[patch] Performance patch for MIPS conditional move in expr.c

[Steve Ellcey]

Back in August of 2011, Richard Biener made a change affecting COND_EXPRs
(r178408).  As a result of that change the GCC MIPS compiler that used
to promote HImode variables to SImode and then put out SImode variables
and assignments for the conditional move (MIPS doesn't support HImode
conditional moves) started putting out HImode variables and assignments and
the resulting code is slower then it was.

The code that used to look like this:

(insn 23 22 24 3 (set (reg/v:SI 231 [ a2+-2 ]) (zero_extend:SI (subreg:HI (reg:SI 320) 2))) x.c:11 -1 (nil))
(insn 27 26 28 3 (set (reg/v:SI 233 [ a2+-2 ]) (zero_extend:SI (subreg:HI (reg:SI 323) 2))) x.c:12 -1 (nil))
IF ()
	(insn 30 241 31 4 (set (reg:SI 324) (reg/v:SI 231 [ a2+-2 ])) -1 (nil))
ELSE
	(insn 34 242 35 5 (set (reg:SI 324) (reg/v:SI 233 [ a2+-2 ])) -1 (nil))
(insn 36 243 37 6 (set (reg/v:SI 234 [ a2+-2 ]) (reg:SI 324)) -1 (nil))



started outputting HI assignments instead:



(insn 23 22 24 3 (set (reg/v:SI 231 [ a2+-2 ]) (zero_extend:SI (subreg:HI (reg:SI 320) 2))) x.c:11 -1 (nil))
(insn 27 26 28 3 (set (reg/v:SI 233 [ a2+-2 ]) (zero_extend:SI (subreg:HI (reg:SI 323) 2))) x.c:12 -1 (nil))
IF ()
	(insn 30 241 31 4 (set (reg:HI 324) (subreg/s/u:HI (reg/v:SI 231 [ a2+-2 ]) 2)) -1 (nil))
ELSE
	(insn 34 242 35 5 (set (reg:HI 324) (subreg/s/u:HI (reg/v:SI 233 [ a2+-2 ]) 2)) -1 (nil))
(insn 36 243 37 6 (set (reg/v:SI 234 [ a2+-2 ]) (zero_extend:SI (reg:HI 324))) -1 (nil))

This resulted in an extra 'andi REG,REG,0xffff' instruction to implement the
final zero_extend instruction and that slowed the code down.  This patch
restores the previous behaviour by modifying expand_cond_expr_using_cmove
so that when we need to promote the mode in order to do a conditional move
we use that promoted mode in the temp that we are creating for the conditional
move.

Tested on mips-mti-elf with no regressions.

OK for checkin?

Steve Ellcey
sellcey@mips.com


2012-11-14  Steve Ellcey  <sellcey@mips.com>

	* expr.c (expand_cond_expr_using_cmove): Use promoted mode for temp.


diff --git a/gcc/expr.c b/gcc/expr.c
index cbf3a40..b1b83d0 100644
--- a/gcc/expr.c
+++ b/gcc/expr.c
@@ -7840,15 +7840,17 @@ expand_cond_expr_using_cmove (tree treeop0 ATTRIBUTE_UNUSED,
   int unsignedp = TYPE_UNSIGNED (type);
   enum machine_mode mode = TYPE_MODE (type);
 
-  temp = assign_temp (type, 0, 1);
-
   /* If we cannot do a conditional move on the mode, try doing it
      with the promoted mode. */
   if (!can_conditionally_move_p (mode))
-    mode = promote_mode (type, mode, &unsignedp);
-
-  if (!can_conditionally_move_p (mode))
-    return NULL_RTX;
+    {
+      mode = promote_mode (type, mode, &unsignedp);
+      if (!can_conditionally_move_p (mode))
+	return NULL_RTX;
+      temp = assign_temp (type, 0, 0); /* Use promoted mode for temp.  */
+    }
+  else
+    temp = assign_temp (type, 0, 1);
 
   start_sequence ();
   expand_operands (treeop1, treeop2,