.o could be COFF object.

tools/determinism/compare_build_artifacts.py

 #!/usr/bin/env python
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Compare the artifacts from two builds."""
 
 import difflib
 import json
 import optparse
@@ skipping 439 matching lines @@
       #   2 -  3: # sections.
       #   4 -  7: timestamp.
       #   ....
       #
       # COFF BigObj header:
       #   0 -  3: signature (0000 FFFF)
       #   4 -  5: version
       #   6 -  7: machine
       #   8 - 11: timestamp.
       COFF_HEADER_TO_COMPARE_SIZE = 12
-      if (sys.platform == 'win32' and first_filepath.endswith('.obj')
+      if (sys.platform == 'win32'
+          and os.path.splitext(first_filepath)[1] in ('.o', '.obj')
           and file_len > COFF_HEADER_TO_COMPARE_SIZE):
         rhs_data = rhs.read(COFF_HEADER_TO_COMPARE_SIZE)
         lhs_data = lhs.read(COFF_HEADER_TO_COMPARE_SIZE)
         if (lhs_data[0:4] == rhs_data[0:4] and lhs_data[4:8] != rhs_data[4:8]
             and lhs_data[8:12] == rhs_data[8:12]):
           offset += COFF_HEADER_TO_COMPARE_SIZE
         elif (lhs_data[0:4] == '\x00\x00\xff\xff' and
               lhs_data[0:8] == rhs_data[0:8] and
               lhs_data[8:12] != rhs_data[8:12]):
           offset += COFF_HEADER_TO_COMPARE_SIZE
@@ skipping 214 matching lines @@
     parser.error('--target-platform is required')
 
   return compare_build_artifacts(os.path.abspath(options.first_build_dir),
                                  os.path.abspath(options.second_build_dir),
                                  options.target_platform,
                                  options.recursive)
 
 
 if __name__ == '__main__':
   sys.exit(main())