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From: cquenel@polyslo.CalPoly.EDU (101 more school days)
Newsgroups: comp.lang.c
Subject: Re: Behaviour of setjmp/longjmp and registers
Message-ID: <7249@polyslo.CalPoly.EDU>
Date: 23 Jan 89 04:51:19 GMT
References: <25@torsqnt.UUCP> <8867@bloom-beacon.MIT.EDU> <7222@polyslo.CalPoly.EDU> <8812@alice.UUCP>
Reply-To: cquenel@polyslo.CalPoly.EDU (101 more school days)
Organization: Blue Blaze Irregulars
Lines: 51

I write:
> Should it ever be necessary/desirable to restore any registers
> on a long-jump BESIDES the frame-pointer and/or stack-pointer ?

Andrew Koenig explains:
[at one time VAX implementations would restore "register" variables]
[this was arguably useful.]

I see this as a BAD thing to try to guarantee.
I agree with the way ANSI did it.
To the best of my understanding the "register"
storage class has always been a "hint" to the compiler.
Since many modern compilers will fairly often out-right ignore
these hints, it makes the use of this feature EXTREMELY unportable at
best.  Consider a very straight-forward example:
I use this restoral feature to restore a register variable
through a longjump.  How many variables can I restore this way ?
How many registers variables can I effectively use ?
Depending on a "register" variable to actually BE in a register 
is (as far as I'm concerned) a BAD thing.

I hope there weren't too many people who were in favor of this 
in the ANSI committees.

Since setjmp/longjmp is such a work-around/violate-all-the-rules/
just-get-me-the-f*ck-back-to-where-I-started/ sorta feature,
(That is to say, a LAST RESORT, to be used almost exclusively
in system dependant routines.) I would say that it SHOULDN'T
actually restore anything.  If you need anything to be restored,
then save it yourself when you do a setjump in a volatile variable.

I think this newer definition of setjmp/longjmp is much cleaner
and provides only what is necessary and most useful.  As a compiler
writer, it certainly makes sense to me to only implement what can
be guaranteed.  That is, to provide well-defined services that
can be relied on, and don't provide anything that might/might-not work.

Comments anyone ?

--chris


If you understand what you're doing, you're not learning anything.
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