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RE: [Patch, MIPS] Enable fp-contract on MIPS and update -mfused-madd

Steve Ellcey <Steve.Ellcey@imgtec.com> writes:
> On Wed, 2015-06-17 at 19:44 +0100, Richard Sandiford wrote:
> 
> >
> > FWIW, to be specific, I think we're talking about every check except
> > the last two in mips.md:
> >
> > and the one mips-ps-3d.md:
> >
> > In particular, the two checks in mips.c should go.
> 
> OK, here is a patch that does that.
> 
> > But like I say, please do add a comment above the unfused patterns to say
> > why we can match NEG here without the HONOR_NANS test.  It's going to be
> > far from obvious to anyone who hasn't read this thread.
> 
> I put these comments in mips.md as part of the patch:
> 
> ;; The various multiply accumulate instructions can be used even when
> ;; HONOR_NANS is true because while IEEE 754-2008 requires the negate
> ;; operation to negate the sign of a NAN and the MIPS neg instruction does
> ;; not do this, the multiply and add (or minus) parts of these instructions
> ;; have no requirement on how the sign of a NAN is handled and so the final
> ;; sign bit of the entire operation is undefined.
> 
> > It's also worth pointing out (although it's probably obvious) that this is
> > effectively going to be a no-op change.  We'd never have a NEG to combine
> > if we were honouring NANs and using the legacy pre-2008 mode.  We should
> > still be accurate though.  Especially when it's also less code :-)
> 
> Actually, this wasn't obvious to me.  I thought this must have some
> affect but I ran through the various options and architectures and, sure
> enough, I found no differences in the generated code.
> 
> Here is my "prequel" patch.  How does this look?
> 
> Steve Ellcey
> sellcey@imgtec.com
> 
> 2015-06-17  Steve Ellcey  <sellcey@imgtec.com>
> 
> 	* config/mips/mips.c (mips_rtx_costs): Remove HONOR_NAN check.
> 	* config/mips/mips.md (*madd4<mode>): Ditto.
> 	(*nmadd3<mode>) Ditto.
> 	(*nmadd4<mode>_fastmath): Ditto.
> 	(*nmadd3<mode>_fastmath): Ditto.
> 	(*nmsub4<mode>): Ditto.
> 	(*nmsub3<mode>): Ditto.
> 	(*nmsub4<mode>_fastmath): Ditto.
> 	(*nmsub3<mode>_fastmath): Ditto.

I'd like to delegate to Maciej to approve this and the actual
fp-contract patch. I have nothing of value to add to this
discussion but to say it is clearly comprehensive.

Thanks,
Matthew

> diff --git a/gcc/config/mips/mips.c b/gcc/config/mips/mips.c
> index d427c0c..1c837cf 100644
> --- a/gcc/config/mips/mips.c
> +++ b/gcc/config/mips/mips.c
> @@ -4069,7 +4069,6 @@ mips_rtx_costs (rtx x, int code, int outer_code, int opno
> ATTRIBUTE_UNUSED,
>        if (float_mode_p
>  	  && (ISA_HAS_NMADD4_NMSUB4 || ISA_HAS_NMADD3_NMSUB3)
>  	  && TARGET_FUSED_MADD
> -	  && !HONOR_NANS (mode)
>  	  && !HONOR_SIGNED_ZEROS (mode))
>  	{
>  	  /* See if we can use NMADD or NMSUB.  See mips.md for the
> @@ -4137,7 +4136,6 @@ mips_rtx_costs (rtx x, int code, int outer_code, int opno
> ATTRIBUTE_UNUSED,
>        if (float_mode_p
>  	  && (ISA_HAS_NMADD4_NMSUB4 || ISA_HAS_NMADD3_NMSUB3)
>  	  && TARGET_FUSED_MADD
> -	  && !HONOR_NANS (mode)
>  	  && HONOR_SIGNED_ZEROS (mode))
>  	{
>  	  /* See if we can use NMADD or NMSUB.  See mips.md for the
> diff --git a/gcc/config/mips/mips.md b/gcc/config/mips/mips.md
> index 0a23fa2..f6912e1 100644
> --- a/gcc/config/mips/mips.md
> +++ b/gcc/config/mips/mips.md
> @@ -2475,6 +2475,13 @@
> 
>  ;; Floating point multiply accumulate instructions.
> 
> +;; The various multiply accumulate instructions can be used even when
> +;; HONOR_NANS is true because while IEEE 754-2008 requires the negate
> +;; operation to negate the sign of a NAN and the MIPS neg instruction does
> +;; not do this, the multiply and add (or minus) parts of these instructions
> +;; have no requirement on how the sign of a NAN is handled and so the final
> +;; sign bit of the entire operation is undefined.
> +
>  (define_insn "*madd4<mode>"
>    [(set (match_operand:ANYF 0 "register_operand" "=f")
>  	(plus:ANYF (mult:ANYF (match_operand:ANYF 1 "register_operand" "f")
> @@ -2533,8 +2540,7 @@
>  		   (match_operand:ANYF 3 "register_operand" "f"))))]
>    "ISA_HAS_NMADD4_NMSUB4
>     && TARGET_FUSED_MADD
> -   && HONOR_SIGNED_ZEROS (<MODE>mode)
> -   && !HONOR_NANS (<MODE>mode)"
> +   && HONOR_SIGNED_ZEROS (<MODE>mode)"
>    "nmadd.<fmt>\t%0,%3,%1,%2"
>    [(set_attr "type" "fmadd")
>     (set_attr "mode" "<UNITMODE>")])
> @@ -2547,8 +2553,7 @@
>  		   (match_operand:ANYF 3 "register_operand" "0"))))]
>    "ISA_HAS_NMADD3_NMSUB3
>     && TARGET_FUSED_MADD
> -   && HONOR_SIGNED_ZEROS (<MODE>mode)
> -   && !HONOR_NANS (<MODE>mode)"
> +   && HONOR_SIGNED_ZEROS (<MODE>mode)"
>    "nmadd.<fmt>\t%0,%1,%2"
>    [(set_attr "type" "fmadd")
>     (set_attr "mode" "<UNITMODE>")])
> @@ -2561,8 +2566,7 @@
>  	 (match_operand:ANYF 3 "register_operand" "f")))]
>    "ISA_HAS_NMADD4_NMSUB4
>     && TARGET_FUSED_MADD
> -   && !HONOR_SIGNED_ZEROS (<MODE>mode)
> -   && !HONOR_NANS (<MODE>mode)"
> +   && !HONOR_SIGNED_ZEROS (<MODE>mode)"
>    "nmadd.<fmt>\t%0,%3,%1,%2"
>    [(set_attr "type" "fmadd")
>     (set_attr "mode" "<UNITMODE>")])
> @@ -2575,8 +2579,7 @@
>  	 (match_operand:ANYF 3 "register_operand" "0")))]
>    "ISA_HAS_NMADD3_NMSUB3
>     && TARGET_FUSED_MADD
> -   && !HONOR_SIGNED_ZEROS (<MODE>mode)
> -   && !HONOR_NANS (<MODE>mode)"
> +   && !HONOR_SIGNED_ZEROS (<MODE>mode)"
>    "nmadd.<fmt>\t%0,%1,%2"
>    [(set_attr "type" "fmadd")
>     (set_attr "mode" "<UNITMODE>")])
> @@ -2589,8 +2592,7 @@
>  		   (match_operand:ANYF 1 "register_operand" "f"))))]
>    "ISA_HAS_NMADD4_NMSUB4
>     && TARGET_FUSED_MADD
> -   && HONOR_SIGNED_ZEROS (<MODE>mode)
> -   && !HONOR_NANS (<MODE>mode)"
> +   && HONOR_SIGNED_ZEROS (<MODE>mode)"
>    "nmsub.<fmt>\t%0,%1,%2,%3"
>    [(set_attr "type" "fmadd")
>     (set_attr "mode" "<UNITMODE>")])
> @@ -2603,8 +2605,7 @@
>  		   (match_operand:ANYF 1 "register_operand" "0"))))]
>    "ISA_HAS_NMADD3_NMSUB3
>     && TARGET_FUSED_MADD
> -   && HONOR_SIGNED_ZEROS (<MODE>mode)
> -   && !HONOR_NANS (<MODE>mode)"
> +   && HONOR_SIGNED_ZEROS (<MODE>mode)"
>    "nmsub.<fmt>\t%0,%1,%2"
>    [(set_attr "type" "fmadd")
>     (set_attr "mode" "<UNITMODE>")])
> @@ -2617,8 +2618,7 @@
>  		    (match_operand:ANYF 3 "register_operand" "f"))))]
>    "ISA_HAS_NMADD4_NMSUB4
>     && TARGET_FUSED_MADD
> -   && !HONOR_SIGNED_ZEROS (<MODE>mode)
> -   && !HONOR_NANS (<MODE>mode)"
> +   && !HONOR_SIGNED_ZEROS (<MODE>mode)"
>    "nmsub.<fmt>\t%0,%1,%2,%3"
>    [(set_attr "type" "fmadd")
>     (set_attr "mode" "<UNITMODE>")])
> @@ -2631,8 +2631,7 @@
>  		    (match_operand:ANYF 3 "register_operand" "0"))))]
>    "ISA_HAS_NMADD3_NMSUB3
>     && TARGET_FUSED_MADD
> -   && !HONOR_SIGNED_ZEROS (<MODE>mode)
> -   && !HONOR_NANS (<MODE>mode)"
> +   && !HONOR_SIGNED_ZEROS (<MODE>mode)"
>    "nmsub.<fmt>\t%0,%1,%2"
>    [(set_attr "type" "fmadd")
>     (set_attr "mode" "<UNITMODE>")])
> 
>
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