Why is an empty function call in python around 15% slower for dynamically compiled python code

Question

This is pretty bad micro-optimizing, but I'm just curious. It usually doesn't make a difference in the "real" world.

So I'm compiling a function (that does nothing) using compile() then calling exec on that code and getting a reference to the function I compiled. Then I'm executing it a couple million times and timing it. Then repeating it with a local function. Why is the dynamically compiled function around 15% slower (on python 2.7.2) for just the call?

import datetime
def getCompiledFunc():
  cc = compile("def aa():pass", '<string>', 'exec')
  dd = {}
  exec cc in dd
  return dd.get('aa')

compiledFunc = getCompiledFunc()  
def localFunc():pass


def testCall(f):
  st = datetime.datetime.now()
  for x in xrange(10000000): f()
  et = datetime.datetime.now()
  return (et-st).total_seconds()

for x in xrange(10):
  lt = testCall(localFunc)
  ct = testCall(compiledFunc)
  print "%s %s %s%% slower" % (lt, ct, int(100.0*(ct-lt)/lt))

The output I'm getting is something like:

1.139 1.319 15% slower

Answer 1

The dis.dis() function shows that the code object for each version is identical:

aa
  1           0 LOAD_CONST               0 (None)
              3 RETURN_VALUE        
localFunc
 10           0 LOAD_CONST               0 (None)
              3 RETURN_VALUE 

So the difference is in the function object. I compared each of the fields (func_doc, func_closure, etc) and the one that is different is func_globals. In other words, localFunc.func_globals != compiledFunc.func_globals.

There is a cost for supplying your own dictionary instead of the built-in globals (the former has to be looked up when a stack frame is created on each call and the latter can be referenced directly by the C code which already knows about the default builtin globals dictionary).

This is easy verified by changing the exec line in your code to:

exec cc in globals(), dd

With that change, the timing difference goes away.

Mystery solved!