Swift: Restore compatibility with old C++ driver

The `-wmo` flag added by commit 6063428de7 (Swift: Update default build
flags, 2022-10-03, v3.26.0-rc1~585^2~1) behaves differently with the old
driver.  Detect when the old driver is being used, and avoid adding that
flag.

Fixes: #24641

3 files changed, 72 insertions, 3 deletions

diff --git a/Modules/CMakeDetermineSwiftCompiler.cmake b/Modules/CMakeDetermineSwiftCompiler.cmake
index aaad5601da..f0a63a8cff 100644
--- a/Modules/CMakeDetermineSwiftCompiler.cmake
+++ b/Modules/CMakeDetermineSwiftCompiler.cmake
@@ -63,6 +63,65 @@ if(NOT CMAKE_Swift_COMPILER_ID_RUN)
   CMAKE_DETERMINE_COMPILER_ID(Swift "" CompilerId/main.swift)
 endif()
 
+# Check if we are using the old compiler driver.
+if(CMAKE_GENERATOR STREQUAL "Xcode")
+  # For Xcode, we can decide driver kind simply by Swift version.
+  if(CMAKE_Swift_COMPILER_VERSION VERSION_GREATER_EQUAL 5.5)
+    set(CMAKE_Swift_COMPILER_USE_OLD_DRIVER FALSE)
+  else()
+    set(CMAKE_Swift_COMPILER_USE_OLD_DRIVER TRUE)
+  endif()
+elseif(NOT DEFINED CMAKE_Swift_COMPILER_USE_OLD_DRIVER)
+  # Dry-run a WMO build to identify the compiler driver.
+
+  # Create a clean directory in which to run the test.
+  set(CMAKE_Swift_COMPILER_DRIVER_TEST_DIR ${CMAKE_PLATFORM_INFO_DIR}/SwiftCompilerDriver)
+  file(REMOVE_RECURSE "${CMAKE_Swift_COMPILER_DRIVER_TEST_DIR}")
+  file(MAKE_DIRECTORY "${CMAKE_Swift_COMPILER_DRIVER_TEST_DIR}")
+
+  # Create a Swift file and an arbitrary linker resource.
+  file(WRITE ${CMAKE_Swift_COMPILER_DRIVER_TEST_DIR}/main.swift "print(\"Hello\")\n")
+  file(WRITE ${CMAKE_Swift_COMPILER_DRIVER_TEST_DIR}/lib.in "\n")
+
+  # Honor user-specified compiler flags.
+  if(DEFINED CMAKE_Swift_FLAGS)
+    separate_arguments(_CMAKE_Swift_COMPILER_FLAGS_LIST NATIVE_COMMAND "${CMAKE_Swift_FLAGS}")
+  else()
+    separate_arguments(_CMAKE_Swift_COMPILER_FLAGS_LIST NATIVE_COMMAND "${CMAKE_Swift_FLAGS_INIT}")
+  endif()
+  set(_CMAKE_Swift_COMPILER_CHECK_COMMAND "${CMAKE_Swift_COMPILER}" ${_CMAKE_Swift_COMPILER_FLAGS_LIST} -wmo main.swift lib.in "-###")
+  unset(_CMAKE_Swift_COMPILER_FLAGS_LIST)
+
+  # Execute in dry-run mode so no compilation will be actually performed.
+  execute_process(COMMAND ${_CMAKE_Swift_COMPILER_CHECK_COMMAND}
+    WORKING_DIRECTORY "${CMAKE_Swift_COMPILER_DRIVER_TEST_DIR}"
+    OUTPUT_VARIABLE _CMAKE_Swift_COMPILER_CHECK_OUTPUT)
+
+  # Check the first frontend execution.  It is on the first line of output.
+  # The old driver treats all inputs as Swift sources while the new driver
+  # can identify "lib.in" as a linker resource.
+  if("${_CMAKE_Swift_COMPILER_CHECK_OUTPUT}" MATCHES "^[^\n]* lib\\.in")
+    set(CMAKE_Swift_COMPILER_USE_OLD_DRIVER TRUE)
+  else()
+    set(CMAKE_Swift_COMPILER_USE_OLD_DRIVER FALSE)
+  endif()
+
+  # Record the check results in the configure log.
+  list(TRANSFORM _CMAKE_Swift_COMPILER_CHECK_COMMAND PREPEND "\"")
+  list(TRANSFORM _CMAKE_Swift_COMPILER_CHECK_COMMAND APPEND "\"")
+  list(JOIN _CMAKE_Swift_COMPILER_CHECK_COMMAND " " _CMAKE_Swift_COMPILER_CHECK_COMMAND)
+  string(REPLACE "\n" "\n  " _CMAKE_Swift_COMPILER_CHECK_OUTPUT "  ${_CMAKE_Swift_COMPILER_CHECK_OUTPUT}")
+  message(CONFIGURE_LOG
+    "Detected CMAKE_Swift_COMPILER_USE_OLD_DRIVER=\"${CMAKE_Swift_COMPILER_USE_OLD_DRIVER}\" from:\n"
+    "  ${_CMAKE_Swift_COMPILER_CHECK_COMMAND}\n"
+    "with output:\n"
+    "${_CMAKE_Swift_COMPILER_CHECK_OUTPUT}"
+    )
+
+  unset(_CMAKE_Swift_COMPILER_CHECK_COMMAND)
+  unset(_CMAKE_Swift_COMPILER_CHECK_OUTPUT)
+endif()
+
 if (NOT _CMAKE_TOOLCHAIN_LOCATION)
   get_filename_component(_CMAKE_TOOLCHAIN_LOCATION "${CMAKE_Swift_COMPILER}" PATH)
 endif ()
diff --git a/Modules/CMakeSwiftCompiler.cmake.in b/Modules/CMakeSwiftCompiler.cmake.in
index 47ada38c2c..b3851908aa 100644
--- a/Modules/CMakeSwiftCompiler.cmake.in
+++ b/Modules/CMakeSwiftCompiler.cmake.in
@@ -13,4 +13,6 @@ set(CMAKE_Swift_COMPILER_ENV_VAR "SWIFTC")
 set(CMAKE_Swift_COMPILER_ID_RUN 1)
 set(CMAKE_Swift_SOURCE_FILE_EXTENSIONS swift)
 
+set(CMAKE_Swift_COMPILER_USE_OLD_DRIVER "@CMAKE_Swift_COMPILER_USE_OLD_DRIVER@")
+
 set(CMAKE_Swift_IMPLICIT_INCLUDE_DIRECTORIES "@CMAKE_Swift_IMPLICIT_INCLUDE_DIRECTORIES@")
diff --git a/Modules/CMakeSwiftInformation.cmake b/Modules/CMakeSwiftInformation.cmake
index a75dfceea1..f524955861 100644
--- a/Modules/CMakeSwiftInformation.cmake
+++ b/Modules/CMakeSwiftInformation.cmake
@@ -78,9 +78,17 @@ if(CMAKE_GENERATOR STREQUAL "Xcode")
   set(CMAKE_Swift_FLAGS_MINSIZEREL_INIT "-Osize")
 else()
   set(CMAKE_Swift_FLAGS_DEBUG_INIT "-Onone -g -incremental")
-  set(CMAKE_Swift_FLAGS_RELEASE_INIT "-O -wmo")
-  set(CMAKE_Swift_FLAGS_RELWITHDEBINFO_INIT "-O -g -wmo")
-  set(CMAKE_Swift_FLAGS_MINSIZEREL_INIT "-Osize -wmo")
+  set(CMAKE_Swift_FLAGS_RELEASE_INIT "-O")
+  set(CMAKE_Swift_FLAGS_RELWITHDEBINFO_INIT "-O -g")
+  set(CMAKE_Swift_FLAGS_MINSIZEREL_INIT "-Osize")
+
+  # Enable Whole Module Optimization by default unless the old
+  # C++ driver is being used, which behaves differently under WMO.
+  if(NOT CMAKE_Swift_COMPILER_USE_OLD_DRIVER)
+    string(APPEND CMAKE_Swift_FLAGS_RELEASE_INIT " -wmo")
+    string(APPEND CMAKE_Swift_FLAGS_RELWITHDEBINFO_INIT " -wmo")
+    string(APPEND CMAKE_Swift_FLAGS_MINSIZEREL_INIT " -wmo")
+  endif()
 endif()
 
 if(CMAKE_EXECUTABLE_FORMAT STREQUAL "ELF")