Ensure lto toolchain consistency by forcing cmake to use llvm-ar

Author: minghangli-uni

CMakeLists.txt

@@ -42,6 +42,9 @@ include(GNUInstallDirs)
 include(FortranLib)
 include(AddPatchedSource)
 
+# Use llvm-ar for IntelLLVM Fortran compiler
+include(UseLlvmArchive)
+
 # Common compiler flags and definitions
 if(CMAKE_Fortran_COMPILER_ID MATCHES "GNU")
   set(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS} -fbacktrace -fconvert=big-endian -ffree-line-length-none -ffixed-line-length-none")

cmake/UseLlvmArchive.cmake

@@ -0,0 +1,35 @@
+# Use llvm-ar with `rcs` so we dont need to setup a separate ranlib.
+
+# Identify compiler is IntelLLVM
+# https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_COMPILER_ID.html
+if (NOT (CMAKE_Fortran_COMPILER_ID MATCHES "IntelLLVM"))
+    return()
+endif()
+
+# set a temp variable to hold the Fortran compiler path
+set(_TMP_VAR "")
+if (CMAKE_Fortran_COMPILER)
+    set(_TMP_VAR "${CMAKE_Fortran_COMPILER}")
+endif()
+
+# try to discover full paths to llvm-ar
+# https://cmake.org/cmake/help/latest/command/execute_process.html
+if (_TMP_VAR)
+    execute_process(
+        COMMAND "${_TMP_VAR}" -print-prog-name=llvm-ar
+        OUTPUT_VARIABLE LLVM_AR_PATH
+        OUTPUT_STRIP_TRAILING_WHITESPACE
+    )
+endif()
+
+# enforce llvm-ar
+set(CMAKE_AR "${LLVM_AR_PATH}" CACHE FILEPATH "" FORCE)
+
+# write index table via `s` flag, which disables separate ranlib step.
+# https://github.com/Kitware/CMake/blob/1c7fe4dc0b47de90bec6918b33f0aca47d688887/Modules/CMakeCInformation.cmake#L152-L162
+# https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_ARCHIVE_FINISH.html
+# 
+set(CMAKE_Fortran_ARCHIVE_CREATE "<CMAKE_AR> rcs <TARGET> <OBJECTS>")
+set(CMAKE_Fortran_ARCHIVE_FINISH "")
+
+unset(_TMP_VAR)