Re: [PATCH], Enable IBM/IEEE long double format to overriden more easily

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

On Wed, Oct 25, 2017 at 07:11:07PM -0500, Segher Boessenkool wrote:
> Hi Mike,
> 
> On Sat, Oct 21, 2017 at 09:09:58AM -0400, Michael Meissner wrote:
> > As Segher and I were discussing off-line, I have some problems with the current
> > -mabi={ieee,ibm}longdouble switches as we start to planning to modify GCC 9 and
> > GLIBC 2.27/2.28 to support __float128 as the default long double format for
> > power server systems.
> > 
> > My gripes are:
> > 
> >     1)	Using Warn() in rs6000.opt means that you get two warning messages when
> > 	you use the switches (one from the driver, and one from cc1).
> > 
> >     2)	I feel you should not get a warning if you select the option that
> > 	reflects the current default behavior (i.e. -mabi=ibmlongdouble
> > 	currently for power server systems).
> > 
> >     3)	There is no way to silenece the warning (i.e. -w doesn't work on
> > 	warnings in the .opt file).  Both GLIBC and LIBGCC will need the
> > 	ability to build support modules with an explicit long double format.
> > 
> >     4)	In the future we will need a little more flexibility in how the default
> > 	is set.
> > 
> >     5)	There is a mis-match between the documentation and rs6000.opt, as these
> > 	switches are documented, but use Undocumented in the rs6000.opt.
> 
> Agreed on all.
> 
> > These patches fix these issues.  If you use -Wno-psabi, it will silence the
> > warning.  I have built these patches on a little endian power8 system, and
> > there were no regressions.  Can I check these patches into the trunk?
> > 
> > 2017-10-21  Michael Meissner  <meissner@linux.vnet.ibm.com>
> > 
> > 	* config/rs6000/aix.h (TARGET_IEEEQUAD_DEFAULT): Set long double
> > 	default to IBM.
> > 	* config/rs6000/darwin.h (TARGET_IEEEQUAD_DEFAULT): Likewise.
> > 	* config/rs6000/rs6000.opt (-mabi=ieeelongdouble): Move the
> > 	warning to rs6000.c.  Remove the Undocumented flag, since it has
> > 	been documented.
> > 	(-mabi=ibmlongdouble): Likewise.
> 
> And more importantly, we _want_ it to be documented (right)?

I would have preferred to not document it until GCC 9 when we start the switch
to long double == _Float128, but since it was already documented (albeit only
for 32 bit), I kept it documented.  If you want me to change it to
undocumented, I can do that.

> > --- gcc/config/rs6000/rs6000.opt	(revision 253961)
> > +++ gcc/config/rs6000/rs6000.opt	(working copy)
> > @@ -381,10 +381,10 @@ mabi=d32
> >  Target RejectNegative Undocumented Warn(using old darwin ABI) Var(rs6000_darwin64_abi, 0)
> >  
> >  mabi=ieeelongdouble
> > -Target RejectNegative Undocumented Warn(using IEEE extended precision long double) Var(rs6000_ieeequad) Save
> > +Target RejectNegative Var(rs6000_ieeequad) Save
> >  
> >  mabi=ibmlongdouble
> > -Target RejectNegative Undocumented Warn(using IBM extended precision long double) Var(rs6000_ieeequad, 0)
> > +Target RejectNegative Var(rs6000_ieeequad, 0)
> 
> Does this need "Save" as well?

No for variables, you want Save on the first instance only.

> > +      if (!warned_change_long_double && warn_psabi)
> > +	{
> > +	  warned_change_long_double = true;
> > +	  if (TARGET_IEEEQUAD)
> > +	    warning (0, "Using IEEE extended precision long double");
> > +	  else
> > +	    warning (0, "Using IBM extended precision long double");
> > +	}
> 
> You can put OPT_Wpsabi in place of that 0, it's what that arg is for :-)

Ah, ok.  Thanks.

> Okay with that changed.  Thanks!
> 
> 
> Segher
> 

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