[cmake] Add support for specifying the number of parallel link jobs for swift.

I am going to use the link job for limiting swift lto compile time.

The reason not to use the same variables as LLVM (i.e. LLVM_PARALLEL_LINK_JOBS)
is that Swift since it is compiling more IR may have a larger memory
representation implying less parallel linking jobs than LLVM can be used on
bots.

Author: gottesmm

CMakeLists.txt

@@ -125,6 +125,10 @@ set(SWIFT_NATIVE_SWIFT_TOOLS_PATH "" CACHE STRING
 option(SWIFT_ENABLE_LTO
   "If set to true, build the swift compiler with link time optimization enabled" FALSE)
 
+# The following only works with the Ninja generator in CMake >= 3.0.
+set(SWIFT_PARALLEL_LINK_JOBS "" CACHE STRING
+  "Define the maximum number of linker jobs for swift.")
+
 #
 # User-configurable Darwin-specific options.
 #
@@ -623,6 +627,15 @@ foreach(sdk ${SWIFT_SDKS})
   unset(UNIVERSAL_LIBRARY_NAMES_${SWIFT_SDK_${sdk}_LIB_SUBDIR} CACHE)
 endforeach()
 
+if(SWIFT_PARALLEL_LINK_JOBS)
+  if(CMAKE_VERSION VERSION_LESS 3.0 OR NOT CMAKE_MAKE_PROGRAM MATCHES "ninja")
+    message(WARNING "Job pooling is only available with Ninja generators and CMake 3.0 and later.")
+  else()
+    set_property(GLOBAL APPEND PROPERTY JOB_POOLS swift_link_job_pool=${SWIFT_PARALLEL_LINK_JOBS})
+    set(CMAKE_JOB_POOL_LINK swift_link_job_pool)
+  endif()
+endif()
+
 message(STATUS "Building host Swift tools for ${SWIFT_HOST_VARIANT_SDK} ${SWIFT_HOST_VARIANT_ARCH}")
 message(STATUS "  Build type: ${CMAKE_BUILD_TYPE}")
 message(STATUS "  Assertions: ${LLVM_ENABLE_ASSERTIONS}")

cmake/modules/AddSwift.cmake

@@ -1814,7 +1814,9 @@ function(_add_swift_executable_single name)
       COMPILE_FLAGS " ${c_compile_flags}")
   set_property(TARGET ${name} APPEND_STRING PROPERTY
       LINK_FLAGS " ${link_flags}")
-
+  if (SWIFT_PARALLEL_LINK_JOBS)
+    set_property(TARGET ${name} PROPERTY JOB_POOL_LINK swift_link_job_pool)
+  endif()
   set_output_directory(${name}
       BINARY_DIR ${SWIFT_RUNTIME_OUTPUT_INTDIR}
       LIBRARY_DIR ${SWIFT_LIBRARY_OUTPUT_INTDIR})

utils/build-script-impl

@@ -203,7 +203,7 @@ KNOWN_SETTINGS=(
     darwin-toolchain-application-cert  ""        "Application Cert name to codesign xctoolchain"
     darwin-toolchain-installer-cert    ""        "Installer Cert name to create installer pkg"
     darwin-toolchain-installer-package ""        "The path to installer pkg"
-    build-jobs ""                                "The number of parallel build jobs to use"
+    build-jobs                         ""        "The number of parallel build jobs to use"
     darwin-toolchain-alias             ""        "Swift alias for toolchain"
     export-compile-commands            ""        "set to generate JSON compilation databases for each build product"
 )
@@ -227,6 +227,37 @@ function set_lldb_build_mode() {
     LLDB_BUILD_MODE="CustomSwift-${LLDB_BUILD_TYPE}"
 }
 
+function num_llvm_parallel_lto_link_jobs() {
+    case "$(uname -s -m)" in
+        Darwin\ x86_64)
+            # Currently with -gline-tables-only clang is ~3.5GB on Darwin. Use
+            # the formula GB Memory/3.5GB to get the number of parallel link
+            # threads we can support.
+            echo $(sysctl hw.memsize | cut -f 2 -d " ")/1000000000.0/3.5 | bc
+        ;;
+        *)
+            echo "Unknown operating system"
+            exit 1
+            ;;
+    esac
+}
+
+function num_swift_parallel_lto_link_jobs() {
+    case "$(uname -s -m)" in
+        Darwin\ x86_64)
+            # Currently with -gline-tables-only swift is ~5-6GB on Darwin. Use
+            # the formula GB Memory/6GB to get the number of parallel link
+            # threads we can support.
+            echo $(sysctl hw.memsize | cut -f 2 -d " ")/1000000000/6.0 | bc
+        ;;
+        *)
+            echo "Unknown operating system"
+            exit 1
+            ;;
+    esac
+}
+
+
 function set_deployment_target_based_options() {
     llvm_cmake_options=()
     swift_cmake_options=()
@@ -438,7 +469,7 @@ function set_deployment_target_based_options() {
 		    "-DCMAKE_CXX_FLAGS=-O2 -flto -gline-tables-only -fno-stack-protector "
 		    "-DCMAKE_C_FLAGS_RELWITHDEBINFO=-O2 -flto -gline-tables-only -fno-stack-protector "
 		    "-DCMAKE_CXX_FLAGS_RELWITHDEBINFO=-O2 -flto -gline-tables-only -fno-stack-protector "
-		    "-DLLVM_PARALLEL_LINK_JOBS=5"
+		    "-DLLVM_PARALLEL_LINK_JOBS=$(num_llvm_parallel_lto_link_jobs)"
 		)
 	    fi
 
@@ -448,6 +479,7 @@ function set_deployment_target_based_options() {
                         "${swift_cmake_options[@]}"
                         "-DCMAKE_C_STANDARD_COMPUTED_DEFAULT=AppleClang"
                         "-DCMAKE_CXX_STANDARD_COMPUTED_DEFAULT=AppleClang"
+                        "-DSWIFT_PARALLEL_LINK_JOBS=$(num_swift_parallel_lto_link_jobs)"
                     )
                 fi
             fi