Make bisect to abort early when the return codes for known good and known bad revisions are same.

tools/auto_bisect/bisect_perf_regression.py

 #!/usr/bin/env python
 # Copyright 2013 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.
 
 """Chromium auto-bisect tool
 
 This script bisects a range of commits using binary search. It starts by getting
 reference values for the specified "good" and "bad" commits. Then, for revisions
 in between, it will get builds, run tests and classify intermediate revisions as
@@ skipping 2289 matching lines @@
 
       if good_results[1]:
         error = ('An error occurred while building and running the \'good\' '
                  'reference value. The bisect cannot continue without '
                  'a working \'good\' revision to start from.\n\nError: %s' %
                  good_results[0])
         return BisectResults(error=error)
 
       # We need these reference values to determine if later runs should be
       # classified as pass or fail.
+
       known_bad_value = bad_results[0]
       known_good_value = good_results[0]
 
+      # Abort bisect early when the return codes for known good
+      # and known bad revisions are same.
+      if (self._IsBisectModeReturnCode() and
+          known_bad_value['mean'] == known_good_value['mean']):
+        return BisectResults(abort_reason=('known good and known bad revisions '
+            'returned same return code (return code=%s). '
+            'Continuing bisect might not yield any results.' %
+            known_bad_value['mean']))
       # Check the direction of improvement only if the improvement_direction
       # option is set to a specific direction (1 for higher is better or -1 for
       # lower is better).
       improvement_dir = self.opts.improvement_direction
       if improvement_dir:
         higher_is_better = improvement_dir > 0
         if higher_is_better:
           message = "Expecting higher values to be better for this metric, "
         else:
           message = "Expecting lower values to be better for this metric, "
@@ skipping 571 matching lines @@
       # bugs. If you change this, please update the perf dashboard as well.
       bisect_utils.OutputAnnotationStepStart('Results')
     print 'Runtime Error: %s' % e
     if opts.output_buildbot_annotations:
       bisect_utils.OutputAnnotationStepClosed()
   return 1
 
 
 if __name__ == '__main__':
   sys.exit(main())