Swift: support multithreaded compilation

Query the number of logical CPUs available to enable parallel
compilation for Swift.

Help/manual/cmake-variables.7.rst

@@ -98,6 +98,7 @@ Variables that Provide Information
    /variable/CMAKE_STATIC_LIBRARY_PREFIX
    /variable/CMAKE_STATIC_LIBRARY_SUFFIX
    /variable/CMAKE_Swift_MODULE_DIRECTORY
+   /variable/CMAKE_Swift_NUM_THREADS
    /variable/CMAKE_TOOLCHAIN_FILE
    /variable/CMAKE_TWEAK_VERSION
    /variable/CMAKE_VERBOSE_MAKEFILE

Help/variable/CMAKE_Swift_NUM_THREADS.rst

@@ -0,0 +1,8 @@
+CMAKE_Swift_NUM_THREADS
+-----------------------
+
+Number of threads for parallel compilation for Swift targets.
+
+This variable controls the number of parallel jobs that the swift driver creates
+for building targets.  If not specified, it will default to the number of
+logical CPUs on the host.

Modules/CMakeSwiftInformation.cmake

@@ -49,11 +49,15 @@ if(NOT CMAKE_Swift_COMPILE_OBJECT)
   set(CMAKE_Swift_COMPILE_OBJECT ":")
 endif()
 
+if(NOT CMAKE_Swift_NUM_THREADS MATCHES "^[0-9]+$")
+  cmake_host_system_information(RESULT CMAKE_Swift_NUM_THREADS QUERY NUMBER_OF_LOGICAL_CORES)
+endif()
+
 if(NOT CMAKE_Swift_CREATE_SHARED_LIBRARY)
   if(CMAKE_Swift_COMPILER_TARGET)
-    set(CMAKE_Swift_CREATE_SHARED_LIBRARY "${CMAKE_Swift_COMPILER} -target <CMAKE_Swift_COMPILER_TARGET> -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -emit-library -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
+    set(CMAKE_Swift_CREATE_SHARED_LIBRARY "${CMAKE_Swift_COMPILER} -target <CMAKE_Swift_COMPILER_TARGET> -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-library -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
   else()
-    set(CMAKE_Swift_CREATE_SHARED_LIBRARY "${CMAKE_Swift_COMPILER} -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -emit-library -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
+    set(CMAKE_Swift_CREATE_SHARED_LIBRARY "${CMAKE_Swift_COMPILER} -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-library -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
   endif()
 endif()
 
@@ -63,17 +67,17 @@ endif()
 
 if(NOT CMAKE_Swift_LINK_EXECUTABLE)
   if(CMAKE_Swift_COMPILER_TARGET)
-    set(CMAKE_Swift_LINK_EXECUTABLE "${CMAKE_Swift_COMPILER} -target <CMAKE_Swift_COMPILER_TARGET> -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -emit-executable -o <TARGET> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
+    set(CMAKE_Swift_LINK_EXECUTABLE "${CMAKE_Swift_COMPILER} -target <CMAKE_Swift_COMPILER_TARGET> -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-executable -o <TARGET> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
   else()
-    set(CMAKE_Swift_LINK_EXECUTABLE "${CMAKE_Swift_COMPILER} -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -emit-executable -o <TARGET> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
+    set(CMAKE_Swift_LINK_EXECUTABLE "${CMAKE_Swift_COMPILER} -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-executable -o <TARGET> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
   endif()
 endif()
 
 if(NOT CMAKE_Swift_CREATE_STATIC_LIBRARY)
   if(CMAKE_Swift_COMPILER_TARGET)
-    set(CMAKE_Swift_CREATE_STATIC_LIBRARY "${CMAKE_Swift_COMPILER} -target <CMAKE_Swift_COMPILER_TARGET> -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -emit-library -static -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
+    set(CMAKE_Swift_CREATE_STATIC_LIBRARY "${CMAKE_Swift_COMPILER} -target <CMAKE_Swift_COMPILER_TARGET> -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-library -static -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
   else()
-    set(CMAKE_Swift_CREATE_STATIC_LIBRARY "${CMAKE_Swift_COMPILER} -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -emit-library -static -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
+    set(CMAKE_Swift_CREATE_STATIC_LIBRARY "${CMAKE_Swift_COMPILER} -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -num-threads ${CMAKE_Swift_NUM_THREADS} -emit-library -static -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <LINK_LIBRARIES>")
   endif()
 
   set(CMAKE_Swift_ARCHIVE_CREATE "<CMAKE_AR> crs <TARGET> <OBJECTS>")