Re: [PATCH] Fix late dwarf generated early from optimized out globals

[Richard Biener <rguenther at suse dot de>]

On Thu, 15 Sep 2016, Richard Biener wrote:

> 
> This addresses sth I needed to address with the early LTO debug patches
> (you might now figure I'm piecemail merging stuff from that patch).
> 
> When the cgraph code optimizes out a global we call the late_global_decl
> debug hook to eventually add a DW_AT_const_value to its DIE (we don't
> really expect a location as that will be invalid after optimizing out
> and will be pruned).
> 
> With the early LTO debug patches I have introduced a early_dwarf_finished
> flag (mainly for consistency checking) and I figured I can use that to
> detect the call to the late hook during the early phase and provide
> the following cleaned up variant of avoiding to create locations that
> require later pruning (which doesn't work with emitting the early DIEs).
> 
> Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
> 
> I verified it does the correct thing for a unit like
> 
> static const int i = 2;
> 
> (but ISTR we do have at least one testcase in the testsuite as well).
> 
> Will commit if testing finishes successfully.

Ok, so it showed issues when merging that back to early-LTO-debug.
Turns out in LTO we never call early_finish and thus early_dwarf_finished
was never set.  Also dwarf2out_late_global_decl itself is a better
place to constrain generating locations.

The following variant is in very late stage of testing.

Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
LTO bootstrap on x86_64-unknown-linux-gnu in stage3.  LTO bootstrap
with early-LTO-debug in stage3, bootstraped with early-LTO-debug,
testing in progress.

Richard.

2016-09-16  Richard Biener  <rguenther@suse.de>

	* dwarf2out.c (early_dwarf_finished): New global.
	(set_early_dwarf::set_early_dwarf): Assert early_dwarf_finished
	is false.
	(dwarf2out_early_finish): Set early_dwarf_finished at the end,
	if called from LTO exit early.
	(dwarf2out_late_global_decl): When being during the early
	debug phase do not add locations but only const value attributes.
	Adjust the way we generate early DIEs for LTO.

	lto/
	* lto.c (lto_main): Invoke early_finish debug hook.

Index: gcc/dwarf2out.c
===================================================================
--- gcc/dwarf2out.c	(revision 240164)
+++ gcc/dwarf2out.c	(working copy)
@@ -2711,9 +2711,14 @@ die_node;
 
 /* Set to TRUE while dwarf2out_early_global_decl is running.  */
 static bool early_dwarf;
+static bool early_dwarf_finished;
 struct set_early_dwarf {
   bool saved;
-  set_early_dwarf () : saved(early_dwarf) { early_dwarf = true; }
+  set_early_dwarf () : saved(early_dwarf)
+    {
+      gcc_assert (! early_dwarf_finished);
+      early_dwarf = true;
+    }
   ~set_early_dwarf () { early_dwarf = saved; }
 };
 
@@ -23878,18 +23883,31 @@ dwarf2out_early_global_decl (tree decl)
 static void
 dwarf2out_late_global_decl (tree decl)
 {
-  /* We have to generate early debug late for LTO.  */
-  if (in_lto_p)
-    dwarf2out_early_global_decl (decl);
-
-    /* Fill-in any location information we were unable to determine
-       on the first pass.  */
+  /* Fill-in any location information we were unable to determine
+     on the first pass.  */
   if (TREE_CODE (decl) == VAR_DECL
       && !POINTER_BOUNDS_P (decl))
     {
       dw_die_ref die = lookup_decl_die (decl);
+
+      /* We have to generate early debug late for LTO.  */
+      if (! die && in_lto_p)
+	{
+	  dwarf2out_decl (decl);
+	  die = lookup_decl_die (decl);
+	}
+
       if (die)
-	add_location_or_const_value_attribute (die, decl, false);
+	{
+	  /* We get called during the early debug phase via the symtab
+	     code invoking late_global_decl for symbols that are optimized
+	     out.  When the early phase is not finished, do not add
+	     locations.  */
+	  if (! early_dwarf_finished)
+	    tree_add_const_value_attribute_for_decl (die, decl);
+	  else
+	    add_location_or_const_value_attribute (die, decl, false);
+	}
     }
 }
 
@@ -28137,6 +28155,14 @@ dwarf2out_early_finish (void)
 {
   set_early_dwarf s;
 
+  /* With LTO early dwarf was really finished at compile-time, so make
+     sure to adjust the phase after annotating the LTRANS CU DIE.  */
+  if (in_lto_p)
+    {
+      early_dwarf_finished = true;
+      return;
+    }
+
   /* Walk through the list of incomplete types again, trying once more to
      emit full debugging info for them.  */
   retry_incomplete_types ();
@@ -28163,6 +28189,9 @@ dwarf2out_early_finish (void)
 	}
     }
   deferred_asm_name = NULL;
+
+  /* The early debug phase is now finished.  */
+  early_dwarf_finished = true;
 }
 
 /* Reset all state within dwarf2out.c so that we can rerun the compiler
Index: gcc/lto/lto.c
===================================================================
--- gcc/lto/lto.c	(revision 240164)
+++ gcc/lto/lto.c	(working copy)
@@ -3315,6 +3315,9 @@ lto_main (void)
 	  if (!flag_ltrans)
 	    lto_promote_statics_nonwpa ();
 
+	  /* Annotate the CU DIE and mark the early debug phase as finished.  */
+	  debug_hooks->early_finish ();
+
 	  /* Let the middle end know that we have read and merged all of
 	     the input files.  */ 
 	  symtab->compile ();