--- a/js/src/devtools/automation/autospider.py
+++ b/js/src/devtools/automation/autospider.py
@@ -34,17 +34,17 @@ DIR = directories(os.path, os.getcwd())
 PDIR = directories(posixpath, os.environ["PWD"],
                    fixup=lambda s: re.sub(r'^(\w):', r'/\1', s))
 
 parser = argparse.ArgumentParser(
     description='Run a spidermonkey shell build job')
 parser.add_argument('--dep', action='store_true',
                     help='do not clobber the objdir before building')
 parser.add_argument('--platform', '-p', type=str, metavar='PLATFORM',
-                    default='', help='build platform')
+                    default='', help='build platform, including a suffix ("-debug" or "") used by buildbot to override the variant\'s "debug" setting. The platform can be used to specify 32 vs 64 bits.')
 parser.add_argument('--timeout', '-t', type=int, metavar='TIMEOUT',
                     default=10800,
                     help='kill job after TIMEOUT seconds')
 parser.add_argument('--objdir', type=str, metavar='DIR',
                     default=env.get('OBJDIR', 'obj-spider'),
                     help='object directory')
 group = parser.add_mutually_exclusive_group()
 group.add_argument('--optimize', action='store_true',
@@ -155,16 +155,19 @@ CONFIGURE_ARGS = variant['configure-args
 
 opt = args.optimize
 if opt is None:
     opt = variant.get('optimize')
 if opt is not None:
     CONFIGURE_ARGS += (" --enable-optimize" if opt else " --disable-optimize")
 
 opt = args.debug
+if opt is None and args.platform:
+    # Override variant['debug'].
+    opt = ('-debug' in args.platform)
 if opt is None:
     opt = variant.get('debug')
 if opt is not None:
     CONFIGURE_ARGS += (" --enable-debug" if opt else " --disable-debug")
 
 # Any jobs that wish to produce additional output can save them into the upload
 # directory if there is such a thing, falling back to OBJDIR.
 env.setdefault('MOZ_UPLOAD_DIR', OBJDIR)