diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index b738e6b0157d5..6e2cef8146043 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -22,6 +22,8 @@ jobs:
           sudo apt-get purge libgcc-9-dev gcc-9 libstdc++-9-dev
           sudo swapoff -a
           sudo rm -f /swapfile
+          sudo rm -rf /opt/hostedtoolcache
+          sudo rm -rf /usr/share/dotnet
           sudo apt clean
           docker rmi $(docker image ls -aq)
           df -h
@@ -93,7 +95,7 @@ jobs:
           name: packaging-scripts
           path: utils/webassembly
       - name: Pack test results
-        run: tar cJf swift-test-results.tar.gz ../build/*/swift-macosx-x86_64/swift-test-results
+        run: tar cJf swift-test-results.tar.gz ../target-build/*/swift-macosx-x86_64/swift-test-results
       - name: Upload test results
         uses: actions/upload-artifact@v1
         with:
diff --git a/lib/ClangImporter/ClangImporter.cpp b/lib/ClangImporter/ClangImporter.cpp
index 7eb56830539b4..ae3b4ca429f00 100644
--- a/lib/ClangImporter/ClangImporter.cpp
+++ b/lib/ClangImporter/ClangImporter.cpp
@@ -458,6 +458,41 @@ getGlibcModuleMapPath(SearchPathOptions& Opts, llvm::Triple triple,
   return None;
 }
 
+static Optional<StringRef>
+getWasiLibcModuleMapPath(SearchPathOptions& Opts, llvm::Triple triple,
+                      SmallVectorImpl<char> &buffer) {
+  StringRef platform = swift::getPlatformNameForTriple(triple);
+  StringRef arch = swift::getMajorArchitectureName(triple);
+
+  if (!Opts.SDKPath.empty()) {
+    buffer.clear();
+    buffer.append(Opts.SDKPath.begin(), Opts.SDKPath.end());
+    llvm::sys::path::append(buffer, "usr", "lib", "swift");
+    llvm::sys::path::append(buffer, platform, arch, "wasi.modulemap");
+
+    // Only specify the module map if that file actually exists.  It may not;
+    // for example in the case that `swiftc -target x86_64-unknown-linux-gnu
+    // -emit-ir` is invoked using a Swift compiler not built for Linux targets.
+    if (llvm::sys::fs::exists(buffer))
+      return StringRef(buffer.data(), buffer.size());
+  }
+
+  if (!Opts.RuntimeResourcePath.empty()) {
+    buffer.clear();
+    buffer.append(Opts.RuntimeResourcePath.begin(),
+                  Opts.RuntimeResourcePath.end());
+    llvm::sys::path::append(buffer, platform, arch, "wasi.modulemap");
+
+    // Only specify the module map if that file actually exists.  It may not;
+    // for example in the case that `swiftc -target x86_64-unknown-linux-gnu
+    // -emit-ir` is invoked using a Swift compiler not built for Linux targets.
+    if (llvm::sys::fs::exists(buffer))
+      return StringRef(buffer.data(), buffer.size());
+  }
+
+  return None;
+}
+
 static void
 getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
                              ASTContext &ctx,
@@ -601,6 +636,10 @@ getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
 
     if (triple.isOSWASI()) {
       invocationArgStrs.insert(invocationArgStrs.end(), {"-D_WASI_EMULATED_MMAN"});
+      SmallString<128> buffer;
+      if (auto path = getWasiLibcModuleMapPath(searchPathOpts, triple, buffer)) {
+        invocationArgStrs.push_back((Twine("-fmodule-map-file=") + *path).str());
+      }
     }
 
     if (triple.isOSWindows()) {
diff --git a/stdlib/private/StdlibUnittest/CMakeLists.txt b/stdlib/private/StdlibUnittest/CMakeLists.txt
index cf2f8f0a2966a..2014d86a1431c 100644
--- a/stdlib/private/StdlibUnittest/CMakeLists.txt
+++ b/stdlib/private/StdlibUnittest/CMakeLists.txt
@@ -41,7 +41,7 @@ add_swift_target_library(swiftStdlibUnittest ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES}
   SWIFT_MODULE_DEPENDS_OPENBSD Glibc SwiftPrivateThreadExtras
   SWIFT_MODULE_DEPENDS_CYGWIN Glibc SwiftPrivateThreadExtras
   SWIFT_MODULE_DEPENDS_HAIKU Glibc SwiftPrivateThreadExtras
-  SWIFT_MODULE_DEPENDS_WASI Glibc
+  SWIFT_MODULE_DEPENDS_WASI WASILibc
   SWIFT_MODULE_DEPENDS_WINDOWS MSVCRT WinSDK SwiftPrivateThreadExtras
   SWIFT_COMPILE_FLAGS ${swift_stdlib_unittest_compile_flags} ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
   INSTALL_IN_COMPONENT stdlib-experimental
diff --git a/stdlib/private/StdlibUnittest/RaceTest.swift b/stdlib/private/StdlibUnittest/RaceTest.swift
index cbd8b460d4cfa..9cfd0677f2129 100644
--- a/stdlib/private/StdlibUnittest/RaceTest.swift
+++ b/stdlib/private/StdlibUnittest/RaceTest.swift
@@ -43,8 +43,10 @@ import SwiftPrivateThreadExtras
 #endif
 #if os(macOS) || os(iOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
 import Glibc
+#elseif os(WASI)
+import WASILibc
 #elseif os(Windows)
 import MSVCRT
 import WinSDK
diff --git a/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift b/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
index db3262166f40e..c612ffbcbd411 100644
--- a/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
+++ b/stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
@@ -14,8 +14,10 @@ import SwiftPrivate
 import SwiftPrivateLibcExtras
 #if os(macOS) || os(iOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
 import Glibc
+#elseif os(WASI)
+import WASILibc
 #elseif os(Windows)
 import MSVCRT
 #endif
diff --git a/stdlib/private/StdlibUnittest/StdlibUnittest.swift b/stdlib/private/StdlibUnittest/StdlibUnittest.swift
index 6628000ac7782..8af0a22f294ef 100644
--- a/stdlib/private/StdlibUnittest/StdlibUnittest.swift
+++ b/stdlib/private/StdlibUnittest/StdlibUnittest.swift
@@ -20,8 +20,10 @@ import SwiftPrivateLibcExtras
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 import Foundation
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
 import Glibc
+#elseif os(WASI)
+import WASILibc
 #elseif os(Windows)
 import MSVCRT
 import WinSDK
diff --git a/stdlib/private/StdlibUnittest/SymbolLookup.swift b/stdlib/private/StdlibUnittest/SymbolLookup.swift
index a403f0381849f..c0d82350a8da4 100644
--- a/stdlib/private/StdlibUnittest/SymbolLookup.swift
+++ b/stdlib/private/StdlibUnittest/SymbolLookup.swift
@@ -12,8 +12,10 @@
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
   import Glibc
+#elseif os(WASI)
+  import WASILibc
 #elseif os(Windows)
   import MSVCRT
   import WinSDK
diff --git a/stdlib/private/SwiftPrivateLibcExtras/CMakeLists.txt b/stdlib/private/SwiftPrivateLibcExtras/CMakeLists.txt
index b9245aef1f272..6e00a29c3ad10 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/CMakeLists.txt
+++ b/stdlib/private/SwiftPrivateLibcExtras/CMakeLists.txt
@@ -18,7 +18,7 @@ add_swift_target_library(swiftSwiftPrivateLibcExtras ${SWIFT_STDLIB_LIBRARY_BUIL
   SWIFT_MODULE_DEPENDS_OPENBSD Glibc
   SWIFT_MODULE_DEPENDS_CYGWIN Glibc
   SWIFT_MODULE_DEPENDS_HAIKU Glibc
-  SWIFT_MODULE_DEPENDS_WASI Glibc
+  SWIFT_MODULE_DEPENDS_WASI WASILibc
   SWIFT_MODULE_DEPENDS_WINDOWS MSVCRT WinSDK
   INSTALL_IN_COMPONENT stdlib-experimental
   DARWIN_INSTALL_NAME_DIR "${SWIFT_DARWIN_STDLIB_PRIVATE_INSTALL_NAME_DIR}")
diff --git a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
index d9c35af07f273..e7e95eee6b73e 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/Subprocess.swift
@@ -13,8 +13,10 @@
 import SwiftPrivate
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
 import Glibc
+#elseif os(WASI)
+import WASILibc
 #elseif os(Windows)
 import MSVCRT
 import WinSDK
diff --git a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
index a72fa0982517e..cfc457de0e84e 100644
--- a/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
+++ b/stdlib/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift
@@ -13,8 +13,10 @@
 import SwiftPrivate
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
 import Glibc
+#elseif os(WASI)
+import WASILibc
 #elseif os(Windows)
 import MSVCRT
 #endif
diff --git a/stdlib/private/SwiftPrivateThreadExtras/CMakeLists.txt b/stdlib/private/SwiftPrivateThreadExtras/CMakeLists.txt
index 10d3ccc73e69c..239cd620ca203 100644
--- a/stdlib/private/SwiftPrivateThreadExtras/CMakeLists.txt
+++ b/stdlib/private/SwiftPrivateThreadExtras/CMakeLists.txt
@@ -15,7 +15,7 @@ add_swift_target_library(swiftSwiftPrivateThreadExtras ${SWIFT_STDLIB_LIBRARY_BU
   SWIFT_MODULE_DEPENDS_OPENBSD Glibc
   SWIFT_MODULE_DEPENDS_CYGWIN Glibc
   SWIFT_MODULE_DEPENDS_HAIKU Glibc
-  SWIFT_MODULE_DEPENDS_WASI Glibc
+  SWIFT_MODULE_DEPENDS_WASI WASILibc
   SWIFT_MODULE_DEPENDS_WINDOWS MSVCRT WinSDK
   SWIFT_COMPILE_FLAGS ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
   TARGET_SDKS ALL_APPLE_PLATFORMS CYGWIN FREEBSD OPENBSD HAIKU LINUX WINDOWS ANDROID
diff --git a/stdlib/private/SwiftPrivateThreadExtras/SwiftPrivateThreadExtras.swift b/stdlib/private/SwiftPrivateThreadExtras/SwiftPrivateThreadExtras.swift
index 086dfd7bb430c..7340425218a4c 100644
--- a/stdlib/private/SwiftPrivateThreadExtras/SwiftPrivateThreadExtras.swift
+++ b/stdlib/private/SwiftPrivateThreadExtras/SwiftPrivateThreadExtras.swift
@@ -17,8 +17,10 @@
 
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
 import Glibc
+#elseif os(WASI)
+import WASILibc
 #elseif os(Windows)
 import MSVCRT
 import WinSDK
diff --git a/stdlib/private/SwiftPrivateThreadExtras/ThreadBarriers.swift b/stdlib/private/SwiftPrivateThreadExtras/ThreadBarriers.swift
index 94bcff325dfc0..d1354854b09c5 100644
--- a/stdlib/private/SwiftPrivateThreadExtras/ThreadBarriers.swift
+++ b/stdlib/private/SwiftPrivateThreadExtras/ThreadBarriers.swift
@@ -12,8 +12,10 @@
 
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
 import Glibc
+#elseif os(WASI)
+import WASILibc
 #elseif os(Windows)
 import MSVCRT
 import WinSDK
diff --git a/stdlib/public/Differentiation/CMakeLists.txt b/stdlib/public/Differentiation/CMakeLists.txt
index cceb6fc6f2560..73c2be3273c3a 100644
--- a/stdlib/public/Differentiation/CMakeLists.txt
+++ b/stdlib/public/Differentiation/CMakeLists.txt
@@ -31,7 +31,7 @@ add_swift_target_library(swift_Differentiation ${SWIFT_STDLIB_LIBRARY_BUILD_TYPE
   SWIFT_MODULE_DEPENDS_OPENBSD Glibc
   SWIFT_MODULE_DEPENDS_CYGWIN Glibc
   SWIFT_MODULE_DEPENDS_HAIKU Glibc
-  SWIFT_MODULE_DEPENDS_WASI Glibc
+  SWIFT_MODULE_DEPENDS_WASI WASILibc
   SWIFT_MODULE_DEPENDS_WINDOWS MSVCRT
 
   SWIFT_COMPILE_FLAGS
diff --git a/stdlib/public/Differentiation/TgmathDerivatives.swift.gyb b/stdlib/public/Differentiation/TgmathDerivatives.swift.gyb
index df807256e15cf..f6843e898921f 100644
--- a/stdlib/public/Differentiation/TgmathDerivatives.swift.gyb
+++ b/stdlib/public/Differentiation/TgmathDerivatives.swift.gyb
@@ -16,8 +16,10 @@ import Swift
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   import Darwin.C.tgmath
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
   import Glibc
+#elseif os(WASI)
+  import WASILibc
 #elseif os(Windows)
   import MSVCRT
 #else
diff --git a/stdlib/public/Platform/CMakeLists.txt b/stdlib/public/Platform/CMakeLists.txt
index d9dfcdf34dbef..aa7d37be2c82f 100644
--- a/stdlib/public/Platform/CMakeLists.txt
+++ b/stdlib/public/Platform/CMakeLists.txt
@@ -44,10 +44,62 @@ add_swift_target_library(swiftGlibc ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_O
       ${SWIFT_RUNTIME_SWIFT_COMPILE_FLAGS}
       ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
     LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
-    TARGET_SDKS ANDROID CYGWIN FREEBSD OPENBSD LINUX HAIKU WASI
+    TARGET_SDKS ANDROID CYGWIN FREEBSD OPENBSD LINUX HAIKU
     INSTALL_IN_COMPONENT sdk-overlay
     DEPENDS glibc_modulemap)
 
+if(WASI IN_LIST SWIFT_SDKS)
+  set(arch_subdir "${SWIFT_SDK_WASI_LIB_SUBDIR}/wasm32")
+  set(module_dir "${SWIFTLIB_DIR}/${arch_subdir}")
+  set(module_dir_static "${SWIFTSTATICLIB_DIR}/${arch_subdir}")
+  set(wasi_libc_modulemap_out "${module_dir}/wasi.modulemap")
+  set(wasi_libc_modulemap_out_static "${module_dir_static}/wasi.modulemap")
+  handle_gyb_source_single(wasi_libc_modulemap_target
+    SOURCE "wasi.modulemap.gyb"
+    OUTPUT "${wasi_libc_modulemap_out}"
+    FLAGS "-DGLIBC_INCLUDE_PATH=${SWIFT_SDK_WASI_ARCH_wasm32_LIBC_INCLUDE_DIRECTORY}"
+          "-DGLIBC_ARCH_INCLUDE_PATH=${SWIFT_SDK_WASI_ARCH_wasm32_LIBC_ARCHITECTURE_INCLUDE_DIRECTORY}")
+  add_custom_command_target(
+    copy_wasi_libc_modulemap_static
+    COMMAND
+      "${CMAKE_COMMAND}" "-E" "make_directory" ${module_dir_static}
+    COMMAND
+      "${CMAKE_COMMAND}" "-E" "copy" ${wasi_libc_modulemap_out} ${wasi_libc_modulemap_out_static}
+    OUTPUT ${wasi_libc_modulemap_out_static}
+    DEPENDS
+      "${wasi_libc_modulemap_target}"
+    COMMENT "Copying WASILlibc modulemap to static resources")
+
+  add_swift_target_library(swiftWASILibc ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_OVERLAY
+      ${swift_platform_sources}
+      POSIXError.swift
+
+      GYB_SOURCES
+        ${swift_platform_gyb_sources}
+        WASI.swift.gyb
+
+      SWIFT_COMPILE_FLAGS
+        ${SWIFT_RUNTIME_SWIFT_COMPILE_FLAGS}
+        ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
+      LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
+      TARGET_SDKS WASI
+      INSTALL_IN_COMPONENT sdk-overlay
+      DEPENDS ${wasi_libc_modulemap_target} ${copy_wasi_libc_modulemap_static})
+
+  swift_install_in_component(FILES "${wasi_libc_modulemap_out}"
+                             DESTINATION "lib/swift/${arch_subdir}"
+                             COMPONENT sdk-overlay)
+
+  swift_install_in_component(FILES "${wasi_libc_modulemap_out}"
+                             DESTINATION "lib/swift_static/${arch_subdir}"
+                             COMPONENT sdk-overlay)
+
+  add_custom_target(wasi_libc_modulemap
+      DEPENDS ${wasi_libc_modulemap_target} ${copy_wasi_libc_modulemap_static})
+  set_property(TARGET wasi_libc_modulemap PROPERTY FOLDER "Miscellaneous")
+  add_dependencies(sdk-overlay wasi_libc_modulemap)
+endif()
+
 add_swift_target_library(swiftMSVCRT ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_OVERLAY
     msvcrt.swift
     ${swift_platform_sources}
@@ -71,8 +123,7 @@ foreach(sdk ${SWIFT_SDKS})
      NOT "${sdk}" STREQUAL "OPENBSD" AND
      NOT "${sdk}" STREQUAL "ANDROID" AND
      NOT "${sdk}" STREQUAL "CYGWIN" AND
-     NOT "${sdk}" STREQUAL "HAIKU" AND
-     NOT "${sdk}" STREQUAL "WASI")
+     NOT "${sdk}" STREQUAL "HAIKU")
     continue()
   endif()
 
diff --git a/stdlib/public/Platform/WASI.swift.gyb b/stdlib/public/Platform/WASI.swift.gyb
new file mode 100644
index 0000000000000..962f1202f70eb
--- /dev/null
+++ b/stdlib/public/Platform/WASI.swift.gyb
@@ -0,0 +1,69 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+@_exported import SwiftWASILibc // Clang module
+
+//  Constants defined by <math.h>
+@available(swift, deprecated: 3.0, message: "Please use 'Double.pi' or '.pi' to get the value of correct type and avoid casting.")
+public let M_PI = Double.pi
+
+@available(swift, deprecated: 3.0, message: "Please use 'Double.pi / 2' or '.pi / 2' to get the value of correct type and avoid casting.")
+public let M_PI_2 = Double.pi / 2
+
+@available(swift, deprecated: 3.0, message: "Please use 'Double.pi / 4' or '.pi / 4' to get the value of correct type and avoid casting.")
+public let M_PI_4 = Double.pi / 4
+
+@available(swift, deprecated: 3.0, message: "Please use '2.squareRoot()'.")
+public let M_SQRT2 = 2.squareRoot()
+
+@available(swift, deprecated: 3.0, message: "Please use '0.5.squareRoot()'.")
+public let M_SQRT1_2 = 0.5.squareRoot()
+
+//  Constants defined by <float.h>
+@available(swift, deprecated: 3.0, message: "Please use 'T.radix' to get the radix of a FloatingPoint type 'T'.")
+public let FLT_RADIX = Double.radix
+
+%for type, prefix in [('Float', 'FLT'), ('Double', 'DBL')]:
+//  Where does the 1 come from? C counts the usually-implicit leading
+//  significand bit, but Swift does not. Neither is really right or wrong.
+@available(swift, deprecated: 3.0, message: "Please use '${type}.significandBitCount + 1'.")
+public let ${prefix}_MANT_DIG = ${type}.significandBitCount + 1
+
+//  Where does the 1 come from? C models floating-point numbers as having a
+//  significand in [0.5, 1), but Swift (following IEEE 754) considers the
+//  significand to be in [1, 2). This rationale applies to ${prefix}_MIN_EXP
+//  as well.
+@available(swift, deprecated: 3.0, message: "Please use '${type}.greatestFiniteMagnitude.exponent + 1'.")
+public let ${prefix}_MAX_EXP = ${type}.greatestFiniteMagnitude.exponent + 1
+
+@available(swift, deprecated: 3.0, message: "Please use '${type}.leastNormalMagnitude.exponent + 1'.")
+public let ${prefix}_MIN_EXP = ${type}.leastNormalMagnitude.exponent + 1
+
+@available(swift, deprecated: 3.0, message: "Please use '${type}.greatestFiniteMagnitude' or '.greatestFiniteMagnitude'.")
+public let ${prefix}_MAX = ${type}.greatestFiniteMagnitude
+
+@available(swift, deprecated: 3.0, message: "Please use '${type}.ulpOfOne' or '.ulpOfOne'.")
+public let ${prefix}_EPSILON = ${type}.ulpOfOne
+
+@available(swift, deprecated: 3.0, message: "Please use '${type}.leastNormalMagnitude' or '.leastNormalMagnitude'.")
+public let ${prefix}_MIN = ${type}.leastNormalMagnitude
+
+@available(swift, deprecated: 3.0, message: "Please use '${type}.leastNonzeroMagnitude' or '.leastNonzeroMagnitude'.")
+public let ${prefix}_TRUE_MIN = ${type}.leastNonzeroMagnitude
+
+%end
+
+public let MAP_FAILED: UnsafeMutableRawPointer! = UnsafeMutableRawPointer(bitPattern: -1)
+
+// WebAssembly's error.h uses a macro that Swift can't import.
+public let EINTR:Int32 = 27
+public let EINVAL:Int32 = 28
diff --git a/stdlib/public/Platform/wasi.modulemap.gyb b/stdlib/public/Platform/wasi.modulemap.gyb
new file mode 100644
index 0000000000000..77951e328f705
--- /dev/null
+++ b/stdlib/public/Platform/wasi.modulemap.gyb
@@ -0,0 +1,231 @@
+//===--- wasi.modulemap.gyb ----------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+/// This is a semi-complete modulemap that maps WASI headers in a roughly
+/// similar way to the Darwin SDK modulemap. We do not take care to list every
+/// single header which may be included by a particular submodule, so there can
+/// still be issues if imported into the same context as one in which someone
+/// included those headers directly.
+///
+/// It's not named just WASI so that it doesn't conflict in the event of a
+/// future official WASI modulemap.
+module SwiftWASILibc [system] {
+  // C standard library
+  module C {
+    module ctype {
+      header "${GLIBC_INCLUDE_PATH}/ctype.h"
+      export *
+    }
+    module errno {
+      header "${GLIBC_INCLUDE_PATH}/errno.h"
+      export *
+    }
+
+    module fenv {
+      header "${GLIBC_INCLUDE_PATH}/fenv.h"
+      export *
+    }
+
+    // note: supplied by compiler
+    // module float {
+    //   header "${GLIBC_INCLUDE_PATH}/float.h"
+    //   export *
+    // }
+
+    module inttypes {
+      header "${GLIBC_INCLUDE_PATH}/inttypes.h"
+      export *
+    }
+
+    // note: potentially supplied by compiler
+    // module iso646 {
+    //   header "${GLIBC_INCLUDE_PATH}/iso646.h"
+    //   export *
+    // }
+    // module limits {
+    //   header "${GLIBC_INCLUDE_PATH}/limits.h"
+    //   export *
+    // }
+
+    module locale {
+      header "${GLIBC_INCLUDE_PATH}/locale.h"
+      export *
+    }
+    module math {
+      header "${GLIBC_INCLUDE_PATH}/math.h"
+      export *
+    }
+    module signal {
+      header "${GLIBC_INCLUDE_PATH}/signal.h"
+      export *
+    }
+
+    // note: supplied by the compiler
+    // module stdarg {
+    //   header "${GLIBC_INCLUDE_PATH}/stdarg.h"
+    //   export *
+    // }
+    // module stdbool {
+    //   header "${GLIBC_INCLUDE_PATH}/stdbool.h"
+    //   export *
+    // }
+    // module stddef {
+    //   header "${GLIBC_INCLUDE_PATH}/stddef.h"
+    //   export *
+    // }
+    // module stdint {
+    //   header "${GLIBC_INCLUDE_PATH}/stdint.h"
+    //   export *
+    // }
+
+    module stdio {
+      header "${GLIBC_INCLUDE_PATH}/stdio.h"
+      export *
+    }
+    module stdlib {
+      header "${GLIBC_INCLUDE_PATH}/stdlib.h"
+      export *
+      export stddef
+    }
+    module string {
+      header "${GLIBC_INCLUDE_PATH}/string.h"
+      export *
+    }
+
+    // note: supplied by the compiler
+    // explicit module tgmath {
+    //   header "${GLIBC_INCLUDE_PATH}/tgmath.h"
+    //   export *
+    // }
+
+    module time {
+      header "${GLIBC_INCLUDE_PATH}/time.h"
+      export *
+    }
+  }
+
+  // POSIX
+  module POSIX {
+    module arpa {
+      module inet {
+        header "${GLIBC_INCLUDE_PATH}/arpa/inet.h"
+        export *
+      }
+      export *
+    }
+    module dirent {
+      header "${GLIBC_INCLUDE_PATH}/dirent.h"
+      export *
+    }
+    module fcntl {
+      header "${GLIBC_INCLUDE_PATH}/fcntl.h"
+      export *
+    }
+    module fnmatch {
+      header "${GLIBC_INCLUDE_PATH}/fnmatch.h"
+      export *
+    }
+    module ioctl {
+      header "${GLIBC_ARCH_INCLUDE_PATH}/sys/ioctl.h"
+      export *
+    }
+    module libgen {
+      header "${GLIBC_INCLUDE_PATH}/libgen.h"
+      export *
+    }
+    module netinet {
+      module in {
+        header "${GLIBC_INCLUDE_PATH}/netinet/in.h"
+        export *
+
+        exclude header "${GLIBC_INCLUDE_PATH}/netinet6/in6.h"
+      }
+      module tcp {
+        header "${GLIBC_INCLUDE_PATH}/netinet/tcp.h"
+        export *
+      }
+    }
+    module poll {
+      header "${GLIBC_INCLUDE_PATH}/poll.h"
+      export *
+    }
+    module regex {
+      header "${GLIBC_INCLUDE_PATH}/regex.h"
+      export *
+    }
+    module sched {
+      header "${GLIBC_INCLUDE_PATH}/sched.h"
+      export *
+    }
+    module semaphore {
+      header "${GLIBC_INCLUDE_PATH}/semaphore.h"
+      export *
+    }
+    module strings {
+      header "${GLIBC_INCLUDE_PATH}/strings.h"
+      export *
+    }
+
+    module sys {
+      export *
+
+      module mman {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/mman.h"
+        export *
+      }
+      module resource {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/resource.h"
+        export *
+      }
+      module select {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/select.h"
+        export *
+      }
+      module socket {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/socket.h"
+        export *
+      }
+      module stat {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/stat.h"
+        export *
+      }
+      module time {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/time.h"
+        export *
+      }
+      module times {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/times.h"
+        export *
+      }
+      module types {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/types.h"
+        export *
+      }
+      module uio {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/uio.h"
+        export *
+      }
+      module un {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/un.h"
+        export *
+      }
+      module utsname {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/utsname.h"
+        export *
+      }
+    }
+    module unistd {
+      header "${GLIBC_INCLUDE_PATH}/unistd.h"
+      export *
+    }
+  }
+}
diff --git a/test/ClangImporter/clang_builtins.swift b/test/ClangImporter/clang_builtins.swift
index ea3fb40684748..49231cb4ab247 100644
--- a/test/ClangImporter/clang_builtins.swift
+++ b/test/ClangImporter/clang_builtins.swift
@@ -2,8 +2,10 @@
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
   import Glibc
+#elseif os(WASI)
+  import WASILibc
 #elseif os(Windows)
   import MSVCRT
 #else
diff --git a/test/IRGen/builtin_math.swift b/test/IRGen/builtin_math.swift
index 5c85dac5e5c20..6bde19c25f12b 100644
--- a/test/IRGen/builtin_math.swift
+++ b/test/IRGen/builtin_math.swift
@@ -2,8 +2,10 @@
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
   import Glibc
+#elseif os(WASI)
+  import WASILibc
 #elseif os(Windows)
   import MSVCRT
 #else
diff --git a/test/stdlib/FloatConstants.swift b/test/stdlib/FloatConstants.swift
index eb03d30e29622..d26b5a06a04f9 100644
--- a/test/stdlib/FloatConstants.swift
+++ b/test/stdlib/FloatConstants.swift
@@ -2,8 +2,10 @@
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
   import Glibc
+#elseif os(WASI)
+  import WASILibc
 #elseif os(Windows)
   import MSVCRT
 #else
diff --git a/test/stdlib/MathConstants.swift b/test/stdlib/MathConstants.swift
index 08284143b1a6d..c20382fff4878 100644
--- a/test/stdlib/MathConstants.swift
+++ b/test/stdlib/MathConstants.swift
@@ -2,8 +2,10 @@
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
   import Glibc
+#elseif os(WASI)
+  import WASILibc
 #elseif os(Windows)
   import MSVCRT
 #else
diff --git a/test/stdlib/PrintFloat.swift.gyb b/test/stdlib/PrintFloat.swift.gyb
index 09d8e9dc49bb2..3bda6d247edb6 100644
--- a/test/stdlib/PrintFloat.swift.gyb
+++ b/test/stdlib/PrintFloat.swift.gyb
@@ -12,8 +12,10 @@
 import StdlibUnittest
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
   import Glibc
+#elseif os(WASI)
+  import WASILibc
 #elseif os(Windows)
   import MSVCRT
 #else
diff --git a/test/stdlib/Runtime.swift.gyb b/test/stdlib/Runtime.swift.gyb
index 8166dfeb255b1..7fa63472b466d 100644
--- a/test/stdlib/Runtime.swift.gyb
+++ b/test/stdlib/Runtime.swift.gyb
@@ -12,8 +12,10 @@ import SwiftShims
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
   import Glibc
+#elseif os(WASI)
+  import WASILibc
 #elseif os(Windows)
   import MSVCRT
   import WinSDK
diff --git a/test/stdlib/VarArgs.swift b/test/stdlib/VarArgs.swift
index ea9317fef5fe2..cb199197748f8 100644
--- a/test/stdlib/VarArgs.swift
+++ b/test/stdlib/VarArgs.swift
@@ -6,9 +6,12 @@ import Swift
 #if _runtime(_ObjC)
   import Darwin
   import CoreGraphics
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
   import Glibc
   typealias CGFloat = Double
+#elseif os(WASI)
+  import WASILibc
+  typealias CGFloat = Double
 #elseif os(Windows)
   import MSVCRT
   #if arch(x86_64) || arch(arm64)
diff --git a/test/stdlib/tgmath.swift.gyb b/test/stdlib/tgmath.swift.gyb
index fede221f148e8..6b0ec2130042f 100644
--- a/test/stdlib/tgmath.swift.gyb
+++ b/test/stdlib/tgmath.swift.gyb
@@ -19,8 +19,10 @@
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   import Darwin.C.tgmath
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
   import Glibc
+#elseif os(WASI)
+  import WASILibc
 #elseif os(Windows)
   import MSVCRT
 #else
diff --git a/test/stdlib/tgmath_optimized.swift b/test/stdlib/tgmath_optimized.swift
index fdf3b75299db1..354243e6819f9 100644
--- a/test/stdlib/tgmath_optimized.swift
+++ b/test/stdlib/tgmath_optimized.swift
@@ -6,8 +6,10 @@
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   import Darwin
-#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku) || os(WASI)
+#elseif os(Linux) || os(FreeBSD) || os(OpenBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
   import Glibc
+#elseif os(WASI)
+  import WASILibc
 #elseif os(Windows)
   import MSVCRT
 #else
diff --git a/utils/build-presets.ini b/utils/build-presets.ini
index 70a3d5125b0ee..a112756c82f38 100644
--- a/utils/build-presets.ini
+++ b/utils/build-presets.ini
@@ -2442,8 +2442,9 @@ skip-build-benchmarks
 llvm-targets-to-build=X86;AArch64;WebAssembly
 install-destdir=%(INSTALL_DESTDIR)s
 swift-install-components=autolink-driver;compiler;clang-builtin-headers;stdlib;sdk-overlay;parser-lib;editor-integration;tools;testsuite-tools;toolchain-tools;license;sourcekit-inproc;swift-remote-mirror;swift-remote-mirror-headers;clang-resource-dir-symlink
-llvm-install-components=clang
+llvm-install-components=clang;compiler-rt;lld;llvm-ar;llvm-ranlib
 install-swift
+install-llvm
 install-prefix=/%(TOOLCHAIN_NAME)s/usr
 
 [preset: webassembly-host]
@@ -2457,7 +2458,6 @@ extra-cmake-options=
 llbuild
 swiftpm
 
-install-llvm
 install-swift
 install-llbuild
 install-swiftpm
diff --git a/utils/build-script-impl b/utils/build-script-impl
index 3ebea70eaa2dc..c26f48c924a47 100755
--- a/utils/build-script-impl
+++ b/utils/build-script-impl
@@ -1610,6 +1610,11 @@ for host in "${ALL_HOSTS[@]}"; do
 
                 llvm_enable_projects=("clang")
 
+                # wasm: lld is necessary to sync compiler llvm version and lld llvm version
+                if [[ ! "${SKIP_BUILD_LLD}" ]]; then
+                    llvm_enable_projects+=("lld")
+                fi
+
                 if [[ ! "${SKIP_BUILD_COMPILER_RT}" && ! $(is_cross_tools_host ${host}) ]]; then
                     llvm_enable_projects+=("compiler-rt")
                 fi
diff --git a/utils/webassembly/build-foundation.sh b/utils/webassembly/build-foundation.sh
index 0f712825160d7..8bb07c1a200a0 100755
--- a/utils/webassembly/build-foundation.sh
+++ b/utils/webassembly/build-foundation.sh
@@ -3,11 +3,7 @@ set -ex
 DESTINATION_TOOLCHAIN=$1
 SOURCE_PATH="$(cd "$(dirname $0)/../../.." && pwd)"
 
-# Remove host CoreFoundation (which can be different from the target) headers
-# to avoid shadowing the wasm32 target CoreFoundation
-rm -rf $DESTINATION_TOOLCHAIN/usr/lib/swift/CoreFoundation
-
-FOUNDATION_BUILD="$SOURCE_PATH/build/Ninja-ReleaseAssert/foundation-wasi-wasm32"
+FOUNDATION_BUILD="$SOURCE_PATH/target-build/Ninja-ReleaseAssert/foundation-wasi-wasm32"
 
 mkdir -p $FOUNDATION_BUILD
 cd $FOUNDATION_BUILD
@@ -19,6 +15,7 @@ cmake -G Ninja \
   -DWASI_SDK_PATH="$SOURCE_PATH/wasi-sdk" \
   -DICU_ROOT="$SOURCE_PATH/icu_out" \
   -DBUILD_SHARED_LIBS=OFF \
+  -DCMAKE_Swift_COMPILER_FORCED=ON \
   "${SOURCE_PATH}/swift-corelibs-foundation"
   
 ninja -v
diff --git a/utils/webassembly/build-toolchain.sh b/utils/webassembly/build-toolchain.sh
index 0129a373ac71c..0418f855a0644 100755
--- a/utils/webassembly/build-toolchain.sh
+++ b/utils/webassembly/build-toolchain.sh
@@ -11,11 +11,13 @@ case $(uname -s) in
     OS_SUFFIX=osx
     HOST_PRESET=webassembly-host
     TARGET_PRESET=webassembly-macos-target
+    HOST_SUFFIX=macosx-x86_64
   ;;
   Linux)
     OS_SUFFIX=linux
     HOST_PRESET=webassembly-linux-host
     TARGET_PRESET=webassembly-linux-target
+    HOST_SUFFIX=linux-x86_64
   ;;
   *)
     echo "Unrecognised platform $(uname -s)"
@@ -37,58 +39,88 @@ BUNDLE_IDENTIFIER="swiftwasm.5.3-${YEAR}${MONTH}${DAY}"
 DISPLAY_NAME_SHORT="Swift for WebAssembly 5.3 Snapshot"
 DISPLAY_NAME="${DISPLAY_NAME_SHORT} ${YEAR}-${MONTH}-${DAY}"
 
+DIST_TOOLCHAIN_DESTDIR=$SOURCE_PATH/dist-toolchain-sdk
 HOST_TOOLCHAIN_DESTDIR=$SOURCE_PATH/host-toolchain-sdk
-HOST_TOOLCHAIN_SDK=$HOST_TOOLCHAIN_DESTDIR/$TOOLCHAIN_NAME
+TARGET_TOOLCHAIN_DESTDIR=$SOURCE_PATH/target-toolchain-sdk
 
-# Avoid clang headers symlink issues
-mkdir -p $HOST_TOOLCHAIN_SDK/usr/lib/clang/10.0.0
-
-# Build the host toolchain and SDK first.
-$SOURCE_PATH/swift/utils/build-script \
-  --preset=$HOST_PRESET \
-  INSTALL_DESTDIR="$HOST_TOOLCHAIN_DESTDIR" \
-  TOOLCHAIN_NAME="$TOOLCHAIN_NAME" \
-  C_CXX_LAUNCHER="$(which sccache)"
-
-# Clean up the host toolchain build directory so that the next
-# `build-script` invocation doesn't pick up wrong CMake config files.
-# For some reason passing `--reconfigure` to `build-script` won't do this.
-rm -rf $SOURCE_PATH/build/Ninja-ReleaseAssert/swift-*
-
-# build the cross-compilled toolchain
-$SOURCE_PATH/swift/utils/build-script \
-  --preset=$TARGET_PRESET \
-  INSTALL_DESTDIR="$SOURCE_PATH/install" \
-  SOURCE_PATH="$SOURCE_PATH" \
-  BUNDLE_IDENTIFIER="${BUNDLE_IDENTIFIER}" \
-  DISPLAY_NAME="${DISPLAY_NAME}" \
-  DISPLAY_NAME_SHORT="${DISPLAY_NAME_SHORT}" \
-  TOOLCHAIN_NAME="${TOOLCHAIN_NAME}" \
-  TOOLCHAIN_VERSION="${TOOLCHAIN_VERSION}" \
-  C_CXX_LAUNCHER="$(which sccache)"
-
-# Merge wasi-sdk and the toolchain
-cp -a $WASI_SDK_PATH/lib/clang $HOST_TOOLCHAIN_SDK/usr/lib
-cp -a $WASI_SDK_PATH/bin/{*ld,llvm-ar} $HOST_TOOLCHAIN_SDK/usr/bin
-cp -r $WASI_SDK_PATH/share/wasi-sysroot $HOST_TOOLCHAIN_SDK/usr/share
-
-# Replace absolute sysroot path with relative path
-sed -i -e "s@\".*/include@\"../../../../share/wasi-sysroot/include@g" $SOURCE_PATH/install/$TOOLCHAIN_NAME/usr/lib/swift/wasi/wasm32/glibc.modulemap
-
-# Copy the target environment stdlib into the toolchain
-
-# Avoid copying usr/lib/swift/clang because our toolchain's one is a directory
-# but nightly's one is symbolic link. A simple copy fails to merge them.
-rsync -v -a $SOURCE_PATH/install/$TOOLCHAIN_NAME/usr/lib/ $HOST_TOOLCHAIN_SDK/usr/lib/ --exclude 'swift/clang'
-
-# FIXME: avoid building foundation for now
-$UTILS_PATH/build-foundation.sh $HOST_TOOLCHAIN_SDK
-$UTILS_PATH/build-xctest.sh $HOST_TOOLCHAIN_SDK
-
-# Cleanup build directory on CI
-if [[ -z "${CI}" ]]; then
-  rm -rf $SOURCE_PATH/build/Ninja-ReleaseAssert/
-fi
-
-cd $HOST_TOOLCHAIN_DESTDIR
+DIST_TOOLCHAIN_SDK=$DIST_TOOLCHAIN_DESTDIR/$TOOLCHAIN_NAME
+HOST_TOOLCHAIN_SDK=$HOST_TOOLCHAIN_DESTDIR/$TOOLCHAIN_NAME
+TARGET_TOOLCHAIN_SDK=$TARGET_TOOLCHAIN_DESTDIR/$TOOLCHAIN_NAME
+
+
+HOST_BUILD_ROOT=$SOURCE_PATH/host-build
+TARGET_BUILD_ROOT=$SOURCE_PATH/target-build
+HOST_BUILD_DIR=$HOST_BUILD_ROOT/Ninja-ReleaseAssert
+TARGET_BUILD_DIR=$TARGET_BUILD_ROOT/Ninja-ReleaseAssert
+
+build_host_toolchain() {
+  # Avoid clang headers symlink issues
+  mkdir -p $HOST_TOOLCHAIN_SDK/usr/lib/clang/10.0.0
+  # Build the host toolchain and SDK first.
+  env SWIFT_BUILD_ROOT="$HOST_BUILD_ROOT" \
+    $SOURCE_PATH/swift/utils/build-script \
+    --preset=$HOST_PRESET \
+    --build-dir="$HOST_BUILD_DIR" \
+    INSTALL_DESTDIR="$HOST_TOOLCHAIN_DESTDIR" \
+    TOOLCHAIN_NAME="$TOOLCHAIN_NAME" \
+    C_CXX_LAUNCHER="$(which sccache)"
+}
+
+build_target_toolchain() {
+  mkdir -p $HOST_BUILD_DIR/
+  # Copy the host build dir to reuse it.
+  if [[ ! -e "$HOST_BUILD_DIR/llvm-$HOST_SUFFIX" ]]; then
+    cp -r "$HOST_BUILD_DIR/llvm-$HOST_SUFFIX" "$TARGET_BUILD_DIR/llvm-$HOST_SUFFIX"
+    # Clean up compiler-rt dir to cross compile it for host and wasm32
+    (cd "$TARGET_BUILD_DIR/llvm-$HOST_SUFFIX" && ninja compiler-rt-clear)
+  fi
+
+  # Avoid clang headers symlink issues
+  mkdir -p $TARGET_TOOLCHAIN_SDK/usr/lib/clang/10.0.0
+
+  # build the cross-compilled toolchain
+  env SWIFT_BUILD_ROOT="$TARGET_BUILD_ROOT" \
+    $SOURCE_PATH/swift/utils/build-script \
+    --preset=$TARGET_PRESET --reconfigure \
+    --build-dir="$TARGET_BUILD_DIR" \
+    INSTALL_DESTDIR="$TARGET_TOOLCHAIN_DESTDIR" \
+    SOURCE_PATH="$SOURCE_PATH" \
+    BUNDLE_IDENTIFIER="${BUNDLE_IDENTIFIER}" \
+    DISPLAY_NAME="${DISPLAY_NAME}" \
+    DISPLAY_NAME_SHORT="${DISPLAY_NAME_SHORT}" \
+    TOOLCHAIN_NAME="${TOOLCHAIN_NAME}" \
+    TOOLCHAIN_VERSION="${TOOLCHAIN_VERSION}" \
+    C_CXX_LAUNCHER="$(which sccache)"
+
+  $UTILS_PATH/build-foundation.sh $TARGET_TOOLCHAIN_SDK
+  $UTILS_PATH/build-xctest.sh $TARGET_TOOLCHAIN_SDK
+
+}
+
+merge_toolchains() {
+  rm -rf "$DIST_TOOLCHAIN_DESTDIR"
+  # Copy the base host toolchain
+  cp -r "$HOST_TOOLCHAIN_DESTDIR" "$DIST_TOOLCHAIN_DESTDIR"
+
+  # Merge wasi-sdk and the toolchain
+  cp -a $WASI_SDK_PATH/lib/clang $DIST_TOOLCHAIN_SDK/usr/lib
+  cp -a $WASI_SDK_PATH/bin/{*ld,llvm-ar} $DIST_TOOLCHAIN_SDK/usr/bin
+  cp -r $WASI_SDK_PATH/share/wasi-sysroot $DIST_TOOLCHAIN_SDK/usr/share
+
+  # Copy the target environment stdlib into the toolchain
+  # Avoid copying usr/lib/swift/clang because our toolchain's one is a directory
+  # but nightly's one is symbolic link. A simple copy fails to merge them.
+  rsync -v -a $TARGET_TOOLCHAIN_SDK/usr/lib/ $DIST_TOOLCHAIN_SDK/usr/lib/ --exclude 'swift/clang'
+  rsync -v -a $TARGET_TOOLCHAIN_SDK/usr/bin/ $DIST_TOOLCHAIN_SDK/usr/bin/
+
+  # Replace absolute sysroot path with relative path
+  sed -i -e "s@\".*/include@\"../../../../share/wasi-sysroot/include@g" $DIST_TOOLCHAIN_SDK/usr/lib/swift/wasi/wasm32/wasi.modulemap
+}
+
+build_host_toolchain
+build_target_toolchain
+
+merge_toolchains
+
+cd $DIST_TOOLCHAIN_DESTDIR
 tar cfz $PACKAGE_ARTIFACT $TOOLCHAIN_NAME
diff --git a/utils/webassembly/build-xctest.sh b/utils/webassembly/build-xctest.sh
index e08e0cef33db1..fe93f244ce0b3 100755
--- a/utils/webassembly/build-xctest.sh
+++ b/utils/webassembly/build-xctest.sh
@@ -3,7 +3,7 @@ set -ex
 DESTINATION_TOOLCHAIN=$1
 SOURCE_PATH="$(cd "$(dirname $0)/../../.." && pwd)"
 
-BUILD_DIR="$SOURCE_PATH/build/Ninja-ReleaseAssert/xctest-wasi-wasm32"
+BUILD_DIR="$SOURCE_PATH/target-build/Ninja-ReleaseAssert/xctest-wasi-wasm32"
 
 mkdir -p $BUILD_DIR
 cd $BUILD_DIR
@@ -14,6 +14,7 @@ cmake -G Ninja \
   -DCMAKE_TOOLCHAIN_FILE="$SOURCE_PATH/swift/utils/webassembly/toolchain-wasi.cmake" \
   -DWASI_SDK_PATH="$SOURCE_PATH/wasi-sdk" \
   -DBUILD_SHARED_LIBS=OFF \
+  -DCMAKE_Swift_COMPILER_FORCED=ON \
   -DSWIFT_FOUNDATION_PATH=$DESTINATION_TOOLCHAIN/usr/lib/swift/wasi/wasm32 \
   "${SOURCE_PATH}/swift-corelibs-xctest"
   
diff --git a/utils/webassembly/ci.sh b/utils/webassembly/ci.sh
index 75c9adb37ff9e..a9ca0d9a11d6f 100755
--- a/utils/webassembly/ci.sh
+++ b/utils/webassembly/ci.sh
@@ -13,7 +13,7 @@ fi
 
 BUILD_SCRIPT=$UTILS_PATH/build-toolchain.sh
 RUN_TEST_BIN=$SWIFT_PATH/utils/run-test
-BUILD_DIR=$SOURCE_PATH/build/Ninja-ReleaseAssert
+TARGET_BUILD_DIR=$SOURCE_PATH/target-build/Ninja-ReleaseAssert
 
 $DEPENDENCIES_SCRIPT
 
@@ -29,14 +29,16 @@ $BUILD_SCRIPT $FLAGS
 
 if [[ "$(uname)" == "Darwin" ]]; then
   # workaround: host target test directory is necessary to use run-test
-  mkdir -p $BUILD_DIR/swift-macosx-x86_64/test-macosx-x86_64
+  mkdir -p $TARGET_BUILD_DIR/swift-macosx-x86_64/test-macosx-x86_64
 fi
 
-$RUN_TEST_BIN --build-dir $BUILD_DIR --target wasi-wasm32 test/stdlib/
-
 if [[ "$(uname)" == "Linux" ]]; then
+  $RUN_TEST_BIN --build-dir $TARGET_BUILD_DIR --target wasi-wasm32 test/stdlib/ || true
   echo "Skip running test suites for Linux"
 else
+
+  $RUN_TEST_BIN --build-dir $TARGET_BUILD_DIR --target wasi-wasm32 test/stdlib/
+
   # Run test but ignore failure temporarily
   $BUILD_SCRIPT $FLAGS -t || true
 fi