Add PROP_VARIABLE_NAME

With -fgnat-encodings=minimal, an internal version (these patches will
be upstreamed in the near future) of the Ada compiler can emit DWARF
for an array where the bound comes from a variable, like:

 <1><12a7>: Abbrev Number: 7 (DW_TAG_array_type)
    <12a8>   DW_AT_name        : (indirect string, offset: 0x1ae9): pck__my_array
[...]
 <2><12b4>: Abbrev Number: 8 (DW_TAG_subrange_type)
    <12b5>   DW_AT_type        : <0x1294>
    <12b9>   DW_AT_upper_bound : <0x1277>

With the upper bound DIE being:

 <1><1277>: Abbrev Number: 2 (DW_TAG_variable)
    <1278>   DW_AT_name        : (indirect string, offset: 0x1a4d): pck__my_length___U
    <127c>   DW_AT_type        : <0x128f>
    <1280>   DW_AT_external    : 1
    <1280>   DW_AT_artificial  : 1
    <1280>   DW_AT_declaration : 1

Note that the variable is just a declaration -- in this situation, the
variable comes from another compilation unit, and must be found when
trying to compute the array bound.

This patch adds a new PROP_VARIABLE_NAME kind, to enable this search.

This same scenario can occur with DW_OP_GNU_variable_value, so this
patch adds support for that as well.

gdb/ChangeLog
2021-06-04  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.h (dwarf2_fetch_die_type_sect_off): Add 'var_name'
	parameter.
	* dwarf2/loc.c (dwarf2_evaluate_property) <case
	PROP_VARIABLE_NAME>: New case.
	(compute_var_value): New function.
	(sect_variable_value): Use compute_var_value.
	* dwarf2/read.c (attr_to_dynamic_prop): Handle DW_TAG_variable.
	(var_decl_name): New function.
	(dwarf2_fetch_die_type_sect_off): Add 'var_name' parameter.
	* gdbtypes.h (enum dynamic_prop_kind) <PROP_VARIABLE_NAME>: New
	constant.
	(union dynamic_prop_data) <variable_name>: New member.
	(struct dynamic_prop) <variable_name, set_variable_name>: New
	methods.

gdb/testsuite/ChangeLog
2021-06-04  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/array_of_symbolic_length.exp: New file.
	* gdb.ada/array_of_symbolic_length/foo.adb: New file.
	* gdb.ada/array_of_symbolic_length/gl.adb: New file.
	* gdb.ada/array_of_symbolic_length/gl.ads: New file.
	* gdb.ada/array_of_symbolic_length/pck.adb: New file.
	* gdb.ada/array_of_symbolic_length/pck.ads: New file.

1 files changed, 36 insertions, 1 deletions

diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
index e816f92..b7e30e3 100644
--- a/gdb/dwarf2/loc.c
+++ b/gdb/dwarf2/loc.c
@@ -620,6 +620,19 @@ per_cu_dwarf_call (struct dwarf_expr_context *ctx, cu_offset die_offset,
   ctx->eval (block.data, block.size);
 }
 
+/* A helper function to find the definition of NAME and compute its
+   value.  Returns nullptr if the name is not found.  */
+
+static value *
+compute_var_value (const char *name)
+{
+  struct block_symbol sym = lookup_symbol (name, nullptr, VAR_DOMAIN,
+					   nullptr);
+  if (sym.symbol != nullptr)
+    return value_of_variable (sym.symbol, sym.block);
+  return nullptr;
+}
+
 /* Given context CTX, section offset SECT_OFF, and compilation unit
    data PER_CU, execute the "variable value" operation on the DIE
    found at SECT_OFF.  */
@@ -629,8 +642,10 @@ sect_variable_value (struct dwarf_expr_context *ctx, sect_offset sect_off,
 		     dwarf2_per_cu_data *per_cu,
 		     dwarf2_per_objfile *per_objfile)
 {
+  const char *var_name = nullptr;
   struct type *die_type
-    = dwarf2_fetch_die_type_sect_off (sect_off, per_cu, per_objfile);
+    = dwarf2_fetch_die_type_sect_off (sect_off, per_cu, per_objfile,
+				      &var_name);
 
   if (die_type == NULL)
     error (_("Bad DW_OP_GNU_variable_value DIE."));
@@ -638,9 +653,18 @@ sect_variable_value (struct dwarf_expr_context *ctx, sect_offset sect_off,
   /* Note: Things still work when the following test is removed.  This
      test and error is here to conform to the proposed specification.  */
   if (die_type->code () != TYPE_CODE_INT
+      && die_type->code () != TYPE_CODE_ENUM
+      && die_type->code () != TYPE_CODE_RANGE
       && die_type->code () != TYPE_CODE_PTR)
     error (_("Type of DW_OP_GNU_variable_value DIE must be an integer or pointer."));
 
+  if (var_name != nullptr)
+    {
+      value *result = compute_var_value (var_name);
+      if (result != nullptr)
+	return result;
+    }
+
   struct type *type = lookup_pointer_type (die_type);
   struct frame_info *frame = get_selected_frame (_("No frame selected."));
   return indirect_synthetic_pointer (sect_off, 0, per_cu, per_objfile, frame,
@@ -2691,6 +2715,17 @@ dwarf2_evaluate_property (const struct dynamic_prop *prop,
 	*value = value_as_address (val);
 	return true;
       }
+
+    case PROP_VARIABLE_NAME:
+      {
+	struct value *val = compute_var_value (prop->variable_name ());
+	if (val != nullptr)
+	  {
+	    *value = value_as_long (val);
+	    return true;
+	  }
+      }
+      break;
     }
 
   return false;