Re: [PATCH 2/4] allow poisoning input_location in ranges it should not be used

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Jul 1, 2021 at 12:16 PM Trevor Saunders <tbsaunde@tbsaunde.org> wrote:
>
> On Wed, Jun 30, 2021 at 11:13:23AM -0400, David Malcolm wrote:
> > On Wed, 2021-06-30 at 01:35 -0400, Trevor Saunders wrote:
> > > This makes it possible to assert if input_location is used during the
> > > lifetime
> > > of a scope.  This will allow us to find places that currently use it
> > > within a
> > > function and its callees, or prevent adding uses within the lifetime
> > > of a
> > > function after all existing uses are removed.
> > >
> > > bootstrapped and regtested on x86_64-linux-gnu, ok?
> > >
> > > Trev
> >
> > [...snip...]
> >
> > > diff --git a/gcc/diagnostic.c b/gcc/diagnostic.c
> > > index d58586f2526..3f68d1d79eb 100644
> > > --- a/gcc/diagnostic.c
> > > +++ b/gcc/diagnostic.c
> > > @@ -1835,7 +1835,7 @@ internal_error (const char *gmsgid, ...)
> > >    auto_diagnostic_group d;
> > >    va_list ap;
> > >    va_start (ap, gmsgid);
> > > -  rich_location richloc (line_table, input_location);
> > > +  rich_location richloc (line_table, UNKNOWN_LOCATION);
> > >    diagnostic_impl (&richloc, NULL, -1, gmsgid, &ap, DK_ICE);
> > >    va_end (ap);
> > >
> >
> > I actually make use of this in the analyzer: the analyzer sets
> > input_location to stmt->location when analyzing a given stmt - that
> > way, if the analyzer ICEs, the ICE is shown at the code construct that
> > crashed the analyzer.
> >
> > This behavior is useful to me, and would be lost with the proposed
> > patch.
>
> I made this change because otherwise if the compiler ICE's while access
> to input_location is blocked we end up infinitely recursing complaining
> we can't access it while trying to say where the last error was.  I was
> nervous about the change before, and now I agree we need something
> else.
>
> > Is there a better way of doing what I'm doing?
> >
> > Is the long-term goal of the patch kit to reduce our reliance on global
> > variables?  Are we ultimately still going to need a variable for "where
> > to show the ICE if gcc crashes"?  (perhaps stashing it in the
> > diagnostic_context???)
>
> Yes, the goal is ultimately removal of global state, however I'm not
> really ure what the better approach to your problem is, after all even
> moving it to the diagnostic context is sort of a global state, and sort
> of dupplicates input_location.  That said it is somewhat more
> constrained, so if it removes usage of input_location perhaps its
> worthwhile?

Reduction of global state is of course good - but in particular input_location
should be something only used during parsing because it's a quite
broken concept otherwise.  And fiddling with it tends to be quite fragile...
for example see g:7d6f7e92c3b737736a2d8ff97a71af9f230c2f88
for the "fun" you can have with "stale" values in input_location ...
IMHO users should have their own "copy", for example the gimplifier
instead of mucking with and using input_location could use a
similar state in its gimplify_ctx.

Richard.

> Sorry I'm not yet sure what to propose here.
>
> Trev
>
> >
> > Hope this is constructive
> > Dave
> >