Re: [PATCH] Optionally trap on impossible devirtualization

[Jakub Jelinek <jakub at redhat dot com>]

On Mon, Apr 28, 2014 at 11:05:06AM +0200, Richard Biener wrote:
> On Fri, Apr 25, 2014 at 5:35 PM, Martin Jambor <mjambor@suse.cz> wrote:
> > Hi,
> >
> > the patch below might be useful for testcase preparation and debugging
> > compiler bugs such as PR 60965.  When
> > -ftrap-on-impossible-devirtualization is supplied on the command line,
> > it makes the devirtualization produce __builtin_trap instead of
> > __builtin_unreachable when it comes to the conclusion that there is no
> > legal target of a virtual call.
> >
> > Apart from dealing with our bugs, it may be even useful to debug
> > compiled programs when a user triggers some sort of illegal
> > devirtualization, typically by missing a type check somewhere.
> > Currently the compiled program might simply take a wrong branch, with
> > the patch it will abort.
> >
> > Bootstrapped and tested (with the option on) on x86_64-linux, I have
> > also successfully LTO built Firefox with it.  If I add some
> > documentation, would like to see this in trunk?
> 
> It's useful for debugging, so yes.  Not sure about the option name though.
> Maybe we should have a generic -ftrap-on-unreachable flag instead
> and handle all __builtin_unreachable () like that (for example by
> folding or by simply make __builtin_unreachable () alias to __builtin_trap ()).

-fsanitize=unreachable should already do that.  With
-fsanitize=unreachable -fsanitize-undefined-trap-on-error
it should fold __builtin_unreachable () to __builtin_trap (), otherwise
to __ubsan_handle_builtin_unreachable () call.

So, from this POV, the new option is redundant.

	Jakub