Handling of arrays with optimized-out bounds

In Ada, sometimes the compiler must emit array bounds by referencing
an artificial variable that's created for this purpose.  However, with
optimization enabled, these variables can be optimized away.

Currently this can result in displays like:

    (gdb) print mumble
    $1 = (warning: unable to get bounds of array, assuming null array
    )

This patch changes this to report that the array is optimized-out,
instead, which is closer to the truth, and more generally useful.  For
example, Python pretty-printers can now recognize this situation.

In order to accomplish this, I introduced a new PROP_OPTIMIZED_OUT
enumerator and changed one place to use it.  Reusing the "unknown"
state wouldn't work properly, because in C it is normal for array
bounds to be unknown.

gdb/ada-lang.c

@@ -679,7 +679,7 @@ ada_discrete_type_high_bound (struct type *type)
 	  return high.const_val ();
 	else
 	  {
-	    gdb_assert (high.kind () == PROP_UNDEFINED);
+	    gdb_assert (!high.is_available ());
 
 	    /* This happens when trying to evaluate a type's dynamic bound
 	       without a live target.  There is nothing relevant for us to
@@ -714,7 +714,7 @@ ada_discrete_type_low_bound (struct type *type)
 	  return low.const_val ();
 	else
 	  {
-	    gdb_assert (low.kind () == PROP_UNDEFINED);
+	    gdb_assert (!low.is_available ());
 
 	    /* This happens when trying to evaluate a type's dynamic bound
 	       without a live target.  There is nothing relevant for us to

gdb/c-varobj.c

@@ -192,7 +192,7 @@ c_number_of_children (const struct varobj *var)
     {
     case TYPE_CODE_ARRAY:
       if (type->length () > 0 && target->length () > 0
-	  && (type->bounds ()->high.kind () != PROP_UNDEFINED))
+	  && type->bounds ()->high.is_available ())
 	children = type->length () / target->length ();
       else
 	/* If we don't know how many elements there are, don't display

gdb/eval.c

@@ -2828,7 +2828,7 @@ var_value_operation::evaluate_for_sizeof (struct expression *exp,
 	  if (type_not_allocated (type) || type_not_associated (type))
 	    return value::zero (size_type, not_lval);
 	  else if (is_dynamic_type (type->index_type ())
-		   && type->bounds ()->high.kind () == PROP_UNDEFINED)
+		   && !type->bounds ()->high.is_available ())
 	    return value::allocate_optimized_out (size_type);
 	}
     }

gdb/f-lang.c

@@ -1372,7 +1372,7 @@ fortran_undetermined::value_subarray (value *array,
 	     have a known upper bound, so don't error check in that
 	     situation.  */
 	  if (index < lb
-	      || (dim_type->index_type ()->bounds ()->high.kind () != PROP_UNDEFINED
+	      || (dim_type->index_type ()->bounds ()->high.is_available ()
 		  && index > ub)
 	      || (array->lval () != lval_memory
 		  && dim_type->index_type ()->bounds ()->high.kind () == PROP_UNDEFINED))

gdb/gdbtypes.c

@@ -2179,21 +2179,35 @@ resolve_dynamic_range (struct type *dyn_range_type,
   gdb_assert (rank >= 0);
 
   const struct dynamic_prop *prop = &dyn_range_type->bounds ()->low;
-  if (resolve_p && dwarf2_evaluate_property (prop, frame, addr_stack, &value,
-					     { (CORE_ADDR) rank }))
-    low_bound.set_const_val (value);
+  if (resolve_p)
+    {
+      if (dwarf2_evaluate_property (prop, frame, addr_stack, &value,
+				    { (CORE_ADDR) rank }))
+	low_bound.set_const_val (value);
+      else if (prop->kind () == PROP_UNDEFINED)
+	low_bound.set_undefined ();
+      else
+	low_bound.set_optimized_out ();
+    }
   else
     low_bound.set_undefined ();
 
   prop = &dyn_range_type->bounds ()->high;
-  if (resolve_p && dwarf2_evaluate_property (prop, frame, addr_stack, &value,
-					     { (CORE_ADDR) rank }))
+  if (resolve_p)
     {
-      high_bound.set_const_val (value);
+      if (dwarf2_evaluate_property (prop, frame, addr_stack, &value,
+				    { (CORE_ADDR) rank }))
+	{
+	  high_bound.set_const_val (value);
 
-      if (dyn_range_type->bounds ()->flag_upper_bound_is_count)
-	high_bound.set_const_val
-	  (low_bound.const_val () + high_bound.const_val () - 1);
+	  if (dyn_range_type->bounds ()->flag_upper_bound_is_count)
+	    high_bound.set_const_val
+	      (low_bound.const_val () + high_bound.const_val () - 1);
+	}
+      else if (prop->kind () == PROP_UNDEFINED)
+	high_bound.set_undefined ();
+      else
+	high_bound.set_optimized_out ();
     }
   else
     high_bound.set_undefined ();

gdb/gdbtypes.h

@@ -271,6 +271,7 @@ enum dynamic_prop_kind
   PROP_VARIANT_PARTS, /* Variant parts.  */
   PROP_TYPE,	   /* Type.  */
   PROP_VARIABLE_NAME, /* Variable name.  */
+  PROP_OPTIMIZED_OUT, /* Optimized out.  */
 };
 
 union dynamic_prop_data
@@ -318,6 +319,18 @@ struct dynamic_prop
     m_kind = PROP_UNDEFINED;
   }
 
+  void set_optimized_out ()
+  {
+    m_kind = PROP_OPTIMIZED_OUT;
+  }
+
+  /* Return true if this property is "available", at least in theory
+     -- meaning it is neither undefined nor optimized out.  */
+  bool is_available () const
+  {
+    return m_kind != PROP_UNDEFINED && m_kind != PROP_OPTIMIZED_OUT;
+  }
+
   LONGEST const_val () const
   {
     gdb_assert (m_kind == PROP_CONST);
@@ -760,6 +773,13 @@ struct range_bounds
       return this->stride.const_val ();
   }
 
+  /* Return true if either bounds is optimized out.  */
+  bool optimized_out () const
+  {
+    return (low.kind () == PROP_OPTIMIZED_OUT
+	    || high.kind () == PROP_OPTIMIZED_OUT);
+  }
+
   /* * Low bound of range.  */
 
   struct dynamic_prop low;
@@ -1135,6 +1155,12 @@ struct type
     this->main_type->flds_bnds.bounds = bounds;
   }
 
+  /* Return true if this type's bounds were optimized out.  */
+  bool bound_optimized_out () const
+  {
+    return bounds ()->optimized_out ();
+  }
+
   ULONGEST bit_stride () const
   {
     if (this->code () == TYPE_CODE_ARRAY && this->field (0).bitsize () != 0)

gdb/testsuite/gdb.dwarf2/arr-opt-out.c

@@ -0,0 +1,22 @@
+/* Copyright 2023 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+int global_array[] = {0, 0};
+
+int
+main (void)
+{
+  return 0;
+}

gdb/testsuite/gdb.dwarf2/arr-opt-out.exp

@@ -0,0 +1,95 @@
+# Copyright 2023 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test that an array whose bounds are optimized out is itself marked
+# as optimized out.
+
+load_lib dwarf.exp
+load_lib gdb-python.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+require dwarf2_support
+
+standard_testfile .c -dw.S
+
+# Make some DWARF for the test.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_Ada95}
+	    {DW_AT_name	    foo.adb}
+	    {DW_AT_comp_dir /tmp}
+	} {
+	    declare_labels integer_label array_label \
+		low_bound_label high_bound_label
+
+	    integer_label: DW_TAG_base_type {
+		{DW_AT_byte_size 4 DW_FORM_sdata}
+		{DW_AT_encoding	 @DW_ATE_signed}
+		{DW_AT_name	 integer}
+	    }
+
+	    # Note that the bounds don't have a location -- they are
+	    # optimized out.  This mimics what it is seen sometimes in
+	    # the wild with optimized Ada code.
+	    low_bound_label: DW_TAG_variable {
+		{DW_AT_name pck__table___L}
+		{DW_AT_type :$integer_label}
+		{DW_AT_declaration 1 flag}
+	    }
+	    high_bound_label: DW_TAG_variable {
+		{DW_AT_name pck__table___U}
+		{DW_AT_type :$integer_label}
+		{DW_AT_declaration 1 flag}
+	    }
+
+	    array_label: DW_TAG_array_type {
+		{DW_AT_name pck__table}
+		{DW_AT_type :$integer_label}
+	    } {
+		DW_TAG_subrange_type {
+		    {DW_AT_type	       :$integer_label}
+		    {DW_AT_lower_bound :$low_bound_label}
+		    {DW_AT_upper_bound :$high_bound_label}
+		}
+	    }
+
+	    DW_TAG_variable {
+		{DW_AT_name the_table}
+		{DW_AT_type :$array_label}
+		{DW_AT_location {
+		    DW_OP_addr [gdb_target_symbol global_array]
+		} SPECIAL_expr}
+		{DW_AT_external 1 flag}
+	    }
+	}
+    }
+}
+
+if {[prepare_for_testing "failed to prepare" ${testfile} \
+	 [list $srcfile $asm_file] {nodebug}]} {
+    return -1
+}
+
+gdb_test_no_output "set language ada"
+
+gdb_test "print the_table" " = <optimized out>"
+
+# The same but in Python.
+if {[allow_python_tests]} {
+    gdb_test "python print(gdb.parse_and_eval('the_table').is_optimized_out)" \
+	True
+}

gdb/value.c

@@ -3603,9 +3603,16 @@ value_from_contents_and_address (struct type *type,
   struct type *resolved_type = resolve_dynamic_type (type, view, address,
 						     &frame);
   struct type *resolved_type_no_typedef = check_typedef (resolved_type);
-  struct value *v;
 
-  if (valaddr == NULL)
+  struct value *v;
+  if (resolved_type_no_typedef->code () == TYPE_CODE_ARRAY
+      && resolved_type_no_typedef->bound_optimized_out ())
+    {
+      /* Resolution found that the bounds are optimized out.  In this
+	 case, mark the array itself as optimized-out.  */
+      v = value::allocate_optimized_out (resolved_type);
+    }
+  else if (valaddr == nullptr)
     v = value::allocate_lazy (resolved_type);
   else
     v = value_from_contents (resolved_type, valaddr);