Handle biased types

In Ada, the programmer can request that a range type with a non-zero
base be stored in the minimal number of bits required for the range.
This is done by biasing the values; so, for example, a range of -7..-4
may be stored as two bits with a bias of -7.

This patch implements this for gdb.  It is done by adding a bias to
struct range_bounds and then adjusting a few spots to handle this.

The test case is written to use -fgnat-encodings=minimal, but a future
compiler patch will change the compiler to emit DW_AT_GNU_bias with
-fgnat-encodings=gdb.  It seemed good to get the gdb patch in first.

Tested on x86-64 Fedora 29; plus a variety of targets using AdaCore's
internal test suite.

gdb/ChangeLog
2019-09-03  Tom Tromey  <tromey@adacore.com>

	* ada-valprint.c (ada_val_print_num): Don't recurse for range
	types.
	(has_negatives): Unbias a range type bound.
	* dwarf2read.c (read_subrange_type): Handle DW_AT_GNU_bias.
	* gdbtypes.c (operator==): Handle new field.
	(create_range_type): Add "bias" parameter.
	(create_static_range_type, resolve_dynamic_range): Update.
	* gdbtypes.h (struct range_bounds) <bias>: New member.
	(create_range_type): Add bias parameter.
	* printcmd.c (print_scalar_formatted): Unbias range types.
	* value.c (unpack_long): Unbias range types.
	(pack_long): Bias range types.

gdb/testsuite/ChangeLog
2019-09-03  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/bias.exp: New file.
	* gdb.ada/bias/bias.adb: New file.
	* gdb.ada/print_chars.exp: Add regression test.
	* gdb.ada/print_chars/foo.adb (My_Character): New type.
	(MC): New variable.

1 files changed, 6 insertions, 1 deletions

diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index af4af19..fb888da 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -17901,6 +17901,11 @@ read_subrange_type (struct die_info *die, struct dwarf2_cu *cu)
 	}
     }
 
+  LONGEST bias = 0;
+  struct attribute *bias_attr = dwarf2_attr (die, DW_AT_GNU_bias, cu);
+  if (bias_attr != nullptr && attr_form_is_constant (bias_attr))
+    bias = dwarf2_get_attr_constant_value (bias_attr, 0);
+
   /* Normally, the DWARF producers are expected to use a signed
      constant form (Eg. DW_FORM_sdata) to express negative bounds.
      But this is unfortunately not always the case, as witnessed
@@ -17917,7 +17922,7 @@ read_subrange_type (struct die_info *die, struct dwarf2_cu *cu)
       && !TYPE_UNSIGNED (base_type) && (high.data.const_val & negative_mask))
     high.data.const_val |= negative_mask;
 
-  range_type = create_range_type (NULL, orig_base_type, &low, &high);
+  range_type = create_range_type (NULL, orig_base_type, &low, &high, bias);
 
   if (high_bound_is_count)
     TYPE_RANGE_DATA (range_type)->flag_upper_bound_is_count = 1;