Fix PR12526: -location watchpoints for bitfield arguments

PR 12526 reports that -location watchpoints against bitfield arguments
trigger false positives when bits around the bitfield, but not the
bitfield itself, are modified.

This happens because -location watchpoints naturally operate at the
byte level, not at the bit level.  When the address of a bitfield
lvalue is taken, information about the bitfield (i.e. its offset and
size) is lost in the process.

This information must first be retained throughout the lifetime of the
-location watchpoint.  This patch achieves this by adding two new
fields to the watchpoint struct: val_bitpos and val_bitsize.  These
fields are set when a watchpoint is first defined in watch_command_1.
They are both equal to zero if the watchpoint is not a -location
watchpoint or if the argument is not a bitfield.

Then these bitfield parameters are used inside update_watchpoint and
watchpoint_check to extract the actual value of the bitfield from the
watchpoint address, with the help of a local helper function
extract_bitfield_from_watchpoint_value.

Finally when creating a HW breakpoint pointing to a bitfield, we
optimize the address and length of the breakpoint.  By skipping over
the bytes that don't cover the bitfield, this step reduces the
frequency at which a read watchpoint for the bitfield is triggered.
It also reduces the number of times a false-positive call to
check_watchpoint is triggered for a write watchpoint.

gdb/
	PR breakpoints/12526
	* breakpoint.h (struct watchpoint): New fields val_bitpos and
	val_bitsize.
	* breakpoint.c (watch_command_1): Use these fields to retain
	bitfield information.
	(extract_bitfield_from_watchpoint_value): New function.
	(watchpoint_check): Use it.
	(update_watchpoint): Use it.  Optimize the address and length of a
	HW watchpoint pointing to a bitfield.
	* value.h (unpack_value_bitfield): New prototype.
	* value.c (unpack_value_bitfield): Make extern.

gdb/testsuite/
	PR breakpoints/12526
	* gdb.base/watch-bitfields.exp: New file.
	* gdb.base/watch-bitfields.c: New file.

1 files changed, 1 insertions, 1 deletions

diff --git a/gdb/value.c b/gdb/value.c
index 6620f96..fdc8858d 100644
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -3231,7 +3231,7 @@ unpack_field_as_long (struct type *type, const gdb_byte *valaddr, int fieldno)
    are unavailable/optimized out, DEST_VAL is correspondingly
    marked unavailable/optimized out.  */
 
-static void
+void
 unpack_value_bitfield (struct value *dest_val,
 		       int bitpos, int bitsize,
 		       const gdb_byte *valaddr, int embedded_offset,