cmake: Use same default optimization level as Makefiles.

Support overriding optimization level in COMMON_OPT, CCOMMON_OPT, and FCOMMON_OPT.

CMakeLists.txt

@@ -87,6 +87,36 @@ if(MSVC AND MSVC_STATIC_CRT)
   endforeach()
 endif()
 
+# Set default optimization level
+if (NOT MSVC)
+  # Filter out cmake's default optimization level from release and debug flags.
+  # They would override any optimization level set in COMMON_OPTs
+  set(CompilerLangs
+        CXX
+        C
+        Fortran
+        ASM
+        )
+  foreach(CompilerLang ${CompilerLangs})
+    string(REGEX REPLACE "-O[0-9]|-Ofast|-Og|-Os"
+         "" CMAKE_${CompilerLang}_FLAGS_RELEASE
+         "${CMAKE_${CompilerLang}_FLAGS_RELEASE}")
+    string(REGEX REPLACE "-O[0-9]|-Ofast|-Og|-Os"
+         "" CMAKE_${CompilerLang}_FLAGS_DEDUG
+         "${CMAKE_${CompilerLang}_FLAGS_DEDUG}")
+  endforeach()
+  # Prepend default optimization level to COMMON_OPTs
+  if (CMAKE_BUILD_TYPE EQUAL "DEBUG")
+    set(COMMON_OPT "-Og ${COMMON_OPT}")
+    set(CCOMMON_OPT "-Og ${CCOMMON_OPT}")
+    set(FCOMMON_OPT "-Og ${FCOMMON_OPT}")
+  else ()
+    set(COMMON_OPT "-O2 ${COMMON_OPT}")
+    set(CCOMMON_OPT "-O2 ${CCOMMON_OPT}")
+    set(FCOMMON_OPT "-O2 ${FCOMMON_OPT}")
+  endif ()
+endif ()
+
 message(WARNING "CMake support is experimental. It does not yet support all build options and may not produce the same Makefiles that OpenBLAS ships with.")
 
 include("${PROJECT_SOURCE_DIR}/cmake/utils.cmake")