DWARF: add a language hook for scalar biased types

Front-ends like GNAT for Ada sometimes use biased encodings for integral
types.  This change creates a new language hook so that the bias
information can make it into the debugging information back-end and
introduces an experimental DWARF attribute to hold it.

gcc/ada/ChangeLog:

	* gcc-interface/misc.c (gnat_get_type_bias): New.
	(LANG_HOOKS_GET_TYPE_BIAS): Redefine macro to implement the
	get_type_bias language hook.

gcc/ChangeLog:

	* langhooks.h (struct lang_hooks_for_types): New get_bias_field.
	* langhooks-def.h (LANG_HOOKS_GET_TYPE_BIAS): New.
	(LANG_HOOKS_FOR_TYPES_INITIALIZER): Initialize the
	get_bias_field.
	* dwarf2out.c
	(base_type_die): In non-strict DWARF mode, invoke the
	get_type_bias language hook for INTEGER_TYPE nodes.  If it
	returns a bias, emit an attribute for it.
	(subrange_type_die): Change signature to handle bias.  If
	non-strict DWARF mode, emit an attribute for it, if one passed.
	(modified_type_die): For subrange types, invoke the
	get_type_bias langage hook and pass the bias to
	subrange_type_die.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@231767 138bc75d-0d04-0410-961f-82ee72b054a4

6 files changed, 65 insertions, 3 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 9736786df31..405d8b0e5a2 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,21 @@
 2015-12-17  Pierre-Marie de Rodat  <derodat@adacore.com>
 
+	* langhooks.h (struct lang_hooks_for_types): New get_bias_field.
+	* langhooks-def.h (LANG_HOOKS_GET_TYPE_BIAS): New.
+	(LANG_HOOKS_FOR_TYPES_INITIALIZER): Initialize the
+	get_bias_field.
+	* dwarf2out.c
+	(base_type_die): In non-strict DWARF mode, invoke the
+	get_type_bias language hook for INTEGER_TYPE nodes.  If it
+	returns a bias, emit an attribute for it.
+	(subrange_type_die): Change signature to handle bias.  If
+	non-strict DWARF mode, emit an attribute for it, if one passed.
+	(modified_type_die): For subrange types, invoke the
+	get_type_bias langage hook and pass the bias to
+	subrange_type_die.
+
+2015-12-17  Pierre-Marie de Rodat  <derodat@adacore.com>
+
 	* dwarf2out.h (DWARF2OUT_ARRAY_DESCR_INFO_MAX_DIMEN): New macro.
 	(struct array_descr_info): Use it for the dimensions array's
 	size.
diff --git a/gcc/ada/ChangeLog b/gcc/ada/ChangeLog
index eb1092380bf..80c5a39d803 100644
--- a/gcc/ada/ChangeLog
+++ b/gcc/ada/ChangeLog
@@ -1,5 +1,11 @@
 2015-12-17  Pierre-Marie de Rodat  <derodat@adacore.com>
 
+	* gcc-interface/misc.c (gnat_get_type_bias): New.
+	(LANG_HOOKS_GET_TYPE_BIAS): Redefine macro to implement the
+	get_type_bias language hook.
+
+2015-12-17  Pierre-Marie de Rodat  <derodat@adacore.com>
+
 	* gcc-interface/misc.c (gnat_get_array_descr_info): When the
 	array has more dimensions than the language hook can handle,
 	fall back to a nested arrays description.  Handle context-less
diff --git a/gcc/ada/gcc-interface/misc.c b/gcc/ada/gcc-interface/misc.c
index 891ca3f338c..269960f917d 100644
--- a/gcc/ada/gcc-interface/misc.c
+++ b/gcc/ada/gcc-interface/misc.c
@@ -977,6 +977,16 @@ gnat_get_subrange_bounds (const_tree gnu_type, tree *lowval, tree *highval)
   *highval = TYPE_MAX_VALUE (gnu_type);
 }
 
+static tree
+gnat_get_type_bias (const_tree gnu_type)
+{
+  if (TREE_CODE (gnu_type) == INTEGER_TYPE
+      && TYPE_BIASED_REPRESENTATION_P (gnu_type)
+      && gnat_encodings == DWARF_GNAT_ENCODINGS_MINIMAL)
+    return TYPE_RM_MIN_VALUE(gnu_type);
+  return NULL_TREE;
+}
+
 /* GNU_TYPE is the type of a subprogram parameter.  Determine if it should be
    passed by reference by default.  */
 
@@ -1276,6 +1286,8 @@ get_lang_specific (tree node)
 #define LANG_HOOKS_GET_ARRAY_DESCR_INFO	gnat_get_array_descr_info
 #undef  LANG_HOOKS_GET_SUBRANGE_BOUNDS
 #define LANG_HOOKS_GET_SUBRANGE_BOUNDS  gnat_get_subrange_bounds
+#undef  LANG_HOOKS_GET_TYPE_BIAS
+#define LANG_HOOKS_GET_TYPE_BIAS	gnat_get_type_bias
 #undef  LANG_HOOKS_DESCRIPTIVE_TYPE
 #define LANG_HOOKS_DESCRIPTIVE_TYPE	gnat_descriptive_type
 #undef  LANG_HOOKS_GET_DEBUG_TYPE
diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 98528c7e062..067c4f23892 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -3288,7 +3288,7 @@ static void output_line_info (bool);
 static void output_file_names (void);
 static dw_die_ref base_type_die (tree);
 static int is_base_type (tree);
-static dw_die_ref subrange_type_die (tree, tree, tree, dw_die_ref);
+static dw_die_ref subrange_type_die (tree, tree, tree, tree, dw_die_ref);
 static int decl_quals (const_tree);
 static dw_die_ref modified_type_die (tree, int, dw_die_ref);
 static dw_die_ref generic_parameter_die (tree, tree, bool, dw_die_ref);
@@ -10809,6 +10809,7 @@ base_type_die (tree type)
   enum dwarf_type encoding;
   bool fpt_used = false;
   struct fixed_point_type_info fpt_info;
+  tree type_bias = NULL_TREE;
 
   if (TREE_CODE (type) == ERROR_MARK || TREE_CODE (type) == VOID_TYPE)
     return 0;
@@ -10859,6 +10860,10 @@ base_type_die (tree type)
 	encoding = DW_ATE_unsigned;
       else
 	encoding = DW_ATE_signed;
+
+      if (!dwarf_strict
+	  && lang_hooks.types.get_type_bias)
+	type_bias = lang_hooks.types.get_type_bias (type);
       break;
 
     case REAL_TYPE:
@@ -10943,6 +10948,12 @@ base_type_die (tree type)
 	  gcc_unreachable ();
 	}
     }
+  if (type_bias != NULL)
+    add_scalar_info (base_type_result, DW_AT_GNU_bias, type_bias,
+		     dw_scalar_form_constant
+		     | dw_scalar_form_exprloc
+		     | dw_scalar_form_reference,
+		     NULL);
   add_pubtype (type, base_type_result);
 
   return base_type_result;
@@ -11044,7 +11055,8 @@ offset_int_type_size_in_bits (const_tree type)
     to a DIE that describes the given type.  */
 
 static dw_die_ref
-subrange_type_die (tree type, tree low, tree high, dw_die_ref context_die)
+subrange_type_die (tree type, tree low, tree high, tree bias,
+		   dw_die_ref context_die)
 {
   dw_die_ref subrange_die;
   const HOST_WIDE_INT size_in_bytes = int_size_in_bytes (type);
@@ -11065,6 +11077,12 @@ subrange_type_die (tree type, tree low, tree high, dw_die_ref context_die)
     add_bound_info (subrange_die, DW_AT_lower_bound, low, NULL);
   if (high)
     add_bound_info (subrange_die, DW_AT_upper_bound, high, NULL);
+  if (bias && !dwarf_strict)
+    add_scalar_info (subrange_die, DW_AT_GNU_bias, bias,
+		     dw_scalar_form_constant
+		     | dw_scalar_form_exprloc
+		     | dw_scalar_form_reference,
+		     NULL);
 
   return subrange_die;
 }
@@ -11279,7 +11297,10 @@ modified_type_die (tree type, int cv_quals, dw_die_ref context_die)
 	   && TREE_TYPE (type) != NULL_TREE
 	   && subrange_type_for_debug_p (type, &low, &high))
     {
-      mod_type_die = subrange_type_die (type, low, high, context_die);
+      tree bias = NULL_TREE;
+      if (lang_hooks.types.get_type_bias)
+	bias = lang_hooks.types.get_type_bias (type);
+      mod_type_die = subrange_type_die (type, low, high, bias, context_die);
       item_type = TREE_TYPE (type);
     }
   else if (is_base_type (type))
diff --git a/gcc/langhooks-def.h b/gcc/langhooks-def.h
index 2d02bf631d0..db96e916bc4 100644
--- a/gcc/langhooks-def.h
+++ b/gcc/langhooks-def.h
@@ -173,6 +173,7 @@ extern tree lhd_make_node (enum tree_code);
 #define LANG_HOOKS_TYPE_HASH_EQ		NULL
 #define LANG_HOOKS_GET_ARRAY_DESCR_INFO	NULL
 #define LANG_HOOKS_GET_SUBRANGE_BOUNDS	NULL
+#define LANG_HOOKS_GET_TYPE_BIAS	NULL
 #define LANG_HOOKS_DESCRIPTIVE_TYPE	NULL
 #define LANG_HOOKS_RECONSTRUCT_COMPLEX_TYPE reconstruct_complex_type
 #define LANG_HOOKS_ENUM_UNDERLYING_BASE_TYPE lhd_enum_underlying_base_type
@@ -195,6 +196,7 @@ extern tree lhd_make_node (enum tree_code);
   LANG_HOOKS_TYPE_HASH_EQ, \
   LANG_HOOKS_GET_ARRAY_DESCR_INFO, \
   LANG_HOOKS_GET_SUBRANGE_BOUNDS, \
+  LANG_HOOKS_GET_TYPE_BIAS, \
   LANG_HOOKS_DESCRIPTIVE_TYPE, \
   LANG_HOOKS_RECONSTRUCT_COMPLEX_TYPE, \
   LANG_HOOKS_ENUM_UNDERLYING_BASE_TYPE, \
diff --git a/gcc/langhooks.h b/gcc/langhooks.h
index f84bdf64bf4..95d5840d0a9 100644
--- a/gcc/langhooks.h
+++ b/gcc/langhooks.h
@@ -127,6 +127,11 @@ struct lang_hooks_for_types
   /* Fill in information for the debugger about the bounds of TYPE.  */
   void (*get_subrange_bounds) (const_tree, tree *, tree *);
 
+  /* Called on INTEGER_TYPEs.  Return NULL_TREE for non-biased types.  For
+     biased types, return as an INTEGER_CST node the value that is represented
+     by a physical zero.  */
+  tree (*get_type_bias) (const_tree);
+
   /* A type descriptive of TYPE's complex layout generated to help the
      debugger to decode variable-length or self-referential constructs.
      This is only used for the AT_GNAT_descriptive_type DWARF attribute.  */