On Sat, Sep 19, 2009 at 9:50 PM, Taylor R Campbell <campb...@mumble.net> wrote:
>
> For SYMBOL?, I thought it was worth integrating because
> GUARANTEE-SYMBOL is called on nearly every I/O operation.
>
> It is? I don't see why it would, unless you are changing a generic
> port's line-ending or using port properties at every I/O operation.
I'm guessing it was an unforseen consequence. Here's what happens:
When reading or writing a string or character, a call is made to
TRANSCRIBE-CHAR or TRANSCRIBE-SUBSTRING which echoes
the character or string to the transcription port, if one exists. In order
to check for a transcription port, these routines call PORT/TRANSCRIPT
which either returns the transcription port or #F if there isn't one.
PORT/TRANSCRIPT calls PORT/GET-PROPERTY which has this
definition:
(define (port/get-property port name default)
(guarantee-symbol name 'PORT/GET-PROPERTY)
(let ((p (assq name (port/properties port))))
(if p
(cdr p)
default)))
So you get an out-of-line call to GUARANTEE-SYMBOL and ASSQ on each
I/O operation.
I think moving the transcript port from the property list to the port
itself would be worthwhile because this call chain happens a lot. It
would make ports a little bit bigger, but I don't think that's enough
to worry about.
Alternatively, these two things would help: have a version of port/get-property
that didn't GUARANTEE-SYMBOL. (If the key isn't a symbol, it won't be
found anyway), and do a quick check on the port/properties to see if it
is NULL? before calling out to ASSQ. (This would avoid a call on the common
case of ports without properties.)
It'd be great to fix these up because port operations are one of the big
time sinks for SF.
--
~jrm
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