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.
