Subject: bug#41354: equal? has no sensible code path for symbols

From: David Kastrup <[email-masked]>
Date: Wed, 27 May 2020 22:49:10 +0200

Ludovic Courtès <[email-masked]> writes:

> Hi David,
>
> David Kastrup <[email-masked]> skribis:
>
>> In Scheme, symbols can be compared using eq? for equality.  However,
>> since they have garbage-collected content attached, they do not meet the
>> predicate SCM_IMP in the short-circuit evaluation at the start of equal?
>> This means that unequal symbols compared using equal? fall through a
>> whole bunch of tests and end up in a general structural comparison
>> comparing their underlying string names.
>
> ‘equal?’ starts by checking for eq-ness, which LGTM:
>
>   SCM
>   scm_equal_p (SCM x, SCM y)
>   #define FUNC_NAME s_scm_i_equal_p
>   {
>     SCM_CHECK_STACK;
>    tailrecurse:
>     SCM_TICK;
>     if (scm_is_eq (x, y))
>       return SCM_BOOL_T;
>
> Or were you referring to something else?

I repeat: "This means that UNEQUAL symbols compared using equal? fall
through a whole bunch of tests and end up in a general structural
comparison comparing their underlying string names".

Lots of searches _end_ with an equal comparison (which is fast) but do a
lot of unequal comparisons before that (which is slow, even though
symbols that are not eq? will also not be equal?, so if you know you are
checking _symbols_, if they are not eq? you are done).

Symbols comparing as _unequal_ have no special path in equal?.

-- 
David Kastrup