[llvm-dev] returns_twice / noreturn

[Sanjoy Das via llvm-dev]

On Fri, Nov 3, 2017 at 6:06 PM, Hal Finkel via llvm-dev
> That's correct. The relevant semantics here come from C's setjmp/longjmp,
> and there's an exception in that language to deal with this situation:
>
> 7.13.2.1p3: "All accessible objects have values, and all other components of
> the abstract machine have state, as of the time the longjmp function was
> called, except that the values of objects of automatic storage duration that
> are local to the function containing the invocation of the corresponding
> setjmp macro that do not have volatile-qualified type and have been changed
> between the setjmp invocation and longjmp call are indeterminate."
>
> We should probably import some version of this into the LangRef to
> more-accurately describe returns_twice/noreturn (because that's what we
> actually implement in this regard).

We do not implement that restricted semantics correctly either -- see
https://bugs.llvm.org/show_bug.cgi?id=27190

IMO the Right(TM) fix is to add a CFG edge from all possibly
longjmp'ing function calls to all setjmps in a function.  We can
probably do this by modeling the possibly longjmp'ing calls as invokes
that unwind to a special "setjmp" landingpad.

-- Sanjoy