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

[David Malcolm <dmalcolm@redhat.com>]

On Thu, 2021-07-01 at 14:53 +0200, Richard Biener wrote:
> 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 ...

Yeah.  Another example, from the analyzer, is
g:2fbea4190e76a59c4880727cf84706fe083c00ae (PR 93349)


> 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.

Some ideas (not necessarily good ones):

(a) the diagnostic_context could have an ice_location field, and use
that in internal_error (and maybe an RAII class for setting/clearing
it).

(b) move input_location to diagnostic_context, and add:
       #define input_location (global_dc->x_input_location)
or:
       #define input_location (global_dc->x_default_location)
which add an indirection everywhere.  I don't love these ideas, in that
we already overuse the preprocessor IMHO.

Trevor: BTW, if you're looking for global state to eliminate, it might
be nice to move the globals in input.c for caching source lines
(fcache_tab etc) into a new source_cache class, and have the
diagnostic_context own it via a new "source_cache *" field.

Dave