Re: [PATCH v2] rs6000: Add buildin for mffscrn instructions

[Carl Love <cel@us.ibm.com>]

On Wed, 2023-05-24 at 13:32 +0800, Kewen.Lin wrote:
> on 2023/5/24 06:30, Peter Bergner wrote:
> > On 5/23/23 12:24 AM, Kewen.Lin wrote:
> > > on 2023/5/23 01:31, Carl Love wrote:
> > > > The builtins were requested for use in GLibC.  As of version
> > > > 2.31 they
> > > > were added as inline asm.  They requested a builtin so the asm
> > > > could be
> > > > removed.
> > > 
> > > So IMHO we also want the similar support for mffscrn, that is to
> > > make
> > > use of mffscrn and mffscrni on Power9 and later, but falls back
> > > to 
> > > __builtin_set_fpscr_rn + mffs similar on older platforms.
> > 
> > So __builtin_set_fpscr_rn everything we want (sets the RN bits) and
> > uses mffscrn/mffscrni on P9 and later and uses older insns on pre-
> > P9.
> > The only problem is we don't return the current FPSCR bits, as the
> > bif
> > is defined to return void.
> 
> Yes.
> 
> > Crazy idea, but could we extend the built-in
> > with an overload that returns the FPSCR bits?  
> 
> So you agree that we should make this proposed new bif handle pre-P9
> just
> like some other existing bifs. :)  I think extending it is good and
> doable,
> but the only concern here is the bif name "__builtin_set_fpscr_rn",
> which
> matches the existing behavior (only set rounding) but doesn't match
> the
> proposed extending behavior (set rounding and get some env bits
> back).
> Maybe it's not a big deal if the documentation clarify it well.

Extending the builtin to pre Power 9 is straight forward and I agree
would make good sense to do.

I am a bit concerned on how to extend __builtin_set_fpscr_rn to add the
new functionality.  Peter suggests overloading the builtin to either
return void or returns FPSCR bits.  It is my understanding that the
return value for a given builtin had to be the same, i.e. you can't
overload the return value. Maybe you can with Bill's new
infrastructure?  I recall having problems trying to overload the return
value in the past and Bill said you couldn't do it.  I play with this
and see if I can overload the return value.
> 
> 
> > To be honest, I like
> > the __builtin_set_fpscr_rn name better than __builtin_mffscrn[i].
> 
> +1
> 
> BR,
> Kewen
> 
> > The built-in machinery can see that the usage is expecting a return
> > value
> > or not and for the pre-P9 code, can skip generating the ending mffs
> > if
> > we don't want the return value.
> > 
> > Peter
> > 
> >