Re: avoid unnecessary register saves for setjmp

[Chris Lattner <sabre@nondot.org>]

On Thu, 27 Nov 2003, Zack Weinberg wrote:

> >From a language-design perspective it's desirable for x to have the
> value 23 in both fragments.  (Leaving out the volatile on x is
> intentional.)  Its being undefined in C is a source of hard-to-find
> bugs.  It happens because longjmp() can't know which registers need
> restoring, whereas the EH unwinder does know and can get it
> (abstractly) right.  So by using the EH unwinder for longjmp we clean
> up a nasty dark corner of the language.

Also, ideally, longjmp should run cleanups in intervening stackframes,
which the LLVM implementation does, just like for pthreads cancellation.

> 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.)

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.

-Chris

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