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Efficiency of local fns and closures

    Date: Fri, 20 Oct 89 05:31:49 EDT
    From: barmar@Think.COM
Barry, this was a good answer, but I need to comment on a couple of
things you said.

       Date: Thu, 19 Oct 89 16:10 EST
       From: GODDEN%gmr.com@RELAY.CS.NET
       (defun foo (x y)
	 (labels ((bar (z)
		   (some-function y z)))		;free reference to y
	   (bar (some-other-function x))))
       I have a two-part question regarding efficiency:
    Your questions can easily be answered by looking at some disassemblies and
    a few simple timings with the TIME macro.
For things this fast you can usually get more accurate results by using
METERING:DEFINE-METERING-FUNCTION.  It runs the test case as many times
as necessary to get a statistically valid result, and filters out
various sources of inaccuracy.  Try clicking middle on the average it
presents.  We probably should make TIME filter out the EVAL overhead,
but I don't think Common Lisp will let us make TIME run the test case
multiple times and do statistical processing, so unfortunately TIME
can never be as accurate as METERING:DEFINE-METERING-FUNCTION.

    A quick
    test I just tried seemed to indicate that it takes about 4 microseconds
    longer to access a lexical variable than a parameter variable; 5
    microseconds rather than 1 microsecond per access.

Those numbers seem quite a lot higher than I expected.  Actual
measurement (admittedly in a prototype of Genera 8.0, but for this
particular operation 7.2 should be the same speed) on a 3640 shows
accessing a parent's lexical variable is 1.3 microseconds slower than
accessing a local lexical variable.  The time to access a local
lexical variable on a 3640 is about 200 ns, which is small enough
to make it difficult to measure accurately.  All the numbers are
faster on Ivory-based machines.
    For compiling to core, I can't think of a hard technical problem, and I
    think the ANSI CL draft requires this to work.  

Do you mean, does (LET ((X 1)) (COMPILE NIL #'(LAMBDA () X))) have a
defined meaning in ANSI Common Lisp?  The answer is that it does not.
This was considered and rejected.  See COMPILE-ARGUMENT-PROBLEMS:CLARIFY.