[gdb] Add gdbarch_dwarf2_reg_piece_offset hook

In rw_pieced_value, when reading/writing part of a register, DW_OP_piece and
DW_OP_bit_piece are handled the same, but the standard tells us:
- DW_OP_piece: if the piece is located in a register, but does not occupy the
  entire register, the placement of the piece within that register is defined
  by the ABI.
- DW_OP_bit_piece: if the location is a register, the offset is from the least
  significant bit end of the register.

Add a new hook gdbarch_dwarf2_reg_piece_offset that allows us to define the
ABI-specific behaviour for DW_OP_piece.

The default implementation of the hook is the behaviour of DW_OP_bit_piece, so
there should not be any functional changes.

Tested on s390x-linux.

Approved-By: Tom Tromey <tom@tromey.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/gdb/findvar.c b/gdb/findvar.c
index f7760aa..1a9d2be 100644
--- a/gdb/findvar.c
+++ b/gdb/findvar.c
@@ -541,6 +541,24 @@ default_value_from_register (gdbarch *gdbarch, type *type, int regnum,
   return value;
 }
 
+/* Default implementation of gdbarch_dwarf2_reg_piece_offset.  Implements
+   DW_OP_bits_piece for DW_OP_piece.  */
+
+ULONGEST
+default_dwarf2_reg_piece_offset (gdbarch *gdbarch, int gdb_regnum, ULONGEST size)
+{
+  ULONGEST reg_size = register_size (gdbarch, gdb_regnum);
+  gdb_assert (size <= reg_size);
+  if (reg_size == size)
+    return 0;
+
+  if (gdbarch_byte_order (gdbarch) == BFD_ENDIAN_BIG)
+    return reg_size - size;
+
+  return 0;
+}
+
+
 /* VALUE must be an lval_register value.  If regnum is the value's
    associated register number, and len the length of the value's type,
    read one or more registers in VALUE's frame, starting with register REGNUM,