[LLVMdev] setjmp - longjmp

[Jakob Stoklund Olesen]

On Oct 4, 2011, at 3:53 PM, Eli Friedman wrote:

> On Tue, Oct 4, 2011 at 3:10 PM, Khaled ElWazeer
> <khalid.alwazeer at gmail.com> wrote:
>> Hi,
>> 
>> I have some code which has sigsetjmp / longjmp. After a longjmp, unreachable
>> is inserted, which is fine. The problem is that in the backend before
>> calling longjmp, some register was spilled to a stack location which is live
>> across the jmp. I mean, it will be live after jumping. The stack location
>> was initialized before the call to setjmp, and is used afterwards.
>> 
>> Is there any bug in handling such a case? I mean, how do LLVM knows about
>> CFG in case of longjmp / setjmp calls?
> 
> No, no handling; we just don't inline anything into functions which
> call setjmp, and hope we get lucky.  In practice, that's generally
> good enough, given the restrictions on functions that call setjmp, but
> there are probably some subtle bugs nobody has discovered yet.

We already disable stack slot sharing in functions that callsFunctionThatReturnsTwice():

  // If there are calls to setjmp or sigsetjmp, don't perform stack slot
  // coloring. The stack could be modified before the longjmp is executed,
  // resulting in the wrong value being used afterwards. (See
  // <rdar://problem/8007500>.)
  if (MF.callsSetJmp())
    return false;

It might be possible for register coalescing to break something as well.

/jakob