[PATCH] Fix PR86321

[Richard Biener <rguenther@suse.de>]

The fortran FE creates array descriptor types via build_distinct_type_copy
which ends up re-using the TYPE_FIELDs chain of FIELD_DECLs between
types in different type-variant chains.  While that seems harmless
in practice it breaks once we try to generate C-like debug info for
it because dwarf2out doesn't expect such sharing to occur (and I
wouldn't be surprised of other odd behavior elsewhere that simply
doesn't manifest in a as fatal way as PR86321).

We generate C-like debug info when you use LTO and -g0 at compile-time
and -g at link-time (that's the way targets w/o debug-copy implementation
end up wired).  For non-LTO we avoid directly generating debug for
the array descriptor types by detecting them via a langhook.

The solution seems to be to adhere to the invariant that TYPE_FIELDs
(and thus FIELD_DECL) sharing is only valid between variant types
and their main variant.  Thus, copy the chain.

Bootstrap / regtest pending on x86_64-unknown-linux-gnu.

I suppose verify_type () could check proper ownership of the
FIELD_DECLs (simply verify that DECL_CONTEXT is TYPE_MAIN_VARIANT).
But I guess this may break in different ways.  Honza - did you
originally try to verify that?  It currently says

      for (tree fld = TYPE_FIELDS (t); fld; fld = TREE_CHAIN (fld))
        {
          /* TODO: verify properties of decls.  */
          if (TREE_CODE (fld) == FIELD_DECL)
            ;
...

OK for trunk?

Thanks,
Richard.

2018-06-28  Richard Biener  <rguenther@suse.de>

	fortran/
	PR lto/86321
	* trans-types.c (gfc_get_array_type_bounds): Unshare TYPE_FIELDs
	for the distinct type copy.

Index: gcc/fortran/trans-types.c
===================================================================
--- gcc/fortran/trans-types.c	(revision 262132)
+++ gcc/fortran/trans-types.c	(working copy)
@@ -1923,6 +1923,14 @@ gfc_get_array_type_bounds (tree etype, i
 
   base_type = gfc_get_array_descriptor_base (dimen, codimen, restricted);
   fat_type = build_distinct_type_copy (base_type);
+  /* Unshare TYPE_FIELDs.  */
+  for (tree *tp = &TYPE_FIELDS (fat_type); *tp; tp = &DECL_CHAIN (*tp))
+    {
+      tree next = DECL_CHAIN (*tp);
+      *tp = copy_node (*tp);
+      DECL_CONTEXT (*tp) = fat_type;
+      DECL_CHAIN (*tp) = next;
+    }
   /* Make sure that nontarget and target array type have the same canonical
      type (and same stub decl for debug info).  */
   base_type = gfc_get_array_descriptor_base (dimen, codimen, false);