Re: [PATCH], Make PowerPC -mcpu=future enable -mpcrel on linux ELFv2

[Michael Meissner <meissner@linux.ibm.com>]

On Mon, Mar 30, 2020 at 05:51:49PM -0500, Segher Boessenkool wrote:
> Hi!
> 
> On Mon, Mar 30, 2020 at 12:50:43PM -0500, will schmidt wrote:
> > On Fri, 2020-03-27 at 21:31 -0400, Michael Meissner via Gcc-patches
> > > 	* config/rs6000/rs6000.c (PCREL_SUPPORTED_BY_OS): New macro.
> > > 	(rs6000_option_override_internal): Set the -mprefixed and
> > > -mpcrel
> > > 	options for -mcpu=future if these options can be used.
> > > 
> > s/can be used/are supported by the platform/ ? 
> 
> The code says
>   /* Enable -mprefixed by default on 64-bit 'future' systems.  */
>   /* If the OS has support for PC-relative relocations, enable it now.  */
> and something like that should go in the changelog as well (two lines in
> changelog is fine -- they are two hunks of patch as well, anyway!)

Ok.

> > > +/* Enable default support for PC-relative addressing on the 'future'
> > > system if
> > > +   we can use the PC-relative instructions.  Currently this support
> > > only exits
> > 
> > exists
> > 
> > > +   for the ELF v2 object file format using the medium code
> > > model.  */
> > 
> > should that be "s/object file format/ABI/" ? 
> 
> Yes.

Ok.

> > > -/* Support for a future processor's features.  Do not enable -mpcrel
> > > until it
> > > -   is fully functional.  */
> > > +/* Support for a future processor's features.  We do not set -mpcrel
> > > or
> > > +   -mprefixed here.  These bits are set in rs6000_option_override if
> > > the system
> > > +   supports those options. */
> > 
> > I'm still not sure the comment here is actually necessary, there are
> > many other places where we also do not set -mpcrel or -mprefixed.  If
> > history of the code here requires a hint to point at those options
> > being set in rs6000_option_override, then it's fine.
> 
> If you really need to say you do *not* do something, you should say why
> not.  Without that it only leaves more questions to the reader :-)
> 
> Hopefully that then also explains why the reader should care about this.

Given this comment is against Will's comment, and not the original code, is
there anything I need to do to the code (other than the ChangeLog and adjusting
object file format to ABI?

-- 
Michael Meissner, IBM
IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
email: meissner@linux.ibm.com, phone: +1 (978) 899-4797