Add an option to build the concurrency library for back deployment.

Introduce an additional build product to build-script to build
back-deployable concurrency libraries. These libraries would need to
be embedded in apps deployed prior to macOS 12/iOS 15 to support
concurrency.

The built-script option `--back-deploy-concurrency` can be provided to
build these back-deployment libraries. They are built in addition to
the normal concurrency libraries, as a separate product that installs
into `lib/swift-5.5/<platform>` within the toolchain. The macro
`SWIFT_CONCURRENCY_BACK_DEPLOYMENT` is set when building the
concurrency library, so that we can adapt the implementation to older
OS's.

Author: DougGregor

CMakeLists.txt

@@ -1057,6 +1057,11 @@ else()
     add_subdirectory(stdlib/toolchain)
   endif()
 
+  if (BUILD_SWIFT_CONCURRENCY_BACK_DEPLOYMENT_LIBRARIES)
+    # Build the back-deployed concurrency library.
+    add_subdirectory(stdlib/public/BackDeployConcurrency)
+  endif()
+
   # Some tools (e.g. swift-reflection-dump) rely on a host swiftReflection, so
   # ensure we build that when building tools.
   if(SWIFT_INCLUDE_TOOLS)

cmake/modules/SwiftComponents.cmake

@@ -46,6 +46,7 @@
 # The set of "defined" swift components are as follows:
 #
 # * autolink-driver -- the Swift driver support tools
+# * back-deployment -- Swift back-deployment libraries
 # * compiler -- the Swift compiler and (on supported platforms) the REPL.
 # * clang-builtin-headers -- install a copy of Clang builtin headers under
 #   'lib/swift/clang'.  This is useful when Swift compiler is installed in
@@ -66,7 +67,7 @@
 # * toolchain-dev-tools -- install development tools useful in a shared toolchain
 # * dev -- headers and libraries required to use Swift compiler as a library.
 set(_SWIFT_DEFINED_COMPONENTS
-  "autolink-driver;compiler;clang-builtin-headers;clang-resource-dir-symlink;clang-builtin-headers-in-clang-resource-dir;stdlib;stdlib-experimental;sdk-overlay;parser-lib;editor-integration;tools;testsuite-tools;toolchain-tools;toolchain-dev-tools;dev;license;sourcekit-xpc-service;sourcekit-inproc;swift-remote-mirror;swift-remote-mirror-headers")
+  "autolink-driver;back-deployment;compiler;clang-builtin-headers;clang-resource-dir-symlink;clang-builtin-headers-in-clang-resource-dir;stdlib;stdlib-experimental;sdk-overlay;parser-lib;editor-integration;tools;testsuite-tools;toolchain-tools;toolchain-dev-tools;dev;license;sourcekit-xpc-service;sourcekit-inproc;swift-remote-mirror;swift-remote-mirror-headers")
 
 # The default install components include all of the defined components, except
 # for the following exceptions.
@@ -85,6 +86,8 @@ else()
   list(REMOVE_ITEM _SWIFT_DEFAULT_COMPONENTS "sourcekit-xpc-service")
 endif()
 list(REMOVE_ITEM _SWIFT_DEFAULT_COMPONENTS "stdlib-experimental")
+# back-deployment libraries are opt-in
+list(REMOVE_ITEM _SWIFT_DEFAULT_COMPONENTS "back-deployment")
 
 macro(swift_configure_components)
   # Set the SWIFT_INSTALL_COMPONENTS variable to the default value if it is not passed in via -D

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -640,6 +640,7 @@ function(_add_swift_target_library_single target name)
         SDK
         DEPLOYMENT_VERSION_MACCATALYST
         MACCATALYST_BUILD_FLAVOR
+        BACK_DEPLOYMENT_LIBRARY
         ENABLE_LTO)
   set(SWIFTLIB_SINGLE_multiple_parameter_options
         C_COMPILE_FLAGS
@@ -771,6 +772,13 @@ function(_add_swift_target_library_single target name)
       -libc;${SWIFT_STDLIB_MSVC_RUNTIME_LIBRARY})
   endif()
 
+  # Don't install the Swift module content for back-deployment libraries.
+  if (SWIFTLIB_SINGLE_BACK_DEPLOYMENT_LIBRARY)
+    set(install_in_component "never_install")
+  else()
+    set(install_in_component "${SWIFTLIB_SINGLE_INSTALL_IN_COMPONENT}")
+  endif()
+
   # FIXME: don't actually depend on the libraries in SWIFTLIB_SINGLE_LINK_LIBRARIES,
   # just any swiftmodule files that are associated with them.
   handle_swift_sources(
@@ -796,7 +804,7 @@ function(_add_swift_target_library_single target name)
       ${embed_bitcode_arg}
       ${SWIFTLIB_SINGLE_STATIC_keyword}
       ENABLE_LTO "${SWIFTLIB_SINGLE_ENABLE_LTO}"
-      INSTALL_IN_COMPONENT "${SWIFTLIB_SINGLE_INSTALL_IN_COMPONENT}"
+      INSTALL_IN_COMPONENT "${install_in_component}"
       MACCATALYST_BUILD_FLAVOR "${SWIFTLIB_SINGLE_MACCATALYST_BUILD_FLAVOR}")
   add_swift_source_group("${SWIFTLIB_SINGLE_EXTERNAL_SOURCES}")
 
@@ -941,21 +949,28 @@ function(_add_swift_target_library_single target name)
         SUFFIX ${LLVM_PLUGIN_EXT})
   endif()
 
+  # For back-deployed libraries, install into lib/swift-<version>.
+  if (SWIFTLIB_SINGLE_BACK_DEPLOYMENT_LIBRARY)
+    set(swiftlib_prefix "${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/lib/swift-${SWIFTLIB_SINGLE_BACK_DEPLOYMENT_LIBRARY}")
+  else()
+    set(swiftlib_prefix ${SWIFTLIB_DIR})
+  endif()
+
   # Install runtime libraries to lib/swift instead of lib. This works around
   # the fact that -isysroot prevents linking to libraries in the system
   # /usr/lib if Swift is installed in /usr.
   set_target_properties("${target}" PROPERTIES
-    LIBRARY_OUTPUT_DIRECTORY ${SWIFTLIB_DIR}/${SWIFTLIB_SINGLE_SUBDIR}
-    ARCHIVE_OUTPUT_DIRECTORY ${SWIFTLIB_DIR}/${SWIFTLIB_SINGLE_SUBDIR})
+    LIBRARY_OUTPUT_DIRECTORY ${swiftlib_prefix}/${SWIFTLIB_SINGLE_SUBDIR}
+    ARCHIVE_OUTPUT_DIRECTORY ${swiftlib_prefix}/${SWIFTLIB_SINGLE_SUBDIR})
   if(SWIFTLIB_SINGLE_SDK STREQUAL WINDOWS AND SWIFTLIB_SINGLE_IS_STDLIB_CORE
       AND libkind STREQUAL SHARED)
     add_custom_command(TARGET ${target} POST_BUILD
-      COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:${target}> ${SWIFTLIB_DIR}/${SWIFTLIB_SINGLE_SUBDIR})
+      COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:${target}> ${swiftlib_prefix}/${SWIFTLIB_SINGLE_SUBDIR})
   endif()
 
   foreach(config ${CMAKE_CONFIGURATION_TYPES})
     string(TOUPPER ${config} config_upper)
-    escape_path_for_xcode("${config}" "${SWIFTLIB_DIR}" config_lib_dir)
+    escape_path_for_xcode("${config}" "${swiftlib_prefix}" config_lib_dir)
     set_target_properties(${target} PROPERTIES
       LIBRARY_OUTPUT_DIRECTORY_${config_upper} ${config_lib_dir}/${SWIFTLIB_SINGLE_SUBDIR}
       ARCHIVE_OUTPUT_DIRECTORY_${config_upper} ${config_lib_dir}/${SWIFTLIB_SINGLE_SUBDIR})
@@ -1390,6 +1405,7 @@ endfunction()
 #     DEPLOYMENT_VERSION_TVOS version
 #     DEPLOYMENT_VERSION_WATCHOS version
 #     MACCATALYST_BUILD_FLAVOR flavor
+#     BACK_DEPLOYMENT_LIBRARY version
 #     source1 [source2 source3 ...])
 #
 # name
@@ -1485,6 +1501,9 @@ endfunction()
 # IS_SDK_OVERLAY
 #   Treat the library as a part of the Swift SDK overlay.
 #
+# BACK_DEPLOYMENT_LIBRARY
+#   Treat this as a back-deployment library to the given Swift version
+#
 # INSTALL_IN_COMPONENT comp
 #   The Swift installation component that this library belongs to.
 #
@@ -1533,7 +1552,8 @@ function(add_swift_target_library name)
         INSTALL_IN_COMPONENT
         DARWIN_INSTALL_NAME_DIR
         DEPLOYMENT_VERSION_MACCATALYST
-        MACCATALYST_BUILD_FLAVOR)
+        MACCATALYST_BUILD_FLAVOR
+        BACK_DEPLOYMENT_LIBRARY)
   set(SWIFTLIB_multiple_parameter_options
         C_COMPILE_FLAGS
         DEPENDS
@@ -1970,6 +1990,12 @@ function(add_swift_target_library name)
         list(APPEND swiftlib_link_flags_all "-Wl,-soname,lib${name}.so")
       endif()
 
+      if (SWIFTLIB_BACK_DEPLOYMENT_LIBRARY)
+        set(back_deployment_library_option BACK_DEPLOYMENT_LIBRARY ${SWIFTLIB_BACK_DEPLOYMENT_LIBRARY})
+      else()
+        set(back_deployment_library_option)
+      endif()
+
       # Add this library variant.
       _add_swift_target_library_single(
         ${variant_name}
@@ -2006,6 +2032,7 @@ function(add_swift_target_library name)
         DEPLOYMENT_VERSION_TVOS "${SWIFTLIB_DEPLOYMENT_VERSION_TVOS}"
         DEPLOYMENT_VERSION_WATCHOS "${SWIFTLIB_DEPLOYMENT_VERSION_WATCHOS}"
         MACCATALYST_BUILD_FLAVOR "${maccatalyst_build_flavor}"
+        ${back_deployment_library_option}
         ENABLE_LTO "${SWIFT_STDLIB_ENABLE_LTO}"
         GYB_SOURCES ${SWIFTLIB_GYB_SOURCES}
       )
@@ -2106,6 +2133,9 @@ function(add_swift_target_library name)
         if("${sdk}" STREQUAL "WINDOWS")
           set(UNIVERSAL_LIBRARY_NAME
             "${SWIFTLIB_DIR}/${library_subdir}/${name}.dll")
+        elseif(SWIFTLIB_BACK_DEPLOYMENT_LIBRARY)
+          set(UNIVERSAL_LIBRARY_NAME
+            "${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/lib/swift-${SWIFTLIB_BACK_DEPLOYMENT_LIBRARY}/${library_subdir}/${CMAKE_SHARED_LIBRARY_PREFIX}${name}${CMAKE_SHARED_LIBRARY_SUFFIX}")
         else()
           set(UNIVERSAL_LIBRARY_NAME
             "${SWIFTLIB_DIR}/${library_subdir}/${CMAKE_SHARED_LIBRARY_PREFIX}${name}${CMAKE_SHARED_LIBRARY_SUFFIX}")
@@ -2185,8 +2215,16 @@ function(add_swift_target_library name)
         # NOTE: ${UNIVERSAL_LIBRARY_NAME} is the output associated with the target
         # ${lipo_target}
         add_dependencies(${SWIFTLIB_INSTALL_IN_COMPONENT} ${lipo_target})
+
+        if (SWIFTLIB_BACK_DEPLOYMENT_LIBRARY)
+          # Back-deployment libraries get installed into a versioned directory.
+          set(install_dest "lib${LLVM_LIBDIR_SUFFIX}/${resource_dir}-${SWIFTLIB_BACK_DEPLOYMENT_LIBRARY}/${resource_dir_sdk_subdir}")
+        else()
+          set(install_dest "lib${LLVM_LIBDIR_SUFFIX}/${resource_dir}/${resource_dir_sdk_subdir}")
+        endif()
+
         swift_install_in_component(FILES "${UNIVERSAL_LIBRARY_NAME}"
-                                   DESTINATION "lib${LLVM_LIBDIR_SUFFIX}/${resource_dir}/${resource_dir_sdk_subdir}"
+                                   DESTINATION ${install_dest}
                                    COMPONENT "${SWIFTLIB_INSTALL_IN_COMPONENT}"
                                    PERMISSIONS ${file_permissions}
                                    "${optional_arg}")

stdlib/public/BackDeployConcurrency/CMakeLists.txt

@@ -0,0 +1,46 @@
+#===--- CMakeLists.txt - Back-deployed concurrency support library -------===#
+#
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2021 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 always build standalone
+include("${CMAKE_CURRENT_SOURCE_DIR}/../../../cmake/modules/StandaloneOverlay.cmake")
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/../../cmake/modules")
+include(AddSwiftStdlib)
+
+# Don't build the libraries for 32-bit iOS targets; there is no back-deployment
+# to them.
+list(REMOVE_ITEM SWIFT_SDK_IOS_ARCHITECTURES "armv7" "armv7s")
+list(REMOVE_ITEM SWIFT_SDK_IOS_SIMULATOR_ARCHITECTURES "i386")
+
+# The back-deployed library can only be shared.
+list(APPEND SWIFT_STDLIB_LIBRARY_BUILD_TYPES SHARED)
+
+# Link aginst the libswiftCore in the SDK. This intentionally avoids using
+# the locally-built libswiftCore.
+foreach(sdk ${SWIFT_SDKS})
+    set(sdk_name ${SWIFT_SDK_${sdk}_LIB_SUBDIR})
+    set(swift_core_target "swiftCore-${sdk_name}")
+    add_library(${swift_core_target} SHARED IMPORTED GLOBAL)
+    set_property(TARGET ${swift_core_target}
+      PROPERTY IMPORTED_LOCATION "${SWIFT_SDK_${sdk}_PUBLIC_PATH}/usr/lib/swift/libswiftCore.tbd")
+    foreach(arch in ${SWIFT_SDK_${sdk}_ARCHITECTURES})
+      add_library("${swift_core_target}-${arch}" ALIAS "${swift_core_target}")
+    endforeach()
+endforeach()
+
+# Build the concurrency library for back deployment.
+add_compile_definitions(SWIFT_CONCURRENCY_BACK_DEPLOYMENT)
+set(swift_concurrency_swift_flags -Xfrontend -enable-experimental-back-deploy-concurrency)
+set(swift_concurrency_install_component back-deployment)
+set(swift_concurrency_options BACK_DEPLOYMENT_LIBRARY 5.5)
+set(swift_concurrency_availability "macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0")
+
+add_subdirectory(../Concurrency stdlib/public/BackDeployConcurrency)

stdlib/public/Concurrency/CMakeLists.txt

@@ -13,8 +13,13 @@
 set(LLVM_OPTIONAL_SOURCES
   ${swift_concurrency_objc_sources})
 
-set(swift_concurrency_link_libraries
-  swiftCore)
+if(NOT swift_concurrency_availability)
+  set(swift_concurrency_availability "macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0")
+endif()
+
+if(NOT swift_concurrency_install_component)
+  set(swift_concurrency_install_component stdlib)
+endif()
 
 if(SWIFT_CONCURRENCY_USES_DISPATCH)
   if(NOT CMAKE_SYSTEM_NAME STREQUAL Darwin)
@@ -92,10 +97,12 @@ add_swift_target_library(swift_Concurrency ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
     -Dswift_Concurrency_EXPORTS
   SWIFT_COMPILE_FLAGS
     ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
+    ${swift_concurrency_swift_flags}
     -parse-stdlib
     -Xfrontend -enable-experimental-concurrency
     -Xfrontend -define-availability
-    -Xfrontend \"SwiftStdlib 5.5:macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0\"
+    -Xfrontend "SwiftStdlib 5.5:${swift_concurrency_availability}"
   LINK_FLAGS "${SWIFT_RUNTIME_CONCURRENCY_SWIFT_LINK_FLAGS}"
-  INSTALL_IN_COMPONENT stdlib
+  ${swift_concurrency_options}
+  INSTALL_IN_COMPONENT ${swift_concurrency_install_component}
 )

utils/build_swift/build_swift/driver_arguments.py

@@ -596,6 +596,9 @@ def create_argument_parser():
     option(['-b', '--llbuild'], toggle_true('build_llbuild'),
            help='build llbuild')
 
+    option(['--back-deploy-concurrency'], toggle_true('build_backdeployconcurrency'),
+           help='build the concurrency back-deployment libraries')
+
     option(['--libcxx'], toggle_true('build_libcxx'),
            help='build libcxx')
 

utils/swift_build_support/swift_build_support/build_script_invocation.py

@@ -553,6 +553,8 @@ def compute_product_pipelines(self):
         # Begin the post build-script-impl build phase.
         builder.begin_pipeline()
 
+        builder.add_product(products.BackDeployConcurrency,
+                            is_enabled=self.args.build_backdeployconcurrency)
         builder.add_product(products.SwiftPM,
                             is_enabled=self.args.build_swiftpm)
         builder.add_product(products.SwiftSyntax,

utils/swift_build_support/swift_build_support/products/__init__.py

@@ -10,6 +10,7 @@
 #
 # ----------------------------------------------------------------------------
 
+from .backdeployconcurrency import BackDeployConcurrency
 from .benchmarks import Benchmarks
 from .cmark import CMark
 from .earlyswiftdriver import EarlySwiftDriver
@@ -36,6 +37,7 @@
 from .xctest import XCTest
 
 __all__ = [
+    'BackDeployConcurrency',
     'CMark',
     'Ninja',
     'Foundation',