Fix bit offset regression

The type-safe attribute patch introduced a regression that can occur
when the DW_AT_bit_offset value is negative.  This can happen with
some Ada programs.

This patch fixes the problem.  It also fixes a minor oddity in the
existing scalar storage test -- this test was intended to assign a
smaller number of bits to the field.

2020-10-09  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.c (dwarf2_add_field): Handle signed offsets.

gdb/testsuite/ChangeLog
2020-10-09  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/scalar_storage/storage.adb (Another_Range): New type.
	(Rec): Add field.  Fix range.
	* gdb.ada/scalar_storage.exp: Update.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2020-10-09  Tom Tromey  <tromey@adacore.com>
+
+	* dwarf2/read.c (dwarf2_add_field): Handle signed offsets.
+
 2020-10-09  Tom Tromey  <tromey@adacore.com>
 
 	* ada-lang.h (ada_encode): Return std::string.

gdb/dwarf2/read.c

@@ -15050,7 +15050,7 @@ dwarf2_add_field (struct field_info *fip, struct die_info *die,
       /* Get bit offset of field.  */
       handle_data_member_location (die, cu, fp);
       attr = dwarf2_attr (die, DW_AT_bit_offset, cu);
-      if (attr != nullptr && attr->form_is_unsigned ())
+      if (attr != nullptr && attr->form_is_constant ())
 	{
 	  if (gdbarch_byte_order (gdbarch) == BFD_ENDIAN_BIG)
 	    {
@@ -15060,7 +15060,7 @@ dwarf2_add_field (struct field_info *fip, struct die_info *die,
 	         have to do anything special since we don't need to
 	         know the size of the anonymous object.  */
 	      SET_FIELD_BITPOS (*fp, (FIELD_BITPOS (*fp)
-				      + attr->as_unsigned ()));
+				      + attr->constant_value (0)));
 	    }
 	  else
 	    {
@@ -15071,15 +15071,15 @@ dwarf2_add_field (struct field_info *fip, struct die_info *die,
 	         the field itself.  The result is the bit offset of
 	         the LSB of the field.  */
 	      int anonymous_size;
-	      int bit_offset = attr->as_unsigned ();
+	      int bit_offset = attr->constant_value (0);
 
 	      attr = dwarf2_attr (die, DW_AT_byte_size, cu);
-	      if (attr != nullptr && attr->form_is_unsigned ())
+	      if (attr != nullptr && attr->form_is_constant ())
 		{
 		  /* The size of the anonymous object containing
 		     the bit field is explicit, so use the
 		     indicated size (in bytes).  */
-		  anonymous_size = attr->as_unsigned ();
+		  anonymous_size = attr->constant_value (0);
 		}
 	      else
 		{

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2020-10-09  Tom Tromey  <tromey@adacore.com>
+
+	* gdb.ada/scalar_storage/storage.adb (Another_Range): New type.
+	(Rec): Add field.  Fix range.
+	* gdb.ada/scalar_storage.exp: Update.
+
 2020-10-09  Hannes Domani  <ssbssa@yahoo.de>
 
 	PR exp/26714

gdb/testsuite/gdb.ada/scalar_storage.exp

@@ -34,5 +34,5 @@ if ![runto "storage.adb:$bp_location" ] then {
   return
 }
 
-gdb_test "print V_LE" "= \\(value => 126\\)"
-gdb_test "print V_BE" "= \\(value => 126\\)"
+gdb_test "print V_LE" "= \\(value => 126, another_value => 12\\)"
+gdb_test "print V_BE" "= \\(value => 126, another_value => 12\\)"

gdb/testsuite/gdb.ada/scalar_storage/storage.adb

@@ -18,13 +18,16 @@ with System.Storage_Elements; use System.Storage_Elements;
 
 procedure Storage is
    subtype Some_Range is Natural range 0..127;
+   subtype Another_Range is Natural range 0..15;
 
    type Rec is record
       Value : Some_Range;
+      Another_Value : Another_Range;
    end record;
    
    for Rec use record
-      Value at 0 range 0..127;
+      Value at 0 range 0..6;
+      Another_Value at 0 range 7..10;
    end record;
 
    type Rec_LE is new Rec;
@@ -39,8 +42,8 @@ procedure Storage is
    V_BE : Rec_BE;
 
 begin
-   V_LE.Value := 126;
-   V_BE.Value := 126;
+   V_LE := (126, 12);
+   V_BE := (126, 12);
 
    Do_Nothing (V_LE'Address);  --  START
    Do_Nothing (V_BE'Address);