Re: Patch RFA: Add option -fcollectible-pointers, use it in ivopts

[Bernd Schmidt <bschmidt at redhat dot com>]

On 01/27/2016 02:59 PM, Ian Lance Taylor wrote:
> > > +This option is disabled by default for most languages, enabled by
> > > +default for languages that use garbage collection.
> >
> >
> > This is not true as of this patch.
>
> Yes.  As I said elsewhere, my intent is to do that as a separate patch.

Then the followup patch should adjust the documentation.

> The option does in principle over-promise.  On the other hand, it's
> probably pretty good in practice; Java and Go have gotten by without
> this option for many years.  I only have a single test case that shows
> a real problem, and it only shows up on PPC64.  That said, I'm fine
> with making the option undocumented if you prefer.
>
> > Or maybe we do need some other form to represent the need to keep a pointer
> > alive in the IL. I suspect that for gcc-6 your patch is probably reasonable,
> > but we should mark the option as internal and likely to go away in the
> > future.
>
> Why would the option  be likely to go away?  The option name is chosen
> (based on Richi's suggestion) so that it can be applied to other
> passes besides ivopts, and it's useful for languages that do not
> normally do garbage collection.  If we improve the implementation as
> you suggest, it would still be reasonable to keep the option.

I think it would be better described in the IL, so that in an LTO 
situation the restrictions would apply only to code which needs it. 
Although I suppose you could then enable it for C and make that generate 
the necessary representation.

Still, I feel uncomfortable about making a promise we don't really 
expect to fully keep yet, so I would prefer this option to be 
undocumented for now. I won't object if someone else wants to approve it 
as a normal option however.


Bernd