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Re: GNU Guile branch, wip-peval-predicates, created. v2.0.5-100-g59c5570

From: Mark H Weaver
Subject: Re: GNU Guile branch, wip-peval-predicates, created. v2.0.5-100-g59c5570
Date: Fri, 13 Apr 2012 13:07:40 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.92 (gnu/linux)

Hi Andy,

Andy Wingo <address@hidden> writes:
> On Fri 13 Apr 2012 07:22, Mark H Weaver <address@hidden> writes:
>>>     * module/language/tree-il/peval.scm (fold-constants): Returns #f instead
>>>       of the expression, as all continuations handle #f themselves.
>>>       (negate, bailout?, extract-facts, infer, infer-defined?)
>>>       (infer-struct-vtable): New helpers.
>> I haven't looked at the code, but it sounds like you are trying to
>> eliminate redundant 'struct-vtable' checks.  Unfortunately, it seems to
>> me that this cannot be done safely.
> The identity of the vtable does not change when it is redefined.  If the
> vtable is redefined, the equality check still succeeds.  There are
> comments in the source.

Okay, I've just reread:

and I see that I had some misconceptions about this.  Point taken: when
a class is redefined, the result of 'struct-vtable' on the instances
remain the same.

Nonetheless, the vtable checks involve comparisons with a mutable
top-level variable.  Whenever unknown code is run (e.g. when a top-level
procedure is called) you must assume the worst: that any top-level
variable might have been 'set!'.


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