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[committed] CRIS: Make rtx-cost 0 for many CONST_INT "quick" operands

Message ID 20230329225727.2D86420433@pchp3.se.axis.com
State New
Headers show
Series [committed] CRIS: Make rtx-cost 0 for many CONST_INT "quick" operands | expand

Commit Message

Hans-Peter Nilsson March 29, 2023, 10:57 p.m. UTC 
Stepping through a gdb session inspecting costs that cause
gcc.dg/tree-ssa/slsr-13.c to fail, exposed that before this
patch, cris_rtx_costs told that a shift of 1 of a register
costs 5, while adding two registers costs 4.

Making the cost of a quick-immediate constant equal to using
a register (default 0) reflects actual performance and
size-cost better.  It also happens to make
gcc.dg/tree-ssa/slsr-13.c pass with what looks like better
code being generated, and improves coremark performance by
0.4%.

But, blindly doing this for *all* valid operands that fit
the "quick-immediate" addressing mode, trips interaction
with other factors*, with the end result mixed at best.  So,
do this only for MINUS and logical operations for the time
being, and only for modes that fit in one register.

*) Examples of "other factors":

- A bad default implementation of insn_cost or actually,
pattern_cost, that looks only at the set_src_cost and
furthermore sees such a cost of 0 as invalid.  (Compare to
the more sane set_rtx_cost.)  This naturally tripped up
combine and ifcvt, causing all sorts of changes, good and
bad.

- Having the same cost, to compare a register with 0 as with
-31..31, means a compare insn of an eliminable form no
longer looks preferable.

	* config/cris/cris.cc (cris_rtx_costs) [CONST_INT]: Return 0
	for many quick operands, for register-sized modes.
---
 gcc/config/cris/cris.cc | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)
diff mbox series

Patch

diff --git a/gcc/config/cris/cris.cc b/gcc/config/cris/cris.cc
index 641e7ea25fb1..05dead9c0778 100644
--- a/gcc/config/cris/cris.cc
+++ b/gcc/config/cris/cris.cc
@@ -1884,7 +1884,28 @@  cris_rtx_costs (rtx x, machine_mode mode, int outer_code, int opno,
 	if (val == 0)
 	  *total = 0;
 	else if (val < 32 && val >= -32)
-	  *total = 1;
+	  switch (outer_code)
+	    {
+	      /* For modes that fit in one register we tell they cost
+		 the same as with register operands.  DImode operations
+		 needs careful consideration for more basic reasons:
+		 shifting by a non-word-size amount needs more
+		 operations than an addition by a register pair.
+		 Deliberately excluding SET, PLUS and comparisons and
+		 also not including the full -64..63 range for (PLUS
+		 and) MINUS.  */
+	    case MINUS: case ASHIFT: case LSHIFTRT:
+	    case ASHIFTRT: case AND: case IOR:
+	      if (GET_MODE_SIZE(mode) <= UNITS_PER_WORD)
+		{
+		  *total = 0;
+		  break;
+		}
+	      /* FALL THROUGH.  */
+	    default:
+	      *total = 1;
+	      break;
+	    }
 	/* Eight or 16 bits are a word and cycle more expensive.  */
 	else if (val <= 32767 && val >= -32768)
 	  *total = 2;