supersize: Fix --tool-prefix autodetection

tools/binary_size/libsupersize/archive.py

 # Copyright 2017 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.
 
 """Main Python API for analyzing binary size."""
 
 import argparse
 import calendar
 import collections
 import datetime
@@ skipping 305 matching lines @@
     if s.size_without_padding == 0:
       continue
     name_list = aliases_by_address.get(s.address)
     if name_list:
       if s.name not in name_list:
         logging.warning('Name missing from aliases: %s %s', s.name, name_list)
         continue
       replacements.append((i, name_list))
       num_new_symbols += len(name_list) - 1
 
+  if float(num_new_symbols) / len(raw_symbols) < .1:
+    logging.warning('Number of aliases is oddly low (%.0f%%). It should '
+                    'usually be around 25%%. Ensure --tool-prefix is correct.',
+                    float(num_new_symbols) / len(raw_symbols) * 100)
+
   # Step 2: Create new symbols as siblings to each existing one.
   logging.debug('Creating %d aliases', num_new_symbols)
   src_cursor_end = len(raw_symbols)
   raw_symbols += [None] * num_new_symbols
   dst_cursor_end = len(raw_symbols)
   for src_index, name_list in reversed(replacements):
     # Copy over symbols that come after the current one.
     chunk_size = src_cursor_end - src_index - 1
     dst_cursor_end -= chunk_size
     src_cursor_end -= chunk_size
@@ skipping 258 matching lines @@
   parser.add_argument('--elf-file',
                       help='Path to input ELF file. Currently used for '
                            'capturing metadata.')
   parser.add_argument('--map-file',
                       help='Path to input .map(.gz) file. Defaults to '
                            '{{elf_file}}.map(.gz)?. If given without '
                            '--elf-file, no size metadata will be recorded.')
   parser.add_argument('--no-source-paths', action='store_true',
                       help='Do not use .ninja files to map '
                            'object_path -> source_path')
-  parser.add_argument('--tool-prefix', default='',
-                      help='Path prefix for c++filt.')
+  parser.add_argument('--tool-prefix',
+                      help='Path prefix for c++filt, nm, readelf.')
   parser.add_argument('--output-directory',
                       help='Path to the root build directory.')
 
 
 def Run(args, parser):
   if not args.size_file.endswith('.size'):
     parser.error('size_file must end with .size')
 
   elf_path = args.elf_file
   map_path = args.map_file
@@ skipping 69 matching lines @@
           logging.warning('Packed section not present: %s', packed_section_name)
         else:
           size_info.section_sizes['%s (unpacked)' % packed_section_name] = (
               unstripped_section_sizes.get(packed_section_name))
 
   logging.info('Recording metadata: \n  %s',
                '\n  '.join(describe.DescribeMetadata(size_info.metadata)))
   logging.info('Saving result to %s', args.size_file)
   file_format.SaveSizeInfo(size_info, args.size_file)
   logging.info('Done')