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

[Segher Boessenkool <segher@kernel.crashing.org>]

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 :-)

> -;; 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.

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)?

> +  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.

> +  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);

and then just generate the one form?


Segher