[x86 PATCH] Correct ix86_rtx_cost for multi-word multiplication.

[x86 PATCH] Correct ix86_rtx_cost for multi-word multiplication.

[Roger Sayle]

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This is the i386 backend specific piece of my revised patch for
PR middle-end/98865, where Richard Biener has suggested that I perform
the desired transformation during RTL expansion where the backend can
control whether it is profitable to convert a multiplication into a
bit-wise AND and a negation.  This works well for x86_64, but alas
exposes a latent bug with -m32, where a DImode multiplication incorrectly
appears to be cheaper than negdi2+anddi3(!?).  The fix to ix86_rtx_costs
is to report that a DImode (multi-word) multiplication actually requires
three SImode multiplications and two SImode additions.  This also corrects
the cost of TImode multiplication on TARGET_64BIT.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}, with
no new failures.  This change avoids the need for a !ia32 target selector
for the upcoming new test case gcc.target/i386/pr98865.c.
Ok for mainline?


2022-05-17  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.cc (ix86_rtx_costs) [MULT]: When mode size
	is wider than word_mode, a multiplication costs three word_mode
	multiplications and two word_mode additions.

Thanks in advance,
Roger
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diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 86752a6..e8a2229 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -20634,7 +20634,17 @@ ix86_rtx_costs (rtx x, machine_mode mode, int outer_code_i, int opno,
 	        op0 = XEXP (op0, 0), mode = GET_MODE (op0);
 	    }
 
-  	  *total = (cost->mult_init[MODE_INDEX (mode)]
+	  int mult_init;
+	  // Double word multiplication requires 3 mults and 2 adds.
+	  if (GET_MODE_SIZE (mode) > UNITS_PER_WORD)
+	    {
+	      mult_init = 3 * cost->mult_init[MODE_INDEX (word_mode)]
+			  + 2 * cost->add;
+	      nbits *= 3;
+	    }
+	  else mult_init = cost->mult_init[MODE_INDEX (mode)];
+
+  	  *total = (mult_init
 		    + nbits * cost->mult_bit
 	            + rtx_cost (op0, mode, outer_code, opno, speed)
 		    + rtx_cost (op1, mode, outer_code, opno, speed));

Re: [x86 PATCH] Correct ix86_rtx_cost for multi-word multiplication.

[Uros Bizjak]

On Tue, May 17, 2022 at 6:44 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This is the i386 backend specific piece of my revised patch for
> PR middle-end/98865, where Richard Biener has suggested that I perform
> the desired transformation during RTL expansion where the backend can
> control whether it is profitable to convert a multiplication into a
> bit-wise AND and a negation.  This works well for x86_64, but alas
> exposes a latent bug with -m32, where a DImode multiplication incorrectly
> appears to be cheaper than negdi2+anddi3(!?).  The fix to ix86_rtx_costs
> is to report that a DImode (multi-word) multiplication actually requires
> three SImode multiplications and two SImode additions.  This also corrects
> the cost of TImode multiplication on TARGET_64BIT.
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32}, with
> no new failures.  This change avoids the need for a !ia32 target selector
> for the upcoming new test case gcc.target/i386/pr98865.c.
> Ok for mainline?
>
>
> 2022-05-17  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         * config/i386/i386.cc (ix86_rtx_costs) [MULT]: When mode size
>         is wider than word_mode, a multiplication costs three word_mode
>         multiplications and two word_mode additions.

LGTM.

Thanks,
Uros.

>
> Thanks in advance,
> Roger
> --
>