From: Hal Fulton <hal9000@...>
Date: 2006-02-15T15:16:27+09:00
Subject: Re: Rescuing blocks?

Dave Cantrell wrote:
> Hal Fulton wrote:
> 
>> Dave Cantrell wrote:
>>
>>> Eric Hodel wrote:
>>>
>>>>
>>>> You're going to get a performance hit setting up an exception trap 
>>>> this way.
>>>
>>>
>>> Time for some down-South edumucation. What kind of performance hit 
>>> are we talking about here? And why the hit at all?
>>>
>>> (I seem to recall reading somewhere once upon a time about memory 
>>> stack stuff, but I'm not an old-school C-programmer so I never had to 
>>> deal with it --- unfortunately for me...)
>>
>>
>> Well, my knowledge is limited. Someone else can answer better.
>>
>> But look at it from a common-sense standpoint. Exceptions aren't
>> magic. Everything Ruby does can be done in assembly language, it's
>> just more verbose.
>>
>> I would think that exceptions, at the lowest level, work something
>> like:
>>
>>    if something_happened_here
>>       goto the_place_where_exceptions_are_caught
>>
>> So at the very least, catching exceptions means that you're doing
>> some sort of comparison or conditional branch. And this is code
>> that doesn't have to be run/generated *unless* you are catching
>> exceptions.
>>
>> Just my naive take.
>>
>>
>> Hal
>>
>>
>>
> 
> That's kind of along the lines of what I was thinking, just better said. 
> But this makes me wonder:
> 
> (a) Worrying about the performance hit from comparison/conditional 
> branching leads to the question "is all branching in blocks bad?" 
> Obviously not.
> 
> (b) Couldn't you say that *any* conditional could be jumping out to 
> another section of code? i.e. not just exception handlers?
> 
>   if this_is_true
>      goto where_we_do_stuff_when_this_is_true
> 
> Hmm. This is something I've read, and it's in my brain somewhere, but I 
> don't know it well enough to grok it yet.
> 
> Especially at midnight... :(
> 
> But your explanation gives me a better idea of what is happening, even 
> with my questions.

Well, I think the point is that there is some non-zero amount of "invisible"
code inserted between each statement. In fact, at a higher level of granularity
than the statement.

For example, if I say:  x = a/(b/(c/d))   there are three possible places that
a ZeroDivision exception (or whatever it's called) might happen. Mentally, I'm
envisioning if-statements sprinkled throughout the evaluation of the expression.
But again, this is all naive talk.


Hal