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Efficiency of local fns and closures
Date: Fri, 20 Oct 89 05:31:49 EDT
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
(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.
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.