[gcc r13-624] Correct ix86_rtx_cost for multi-word multiplication.

[Roger Sayle <sayle@gcc.gnu.org>]

https://gcc.gnu.org/g:4a9be8d51182076222d707d9d68f6eda78e8ee2c

commit r13-624-g4a9be8d51182076222d707d9d68f6eda78e8ee2c
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Wed May 18 16:23:01 2022 +0100

    Correct ix86_rtx_cost for multi-word multiplication.
    
    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.
    
    2022-05-18  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.

Diff:
---
 gcc/config/i386/i386.cc | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 86752a6516a..30a9cd006e8 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));