[PATCH] Fix dwarf2out regression introduced with LVUs (PR debug/84456)

[Jakub Jelinek <jakub@redhat.com>]

Hi!

GCC 7 and earlier had in this spot
  if (list && loc_list->first->next)
    gen_llsym (list);
where we wanted to force loclist if we've seen more than one
NOTE_INSN_VAR_LOCATION for the decl, which means it is not possible to use
a block form DW_AT_location.

Alex has changed this to maybe_gen_llsym (list) call, which does:
  if (!list || (!list->dw_loc_next && !loc_list_has_views (list)))
    return;
   
  gen_llsym (list);
This matches what the other caller of gen_llsym did:
  if (list && list->dw_loc_next)
    gen_llsym (list);
but not the dw_loc_list spot.  The difference between the two is
(when -gno-variable-location-views) that maybe_gen_llsym will not force
loclist form if we transform 2+ NOTE_INSN_VAR_LOCATION notes into a single
defined location expression, which can happen e.g. when the first
note is some usable location and the second one is <optimized away>.
In that case we must use loclist to force the variable to be <optimized away>
in the second part of the function, because the first DWARF expression would
give wrong results there.

Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux, ok for
trunk?

2018-03-08  Jakub Jelinek  <jakub@redhat.com>

	PR debug/84456
	* dwarf2out.c (dw_loc_list): If list && loc_list->first->next, call
	gen_llsym, otherwise call maybe_gen_llsym.

--- gcc/dwarf2out.c.jj	2018-03-02 00:15:54.704780976 +0100
+++ gcc/dwarf2out.c	2018-03-08 12:54:01.794054675 +0100
@@ -17076,7 +17076,10 @@ dw_loc_list (var_loc_list *loc_list, tre
      representable, we don't want to pretend a single entry that was
      applies to the entire scope in which the variable is
      available.  */
-  maybe_gen_llsym (list);
+  if (list && loc_list->first->next)
+    gen_llsym (list);
+  else
+    maybe_gen_llsym (list);
 
   return list;
 }

	Jakub