Re: [MIT-Scheme-devel] two changes to push

[Chris Hanson]

Arggh....   Good catch.  Let's move the transcript operations as you suggest.

On Sun, Sep 20, 2009 at 12:14 PM, Joe Marshall <address@hidden> wrote:
> On Sat, Sep 19, 2009 at 9:50 PM, Taylor R Campbell <address@hidden> 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
>