wasm: Add support for WASI threads in CMake build system

This patch adds a new CMake option, `SWIFT_ENABLE_WASI_THREADS`, to
enable building the Standard Library using WASI threads primitives
(https://github.com/WebAssembly/wasi-threads). With this option, the
Standard Library will be built for the new "wasm32-unknown-wasi-threads"
target. We add the new triple because the WASI threads proposal requires
extra syscalls and object files compiled to "wasm32-unknown-wasi" and
"wasm32-unknown-wasi-threads" are not compatible and cannot be linked
together.

Author: kateinoigakukun

CMakeLists.txt

@@ -487,6 +487,14 @@ set(SWIFT_ANDROID_NDK_PATH "" CACHE STRING
 set(SWIFT_ANDROID_DEPLOY_DEVICE_PATH "" CACHE STRING
   "Path on an Android device where build products will be pushed. These are used when running the test suite against the device")
 
+#
+# User-configurable WebAssembly specific options.
+#
+
+option(SWIFT_ENABLE_WASI_THREADS
+  "Build the Standard Library with WASI threads support"
+  FALSE)
+
 #
 # User-configurable Darwin-specific options.
 #

cmake/modules/SwiftConfigureSDK.cmake

@@ -329,7 +329,11 @@ macro(configure_sdk_unix name architectures)
   if("${prefix}" STREQUAL "LINUX")
     set(_default_threading_package "linux")
   elseif("${prefix}" STREQUAL "WASI")
-    set(_default_threading_package "none")
+    if(SWIFT_ENABLE_WASI_THREADS)
+      set(_default_threading_package "pthreads")
+    else()
+      set(_default_threading_package "none")
+    endif()
   endif()
   get_threading_package(${prefix} ${_default_threading_package}
     SWIFT_SDK_${prefix}_THREADING_PACKAGE)
@@ -418,7 +422,11 @@ macro(configure_sdk_unix name architectures)
           message(FATAL_ERROR "unsupported arch for WebAssembly: ${arch}")
         endif()
         set(SWIFT_SDK_WASI_ARCH_wasm32_PATH "${SWIFT_WASI_SYSROOT_PATH}")
-        set(SWIFT_SDK_WASI_ARCH_wasm32_TRIPLE "wasm32-unknown-wasi")
+        if(SWIFT_ENABLE_WASI_THREADS)
+          set(SWIFT_SDK_WASI_ARCH_wasm32_TRIPLE "wasm32-unknown-wasi-threads")
+        else()
+          set(SWIFT_SDK_WASI_ARCH_wasm32_TRIPLE "wasm32-unknown-wasi")
+        endif()
       else()
         message(FATAL_ERROR "unknown Unix OS: ${prefix}")
       endif()