Re: [PATCH v4 1/3] c++: Track lifetimes in constant evaluation [PR70331,PR96630,PR98675]

[Nathaniel Shead <nathanieloshead@gmail.com>]

On Thu, Jul 20, 2023 at 10:42:29AM -0400, Jason Merrill wrote:
> On 7/20/23 05:35, Nathaniel Shead wrote:
> > This adds rudimentary lifetime tracking in C++ constexpr contexts,
> > allowing the compiler to report errors with using values after their
> > backing has gone out of scope. We don't yet handle other ways of
> > accessing values outside their lifetime (e.g. following explicit
> > destructor calls).
> 
> Incidentally, much of that should be straightforward to handle by no longer
> ignoring clobbers here:
> 
> >     case MODIFY_EXPR:
> >       if (cxx_dialect < cxx14)
> >         goto fail;
> >       if (!RECUR (TREE_OPERAND (t, 0), any))
> >         return false;
> >       /* Just ignore clobbers.  */
> >       if (TREE_CLOBBER_P (TREE_OPERAND (t, 1)))
> >         return true;
> 
> Assignment from a clobber represents end of lifetime to the middle-end. This
> can be a follow-up patch.

Thanks, this is very helpful to know. I'll keep this in mind.

> > @@ -7051,10 +7065,17 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
> >   	return ctx->ctor;
> >         if (VAR_P (t))
> >   	if (tree v = ctx->global->get_value (t))
> > -	    {
> > -	      r = v;
> > -	      break;
> > -	    }
> > +	  {
> > +	    r = v;
> > +	    break;
> > +	  }
> > +      if (ctx->global->is_outside_lifetime (t))
> > +	{
> > +	  if (!ctx->quiet)
> > +	    outside_lifetime_error (loc, t);
> > +	  *non_constant_p = true;
> > +	  break;
> > +	}
> 
> Shouldn't this new check also be under the if (VAR_P (t))?  A CONST_DECL
> can't go out of scope.
> 
> Jason
> 

Yup you're right; I didn't properly read the documentation on what a
CONST_DECL was and misunderstood. I'll fix this up for the next version.