Re: [PATCH] Prevent downgrading of hardware watchpoints if possible

[Hannes Domani <ssbssa@yahoo.de>]

 Am Freitag, 3. Juli 2026 um 05:18:19 MESZ hat Thiago Jung Bauermann <thiago.bauermann@linaro.org> Folgendes geschrieben:

> Hannes Domani <ssbssa@yahoo.de> writes:
> 
> > The lazy flag of a value can tell us if the value contents are available.
> > But this could be either because it was simply never actually accessed,
> > or it tried to be accessed, and failed.
> > The latter are interesting for watchpoints, the former are not.
> >
> > Currently it only uses lazy values if they are at the head of the value
> > chain, but this is fragile logic, and degrades some watchpoints to
> > software watchpoints where it's actually not necessary.
> >
> > So this adds a new value flag 'm_fetch_lazy_failed' which tells if the
> > watchpoint expression actually tried to access the value and failed.
> > And this is used instead of the value chain location to tell if a lazy
> > value should be used as a watchpoint location.
> >
> > Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27423
> > ---
> >  gdb/breakpoint.c                              | 11 ++--
> >  .../gdb.base/watchpoint-hw-no-degradation.c  | 51 +++++++++++++++++++
> >  .../gdb.base/watchpoint-hw-no-degradation.exp | 38 ++++++++++++++
> >  gdb/value.c                                  |  3 ++
> >  gdb/value.h                                  | 12 ++++-
> >  5 files changed, 108 insertions(+), 7 deletions(-)
> >  create mode 100644 gdb/testsuite/gdb.base/watchpoint-hw-no-degradation.c
> >  create mode 100644 gdb/testsuite/gdb.base/watchpoint-hw-no-degradation.exp
> 
> I like the solution, thanks!
> 
> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
> 
> Just one nit in the testcase:
> 
> > diff --git a/gdb/testsuite/gdb.base/watchpoint-hw-no-degradation.exp b/gdb/testsuite/gdb.base/watchpoint-hw-no-degradation.exp
> > new file mode 100644
> > index 00000000000..94ef335c77f
> > --- /dev/null
> > +++ b/gdb/testsuite/gdb.base/watchpoint-hw-no-degradation.exp
> > @@ -0,0 +1,38 @@
> > +# Copyright 2009-2026 Free Software Foundation, Inc.
> > +
> > +# This program is free software; you can redistribute it and/or modify
> > +# it under the terms of the GNU General Public License as published by
> > +# the Free Software Foundation; either version 3 of the License, or
> > +# (at your option) any later version.
> > +#
> > +# This program is distributed in the hope that it will be useful,
> > +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> > +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > +# GNU General Public License for more details.
> > +#
> > +# You should have received a copy of the GNU General Public License
> > +# along with this program.  If not, see <http://www.gnu.org/licenses/>.
> > +
> > +# Test if watchpoint doesn't degrade to a software watchpoint if part of
> > +# expression isn't accessible at time of watchpoint creation.
> > +
> > +require allow_hw_watchpoint_access_tests
> > +
> > +standard_testfile
> > +
> > +if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
> > +    return -1
> > +}
> > +
> > +if {![runto_main]} {
> > +    return -1
> > +}
> 
> There's no need to return a value from the top-level. Please change the
> returns above to just "return".

I've made these changes.


Hannes