added optimization for normal static libraries Tokutek/mongo#983

cmake/merge_archives_unix.cmake.in

@@ -40,6 +40,20 @@ FOREACH(LIB ${STATIC_LIBS})
     )
   STRING(REGEX REPLACE "\n" ";" LIB_OBJ_LIST "${LIB_OBJS}")
   STRING(REGEX REPLACE ";$" "" LIB_OBJ_LIST "${LIB_OBJ_LIST}")
+
+  LIST(LENGTH LIB_OBJ_LIST LENGTH_WITH_DUPS)
+  SET(LIB_OBJ_LIST_NO_DUPS ${LIB_OBJ_LIST})
+  LIST(REMOVE_DUPLICATES LIB_OBJ_LIST_NO_DUPS)
+  LIST(LENGTH LIB_OBJ_LIST_NO_DUPS LENGTH_WITHOUT_DUPS)
+
+  IF(LENGTH_WITH_DUPS EQUAL LENGTH_WITHOUT_DUPS)
+    # Optimization for when lib doesn't actually have duplicate object
+    # names, we can just extract everything.
+    EXECUTE_PROCESS(
+      COMMAND ${CMAKE_AR} -x ${LIB}
+      WORKING_DIRECTORY ${TEMP_SUBDIR}
+      )
+  ELSE()
     LIST(SORT LIB_OBJ_LIST)
     SET(SAME_OBJ_COUNT 1)
     SET(LAST_OBJ_NAME)
@@ -57,6 +71,7 @@ FOREACH(LIB ${STATIC_LIBS})
         WORKING_DIRECTORY ${TEMP_SUBDIR}
         )
     ENDFOREACH()
+  ENDIF()
 
   FILE(GLOB_RECURSE LIB_OBJECTS "${TEMP_SUBDIR}/*.o")
   SET(OBJECTS ${OBJECTS} ${LIB_OBJECTS})