Re: [RS6000] rs6000_rtx_costs reduce cost for SETs

[Alan Modra <amodra@gmail.com>]

On Fri, Sep 18, 2020 at 01:13:18PM -0500, Segher Boessenkool wrote:
> Thanks (to both of you).  Interesting!  Which of these unrelated changes
> does this come from?

Most of the changes I saw in code generation (not in spec, I didn't
look there, but in gcc) came down to this change to the cost for SETs,
and "rs6000_rtx_costs multi-insn constants".  I expect they were the
changes that made most difference to spec results, with this patch
likely resulting in more if-conversion.

So here is the patch again, this time without any distracting other
changes.  With a further revised comment.

	* config/rs6000/rs6000.c (rs6000_rtx_costs): Reduce cost of SET
	operands.

diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
index 8969baa4dcf..2d770afd8fe 100644
--- a/gcc/config/rs6000/rs6000.c
+++ b/gcc/config/rs6000/rs6000.c
@@ -21599,6 +21599,35 @@ rs6000_rtx_costs (rtx x, machine_mode mode, int outer_code,
 	}
       break;
 
+    case SET:
+      /* On entry the value in *TOTAL is the number of general purpose
+	 regs being set, multiplied by COSTS_N_INSNS (1).  Handle
+	 costing of set operands specially since in most cases we have
+	 an instruction rather than just a piece of RTL and should
+	 return a cost comparable to insn_cost.  That's a little
+	 complicated because in some cases the cost of SET operands is
+	 non-zero, see point 5 above and cost of PLUS for example, and
+	 in others it is zero, for example for (set (reg) (reg)).
+	 But (set (reg) (reg)) has the same insn_cost as
+	 (set (reg) (plus (reg) (reg))).  Hack around this by
+	 subtracting COSTS_N_INSNS (1) from the operand cost in cases
+	 were we add at least COSTS_N_INSNS (1) for some operation.
+	 However, don't do so for constants.  Constants might cost
+	 more than zero when they require more than one instruction,
+	 and we do want the cost of extra instructions.  */
+      {
+	rtx_code src_code = GET_CODE (SET_SRC (x));
+	if (src_code == CONST_INT
+	    || src_code == CONST_DOUBLE
+	    || src_code == CONST_WIDE_INT)
+	  return false;
+	int set_cost = (rtx_cost (SET_SRC (x), mode, SET, 1, speed)
+			+ rtx_cost (SET_DEST (x), mode, SET, 0, speed));
+	if (set_cost >= COSTS_N_INSNS (1))
+	  *total += set_cost - COSTS_N_INSNS (1);
+	return true;
+      }
+
     default:
       break;
     }

-- 
Alan Modra
Australia Development Lab, IBM