Re: [patch] DWARF-3+ DW_AT_accessibility defaults #2 (GCC PR debug/45124)

[Jan Kratochvil <jan.kratochvil@redhat.com>]

On Tue, 22 Mar 2011 19:16:20 +0100, Jan Kratochvil wrote:
> If GCC 4.6.0 GA still will not produce DW_AT_producer for DW_TAG_type_unit
> there will be no way to distinguish what does DW_TAG_type_unit assume as
> a default for DW_AT_accessibility.

which has happened now with 4.6.0 GA.

Posted now:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48229#c5

# It cannot work as GDB often looks up the type without any referrer from
# DW_TAG_compile_unit, such as during the `ptype' GDB command.  A draft patch
# thus has many regressions such as:
# -PASS: gdb.base/nofield.exp: ptype struct not_empty
# +FAIL: gdb.base/nofield.exp: ptype struct not_empty (GDB internal error)
# 
# This means GDB will have to start full read (like -readnow) of CUs till it
# finds some CU referencing the specific type to find its DW_AT_producer.  During
# scan of .debug_info for GDB partial symbols GDB currently skips over subtrees
# of DIEs which reference the type signature for performance reasons, it no
# longer can.
# 
# With .gdb_index there is no GDB partial symbols scan but .gdb_index also
# indexes types by their name and there is no referrer ever seen during the
# `ptype' GDB command.
# 
# So it means performance regression for non-.gdb_index case and a possible new
# extension/version of .gdb_index to handle it.

So any solution is not easy now.  While playing with it one should have:
	[patch] Fix -readnow for -gdwarf-4 unused type units
	http://sourceware.org/ml/gdb-patches/2011-03/msg01119.html


Thanks,
Jan


--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -446,6 +446,11 @@ struct dwarf2_per_cu_data
     /* Data needed by the "quick" functions.  */
     struct dwarf2_per_cu_quick_data *quick;
   } v;
+
+  /* Track the .debug_info CUs referring to .debug_types CUs, even
+     transitively.  If unknown it is NULL, otherwise it points to
+     a .debug_info CU.  */
+  struct dwarf2_per_cu_data *referrer;
 };
 
 /* Entry in the signatured_types hash table.  */
@@ -2446,8 +2451,22 @@ dw2_expand_all_symtabs (struct objfile *objfile)
 
   dw2_setup (objfile);
 
-  for (i = 0; i < (dwarf2_per_objfile->n_comp_units
-		   + dwarf2_per_objfile->n_type_comp_units); ++i)
+  /* N_TYPE_COMP_UNITS - .debug_types CUs - do not have to be read in as each
+     such PER_CU belongs into one of these cases:
+     (1) per_cu->referrer == NULL
+	 It is an unused DW_TAG_type_unit which no referrers.  Such unit
+	 cannot be read-in even just for a verification by OBJF_READNOW as
+	 producer_is_gxx_lt_4_6 would not know how to read it.
+     (2) per_cu->referrer->cu == NULL
+	 This DW_TAG_type_unit has been already read it before as a dependency
+	 for some DW_TAG_compile_unit.  producer_is_gxx_lt_4_6 would no longer
+	 know how to read it with DW_TAG_compile_unit no longer available.
+	 And also:
+     (3) per_cu->referrer->cu != NULL
+	 We can read this DW_TAG_type_unit but it would be read in the second
+	 time, there is no need to do so.  */
+
+  for (i = 0; i < dwarf2_per_objfile->n_comp_units; ++i)
     {
       struct dwarf2_per_cu_data *per_cu = dw2_get_cu (i);
 
@@ -6216,6 +6235,83 @@ dwarf2_record_block_ranges (struct die_info *die, struct block *block,
     }
 }
 
+/* Check for GCC PR debug/45124 fix which is not present in any G++ version up
+   to 4.5.any while it is present already in G++ 4.6.0 - the PR has been fixed
+   during 4.6.0 experimental.  */
+
+static int
+producer_is_gxx_lt_4_6 (struct dwarf2_cu *cu)
+{
+  int major, minor;
+
+  if (cu->producer == NULL && cu->per_cu->from_debug_types)
+    {
+      struct dwarf2_per_cu_data *referrer;
+
+      /* We may get to a .debug_types CU only through a pointer from
+	 a .debug_info CU.  .debug_types CUs without such pointer are skipped
+	 in dw2_expand_all_symtabs.  */
+      referrer = cu->per_cu->referrer;
+      gdb_assert (referrer != NULL);
+
+      /* On a second and further referrer the first referrer stored in
+	 referrer may have its CU already freed.  But that time
+	 DEBUG_TYPES_TYPE_HASH is used to look up the type and the
+	 .debug_types CU is not read in the second time at all.  */
+      gdb_assert (referrer->cu != NULL);
+
+      cu = referrer->cu;
+    }
+
+  if (cu->producer == NULL)
+    {
+      /* For unknown compilers expect their behavior is DWARF version
+         compliant.  */
+
+      return 0;
+    }
+
+  /* Whitespaces are ignored in both PRODUCER and the format string.  */
+  if (sscanf (cu->producer, "GNU C++ %d.%d", &major, &minor) != 2)
+    {
+      /* For non-GCC compilers expect their behavior is DWARF version
+	 compliant.  */
+
+      return 0;
+    }
+
+  return major < 4 || (major == 4 && minor < 6);
+}
+
+/* Return the default accessibility type if it is not overriden by
+   DW_AT_accessibility.  */
+
+static enum dwarf_access_attribute
+dwarf2_default_access_attribute (struct die_info *die, struct dwarf2_cu *cu)
+{
+  if (cu->header.version < 3 || producer_is_gxx_lt_4_6 (cu))
+    {
+      /* The default DWARF 2 accessibility for members is public, the default
+	 accessibility for inheritance is private.  */
+
+      if (die->tag != DW_TAG_inheritance)
+	return DW_ACCESS_public;
+      else
+	return DW_ACCESS_private;
+    }
+  else
+    {
+      /* DWARF 3+ defines the default accessibility a different way.  The same
+	 rules apply now for DW_TAG_inheritance as for the members and it only
+	 depends on the container kind.  */
+
+      if (die->parent->tag == DW_TAG_class_type)
+	return DW_ACCESS_private;
+      else
+	return DW_ACCESS_public;
+    }
+}
+
 /* Add an aggregate field to the field list.  */
 
 static void
@@ -6246,23 +6342,19 @@ dwarf2_add_field (struct field_info *fip, struct die_info *die,
     }
   fip->nfields++;
 
-  /* Handle accessibility and virtuality of field.
-     The default accessibility for members is public, the default
-     accessibility for inheritance is private.  */
-  if (die->tag != DW_TAG_inheritance)
-    new_field->accessibility = DW_ACCESS_public;
-  else
-    new_field->accessibility = DW_ACCESS_private;
-  new_field->virtuality = DW_VIRTUALITY_none;
-
   attr = dwarf2_attr (die, DW_AT_accessibility, cu);
   if (attr)
     new_field->accessibility = DW_UNSND (attr);
+  else
+    new_field->accessibility = dwarf2_default_access_attribute (die, cu);
   if (new_field->accessibility != DW_ACCESS_public)
     fip->non_public_fields = 1;
+
   attr = dwarf2_attr (die, DW_AT_virtuality, cu);
   if (attr)
     new_field->virtuality = DW_UNSND (attr);
+  else
+    new_field->virtuality = DW_VIRTUALITY_none;
 
   fp = &new_field->field;
 
@@ -6578,6 +6670,7 @@ dwarf2_add_member_fn (struct field_info *fip, struct die_info *die,
   char *fieldname;
   struct nextfnfield *new_fnfield;
   struct type *this_type;
+  enum dwarf_access_attribute accessibility;
 
   if (cu->language == language_ada)
     error (_("unexpected member function in Ada type"));
@@ -6676,16 +6769,17 @@ dwarf2_add_member_fn (struct field_info *fip, struct die_info *die,
   /* Get accessibility.  */
   attr = dwarf2_attr (die, DW_AT_accessibility, cu);
   if (attr)
+    accessibility = DW_UNSND (attr);
+  else
+    accessibility = dwarf2_default_access_attribute (die, cu);
+  switch (accessibility)
     {
-      switch (DW_UNSND (attr))
-	{
-	case DW_ACCESS_private:
-	  fnp->is_private = 1;
-	  break;
-	case DW_ACCESS_protected:
-	  fnp->is_protected = 1;
-	  break;
-	}
+    case DW_ACCESS_private:
+      fnp->is_private = 1;
+      break;
+    case DW_ACCESS_protected:
+      fnp->is_protected = 1;
+      break;
     }
 
   /* Check for artificial methods.  */
@@ -13254,6 +13348,17 @@ static int
 maybe_queue_comp_unit (struct dwarf2_cu *this_cu,
 		       struct dwarf2_per_cu_data *per_cu)
 {
+  /* Try to fill-in referrer for .debug_types CUs.  While
+     THIS_CU->CU may become NULL soon it should be already processed by
+     producer_is_gxx_lt_4_6 that time.  */
+  if (per_cu->from_debug_types && per_cu->referrer == NULL)
+    {
+      if (!this_cu->per_cu->from_debug_types)
+	per_cu->referrer = this_cu->per_cu;
+      else
+	per_cu->referrer = this_cu->per_cu->referrer;
+    }
+
   /* We may arrive here during partial symbol reading, if we need full
      DIEs to process an unusual case (e.g. template arguments).  Do
      not queue PER_CU, just tell our caller to load its DIEs.  */