Partitioned LTO bug with cdtor aliases

[Nathan Sidwell <nathan at acm dot org>]

Jan,
I think I've located a bug in LTO handling.  I'm working on a gcc 5 
source base, but AFAICT the findings are still current.  Except that on 
trunk the fix for 68881 hides this problem (not confirmed).

I have a large source base, invoke LTO in partitioned wpa mode, and end 
up with a local alias defined for a destructor, that is therefore not 
visible in the other partitions. (it should be a global hidden symbol)

In the original TUs we emit a comdat instantiation of a base and 
complete dtor pair.  The comp_dtor is a weak alias to the weakly defined 
base_dtor.  So far so good.

During wpa analysis we first determine prevailing decls.  That's fine. 
We pick base and comp dtor decls from the same comdat group from one of 
the input TUs.

Then we determine function_and_variable_visibility.  We encounter the 
comp_dtor alias first (not sure if that's an invariant, or just accident 
of test case).  Anyway its symtab node marks it correctly as an 
externally_visible alias in the same comdat group as the base dtor. It 
has a resolution of LDPR_PREVAILING_DEF, and we therefore think 
cgraph_externally_visible_p (node, whole_program) is true and keep it 
externally visible.

Next we encounter the base dtor.  That has resolution 
LDPR_PREVAILING_DEF_IRONLY, and we do not think it need remain 
externally visible.  We clear node->externally_visible, and then proceed 
to make it local.  During that process we iterate over the members of 
the same comdat group, making them all local.  That encounters the 
comp_dtor, which we apply make_decl_local to.  That clears TREE_PUBLIC 
on the decl, but leaves its node->externally_visible set.  This is 
inconsistent.

That inconsistency bites us in partitioning, because we then don't 
promote the comp_dtor decl as we think it already visible.  But as its 
TREE_PUBLIC is clear, it gets output as a local symbol in the partition 
defining it. (the inconsistency is harmless in non-partitioned lto mode)

Not sure if the right fix is to
a) clear its externally_visible flag too in 
function_and_variable_visibility.  (What if we encounter the nodes in 
the other order?)  [Perhaps the first loop should be split into one 
setting externally_visible as appropriate and then a second loop fixing 
up mismatch between externally_visible and TREE_PUBLIC?]

b) check in lto_promote_cross_file_statics for both externally_visible 
&& TREE_PUBLIC:
	  /* No need to promote if symbol already is externally visible ... */
	  if ((node->externally_visible && TREE_PUBLIC(node->decl))

c) Something else?

clues?

nathan
--
Nathan Sidwell