Hardware watchpoints; dealing with false triggers?

[Orjan Friberg <orjan.friberg@axis.com>]

I'm wondering how/if gdb deals with false hardware watchpoint triggers. 
Depending on how your hardware watchpoints are implemented, you could
run the risk of having to watch a larger area than what the watched
variable actually occupies in memory.

I've looked at the i386 hardware watchpoint implementation in gdb, and
read chapter 15 of the Intel Architecture Software Developer's Manual
Volume 3 about the debug registers, but I can't tell if it's susceptible
to false triggers.  The i386 breakpoint registers can only deal with 1,
2, and 4-byte sized areas, so watching a 4-byte aligned 3-byte struct
seems to use two of the i386's debug registers (watching 2 and 1 bytes,
respectively).

But consider the following: say your watchpoint registers can only watch
4-byte aligned areas of 4 bytes, but you want to rwatch (or awatch) an
unaligned variable of size 4 bytes.  You'd have to use two watchpoint
registers, both covering too much, like this:

Variable to watch:      | 0 1   2 3 |
Watchpoints:        | 0 1 2 3 | 0 1 2 3 |
                        wp1       wp2

Now, say a there's a read of wp1's byte 0.  The hardware would trigger,
but it would be a false trigger.  Gdb would somehow have to find out the
actual address that was read and if it was found to be outside of the
variable's range it would not trigger the watchpoint.  (For the current
write watchpoint implementation in gdb, false triggers wouldn't be a
problem, since the value of the variable wouldn't change.)

I guess some new target hooks could be needed, but besides that: are
there any major obstacles for implementing such target-dependent false
trigger handling in gdb?

-- 
Orjan Friberg
Axis Communications AB