Re: Native compilation on MPS branch

[Eli Zaretskii <eliz@gnu.org>]

> From: Andrea Corallo <acorallo@gnu.org>
> Cc: gerd.moellmann@gmail.com,  emacs-devel@gnu.org
> Date: Thu, 25 Apr 2024 01:48:43 -0400
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Andrea Corallo <acorallo@gnu.org>
> >> Cc: Eli Zaretskii <eliz@gnu.org>,  emacs-devel@gnu.org
> >> Date: Wed, 24 Apr 2024 16:54:30 -0400
> >> 
> >> IME rr is invaluable for debugging GC bugs, one can do things like
> >> placing watch-points while doing reverse execution and discover
> >> immediately where something was modified the last time.  You did a very
> >> good job reaching this point of the development without rr.
> >
> > We have the last_marked[] array to facilitate debugging of GC without
> > rr.  I hope we will be able to do something similar with MPS, because
> > rr is not universally available (e.g., not on Windows).
> >
> >> Anyway I'll mention a trick I find extremely usefull to discover where
> >> something was modified last time before a certain event without rr, I
> >> apologize if you know it already but in case it might help some other
> >> reader :)
> >> 
> >> 1 When we crash we set a watch point on the memory we see got
> >> unexpectedly modified
> >> 2 We set the ignore number of the watch-point to a very large number
> >> 3 We restart the execution of the program
> >> 4 When the program crashes again we can see how many time the watch point
> >> was actually ignored
> >> 5 We subtract 1 to this number and we set it again as the ignore number
> >> of the watch-point
> >> 6 We restart again and tada!! We are where that piece of memory was
> >> modified for the last time :)
> >
> > This only works if you disable ASLR, AFAIU.
> 
> Correct, on GNU/Linux gdb does that by default, no idea on other
> systems.

It does it also on Windows, if the version of the OS is new enough to
support that.