From: Simon Marchi <simon.marchi@polymtl.ca>
To: Joel Brobecker <brobecker@adacore.com>
Cc: Tim Newsome <tim@sifive.com>, gdb <gdb@sourceware.org>
Subject: Re: gdb requires watchpoints to fire after the write
Date: Wed, 29 Aug 2018 16:02:00 -0000 [thread overview]
Message-ID: <f7737cfa92e27db8254934d9efd6f49d@polymtl.ca> (raw)
In-Reply-To: <20180829154739.GB2521@adacore.com>
On 2018-08-29 11:47, Joel Brobecker wrote:
>> I don't have experience with many different architectures, but as far
>> as I
>> know, the expectation of the GDB is that the watchpoint is reported
>> after
>> the write. Otherwise it wouldn't need to save the value of the
>> watched
>> expression. That's also how software watchpoints seem to work.
>>
>> The easiest way to deal with this would be to match GDB's expectation.
>> But
>> if you really prefer the behavior of reporting the watchpoint before
>> the
>> event, I suppose it's always possible to teach GDB about this, but
>> it's a
>> less trivial task. Especially that when you GDB evaluates whether the
>> watch
>> expression has changed value, it would need to consider the
>> not-yet-written
>> value in memory.
>>
>> I'm also curious to know if other architectures work in this way
>> (report the
>> event before the write actually take place).
>
> I seem to remember some architectures having different behaviors,
> and so we have a couple of entry points in GDB. For
> architecture-specific
> settings, we have gdbarch_have_nonsteppable_watchpoint. For
> target-specific
> settings, you would use target_have_steppable_watchpoint. (IIRC)
Indeed, the comment at infrun.c:5805 seems to hint that some (or all?)
targets/arches do work like that? And the fix is that GDB does a single
step to execute the instruction that modifies the memory, and then
evaluates the expression. I hadn't thought about that.
See:
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=gdb/infrun.c;h=7731ccda68343b0118b9806615ff45b9f4d56c63;hb=HEAD#l5805
I'm just confused by this condition:
if (stopped_by_watchpoint
&& (target_have_steppable_watchpoint
|| gdbarch_have_nonsteppable_watchpoint (gdbarch)))
I don't understand why we check for target_have_steppable_watchpoint OR
gdbarch_have_nonsteppable_watchpoint, they seem to mean opposite things.
Simon
next prev parent reply other threads:[~2018-08-29 16:02 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-28 22:08 Tim Newsome
2018-08-29 15:33 ` Simon Marchi
2018-08-29 15:47 ` Joel Brobecker
2018-08-29 15:56 ` Pedro Alves
2018-08-29 16:02 ` Simon Marchi [this message]
2018-08-29 17:29 ` Pedro Alves
2018-08-29 20:13 ` Tim Newsome
2018-08-29 20:58 ` Tom Tromey
2018-08-30 8:05 ` Joel Brobecker
2018-08-31 15:37 ` Pedro Alves
2018-08-31 15:13 ` Pedro Alves
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