[PATCH, PR 59176] Mark "zombie" call graph nodes to remove verifier false positive

[Jan Hubicka]

> On Thu, 20 Mar 2014, Martin Jambor wrote:
> 
> > Hi,
> > 
> > On Thu, Mar 20, 2014 at 07:40:56PM +0100, Jakub Jelinek wrote:
> > > On Thu, Mar 20, 2014 at 05:07:32PM +0100, Martin Jambor wrote:
> > > > in the PR, verifier claims an edge is pointing to a wrong declaration
> > > > even though it has successfully verified the edge multiple times
> > > > before.  The reason is that symtab_remove_unreachable_nodes decides to
> > > > "remove the body" of a node and also clear any information that it is
> > > > an alias of another in the process (more detailed analysis in comment
> > > > #9 of the bug).
> > > > 
> > > > In bugzilla Honza wrote that "silencing the verifier" is the way to
> > > > go.  Either we can dedicate a new flag in each cgraph_node or
> > > > symtab_node just for the purpose of verification or do something more
> > > > hackish like the patch below which re-uses the former_clone_of field
> > > > for this purpose.  Since clones are always private nodes, they should
> > > > always either survive removal of unreachable nodes or be completely
> > > > killed by it and should never enter the in_border zombie state.
> > > > Therefore their former_clone_of must always be NULL.  So I added a new
> > > > special value, error_mark_node, to mark this zombie state and taught
> > > > the verifier to be happy with such nodes.
> > > > 
> > > > Bootstrapped and tested on x86_64-linux.  What do you think?
> > > 
> > > Don't we have like 22 spare bits in cgraph_node and 20 spare bits in
> > > symtab_node?  I'd find it clearer if you just used a new flag to mark the
> > > zombie nodes.  Though, I'll let Richard or Honza to decide, don't feel
> > > strongly about it.
> > > 
> > 
> > I guess you are right, here is the proper version which is currently
> > undergoing bootstrap and testing.
> 
> I agree with Jakub, the following variant is ok.

With the extra bit, you probably will need to LTO pickle it, too.
I would go with just clerning the thunk flag: this makes thunk to behave like
external function that is safe to do.
(I am back in civilization from Alaska camping, will catch up with email
early next week)

Honza