test("customMirror") {
   expectEqual(1, mirror.children.count)
 #if arch(i386) || arch(arm)
   expectEqual("18446744071562067968", String(mirror.children.first!.1))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   expectEqual("9223372036854775808", String(mirror.children.first!.1))
 #else
   fatalError("Unimplemented")
@@ -346,7 +346,7 @@ UnsafePointerTestSuite.test("customPlaygroundQuickLook") {
   if case let .text(desc) = ptr.customPlaygroundQuickLook {
 #if arch(i386) || arch(arm)
     expectEqual("UnsafePointer(0xFFFFFFFF80000000)", desc)
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
     expectEqual("UnsafePointer(0x8000000000000000)", desc)
 #else
     fatalError("Unimplemented")
@@ -363,7 +363,7 @@ UnsafeMutablePointerTestSuite.test("customMirror") {
   expectEqual(1, mirror.children.count)
 #if arch(i386) || arch(arm)
   expectEqual("18446744071562067968", String(mirror.children.first!.1))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   expectEqual("9223372036854775808", String(mirror.children.first!.1))
 #else
   fatalError("Unimplemented")
@@ -377,7 +377,7 @@ UnsafeMutablePointerTestSuite.test("customPlaygroundQuickLook") {
   if case let .text(desc) = ptr.customPlaygroundQuickLook {
 #if arch(i386) || arch(arm)
     expectEqual("UnsafeMutablePointer(0xFFFFFFFF80000000)", desc)
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
     expectEqual("UnsafeMutablePointer(0x8000000000000000)", desc)
 #else
     fatalError("Unimplemented")
diff --git a/test/1_stdlib/tgmath.swift b/test/1_stdlib/tgmath.swift
index c017031631ffc..6353dd8174065 100644
--- a/test/1_stdlib/tgmath.swift
+++ b/test/1_stdlib/tgmath.swift
@@ -7,7 +7,7 @@
   // where they doesn't have CoreGraphics module.
   #if arch(i386) || arch(arm)
     typealias CGFloat = Float
-  #elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+  #elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
     typealias CGFloat = Double
   #endif
 #else
diff --git a/test/ClangModules/ctypes_parse.swift b/test/ClangModules/ctypes_parse.swift
index a6f2a890597bf..30ce40c9c9ef7 100644
--- a/test/ClangModules/ctypes_parse.swift
+++ b/test/ClangModules/ctypes_parse.swift
@@ -32,7 +32,7 @@ func testAnonEnum() {
   a = AnonConst2
 #if arch(i386) || arch(arm)
   _ = a as CUnsignedLongLong
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   _ = a as CUnsignedLong
 #else
   __portMe()
diff --git a/test/IRGen/c_layout.sil b/test/IRGen/c_layout.sil
index bf30abf0da5d3..9ac50a6afc879 100644
--- a/test/IRGen/c_layout.sil
+++ b/test/IRGen/c_layout.sil
@@ -237,6 +237,22 @@ bb0:
 // CHECK-powerpc64le-LABEL: declare signext i32 @ints(i32 signext)
 // CHECK-powerpc64le-LABEL: declare zeroext i32 @unsigneds(i32 zeroext)
 
+// CHECK-s390x-LABEL: define{{( protected)?}} void @testIntegerExtension
+// CHECK-s390x:         call zeroext i8 @chareth(i8 zeroext %0)
+// CHECK-s390x:         call zeroext i8 @signedChareth(i8 zeroext %1)
+// CHECK-s390x:         call zeroext i8 @unsignedChareth(i8 zeroext %2)
+// CHECK-s390x:         call signext i16 @eatMyShorts(i16 signext %3)
+// CHECK-s390x:         call zeroext i16 @eatMyUnsignedShorts(i16 zeroext %4)
+// CHECK-s390x:         call signext i32 @ints(i32 signext %5)
+// CHECK-s390x:         call zeroext i32 @unsigneds(i32 zeroext %6)
+// CHECK-s390x-LABEL: declare zeroext i8 @chareth(i8 zeroext)
+// CHECK-s390x-LABEL: declare zeroext i8 @signedChareth(i8 zeroext)
+// CHECK-s390x-LABEL: declare zeroext i8 @unsignedChareth(i8 zeroext)
+// CHECK-s390x-LABEL: declare signext i16 @eatMyShorts(i16 signext)
+// CHECK-s390x-LABEL: declare zeroext i16 @eatMyUnsignedShorts(i16 zeroext)
+// CHECK-s390x-LABEL: declare signext i32 @ints(i32 signext)
+// CHECK-s390x-LABEL: declare zeroext i32 @unsigneds(i32 zeroext)
+
 sil @testIntegerExtension : $@convention(thin) (CChar, CSignedChar, CUnsignedChar, CShort, CUnsignedShort, CInt, CUnsignedInt) -> () {
 entry(%a : $CChar, %b : $CSignedChar, %c : $CUnsignedChar, %d : $CShort, %e : $CUnsignedShort, %f : $CInt, %g : $CUnsignedInt):
   %h = function_ref @chareth : $@convention(c) (CChar) -> CChar
diff --git a/test/IRGen/class.sil b/test/IRGen/class.sil
index 87610530a0566..702c6dd337678 100644
--- a/test/IRGen/class.sil
+++ b/test/IRGen/class.sil
@@ -97,9 +97,9 @@ bb0(%0 : $@thick C.Type):
   // CHECK:   [[SIZE32:%.*]] = load i32, i32* [[T1]], align 8
   // CHECK:   [[SIZE:%.*]] = zext i32 [[SIZE32]] to i64
   // CHECK:   [[T0:%.*]] = getelementptr inbounds i8, i8* [[META_PTR]], i32 52
-  // CHECK:   [[T1:%.*]] = bitcast i8* [[T0]] to i32*
-  // CHECK:   [[ALIGN32:%.*]] = load i32, i32* [[T1]], align 4
-  // CHECK:   [[ALIGN:%.*]] = zext i32 [[ALIGN32]] to i64
+  // CHECK:   [[T1:%.*]] = bitcast i8* [[T0]] to i16*
+  // CHECK:   [[ALIGN16:%.*]] = load i16, i16* [[T1]], align 4
+  // CHECK:   [[ALIGN:%.*]] = zext i16 [[ALIGN16]] to i64
   // CHECK:   [[RESULT:%[0-9]+]] = call noalias %swift.refcounted* @rt_swift_allocObject(%swift.type* %0, i64 [[SIZE]], i64 [[ALIGN]])
   %1 = alloc_ref_dynamic %0 : $@thick C.Type, $C
   return %1 : $C
diff --git a/test/IRGen/condfail.sil b/test/IRGen/condfail.sil
index cf3c772ca98a9..eafaa7111b3d8 100644
--- a/test/IRGen/condfail.sil
+++ b/test/IRGen/condfail.sil
@@ -13,6 +13,7 @@ import Swift
 // CHECK-armv7:       trap
 // CHECK-powerpc64:   trap
 // CHECK-powerpc64le: trap
+// CHECK-s390x:       trap
 // CHECK-NOT:         .cfi_endproc
 // CHECK-x86_64:      ud2
 // CHECK-i386:        ud2
@@ -20,6 +21,7 @@ import Swift
 // CHECK-armv7:       trap
 // CHECK-powerpc64:   trap
 // CHECK-powerpc64le: trap
+// CHECK-s390x:       trap
 // CHECK:             .cfi_endproc
 sil hidden @test_cond_fail : $@convention(thin) (Int32) -> Int32 {
 bb0(%0 : $Int32):
diff --git a/test/IRGen/generic_classes.sil b/test/IRGen/generic_classes.sil
index fc17c994063d2..1a7a0ea609103 100644
--- a/test/IRGen/generic_classes.sil
+++ b/test/IRGen/generic_classes.sil
@@ -211,9 +211,9 @@ sil @_TFC15generic_classes31RecursiveGenericInheritsGenericD : $@convention(meth
 // CHECK:   [[SIZE32:%.*]] = load i32, i32* [[T1]], align 8
 // CHECK:   [[SIZE:%.*]] = zext i32 [[SIZE32]] to i64
 // CHECK:   [[T0:%.*]] = getelementptr inbounds i8, i8* [[METADATA_ARRAY]], i32 52
-// CHECK:   [[T1:%.*]] = bitcast i8* [[T0]] to i32*
-// CHECK:   [[ALIGN32:%.*]] = load i32, i32* [[T1]], align 4
-// CHECK:   [[ALIGN:%.*]] = zext i32 [[ALIGN32]] to i64
+// CHECK:   [[T1:%.*]] = bitcast i8* [[T0]] to i16*
+// CHECK:   [[ALIGN16:%.*]] = load i16, i16* [[T1]], align 4
+// CHECK:   [[ALIGN:%.*]] = zext i16 [[ALIGN16]] to i64
 // CHECK:   call noalias %swift.refcounted* @rt_swift_allocObject(%swift.type* [[METADATA]], i64 [[SIZE]], i64 [[ALIGN]])
 sil @RootGeneric_fragile_dependent_alloc : $<G> () -> RootGeneric<G> {
 entry:
diff --git a/test/IRGen/objc_simd.sil b/test/IRGen/objc_simd.sil
index e346dfa59f4db..676d2f7353783 100644
--- a/test/IRGen/objc_simd.sil
+++ b/test/IRGen/objc_simd.sil
@@ -22,6 +22,7 @@ func forceStuff(x: float4, y: float3) -> (Float, Float, Float, Float) {
 // armv7k-LABEL: define{{( protected)?}} <4 x float> @simd_c_args(<4 x float>)
 // powerpc64-LABEL: define{{( protected)?}} <4 x float> @simd_c_args(<4 x float>)
 // powerpc64le-LABEL: define{{( protected)?}} <4 x float> @simd_c_args(<4 x float>)
+// s390x-LABEL: define{{( protected)?}} <4 x float> @simd_c_args(<4 x float>)
 sil @simd_c_args : $@convention(c) (float4) -> float4 {
 entry(%x : $float4):
   return %x : $float4
@@ -36,6 +37,7 @@ entry(%x : $float4):
 // armv7k-LABEL: define{{( protected)?}} <3 x float> @simd_c_args_float3(<4 x i32>)
 // powerpc64-LABEL: define{{( protected)?}} <3 x float> @simd_c_args_float3(<3 x float>)
 // powerpc64le-LABEL: define{{( protected)?}} <3 x float> @simd_c_args_float3(<3 x float>)
+// s390x-LABEL: define{{( protected)?}} <3 x float> @simd_c_args_float3(<3 x float>)
 sil @simd_c_args_float3 : $@convention(c) (float3) -> float3 {
 entry(%x : $float3):
 // x86_64: [[COERCE:%.*]] = alloca <3 x float>, align 16
@@ -53,6 +55,7 @@ entry(%x : $float3):
 // armv7k-LABEL: define{{( protected)?}} void @simd_native_args(%V4simd6float4* noalias nocapture sret, %V4simd6float4* noalias nocapture dereferenceable({{.*}}))
 // powerpc64-LABEL: define{{( protected)?}} void @simd_native_args(%V4simd6float4* noalias nocapture sret, %V4simd6float4* noalias nocapture dereferenceable({{.*}}))
 // powerpc64le-LABEL: define{{( protected)?}} void @simd_native_args(%V4simd6float4* noalias nocapture sret, %V4simd6float4* noalias nocapture dereferenceable({{.*}}))
+// s390x-LABEL: define{{( protected)?}} void @simd_native_args(%V4simd6float4* noalias nocapture sret, %V4simd6float4* noalias nocapture dereferenceable({{.*}}))
 sil @simd_native_args : $@convention(thin) (float4) -> float4 {
 entry(%x : $float4):
   %f = function_ref @simd_c_args : $@convention(c) (float4) -> float4
@@ -69,6 +72,7 @@ entry(%x : $float4):
 // armv7k-LABEL: define{{( protected)?}} { float, float, float } @simd_native_args_float3(float, float, float)
 // powerpc64-LABEL: define{{( protected)?}} { float, float, float } @simd_native_args_float3(float, float, float)
 // powerpc64le-LABEL: define{{( protected)?}} { float, float, float } @simd_native_args_float3(float, float, float)
+// s390x-LABEL: define{{( protected)?}} { float, float, float } @simd_native_args_float3(float, float, float)
 sil @simd_native_args_float3 : $@convention(thin) (float3) -> float3 {
 entry(%x : $float3):
   %f = function_ref @simd_c_args_float3 : $@convention(c) (float3) -> float3
diff --git a/test/Interpreter/builtin_bridge_object.swift b/test/Interpreter/builtin_bridge_object.swift
index f58562fada22d..427c90c51e2af 100644
--- a/test/Interpreter/builtin_bridge_object.swift
+++ b/test/Interpreter/builtin_bridge_object.swift
@@ -37,6 +37,12 @@ let OBJC_TAGGED_POINTER_BITS: UInt = 0x8000_0000_0000_0000
 let NATIVE_SPARE_BITS: UInt = 0x0000_0000_0000_0007
 let OBJC_TAGGED_POINTER_BITS: UInt = 0
 
+#elseif arch(s390x)
+
+// We have no ObjC tagged pointers, and three low spare bits due to alignment.
+let NATIVE_SPARE_BITS: UInt = 0x0000_0000_0000_0007
+let OBJC_TAGGED_POINTER_BITS: UInt = 0
+
 #endif
 
 func bitPattern(_ x: Builtin.BridgeObject) -> UInt {
diff --git a/test/Parse/ConditionalCompilation/arm64AppleTVOSTarget.swift b/test/Parse/ConditionalCompilation/arm64AppleTVOSTarget.swift
index 43259400cc419..71e8574953fca 100644
--- a/test/Parse/ConditionalCompilation/arm64AppleTVOSTarget.swift
+++ b/test/Parse/ConditionalCompilation/arm64AppleTVOSTarget.swift
@@ -7,7 +7,7 @@
 let i: Int = "Hello"
 #endif
 
-#if arch(arm64) && os(tvOS) && _runtime(_ObjC)
+#if arch(arm64) && os(tvOS) && _runtime(_ObjC) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/arm64IOSTarget.swift b/test/Parse/ConditionalCompilation/arm64IOSTarget.swift
index b6087aaa13665..6f744fc6e32d3 100644
--- a/test/Parse/ConditionalCompilation/arm64IOSTarget.swift
+++ b/test/Parse/ConditionalCompilation/arm64IOSTarget.swift
@@ -7,7 +7,7 @@
 let i: Int = "Hello"
 #endif
 
-#if arch(arm64) && os(iOS) && _runtime(_ObjC)
+#if arch(arm64) && os(iOS) && _runtime(_ObjC) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/armAndroidTarget.swift b/test/Parse/ConditionalCompilation/armAndroidTarget.swift
index de4bcb4032946..975c33f190c99 100644
--- a/test/Parse/ConditionalCompilation/armAndroidTarget.swift
+++ b/test/Parse/ConditionalCompilation/armAndroidTarget.swift
@@ -7,7 +7,7 @@
 let i: Int = "Hello"
 #endif
 
-#if arch(arm) && os(Android) && _runtime(_Native)
+#if arch(arm) && os(Android) && _runtime(_Native) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/armIOSTarget.swift b/test/Parse/ConditionalCompilation/armIOSTarget.swift
index f8dbdec68b855..3d01c07a4407d 100644
--- a/test/Parse/ConditionalCompilation/armIOSTarget.swift
+++ b/test/Parse/ConditionalCompilation/armIOSTarget.swift
@@ -7,7 +7,7 @@
 let i: Int = "Hello"
 #endif
 
-#if arch(arm) && os(iOS) && _runtime(_ObjC)
+#if arch(arm) && os(iOS) && _runtime(_ObjC) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/armWatchOSTarget.swift b/test/Parse/ConditionalCompilation/armWatchOSTarget.swift
index 2d382fff8f433..7df504d7eb27f 100644
--- a/test/Parse/ConditionalCompilation/armWatchOSTarget.swift
+++ b/test/Parse/ConditionalCompilation/armWatchOSTarget.swift
@@ -7,7 +7,7 @@
 let i: Int = "Hello"
 #endif
 
-#if arch(arm) && os(watchOS) && _runtime(_ObjC)
+#if arch(arm) && os(watchOS) && _runtime(_ObjC) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/basicParseErrors.swift b/test/Parse/ConditionalCompilation/basicParseErrors.swift
index 206710f36e51f..3462756e1ef37 100644
--- a/test/Parse/ConditionalCompilation/basicParseErrors.swift
+++ b/test/Parse/ConditionalCompilation/basicParseErrors.swift
@@ -61,3 +61,6 @@ struct S {
 
 #if arch(leg) // expected-warning {{unknown architecture for build configuration 'arch'}}
 #endif
+
+#if _endian(mid) // expected-warning {{unknown endianness for build configuration '_endian'}}
+#endif
diff --git a/test/Parse/ConditionalCompilation/i386AppleTVOSTarget.swift b/test/Parse/ConditionalCompilation/i386AppleTVOSTarget.swift
index 0a4183945bd52..94273fb2f3c3a 100644
--- a/test/Parse/ConditionalCompilation/i386AppleTVOSTarget.swift
+++ b/test/Parse/ConditionalCompilation/i386AppleTVOSTarget.swift
@@ -7,7 +7,7 @@
 let i: Int = "Hello"
 #endif
 
-#if arch(i386) && os(tvOS) && _runtime(_ObjC)
+#if arch(i386) && os(tvOS) && _runtime(_ObjC) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/i386IOSTarget.swift b/test/Parse/ConditionalCompilation/i386IOSTarget.swift
index 73866cd92d950..9c965c5957535 100644
--- a/test/Parse/ConditionalCompilation/i386IOSTarget.swift
+++ b/test/Parse/ConditionalCompilation/i386IOSTarget.swift
@@ -7,7 +7,7 @@
 let i: Int = "Hello"
 #endif
 
-#if arch(i386) && os(iOS) && _runtime(_ObjC)
+#if arch(i386) && os(iOS) && _runtime(_ObjC) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/i386WatchOSTarget.swift b/test/Parse/ConditionalCompilation/i386WatchOSTarget.swift
index 9d855322207ea..0e4614ac14ba9 100644
--- a/test/Parse/ConditionalCompilation/i386WatchOSTarget.swift
+++ b/test/Parse/ConditionalCompilation/i386WatchOSTarget.swift
@@ -7,7 +7,7 @@
 let i: Int = "Hello"
 #endif
 
-#if arch(i386) && os(watchOS) && _runtime(_ObjC)
+#if arch(i386) && os(watchOS) && _runtime(_ObjC) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/powerpc64LinuxTarget.swift b/test/Parse/ConditionalCompilation/powerpc64LinuxTarget.swift
index 186766473e2cc..874ad8dec39be 100644
--- a/test/Parse/ConditionalCompilation/powerpc64LinuxTarget.swift
+++ b/test/Parse/ConditionalCompilation/powerpc64LinuxTarget.swift
@@ -1,7 +1,7 @@
 // RUN: %swift -parse %s -verify -D FOO -D BAR -target powerpc64-unknown-linux-gnu -disable-objc-interop -D FOO -parse-stdlib
 // RUN: %swift-ide-test -test-input-complete -source-filename=%s -target powerpc64-unknown-linux-gnu
 
-#if arch(powerpc64) && os(Linux) && _runtime(_Native)
+#if arch(powerpc64) && os(Linux) && _runtime(_Native) && _endian(big)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/powerpc64leLinuxTarget.swift b/test/Parse/ConditionalCompilation/powerpc64leLinuxTarget.swift
index 586972fa83706..d8973a8e357ed 100644
--- a/test/Parse/ConditionalCompilation/powerpc64leLinuxTarget.swift
+++ b/test/Parse/ConditionalCompilation/powerpc64leLinuxTarget.swift
@@ -1,7 +1,7 @@
 // RUN: %swift -parse %s -verify -D FOO -D BAR -target powerpc64le-unknown-linux-gnu -disable-objc-interop -D FOO -parse-stdlib
 // RUN: %swift-ide-test -test-input-complete -source-filename=%s -target powerpc64le-unknown-linux-gnu
 
-#if arch(powerpc64le) && os(Linux) && _runtime(_Native)
+#if arch(powerpc64le) && os(Linux) && _runtime(_Native) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/s390xLinuxTarget.swift b/test/Parse/ConditionalCompilation/s390xLinuxTarget.swift
new file mode 100644
index 0000000000000..d2c56f3373e98
--- /dev/null
+++ b/test/Parse/ConditionalCompilation/s390xLinuxTarget.swift
@@ -0,0 +1,8 @@
+// RUN: %swift -parse %s -verify -D FOO -D BAR -target s390x-unknown-linux-gnu -disable-objc-interop -D FOO -parse-stdlib
+// RUN: %swift-ide-test -test-input-complete -source-filename=%s -target s390x-unknown-linux-gnu
+
+#if arch(s390x) && os(Linux) && _runtime(_Native) && _endian(big)
+class C {}
+var x = C()
+#endif
+var y = x
diff --git a/test/Parse/ConditionalCompilation/x64AppleTVOSTarget.swift b/test/Parse/ConditionalCompilation/x64AppleTVOSTarget.swift
index 4f240ae010f5b..2c82025321dde 100644
--- a/test/Parse/ConditionalCompilation/x64AppleTVOSTarget.swift
+++ b/test/Parse/ConditionalCompilation/x64AppleTVOSTarget.swift
@@ -7,7 +7,7 @@
 let i: Int = "Hello"
 #endif
 
-#if arch(x86_64) && os(tvOS) && _runtime(_ObjC)
+#if arch(x86_64) && os(tvOS) && _runtime(_ObjC) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/x64FreeBSDTarget.swift b/test/Parse/ConditionalCompilation/x64FreeBSDTarget.swift
index 284060fa6eb49..cd81bddcd2a6c 100644
--- a/test/Parse/ConditionalCompilation/x64FreeBSDTarget.swift
+++ b/test/Parse/ConditionalCompilation/x64FreeBSDTarget.swift
@@ -1,7 +1,7 @@
 // RUN: %swift -parse %s -verify -D FOO -D BAR -target x86_64-unknown-freebsd10 -disable-objc-interop -D FOO -parse-stdlib
 // RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-freebsd10
 
-#if arch(x86_64) && os(FreeBSD) && _runtime(_Native)
+#if arch(x86_64) && os(FreeBSD) && _runtime(_Native) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/x64IOSTarget.swift b/test/Parse/ConditionalCompilation/x64IOSTarget.swift
index 65989f04a3733..9d8776a70fb01 100644
--- a/test/Parse/ConditionalCompilation/x64IOSTarget.swift
+++ b/test/Parse/ConditionalCompilation/x64IOSTarget.swift
@@ -7,7 +7,7 @@
 let i: Int = "Hello"
 #endif
 
-#if arch(x86_64) && os(iOS) && _runtime(_ObjC)
+#if arch(x86_64) && os(iOS) && _runtime(_ObjC) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/x64LinuxTarget.swift b/test/Parse/ConditionalCompilation/x64LinuxTarget.swift
index 58c5bbb5e28c1..94e5d5856b323 100644
--- a/test/Parse/ConditionalCompilation/x64LinuxTarget.swift
+++ b/test/Parse/ConditionalCompilation/x64LinuxTarget.swift
@@ -1,7 +1,7 @@
 // RUN: %swift -parse %s -verify -D FOO -D BAR -target x86_64-unknown-linux-gnu -disable-objc-interop -D FOO -parse-stdlib
 // RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-linux-gnu
 
-#if arch(x86_64) && os(Linux) && _runtime(_Native)
+#if arch(x86_64) && os(Linux) && _runtime(_Native) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/x64OSXTarget.swift b/test/Parse/ConditionalCompilation/x64OSXTarget.swift
index 196fcbf060595..db6411f2de1be 100644
--- a/test/Parse/ConditionalCompilation/x64OSXTarget.swift
+++ b/test/Parse/ConditionalCompilation/x64OSXTarget.swift
@@ -1,7 +1,7 @@
 // RUN: %swift -parse %s -verify -D FOO -D BAR -target x86_64-apple-macosx10.9 -D FOO -parse-stdlib
 // RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-apple-macosx10.9
 
-#if arch(x86_64) && os(OSX) && _runtime(_ObjC)
+#if arch(x86_64) && os(OSX) && _runtime(_ObjC) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/Parse/ConditionalCompilation/x64WindowsTarget.swift b/test/Parse/ConditionalCompilation/x64WindowsTarget.swift
index b4bd7d590eaa7..40a5552514a75 100644
--- a/test/Parse/ConditionalCompilation/x64WindowsTarget.swift
+++ b/test/Parse/ConditionalCompilation/x64WindowsTarget.swift
@@ -1,7 +1,7 @@
 // RUN: %swift -parse %s -verify -D FOO -D BAR -target x86_64-unknown-windows-cygnus -disable-objc-interop -D FOO -parse-stdlib
 // RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-windows-cygnus
 
-#if arch(x86_64) && os(Windows) && _runtime(_Native)
+#if arch(x86_64) && os(Windows) && _runtime(_Native) && _endian(little)
 class C {}
 var x = C()
 #endif
diff --git a/test/SILOptimizer/diagnostic_constant_propagation_int.swift b/test/SILOptimizer/diagnostic_constant_propagation_int.swift
index 65053fd98310d..b6b53fe573c49 100644
--- a/test/SILOptimizer/diagnostic_constant_propagation_int.swift
+++ b/test/SILOptimizer/diagnostic_constant_propagation_int.swift
@@ -265,7 +265,7 @@ func testArithmeticOverflow_UInt_32bit() {
   }
 }
 
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
 
 func testArithmeticOverflow_Int_64bit() {
   do {
diff --git a/test/lit.cfg b/test/lit.cfg
index d5347e07467a9..74254712ea7e9 100644
--- a/test/lit.cfg
+++ b/test/lit.cfg
@@ -305,7 +305,7 @@ lit_config.note('Using resource dir: ' + test_resource_dir)
 
 # Parse the variant triple.
 (run_cpu, run_vendor, run_os, run_vers) = re.match('([^-]+)-([^-]+)-([^0-9]+)(.*)', config.variant_triple).groups()
-run_ptrsize = '64' if '64' in run_cpu else '32'
+run_ptrsize = '64' if ('64' in run_cpu or run_cpu == "s390x") else '32'
 
 sdk_overlay_link_path = ""
 sdk_overlay_linker_opt = ""
diff --git a/unittests/runtime/Enum.cpp b/unittests/runtime/Enum.cpp
index 2bf7c0168513e..c30603946c441 100644
--- a/unittests/runtime/Enum.cpp
+++ b/unittests/runtime/Enum.cpp
@@ -76,9 +76,15 @@ TEST(EnumTest, getEnumCaseSinglePayload) {
                                                128*1024));
   ASSERT_EQ(-1, test_getEnumCaseSinglePayload({255, 0, 0}, _TMBi8_,
                                                128*1024));
+#if defined(__BIG_ENDIAN__)
+  ASSERT_EQ(65535 - 255,
+            test_getEnumCaseSinglePayload({0, 1, 0}, _TMBi8_,
+                                           128*1024));
+#else
   ASSERT_EQ(65535 - 255,
             test_getEnumCaseSinglePayload({0, 0, 1}, _TMBi8_,
                                            128*1024));
+#endif
 
   // Test with XI.
   ASSERT_EQ(-1, test_getEnumCaseSinglePayload({0}, XI_TMBi8_, 2));
@@ -126,6 +132,16 @@ TEST(EnumTest, storeEnumTagSinglePayload) {
 
   ASSERT_TRUE(test_storeEnumTagSinglePayload({219, 0, 0}, {219, 123, 77},
                                               _TMBi8_, -1, 128*1024));
+#if defined(__BIG_ENDIAN__)
+  ASSERT_TRUE(test_storeEnumTagSinglePayload({0, 0, 1}, {219, 123, 77},
+                                              _TMBi8_, 0, 128*1024));
+  ASSERT_TRUE(test_storeEnumTagSinglePayload({255, 0, 1}, {219, 123, 77},
+                                              _TMBi8_, 255, 128*1024));
+  ASSERT_TRUE(test_storeEnumTagSinglePayload({0, 0, 2}, {219, 123, 77},
+                                              _TMBi8_, 256, 128*1024));
+  ASSERT_TRUE(test_storeEnumTagSinglePayload({255, 2, 0}, {219, 123, 77},
+                                              _TMBi8_, 128*1024 - 1, 128*1024));
+#else
   ASSERT_TRUE(test_storeEnumTagSinglePayload({0, 1, 0}, {219, 123, 77},
                                               _TMBi8_, 0, 128*1024));
   ASSERT_TRUE(test_storeEnumTagSinglePayload({255, 1, 0}, {219, 123, 77},
@@ -134,6 +150,7 @@ TEST(EnumTest, storeEnumTagSinglePayload) {
                                               _TMBi8_, 256, 128*1024));
   ASSERT_TRUE(test_storeEnumTagSinglePayload({255, 0, 2}, {219, 123, 77},
                                               _TMBi8_, 128*1024 - 1, 128*1024));
+#endif
 
   // Test with XI.
   ASSERT_TRUE(test_storeEnumTagSinglePayload({219}, {219},
diff --git a/utils/build-script-impl b/utils/build-script-impl
index 54f34978de600..ba249ebab4678 100755
--- a/utils/build-script-impl
+++ b/utils/build-script-impl
@@ -30,7 +30,7 @@ umask 0022
 # need to change any of these, you should do so on trunk or in a branch.
 #
 
-LLVM_TARGETS_TO_BUILD="X86;ARM;AArch64;PowerPC"
+LLVM_TARGETS_TO_BUILD="X86;ARM;AArch64;PowerPC;SystemZ"
 
 #
 # End of configurable options.
@@ -346,6 +346,9 @@ function set_deployment_target_based_options() {
             USE_GOLD_LINKER=1
             SWIFT_HOST_VARIANT_ARCH="powerpc64le"
             ;;
+        linux-s390x)
+            SWIFT_HOST_VARIANT_ARCH="s390x"
+            ;;
         cygwin-x86_64)
             SWIFT_HOST_VARIANT_ARCH="x86_64"
             ;;
diff --git a/utils/swift-bench.py b/utils/swift-bench.py
index 33a7bd6587e92..e783d7d6776c6 100644
--- a/utils/swift-bench.py
+++ b/utils/swift-bench.py
@@ -118,7 +118,7 @@ def process_source(self, name):
 public func getInt(x: Int) -> Int {
 #if arch(i386) || arch(arm)
   return _opaqueGetInt32(x)
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   return _opaqueGetInt64(x)
 #else
   return x
diff --git a/utils/swift_build_support/swift_build_support/targets.py b/utils/swift_build_support/swift_build_support/targets.py
index b2a2f4513ba1a..8cfe2a0f4e1bf 100644
--- a/utils/swift_build_support/swift_build_support/targets.py
+++ b/utils/swift_build_support/swift_build_support/targets.py
@@ -35,6 +35,8 @@ def host_target():
             return 'linux-powerpc64'
         elif machine == 'ppc64le':
             return 'linux-powerpc64le'
+        elif machine == 's390x':
+            return 'linux-s390x'
 
     elif system == 'Darwin':
         if machine == 'x86_64':
@@ -77,6 +79,8 @@ def stdlib_deployment_targets():
             return ['linux-powerpc64']
         elif machine == 'ppc64le':
             return ['linux-powerpc64le']
+        elif machine == 's390x':
+            return ['linux-s390x']
     elif system == 'Darwin':
         if machine == 'x86_64':
             return [
diff --git a/validation-test/stdlib/FixedPoint.swift.gyb b/validation-test/stdlib/FixedPoint.swift.gyb
index e917977d3736b..ba0b888d29140 100644
--- a/validation-test/stdlib/FixedPoint.swift.gyb
+++ b/validation-test/stdlib/FixedPoint.swift.gyb
@@ -118,7 +118,7 @@ FixedPoint.test("${Dst}(truncatingBitPattern:) from ${Src}(${bit_pattern})") {
       test_bit_patterns=test_bit_patterns,
       word_bits=32)}
 
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
 
   ${gyb.execute_template(
       truncating_bit_pattern_test_template,
@@ -190,7 +190,7 @@ FixedPoint.test("${Dst}(bitPattern:) from ${Src}(${bit_pattern})") {
       test_bit_patterns=test_bit_patterns,
       word_bits=32)}
 
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
 
   ${gyb.execute_template(
       bit_pattern_test_template,
@@ -248,7 +248,7 @@ FixedPoint.test("${Self}.hashValue") {
       test_bit_patterns=test_bit_patterns,
       word_bits=32)}
 
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
 
   ${gyb.execute_template(
       hash_value_test_template,
@@ -307,10 +307,17 @@ FixedPoint.test("OverflowCheck") {
 
 FixedPoint.test("String.init") {
   let x: UInt32 = 0xdeadbeef
+#if _endian(little)
   expectEqual("efbeadde", String(x.bigEndian, radix: 16))
 
   let y = UInt32(bigEndian: 0xdeadbeef)
   expectEqual("deadbeef", String(y.bigEndian, radix: 16))
+#else
+  expectEqual("efbeadde", String(x.littleEndian, radix: 16))
+
+  let y = UInt32(littleEndian: 0xdeadbeef)
+  expectEqual("deadbeef", String(y.littleEndian, radix: 16))
+#endif
 }
 
 FixedPoint.test("byteSwapped") {
@@ -426,5 +433,4 @@ BoolTestSuite.test("||") {
   expectFalse(getBool(false) || getBool(false))
 }
 
-runAllTests()
-
+runAllTests()
\ No newline at end of file
diff --git a/validation-test/stdlib/Hashing.swift b/validation-test/stdlib/Hashing.swift
index b7c5a7076f58a..2f0b03f762b53 100644
--- a/validation-test/stdlib/Hashing.swift
+++ b/validation-test/stdlib/Hashing.swift
@@ -53,7 +53,7 @@ HashingTestSuite.test("_mixUInt64/GoldenValues") {
 HashingTestSuite.test("_mixUInt/GoldenValues") {
 #if arch(i386) || arch(arm)
   expectEqual(0x11b8_82c9, _mixUInt(0x0))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   expectEqual(0xb2b2_4f68_8dc4_164d, _mixUInt(0x0))
 #else
   fatalError("unimplemented")
@@ -63,7 +63,7 @@ HashingTestSuite.test("_mixUInt/GoldenValues") {
 HashingTestSuite.test("_mixInt/GoldenValues") {
 #if arch(i386) || arch(arm)
   expectEqual(Int(bitPattern: 0x11b8_82c9), _mixInt(0x0))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   expectEqual(Int(bitPattern: 0xb2b2_4f68_8dc4_164d), _mixInt(0x0))
 #else
   fatalError("unimplemented")
@@ -93,7 +93,7 @@ HashingTestSuite.test("_squeezeHashValue/Int") {
 #if arch(i386) || arch(arm)
   expectEqual(-0x6e477d37, _squeezeHashValue(0, Int.min..<(Int.max - 1)))
   expectEqual(0x38a3ea26, _squeezeHashValue(2, Int.min..<(Int.max - 1)))
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   expectEqual(0x32b24f688dc4164d, _squeezeHashValue(0, Int.min..<(Int.max - 1)))
   expectEqual(-0x6d1cc14f97aa822, _squeezeHashValue(1, Int.min..<(Int.max - 1)))
 #else
diff --git a/validation-test/stdlib/UnsafeBitMap.swift b/validation-test/stdlib/UnsafeBitMap.swift
index 5f471da7499cf..a27cf495185d4 100644
--- a/validation-test/stdlib/UnsafeBitMap.swift
+++ b/validation-test/stdlib/UnsafeBitMap.swift
@@ -23,7 +23,7 @@ UnsafeBitMapTests.test("wordIndex(_:), bitIndex(_:)") {
     expectEqual(3, _UnsafeBitMap.wordIndex(i), "i=\(i)")
     expectEqual(i - 96, Int(_UnsafeBitMap.bitIndex(i)), "i=\(i)")
   }
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   for i in 0...63 {
     expectEqual(0, _UnsafeBitMap.wordIndex(i), "i=\(i)")
     expectEqual(i, Int(_UnsafeBitMap.bitIndex(i)), "i=\(i)")
@@ -56,7 +56,7 @@ UnsafeBitMapTests.test("sizeInWords(forSizeInBits:)") {
   for i in 97...128 {
     expectEqual(4, _UnsafeBitMap.sizeInWords(forSizeInBits: i), "i=\(i)")
   }
-#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+#elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   for i in 1...64 {
     expectEqual(1, _UnsafeBitMap.sizeInWords(forSizeInBits: i), "i=\(i)")
   }