Re: avoid unnecessary register saves for setjmp

[Jan Vroonhof <jvlists@ntlworld.com>]

Chris Lattner <sabre@nondot.org> writes:

>> To be fair, an argument against this change is that it would break
>> code that uses setjmp and longjmp to implement coroutine linkage.
>> (The set/get/make/swapcontext calls are better for this, but that
>> doesn't mean everyone uses them.)

Doesn't it also convert longjmp from an O(1) in to an O(nr off stack frames)
operation?

>
> I don't believe that this is actually legal ANSI C.  You can only return
> to a place that has been setjmp'd to, there is no way to 'construct' a
> jmpbuf.  Likewise, you cannot setjmp, return from the setjmp'ing function,
> then longjmp back to the 'dead' jmpbuf.

It might not be ANSI C, but it does work on most systems for user
level threading (essentially implementing swapcontext by direct
manipulation of the jmbuf). There is lots of useful things people do
with C that are not strictly ANSI/ISO C. In fact that kind of low
level control is often exactly the reason to use C in the first place.

Jan