Fix handling of DWARF register pieces on big-endian targets

For big-endian targets the logic in read/write_pieced_value tries to take
a register piece from the LSB end.  This requires offsets and sizes to be
adjusted accordingly, and that's where the current implementation has some
issues:

* The formulas for recalculating the bit- and byte-offsets into the
  register are wrong.  They just happen to yield correct results if
  everything is byte-aligned and the piece's last byte belongs to the
  given value.

* After recalculating the bit offset into the register, the number of
  bytes to be copied from the register is not recalculated.  Of course
  this does not matter if everything (particularly the piece size) is
  byte-aligned.

These issues are fixed.  The size calculation is performed with a new
helper function bits_to_bytes().

gdb/ChangeLog:

	* dwarf2loc.c (bits_to_bytes): New function.
	(read_pieced_value): Fix offset calculations for register pieces
	on big-endian targets.
	(write_pieced_value): Likewise.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/var-access.exp: Add test for non-byte-aligned
	register pieces.

1 files changed, 25 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.dwarf2/var-access.exp b/gdb/testsuite/gdb.dwarf2/var-access.exp
index 3ba2c08..c533b8d 100644
--- a/gdb/testsuite/gdb.dwarf2/var-access.exp
+++ b/gdb/testsuite/gdb.dwarf2/var-access.exp
@@ -215,6 +215,19 @@ Dwarf::assemble $asm_file {
 			piece 1
 		    } SPECIAL_expr}
 		}
+		# Register pieces for bitfield access: 4 bytes optimized
+		# out, 3 bytes from r0, and 1 byte from r1.
+		DW_TAG_variable {
+		    {name "t2"}
+		    {type :$struct_t_label}
+		    {location {
+			piece 4
+			regx [lindex $dwarf_regnum 0]
+			piece 3
+			regx [lindex $dwarf_regnum 1]
+			piece 1
+		    } SPECIAL_expr}
+		}
 	    }
 	}
     }
@@ -279,3 +292,15 @@ switch $endian {
 #                             val
 gdb_test "print/x a" " = \\{0x0, ${val}, 0x0, 0x0\\}" \
     "verify st1 through a"
+
+switch $endian { big {set val 0x7ffc} little {set val 0x3ffe00} }
+gdb_test_no_output "set var \$[lindex $regname 0] = $val" \
+    "init t2, first piece"
+gdb_test_no_output "set var \$[lindex $regname 1] = 0" \
+    "init t2, second piece"
+gdb_test "print/d t2" " = \\{u = <optimized out>, x = 0, y = -1, z = 0\\}" \
+    "initialized t2 from regs"
+gdb_test_no_output "set var t2.y = 2641"
+gdb_test_no_output "set var t2.z = -400"
+gdb_test_no_output "set var t2.x = 200"
+gdb_test "print t2.x + t2.y + t2.z" " = 2441"