[build] Add initial support for Emscripten SDK

This patch adds initial support for Emscripten SDK alongside the existing
support for WASI SDK. This is a first step towards building a part of
Swift compiler for Emscripten target (which will be used to build LLDB
with Swift to WebAssembly target).

Author: kateinoigakukun

CMakeLists.txt

@@ -157,6 +157,8 @@ else()
     set(SWIFT_HOST_VARIANT_SDK_default "OSX")
   elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "WASI")
     set(SWIFT_HOST_VARIANT_SDK_default "WASI")
+  elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Emscripten")
+    set(SWIFT_HOST_VARIANT_SDK_default "EMSCRIPTEN")
   else()
     message(FATAL_ERROR "Unable to detect SDK for host system: ${CMAKE_SYSTEM_NAME}")
   endif()
@@ -1248,6 +1250,15 @@ elseif("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "WASI")
   set(SWIFT_PRIMARY_VARIANT_SDK_default  "${SWIFT_HOST_VARIANT_SDK}")
   set(SWIFT_PRIMARY_VARIANT_ARCH_default "${SWIFT_HOST_VARIANT_ARCH}")
 
+elseif("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "EMSCRIPTEN")
+
+  set(SWIFT_HOST_VARIANT "emscripten" CACHE STRING
+      "Deployment OS for Swift host tools (the compiler) [emscripten]")
+
+  configure_sdk_unix("Emscripten" "wasm32")
+  set(SWIFT_PRIMARY_VARIANT_SDK_default  "${SWIFT_HOST_VARIANT_SDK}")
+  set(SWIFT_PRIMARY_VARIANT_ARCH_default "${SWIFT_HOST_VARIANT_ARCH}")
+
 elseif("${SWIFT_HOST_VARIANT_SDK}" MATCHES "(OSX|IOS*|TVOS*|WATCHOS*|XROS*)")
 
   set(SWIFT_HOST_VARIANT "macosx" CACHE STRING

cmake/modules/AddSwift.cmake

@@ -137,6 +137,12 @@ function(_add_host_variant_c_compile_link_flags name)
     if("${CMAKE_C_COMPILER_FRONTEND_VARIANT}" STREQUAL "MSVC") # clang-cl options
       target_compile_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:--target=${SWIFT_HOST_TRIPLE}>)
       target_link_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:--target=${SWIFT_HOST_TRIPLE}>)
+    elseif("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "EMSCRIPTEN") # emcc options
+      # some older emcc don't understand -target=<triple>
+      # FIXME: remove this when we no longer support Emscripten < 3.1.44
+      # https://github.com/emscripten-core/emscripten/pull/19840
+      target_compile_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:--target=${SWIFT_HOST_TRIPLE}>)
+      target_link_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:-target=${SWIFT_HOST_TRIPLE}>)
     else()
       target_compile_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:-target;${SWIFT_HOST_TRIPLE}>)
       target_link_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:-target;${SWIFT_HOST_TRIPLE}>)

cmake/modules/SwiftConfigureSDK.cmake

@@ -326,6 +326,7 @@ macro(configure_sdk_unix name architectures)
   # Static linking is supported on Linux and WASI
   if("${prefix}" STREQUAL "LINUX"
       OR "${prefix}" STREQUAL "LINUX_STATIC"
+      OR "${prefix}" STREQUAL "EMSCRIPTEN"
       OR "${prefix}" STREQUAL "WASI")
     set(SWIFT_SDK_${prefix}_STATIC_LINKING_SUPPORTED TRUE)
   else()
@@ -465,6 +466,8 @@ macro(configure_sdk_unix name architectures)
         else()
           set(SWIFT_SDK_WASI_ARCH_wasm32_TRIPLE "wasm32-unknown-wasi")
         endif()
+      elseif("${prefix}" STREQUAL "EMSCRIPTEN")
+        set(SWIFT_SDK_EMSCRIPTEN_ARCH_${arch}_TRIPLE "${arch}-unknown-emscripten")
       elseif("${prefix}" STREQUAL "LINUX_STATIC")
         set(SWIFT_SDK_LINUX_STATIC_ARCH_${arch}_TRIPLE "${arch}-swift-linux-musl")
         set(SWIFT_SDK_LINUX_STATIC_ARCH_${arch}_PATH "${SWIFT_MUSL_PATH}/${arch}")