diff --git a/include/swift/Basic/Lazy.h b/include/swift/Basic/Lazy.h
index 594e2ed8b9fcd..6c899abeeffd6 100644
--- a/include/swift/Basic/Lazy.h
+++ b/include/swift/Basic/Lazy.h
@@ -16,6 +16,8 @@
 #include <memory>
 #ifdef __APPLE__
 #include <dispatch/dispatch.h>
+#elif defined(__wasi__)
+// No pthread on wasi, see https://bugs.swift.org/browse/SR-12097 for more details.
 #else
 #include <mutex>
 #endif
diff --git a/include/swift/SwiftRemoteMirror/Platform.h b/include/swift/SwiftRemoteMirror/Platform.h
index cefdea2c4de6f..96ed451f854eb 100644
--- a/include/swift/SwiftRemoteMirror/Platform.h
+++ b/include/swift/SwiftRemoteMirror/Platform.h
@@ -18,7 +18,7 @@ extern "C" {
 #endif
 
 #if defined(swiftRemoteMirror_EXPORTS)
-# if defined(__ELF__)
+# if defined(__ELF__) || defined(__WASM__)
 #   define SWIFT_REMOTE_MIRROR_LINKAGE __attribute__((__visibility__("protected")))
 # elif defined(__MACH__)
 #   define SWIFT_REMOTE_MIRROR_LINKAGE __attribute__((__visibility__("default")))
@@ -30,9 +30,7 @@ extern "C" {
 #   endif
 # endif
 #else
-# if defined(__ELF__)
-#   define SWIFT_REMOTE_MIRROR_LINKAGE __attribute__((__visibility__("default")))
-# elif defined(__MACH__)
+# if defined(__ELF__) || defined(__MACH__) || defined(__WASM__)
 #   define SWIFT_REMOTE_MIRROR_LINKAGE __attribute__((__visibility__("default")))
 # else
 #   if defined(_WINDLL)
diff --git a/lib/IRGen/IRGen.cpp b/lib/IRGen/IRGen.cpp
index a39ff7e63e4cd..b2d9b38b7bd50 100644
--- a/lib/IRGen/IRGen.cpp
+++ b/lib/IRGen/IRGen.cpp
@@ -165,6 +165,12 @@ swift::getIRTargetOptions(const IRGenOptions &Opts, ASTContext &Ctx) {
   TargetOpts.FunctionSections = Opts.FunctionSections;
 
   auto *Clang = static_cast<ClangImporter *>(Ctx.getClangModuleLoader());
+
+  // WebAssembly doesn't support atomics yet, see https://bugs.swift.org/browse/SR-12097
+  // for more details.
+  if (Clang->getTargetInfo().getTriple().isOSBinFormatWasm())
+    TargetOpts.ThreadModel = llvm::ThreadModel::Single;
+
   clang::TargetOptions &ClangOpts = Clang->getTargetInfo().getTargetOpts();
   return std::make_tuple(TargetOpts, ClangOpts.CPU, ClangOpts.Features, ClangOpts.Triple);
 }
diff --git a/stdlib/private/StdlibUnittest/InterceptTraps.cpp b/stdlib/private/StdlibUnittest/InterceptTraps.cpp
index 377397bebb9bf..80c3fa5979571 100644
--- a/stdlib/private/StdlibUnittest/InterceptTraps.cpp
+++ b/stdlib/private/StdlibUnittest/InterceptTraps.cpp
@@ -10,6 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// No signals support on WASI yet, see https://github.com/WebAssembly/WASI/issues/166.
+#if !defined(__wasi__)
 #include <stdio.h>
 #include <signal.h>
 #include <string.h>
@@ -48,6 +50,8 @@ static void CrashCatcher(int Sig) {
   _exit(0);
 }
 
+#endif // __wasi__
+
 #if defined(_WIN32)
 static LONG WINAPI
 VectoredCrashHandler(PEXCEPTION_POINTERS ExceptionInfo) {
@@ -63,13 +67,16 @@ VectoredCrashHandler(PEXCEPTION_POINTERS ExceptionInfo) {
 
   return EXCEPTION_CONTINUE_SEARCH;
 }
-#endif
+#endif // _WIN32
 
 SWIFT_CC(swift) SWIFT_RUNTIME_LIBRARY_VISIBILITY extern "C"
 void installTrapInterceptor() {
   // Disable buffering on stdout so that everything is printed before crashing.
   setbuf(stdout, 0);
 
+// No signals support on WASI yet, see https://github.com/WebAssembly/WASI/issues/166.
+#if !defined(__wasi__)
+
 #if defined(_WIN32)
   _set_abort_behavior(0, _WRITE_ABORT_MSG);
 #endif
@@ -87,3 +94,4 @@ void installTrapInterceptor() {
 #endif
 }
 
+#endif // !defined(__wasi__)
diff --git a/stdlib/private/StdlibUnittest/RaceTest.swift b/stdlib/private/StdlibUnittest/RaceTest.swift
index 3bdee4f03d32f..c883b03a8b85b 100644
--- a/stdlib/private/StdlibUnittest/RaceTest.swift
+++ b/stdlib/private/StdlibUnittest/RaceTest.swift
@@ -41,7 +41,7 @@ import SwiftPrivateLibcExtras
 import SwiftPrivateThreadExtras
 #if os(macOS) || os(iOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
 import Glibc
 #elseif os(Windows)
 import MSVCRT
@@ -562,7 +562,9 @@ class _InterruptibleSleep {
       return
     }
 
-    var timeout = timeval(tv_sec: duration, tv_usec: 0)
+    // WebAssembly/WASI on wasm32 is the only 32-bit platform with Int64 time_t,
+    // needs an explicit conversion to `time_t` because of this.
+    var timeout = timeval(tv_sec: time_t(duration), tv_usec: 0)
 
     var readFDs = _stdlib_fd_set()
     var writeFDs = _stdlib_fd_set()
diff --git a/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift b/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
index 20bb2b2c9376f..bc8bd4fe980e0 100644
--- a/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
+++ b/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
@@ -14,7 +14,7 @@ import SwiftPrivate
 import SwiftPrivateLibcExtras
 #if os(macOS) || os(iOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
 import Glibc
 #elseif os(Windows)
 import MSVCRT
diff --git a/stdlib/private/StdlibUnittest/StdlibUnittest.swift b/stdlib/private/StdlibUnittest/StdlibUnittest.swift
index c67f1778d8a3b..353fed99deb67 100644
--- a/stdlib/private/StdlibUnittest/StdlibUnittest.swift
+++ b/stdlib/private/StdlibUnittest/StdlibUnittest.swift
@@ -18,7 +18,7 @@ import SwiftPrivateLibcExtras
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 import Foundation
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
 import Glibc
 #elseif os(Windows)
 import MSVCRT
@@ -748,6 +748,9 @@ extension ProcessTerminationStatus {
     case .signal(let signal):
 #if os(Windows)
       return CInt(signal) == SIGILL
+#elseif os(WASI)
+      // No signals support on WASI yet, see https://github.com/WebAssembly/WASI/issues/166.
+      return false
 #else
       return CInt(signal) == SIGILL || CInt(signal) == SIGTRAP
 #endif
@@ -1746,6 +1749,7 @@ public enum OSVersion : CustomStringConvertible {
   case windowsCygnus
   case windows
   case haiku
+  case wasi
 
   public var description: String {
     switch self {
@@ -1777,6 +1781,8 @@ public enum OSVersion : CustomStringConvertible {
       return "Windows"
     case .haiku:
       return "Haiku"
+    case .wasi:
+      return "WASI"
     }
   }
 }
@@ -1821,6 +1827,8 @@ func _getOSVersion() -> OSVersion {
   return .windows
 #elseif os(Haiku)
   return .haiku
+#elseif os(WASI)
+  return .wasi
 #else
   let productVersion = _getSystemVersionPlistProperty("ProductVersion")!
   let (major, minor, bugFix) = _parseDottedVersionTriple(productVersion)
diff --git a/stdlib/private/StdlibUnittest/SymbolLookup.swift b/stdlib/private/StdlibUnittest/SymbolLookup.swift
index 2e9c627487a4f..ef211aa3db7ba 100644
--- a/stdlib/private/StdlibUnittest/SymbolLookup.swift
+++ b/stdlib/private/StdlibUnittest/SymbolLookup.swift
@@ -12,7 +12,7 @@
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
   import Glibc
 #elseif os(Windows)
   import MSVCRT
@@ -35,6 +35,8 @@
   #endif
 #elseif os(Windows)
   let hStdlibCore: HMODULE = GetModuleHandleA("swiftCore.dll")!
+#elseif os(WASI)
+// WASI doesn't support dynamic linking yet.
 #else
   #error("Unsupported platform")
 #endif
@@ -43,6 +45,8 @@ public func pointerToSwiftCoreSymbol(name: String) -> UnsafeMutableRawPointer? {
 #if os(Windows)
   return unsafeBitCast(GetProcAddress(hStdlibCore, name),
                        to: UnsafeMutableRawPointer?.self)
+#elseif os(WASI)
+  fatalError("\(#function) is not supported on WebAssembly/WASI")
 #else
   return dlsym(RTLD_DEFAULT, name)
 #endif
diff --git a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.c b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.c
index 4ef6fdbff8537..877dcd847b719 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.c
+++ b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.c
@@ -10,8 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-// posix_spawn is not available on Android or Windows (MSVC).
-#if !defined(__ANDROID__) && !defined(__HAIKU__) && (!defined(_WIN32) || defined(__CYGWIN__))
+// posix_spawn is not available on Android, HAIKU, WASI or Windows (MSVC).
+#if !defined(__ANDROID__) && !defined(__HAIKU__) && (!defined(_WIN32) || defined(__CYGWIN__)) && !defined(__wasi__)
 
 #include "swift/Runtime/Config.h"
 
@@ -54,5 +54,5 @@ int _stdlib_posix_spawn(pid_t *__restrict pid, const char * __restrict path,
   return posix_spawn(pid, path, file_actions, attrp, argv, envp);
 }
 
-#endif // !defined(__ANDROID__) && !defined(__HAIKU__) && (!defined(_WIN32) || defined(__CGYWIN__))
+#endif // !defined(__ANDROID__) && !defined(__HAIKU__) && (!defined(_WIN32) || defined(__CGYWIN__)) && !defined(__wasi__)
 
diff --git a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
index e95e07e142c3c..7142e1750fcf4 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
@@ -13,13 +13,15 @@
 import SwiftPrivate
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
 import Glibc
 #elseif os(Windows)
 import MSVCRT
 import WinSDK
 #endif
 
+#if !os(WASI)
+// No signals support on WASI yet, see https://github.com/WebAssembly/WASI/issues/166.
 internal func _signalToString(_ signal: Int) -> String {
   switch CInt(signal) {
   case SIGILL:  return "SIGILL"
@@ -34,6 +36,7 @@ internal func _signalToString(_ signal: Int) -> String {
   default:      return "SIG???? (\(signal))"
   }
 }
+#endif
 
 public enum ProcessTerminationStatus : CustomStringConvertible {
   case exit(Int)
@@ -44,7 +47,12 @@ public enum ProcessTerminationStatus : CustomStringConvertible {
     case .exit(let status):
       return "Exit(\(status))"
     case .signal(let signal):
+#if os(WASI)
+      // No signals support on WASI yet, see https://github.com/WebAssembly/WASI/issues/166.
+      fatalError("Signals are not supported on WebAssembly/WASI")
+#else
       return "Signal(\(_signalToString(signal)))"
+#endif
     }
   }
 }
@@ -141,6 +149,15 @@ public func waitProcess(_ process: HANDLE) -> ProcessTerminationStatus {
   }
   return .exit(Int(status))
 }
+#elseif os(WASI)
+// WASI doesn't support child processes
+public func spawnChild(_ args: [String])
+  -> (pid: pid_t, stdinFD: CInt, stdoutFD: CInt, stderrFD: CInt) {
+  fatalError("\(#function) is not supported on WebAssembly/WASI")
+}
+public func posixWaitpid(_ pid: pid_t) -> ProcessTerminationStatus {
+  fatalError("\(#function) is not supported on WebAssembly/WASI")
+}
 #else
 // posix_spawn is not available on Android.
 // posix_spawn is not available on Haiku.
diff --git a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
index 6fa2c06b6f6ee..ff2e2d9443ed0 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
@@ -13,14 +13,14 @@
 import SwiftPrivate
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
 import Glibc
 #elseif os(Windows)
 import MSVCRT
 #endif
 
 public func _stdlib_mkstemps(_ template: inout String, _ suffixlen: CInt) -> CInt {
-#if os(Android) || os(Haiku) || os(Windows)
+#if os(Android) || os(Haiku) || os(Windows) || os(WASI)
   preconditionFailure("mkstemps doesn't work on your platform")
 #else
   var utf8CStr = template.utf8CString
@@ -125,6 +125,8 @@ public func _stdlib_pipe() -> (readEnd: CInt, writeEnd: CInt, error: CInt) {
   let ret = fds.withUnsafeMutableBufferPointer { unsafeFds -> CInt in
 #if os(Windows)
     return _pipe(unsafeFds.baseAddress, 0, 0)
+#elseif os(WASI)
+    preconditionFailure("No pipes available on WebAssembly/WASI")
 #else
     return pipe(unsafeFds.baseAddress)
 #endif
diff --git a/stdlib/private/SwiftPrivateThreadExtras/CMakeLists.txt b/stdlib/private/SwiftPrivateThreadExtras/CMakeLists.txt
index 97e2cc0d2af5b..3a72dd196729f 100644
--- a/stdlib/private/SwiftPrivateThreadExtras/CMakeLists.txt
+++ b/stdlib/private/SwiftPrivateThreadExtras/CMakeLists.txt
@@ -16,6 +16,7 @@ add_swift_target_library(swiftSwiftPrivateThreadExtras ${SWIFT_STDLIB_LIBRARY_BU
   SWIFT_MODULE_DEPENDS_HAIKU Glibc
   SWIFT_MODULE_DEPENDS_WINDOWS MSVCRT WinSDK
   SWIFT_COMPILE_FLAGS ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
+  TARGET_SDKS ALL_APPLE_PLATFORMS CYGWIN FREEBSD HAIKU LINUX WINDOWS
   INSTALL_IN_COMPONENT stdlib-experimental
   DARWIN_INSTALL_NAME_DIR "${SWIFT_DARWIN_STDLIB_PRIVATE_INSTALL_NAME_DIR}")
 
diff --git a/stdlib/public/Platform/Platform.swift b/stdlib/public/Platform/Platform.swift
index f5e54c7da1283..b9a2abaca082f 100644
--- a/stdlib/public/Platform/Platform.swift
+++ b/stdlib/public/Platform/Platform.swift
@@ -366,6 +366,8 @@ public var SIG_IGN: _crt_signal_t {
 public var SIG_ERR: _crt_signal_t {
   return unsafeBitCast(-1, to: _crt_signal_t.self)
 }
+#elseif os(WASI)
+// No signals support on WASI yet, see https://github.com/WebAssembly/WASI/issues/166.
 #else
 internal var _ignore = _UnsupportedPlatformError()
 #endif
@@ -380,7 +382,7 @@ public var SEM_FAILED: UnsafeMutablePointer<sem_t>? {
 #if os(macOS) || 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) || os(Cygwin) || os(Haiku)
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
   // The value is ABI.  Value verified to be correct on Glibc.
   return UnsafeMutablePointer<sem_t>(bitPattern: 0)
 #else
diff --git a/stdlib/public/Platform/glibc.modulemap.gyb b/stdlib/public/Platform/glibc.modulemap.gyb
index 12de1073b6317..9a88a108c74c3 100644
--- a/stdlib/public/Platform/glibc.modulemap.gyb
+++ b/stdlib/public/Platform/glibc.modulemap.gyb
@@ -126,10 +126,12 @@ module SwiftGlibc [system] {
       header "${GLIBC_INCLUDE_PATH}/math.h"
       export *
     }
+% if CMAKE_SDK != "WASI":
     module setjmp {
       header "${GLIBC_INCLUDE_PATH}/setjmp.h"
       export *
     }
+% end
     module signal {
       header "${GLIBC_INCLUDE_PATH}/signal.h"
       export *
@@ -319,6 +321,7 @@ module SwiftGlibc [system] {
       header "${GLIBC_INCLUDE_PATH}/dirent.h"
       export *
     }
+% if CMAKE_SDK != "WASI":
     module dl {
       header "${GLIBC_INCLUDE_PATH}/link.h"
       export *
@@ -327,6 +330,7 @@ module SwiftGlibc [system] {
       header "${GLIBC_INCLUDE_PATH}/dlfcn.h"
       export *
     }
+% end
     module fcntl {
       header "${GLIBC_INCLUDE_PATH}/fcntl.h"
       export *
@@ -335,10 +339,12 @@ module SwiftGlibc [system] {
       header "${GLIBC_INCLUDE_PATH}/fnmatch.h"
       export *
     }
+% if CMAKE_SDK != "WASI":
     module grp {
       header "${GLIBC_INCLUDE_PATH}/grp.h"
       export *
     }
+% end
     module ioctl {
       header "${GLIBC_ARCH_INCLUDE_PATH}/sys/ioctl.h"
       export *
@@ -347,12 +353,14 @@ module SwiftGlibc [system] {
       header "${GLIBC_INCLUDE_PATH}/libgen.h"
       export *
     }
+% if CMAKE_SDK != "WASI":
     module net {
       module if {
         header "${GLIBC_INCLUDE_PATH}/net/if.h"
         export *
       }
     }
+% end
     module netinet {
       module in {
         header "${GLIBC_INCLUDE_PATH}/netinet/in.h"
@@ -369,10 +377,12 @@ module SwiftGlibc [system] {
       header "${GLIBC_INCLUDE_PATH}/poll.h"
       export *
     }
+% if CMAKE_SDK != "WASI":
     module pthread {
       header "${GLIBC_INCLUDE_PATH}/pthread.h"
       export *
     }
+% end
     module pwd {
       header "${GLIBC_INCLUDE_PATH}/pwd.h"
       export *
@@ -422,18 +432,22 @@ module SwiftGlibc [system] {
       }
 % end
 
+% if CMAKE_SDK != "WASI":
       module ipc {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/ipc.h"
         export *
       }
+% end
       module mman {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/mman.h"
         export *
       }
+% if CMAKE_SDK != "WASI":
       module msg {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/msg.h"
         export *
       }
+% end
       module resource {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/resource.h"
         export *
@@ -442,7 +456,7 @@ module SwiftGlibc [system] {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/select.h"
         export *
       }
-% if CMAKE_SDK != "FREEBSD" and CMAKE_SDK != "HAIKU":
+% if CMAKE_SDK != "FREEBSD" and CMAKE_SDK != "HAIKU" and CMAKE_SDK != "WASI":
       module sendfile {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/sendfile.h"
         export *
@@ -492,34 +506,42 @@ module SwiftGlibc [system] {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/utsname.h"
         export *
       }
+% if CMAKE_SDK != "WASI":
       module wait {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/wait.h"
         export *
       }
     }
+% end
 % if CMAKE_SDK in ["LINUX", "FREEBSD"]:
     module sysexits {
       header "${GLIBC_INCLUDE_PATH}/sysexits.h"
       export *
     }
 % end
+% if CMAKE_SDK != "WASI":
     module termios {
       header "${GLIBC_INCLUDE_PATH}/termios.h"
       export *
     }
+% end
     module unistd {
       header "${GLIBC_INCLUDE_PATH}/unistd.h"
       export *
     }
+% if CMAKE_SDK != "WASI":
     module utime {
       header "${GLIBC_INCLUDE_PATH}/utime.h"
       export *
     }
   }
+% end
 }
 
+% if CMAKE_SDK != "WASI":
 module CUUID [system] {
   header "${GLIBC_INCLUDE_PATH}/uuid/uuid.h"
   link "uuid"
   export *
 }
+% end
diff --git a/stdlib/public/SwiftShims/Visibility.h b/stdlib/public/SwiftShims/Visibility.h
index 8577fad1653b9..ebe73edd650fd 100644
--- a/stdlib/public/SwiftShims/Visibility.h
+++ b/stdlib/public/SwiftShims/Visibility.h
@@ -76,7 +76,7 @@
 // SWIFT_RUNTIME_EXPORT on the library it's exported from.
 
 /// Attribute used to export symbols from the runtime.
-#if defined(__MACH__)
+#if defined(__MACH__) || defined(__wasi__)
 
 # define SWIFT_EXPORT_ATTRIBUTE __attribute__((__visibility__("default")))
 
diff --git a/stdlib/public/core/AtomicInt.swift.gyb b/stdlib/public/core/AtomicInt.swift.gyb
index 62217f282ccc5..80514f42c147c 100644
--- a/stdlib/public/core/AtomicInt.swift.gyb
+++ b/stdlib/public/core/AtomicInt.swift.gyb
@@ -65,7 +65,7 @@ internal func _swift_stdlib_atomicCompareExchangeStrongInt(
   object target: UnsafeMutablePointer<Int>,
   expected: UnsafeMutablePointer<Int>,
   desired: Int) -> Bool {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
   let (oldValue, won) = Builtin.cmpxchg_seqcst_seqcst_Int32(
     target._rawValue, expected.pointee._value, desired._value)
 #elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
@@ -82,7 +82,7 @@ internal func _swift_stdlib_atomicCompareExchangeStrongInt(
 public // Existing uses outside stdlib
 func _swift_stdlib_atomicLoadInt(
   object target: UnsafeMutablePointer<Int>) -> Int {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
   let value = Builtin.atomicload_seqcst_Int32(target._rawValue)
   return Int(value)
 #elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
@@ -95,7 +95,7 @@ func _swift_stdlib_atomicLoadInt(
 internal func _swift_stdlib_atomicStoreInt(
   object target: UnsafeMutablePointer<Int>,
   desired: Int) {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
   Builtin.atomicstore_seqcst_Int32(target._rawValue, desired._value)
 #elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
   Builtin.atomicstore_seqcst_Int64(target._rawValue, desired._value)
@@ -111,7 +111,7 @@ func _swift_stdlib_atomicFetch${operation}Int(
   object target: UnsafeMutablePointer<Int>,
   operand: Int) -> Int {
   let rawTarget = UnsafeMutableRawPointer(target)
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
   let value = _swift_stdlib_atomicFetch${operation}Int32(
     object: rawTarget.assumingMemoryBound(to: Int32.self),
     operand: Int32(operand))
diff --git a/stdlib/public/core/BridgeStorage.swift b/stdlib/public/core/BridgeStorage.swift
index 4b100f28f7cfe..fcc971f8b6f76 100644
--- a/stdlib/public/core/BridgeStorage.swift
+++ b/stdlib/public/core/BridgeStorage.swift
@@ -61,7 +61,7 @@ internal struct _BridgeStorage<NativeClass: AnyObject> {
     rawValue = Builtin.reinterpretCast(native)
   }
 
-#if !(arch(i386) || arch(arm))
+#if !(arch(i386) || arch(arm) || arch(wasm32))
   @inlinable
   @inline(__always)
   internal init(taggedPayload: UInt) {
diff --git a/stdlib/public/core/Builtin.swift b/stdlib/public/core/Builtin.swift
index 07c9ce14af6a7..8aceb9014df08 100644
--- a/stdlib/public/core/Builtin.swift
+++ b/stdlib/public/core/Builtin.swift
@@ -378,8 +378,8 @@ internal var _objectPointerLowSpareBitShift: UInt {
     }
 }
 
-#if arch(i386) || arch(arm) || arch(powerpc64) || arch(powerpc64le) || arch(
-  s390x)
+#if arch(i386) || arch(arm) || arch(wasm32) || arch(powerpc64) || arch(
+  powerpc64le) || arch(s390x)
 @inlinable
 internal var _objectPointerIsObjCBit: UInt {
     @inline(__always) get { return 0x0000_0002 }
diff --git a/stdlib/public/core/DictionaryVariant.swift b/stdlib/public/core/DictionaryVariant.swift
index 988ef9f7d0679..5ffaacc45ef32 100644
--- a/stdlib/public/core/DictionaryVariant.swift
+++ b/stdlib/public/core/DictionaryVariant.swift
@@ -46,7 +46,7 @@ extension Dictionary {
     @inlinable
     @inline(__always)
     init(dummy: Void) {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
       self.init(native: _NativeDictionary())
 #else
       self.object = _BridgeStorage(taggedPayload: 0)
diff --git a/stdlib/public/core/Hasher.swift b/stdlib/public/core/Hasher.swift
index f7098db934aa3..202c2830610ab 100644
--- a/stdlib/public/core/Hasher.swift
+++ b/stdlib/public/core/Hasher.swift
@@ -160,7 +160,7 @@ extension Hasher {
 
     @inline(__always)
     internal mutating func combine(_ value: UInt) {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
       combine(UInt32(truncatingIfNeeded: value))
 #else
       combine(UInt64(truncatingIfNeeded: value))
@@ -423,7 +423,7 @@ public struct Hasher {
   @usableFromInline
   internal static func _hash(seed: Int, _ value: UInt) -> Int {
     var state = _State(seed: seed)
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     _internalInvariant(UInt.bitWidth < UInt64.bitWidth)
     let tbc = _TailBuffer(
       tail: UInt64(truncatingIfNeeded: value),
diff --git a/stdlib/public/core/SetVariant.swift b/stdlib/public/core/SetVariant.swift
index 0092fc74ca45e..ae3149002c7c4 100644
--- a/stdlib/public/core/SetVariant.swift
+++ b/stdlib/public/core/SetVariant.swift
@@ -36,7 +36,7 @@ extension Set {
     @inlinable
     @inline(__always)
     init(dummy: ()) {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
       self.init(native: _NativeSet())
 #else
       self.object = _BridgeStorage(taggedPayload: 0)
diff --git a/stdlib/public/core/StringBridge.swift b/stdlib/public/core/StringBridge.swift
index 1896d974204b6..2b892c2d52867 100644
--- a/stdlib/public/core/StringBridge.swift
+++ b/stdlib/public/core/StringBridge.swift
@@ -285,7 +285,7 @@ internal enum _KnownCocoaString {
   case storage
   case shared
   case cocoa
-#if !(arch(i386) || arch(arm))
+#if !(arch(i386) || arch(arm) || arch(wasm32))
   case tagged
 #endif
 
diff --git a/stdlib/public/core/StringGuts.swift b/stdlib/public/core/StringGuts.swift
index 2b7765cd4093e..cc6ed08226e67 100644
--- a/stdlib/public/core/StringGuts.swift
+++ b/stdlib/public/core/StringGuts.swift
@@ -178,7 +178,7 @@ extension _StringGuts {
   #else
   @usableFromInline @inline(never) @_effects(releasenone)
   internal func _invariantCheck() {
-    #if arch(i386) || arch(arm)
+    #if arch(i386) || arch(arm) || arch(wasm32)
     _internalInvariant(MemoryLayout<String>.size == 12, """
     the runtime is depending on this, update Reflection.mm and \
     this if you change it
diff --git a/stdlib/public/core/StringObject.swift b/stdlib/public/core/StringObject.swift
index 501d81b66ff53..7173d8d8b55be 100644
--- a/stdlib/public/core/StringObject.swift
+++ b/stdlib/public/core/StringObject.swift
@@ -169,7 +169,7 @@ extension _StringObject {
   @usableFromInline
   internal typealias RawBitPattern = (UInt64, UInt64)
 
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
   // On 32-bit platforms, raw bit conversion is one-way only and uses the same
   // layout as on 64-bit platforms.
   @usableFromInline
@@ -245,7 +245,7 @@ extension _StringObject {
 
   @inlinable @_transparent
   internal var discriminatedObjectRawBits: UInt64 {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     let low32: UInt
     switch _variant {
     case .immortal(let bitPattern):
@@ -387,7 +387,7 @@ extension _StringObject.Nibbles {
 extension _StringObject {
   @inlinable @inline(__always)
   internal static var nativeBias: UInt {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     return 20
 #else
     return 32
@@ -556,7 +556,7 @@ extension _StringObject {
   @inlinable @inline(__always)
   internal init(empty:()) {
     // Canonical empty pattern: small zero-length string
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     self.init(
       count: 0,
       variant: .immortal(0),
@@ -832,7 +832,7 @@ extension _StringObject {
 
   @inline(__always)
   internal var sharedStorage: __SharedStringStorage {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     guard case .native(let storage) = _variant else {
       _internalInvariantFailure()
     }
@@ -846,7 +846,7 @@ extension _StringObject {
 
   @inline(__always)
   internal var cocoaObject: AnyObject {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     guard case .bridged(let object) = _variant else {
       _internalInvariantFailure()
     }
@@ -935,7 +935,7 @@ extension _StringObject {
   internal init(immortal bufPtr: UnsafeBufferPointer<UInt8>, isASCII: Bool) {
     let countAndFlags = CountAndFlags(
       immortalCount: bufPtr.count, isASCII: isASCII)
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     self.init(
       variant: .immortal(start: bufPtr.baseAddress._unsafelyUnwrappedUnchecked),
       discriminator: Nibbles.largeImmortal(),
@@ -955,7 +955,7 @@ extension _StringObject {
 
   @inline(__always)
   internal init(_ storage: __StringStorage) {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     self.init(
       variant: .native(storage),
       discriminator: Nibbles.largeMortal(),
@@ -969,7 +969,7 @@ extension _StringObject {
   }
 
   internal init(_ storage: __SharedStringStorage) {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     self.init(
       variant: .native(storage),
       discriminator: Nibbles.largeMortal(),
@@ -1009,7 +1009,7 @@ extension _StringObject {
   #else
   @usableFromInline @inline(never) @_effects(releasenone)
   internal func _invariantCheck() {
-    #if arch(i386) || arch(arm)
+    #if arch(i386) || arch(arm) || arch(wasm32)
     _internalInvariant(MemoryLayout<_StringObject>.size == 12)
     _internalInvariant(MemoryLayout<_StringObject>.stride == 12)
     _internalInvariant(MemoryLayout<_StringObject>.alignment == 4)
@@ -1079,7 +1079,7 @@ extension _StringObject {
       }
     }
 
-    #if arch(i386) || arch(arm)
+    #if arch(i386) || arch(arm) || arch(wasm32)
     switch _variant {
     case .immortal:
       _internalInvariant(isImmortal)
@@ -1099,7 +1099,7 @@ extension _StringObject {
     let raw = self.rawBits
     let word0 = ("0000000000000000" + String(raw.0, radix: 16)).suffix(16)
     let word1 = ("0000000000000000" + String(raw.1, radix: 16)).suffix(16)
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     print("""
       StringObject(\
       <\(word0) \(word1)> \
diff --git a/stdlib/public/core/StringStorage.swift b/stdlib/public/core/StringStorage.swift
index 8e8a3d29d2133..a80937d1f7141 100644
--- a/stdlib/public/core/StringStorage.swift
+++ b/stdlib/public/core/StringStorage.swift
@@ -46,7 +46,7 @@ private typealias CountAndFlags = _StringObject.CountAndFlags
 // renamed. The old name must not be used in the new runtime.
 final internal class __StringStorage
   : __SwiftNativeNSString, _AbstractStringStorage {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
   // The total allocated storage capacity. Note that this includes the required
   // nul-terminator.
   internal var _realCapacity: Int
@@ -106,7 +106,7 @@ final internal class __StringStorage
 // for Strings ~1KB or larger, though at this point we're well into our growth
 // curve.
 private func determineCodeUnitCapacity(_ desiredCapacity: Int) -> Int {
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
   // FIXME: Adapt to actual 32-bit allocator. For now, let's arrange things so
   // that the instance size will be a multiple of 4.
   let bias = Int(bitPattern: _StringObject.nativeBias)
@@ -139,7 +139,7 @@ extension __StringStorage {
       __StringStorage.self,
       realCodeUnitCapacity._builtinWordValue, UInt8.self,
       1._builtinWordValue, Optional<_StringBreadcrumbs>.self)
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     storage._realCapacity = realCodeUnitCapacity
     storage._count = countAndFlags.count
     storage._flags = countAndFlags.flags
@@ -319,7 +319,7 @@ extension __StringStorage {
   internal func _updateCountAndFlags(newCount: Int, newIsASCII: Bool) {
     let countAndFlags = CountAndFlags(
       mortalCount: newCount, isASCII: newIsASCII)
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     self._count = countAndFlags.count
     self._flags = countAndFlags.flags
 #else
@@ -463,7 +463,7 @@ final internal class __SharedStringStorage
   internal var _owner: AnyObject?
   internal var start: UnsafePointer<UInt8>
 
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
   internal var _count: Int
   internal var _flags: UInt16
 
@@ -485,7 +485,7 @@ final internal class __SharedStringStorage
   ) {
     self._owner = nil
     self.start = ptr
-#if arch(i386) || arch(arm)
+#if arch(i386) || arch(arm) || arch(wasm32)
     self._count = countAndFlags.count
     self._flags = countAndFlags.flags
 #else
diff --git a/stdlib/public/runtime/Casting.cpp b/stdlib/public/runtime/Casting.cpp
index 2c516a9952fb4..5058f7cb6eb9c 100644
--- a/stdlib/public/runtime/Casting.cpp
+++ b/stdlib/public/runtime/Casting.cpp
@@ -30,7 +30,12 @@
 #include "swift/Runtime/ExistentialContainer.h"
 #include "swift/Runtime/HeapObject.h"
 #include "swift/Runtime/Metadata.h"
-#include "swift/Runtime/Mutex.h"
+#if defined(__wasi__)
+# define SWIFT_CASTING_SUPPORTS_MUTEX 0
+#else
+# define SWIFT_CASTING_SUPPORTS_MUTEX 1
+# include "swift/Runtime/Mutex.h"
+#endif
 #include "swift/Runtime/Unreachable.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/PointerIntPair.h"
@@ -125,7 +130,9 @@ TypeNamePair
 swift::swift_getTypeName(const Metadata *type, bool qualified) {
   using Key = llvm::PointerIntPair<const Metadata *, 1, bool>;
 
+  #if SWIFT_CASTING_SUPPORTS_MUTEX
   static StaticReadWriteLock TypeNameCacheLock;
+  #endif
   static Lazy<llvm::DenseMap<Key, std::pair<const char *, size_t>>>
     TypeNameCache;
   
@@ -134,7 +141,9 @@ swift::swift_getTypeName(const Metadata *type, bool qualified) {
 
   // Attempt read-only lookup of cache entry.
   {
+    #if SWIFT_CASTING_SUPPORTS_MUTEX
     StaticScopedReadLock guard(TypeNameCacheLock);
+    #endif
 
     auto found = cache.find(key);
     if (found != cache.end()) {
@@ -145,7 +154,9 @@ swift::swift_getTypeName(const Metadata *type, bool qualified) {
 
   // Read-only lookup failed to find item, we may need to create it.
   {
+    #if SWIFT_CASTING_SUPPORTS_MUTEX
     StaticScopedWriteLock guard(TypeNameCacheLock);
+    #endif
 
     // Do lookup again just to make sure it wasn't created by another
     // thread before we acquired the write lock.
diff --git a/stdlib/public/runtime/Errors.cpp b/stdlib/public/runtime/Errors.cpp
index 6e202434750f0..48201d54d6a2a 100644
--- a/stdlib/public/runtime/Errors.cpp
+++ b/stdlib/public/runtime/Errors.cpp
@@ -14,7 +14,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if defined(__CYGWIN__) || defined(__HAIKU__)
+#if defined(__CYGWIN__) || defined(__HAIKU__) || defined(__wasi__)
 #define SWIFT_SUPPORTS_BACKTRACE_REPORTING 0
 #else
 #define SWIFT_SUPPORTS_BACKTRACE_REPORTING 1
diff --git a/stdlib/public/runtime/Metadata.cpp b/stdlib/public/runtime/Metadata.cpp
index 2546d90dec5cd..7a23376cf8f11 100644
--- a/stdlib/public/runtime/Metadata.cpp
+++ b/stdlib/public/runtime/Metadata.cpp
@@ -44,7 +44,10 @@
 #else
 #include <sys/mman.h>
 #include <unistd.h>
+// WASI doesn't support dynamic linking yet.
+#if !defined(__wasi__)
 #include <dlfcn.h>
+#endif // !defined(__wasi__)
 #endif
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Hashing.h"
diff --git a/stdlib/public/runtime/MutexPThread.cpp b/stdlib/public/runtime/MutexPThread.cpp
index 62e718c16abaa..92db58226cd09 100644
--- a/stdlib/public/runtime/MutexPThread.cpp
+++ b/stdlib/public/runtime/MutexPThread.cpp
@@ -15,7 +15,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if !defined(_WIN32)
+#if !defined(_WIN32) && !defined(__wasi__)
 #include "swift/Runtime/Mutex.h"
 
 #include "swift/Runtime/Debug.h"