Re: catch and throw, was Is multi-level function return possible?

On 3/28/2014 7:47 PM, Kaz Kylheku wrote:
> On 2014-03-29, Ivan Godard <ivan@ootbcomp.com> wrote:


>> Useful or no, the ability to resume requires retaining the illusion of
>> sequential execution. You have no idea how expensive, in power and
>> performance, that illusion is. And it's flat-out impossible on wide
>> issue machine such as VLIWs.
>
> I suspect you're may be thinking about resuming the exact instruction with the
> exact machine state where some exception occured.
>
> That's not even necessarily the best, most useful way to resume.


<massive snip>


In machine terms, you are describing throw/catch, not resume. It
happens that your catch does a programming-language-level resume, as
opposed to a hardware resume. However, programming languages consume
zero power; my note was about machines, which unfortunately do a bit
worse :-)


Even in your throw/catch, it is extremely difficult to provide a
consistent model of side-effects that have, or have not, happened at
resumption. Of course, the language level (which runs infinitely fast)
can say that the program state is memoized at try and restored at
catch. That's not a recommended practice on finite machines though.

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