Re: How much tuning does regular lisp compilers do?

Don Geddis <don@xxxxxxxxxx> wrote:
+---------------
| rpw3@xxxxxxxx (Rob Warnock) wrote on Fri, 29 Aug 2008:
| > For example, in CMUCL, consider these successive trials:
| > cmu> (time (dotimes (i 2000000000)))
| > Note that the same loop took ~4, ~4, ~2, & ~4 CPU cycles/iteration.
|
| Shouldn't you first compile code, that you want to time?
+---------------

WQell, what I wonder is why you failed to quote the lines from my post
that showed that each run *WAS* in fact being compiled [which CMUCL does
by default for each call to TIME]?!?!?

cmu> (time (dotimes (i 2000000000)))
; Compiling LAMBDA NIL: <=== THESE TWO LINES
; Compiling Top-Level Form: <=== THESE TWO LINES

; Evaluation took:
; 2.73f0 seconds of real time
; 2.720226f0 seconds of user run time
; 0.0f0 seconds of system run time
; 6,045,046,240 CPU cycles
; 0 page faults and
; 0 bytes consed.
;
NIL
cmu>

+---------------
| > Why? Because the third compilation was more optimally placed for
| > the CPU's branch prediction (Athlon-32, as it happens).
|
| If you're running interpreted code, how can you be so certain that
| this is the correct explanation for the difference in run times?
+---------------

1. Because it's *NOT* "interpreted code" (see above).

2. Because I spent quite a bit of time one afternoon examining this
situation at length. [See my parallel reply to "verec" for details.]


-Rob

-----
Rob Warnock <rpw3@xxxxxxxx>
627 26th Avenue <URL:http://rpw3.org/>
San Mateo, CA 94403 (650)572-2607

.

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