Re: [PATCH], Add PowerPC ISA 3.0 min/max support

[Michael Meissner <meissner at linux dot vnet dot ibm dot com>]

On Mon, May 09, 2016 at 09:31:43AM -0500, Segher Boessenkool wrote:
> On Thu, May 05, 2016 at 03:18:39PM -0400, Michael Meissner wrote:
> > At the present time, the code does not support comparisons involving >= and <=
> > unless the -ffast-math option is used. I hope eventually to support generating
> > these instructions without having -ffast-math used.
> > 
> > The underlying reason is when fast math is not used, we change the condition
> > from:
> > 
> > 	(ge:SI (reg:CCFP <reg>) (const_int 0))
> > 
> > to:
> > 
> > 	(ior:SI (gt:SI (reg:CCFP <reg>) (const_int 0))
> > 		(eq:SI (reg:CCFP <reg>) (const_int 0)))
> > 
> > The machine independent portion of the compiler does not recognize this when
> > trying to generate conditional moves.
> > 
> > I would imagine the 'fix' is to generate GE/LE all of the time, and then have a
> > splitter that converts it to IOR of GT/EQ if it is not a conditional move with
> > ISA 3.0 instructions.
> 
> That sounds like a plan :-)

Sure, but at the moment it is lower priority to do it than all of the other
things that I'm doing at the moment.  Patches by other people are welcome.

> > -;; Return true if operand is MIN or MAX operator.
> > +;; Return true if operand is MIN or MAX operator.  Since this is only used to
> > +;; convert floating point MIN/MAX operations into FSEL on pre-vsx systems,
> > +;; don't include UMIN or UMAX.
> >  (define_predicate "min_max_operator"
> > -  (match_code "smin,smax,umin,umax"))
> > +  (match_code "smin,smax"))
> 
> Please name it signed_min_max_operator instead?
> 
> > --- gcc/config/rs6000/rs6000.c	(.../svn+ssh://meissner@gcc.gnu.org/svn/gcc/trunk/gcc/config/rs6000)	(revision 235831)
> > +++ gcc/config/rs6000/rs6000.c	(.../gcc/config/rs6000)	(working copy)
> > @@ -20534,6 +20534,12 @@ print_operand (FILE *file, rtx x, int co
> >  				"local dynamic TLS references");
> >        return;
> >  
> > +    case '@':
> > +      /* If -mpower9-minmax, use xsmaxcpdp instead of xsmaxdp.  */
> > +      if (TARGET_P9_MINMAX)
> > +	putc ('c', file);
> > +      return;
> 
> I don't think @ is very mnemonic, nor is this special enough for such
> a nice letter.

I don't care what punctuation letter is used, but it needs to be one. What do
you prefer?

> 
> Form looking at how it is used, it seems you can make it part of code_attr
> minmax (and give that a better name, minmax_fp or such)?

It is used to distinguish between generating

	XSMAXDP		on power7 with -ffast-math
and	XSMAXCDP	on power9 with/without -ffast-math

I would prefer not to have to change this to C code, hence the use of a
punctuation print operand code. But if you insist, I can just do it with if's.

> > +  rs6000_emit_minmax (dest, (max_p) ? SMAX : SMIN, op0, op1);
> 
> Superfluous parentheses.
> 
> > +rs6000_emit_power9_cmove (rtx dest, rtx op, rtx true_cond, rtx false_cond)
> 
> Maybe put some "fp" in the name?  For "minmax" as well.

Sigh, ok.

> > +  if (swap_p)
> > +    compare_rtx = gen_rtx_fmt_ee (code, CCFPmode, op1, op0);
> > +  else
> > +    compare_rtx = gen_rtx_fmt_ee (code, CCFPmode, op0, op1);
> 
> if (swap_p)
>   std::swap (op0, op1);

I'll look into it.

-- 
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