[patch] Do not generate useless branches for multi-word comparison

[Eric Botcazou]

Hi,

this improves the RTL generated for multi-word comparison to avoid generating 
useless branches.  In do_jump_by_parts_greater_rtx:
  1) do not generate the last cond jump,
  2) do not generate the uncond branch to the drop-through label, if any,
  3) generate only one comparison for the special (0  x) case.

Tested on i586-suse-linux, OK for the mainline?


2011-03-24  Eric Botcazou  ebotca...@adacore.com

* dojump.c (do_jump_by_parts_greater_rtx): Optimize in specific cases.


-- 
Eric Botcazou
Index: dojump.c
===
--- dojump.c	(revision 171345)
+++ dojump.c	(working copy)
@@ -637,14 +637,32 @@ do_jump_by_parts_greater_rtx (enum machi
 {
   int nwords = (GET_MODE_SIZE (mode) / UNITS_PER_WORD);
   rtx drop_through_label = 0;
+  bool drop_through_if_true = false, drop_through_if_false = false;
+  enum rtx_code code = GT;
   int i;
 
   if (! if_true_label || ! if_false_label)
 drop_through_label = gen_label_rtx ();
   if (! if_true_label)
-if_true_label = drop_through_label;
+{
+  if_true_label = drop_through_label;
+  drop_through_if_true = true;
+}
   if (! if_false_label)
-if_false_label = drop_through_label;
+{
+  if_false_label = drop_through_label;
+  drop_through_if_false = true;
+}
+
+  /* Deal with the special case 0  x: only one comparison is necessary and
+ we reverse it to avoid jumping to the drop-through label.  */
+  if (op0 == const0_rtx  drop_through_if_true  !drop_through_if_false)
+{
+  code = LE;
+  if_true_label = if_false_label;
+  if_false_label = drop_through_label;
+  drop_through_if_false = true;
+}
 
   /* Compare a word at a time, high order first.  */
   for (i = 0; i  nwords; i++)
@@ -663,17 +681,20 @@ do_jump_by_parts_greater_rtx (enum machi
 }
 
   /* All but high-order word must be compared as unsigned.  */
-  do_compare_rtx_and_jump (op0_word, op1_word, GT,
-   (unsignedp || i  0), word_mode, NULL_RTX,
-			   NULL_RTX, if_true_label, prob);
+  do_compare_rtx_and_jump (op0_word, op1_word, code, (unsignedp || i  0),
+			   word_mode, NULL_RTX, NULL_RTX, if_true_label,
+			   prob);
+
+  /* Emit only one comparison for 0.  Do not emit the last cond jump.  */
+  if (op0 == const0_rtx || i == nwords - 1)
+	break;
 
   /* Consider lower words only if these are equal.  */
   do_compare_rtx_and_jump (op0_word, op1_word, NE, unsignedp, word_mode,
-			   NULL_RTX, NULL_RTX, if_false_label,
-			   inv (prob));
+			   NULL_RTX, NULL_RTX, if_false_label, inv (prob));
 }
 
-  if (if_false_label)
+  if (!drop_through_if_false)
 emit_jump (if_false_label);
   if (drop_through_label)
 emit_label (drop_through_label);

Re: [patch] Do not generate useless branches for multi-word comparison

[Richard Henderson]

On 03/24/2011 07:38 AM, Eric Botcazou wrote:
 +  /* Deal with the special case 0  x: only one comparison is necessary and
 + we reverse it to avoid jumping to the drop-through label.  */
 +  if (op0 == const0_rtx  drop_through_if_true  !drop_through_if_false)
 +{
 +  code = LE;
 +  if_true_label = if_false_label;
 +  if_false_label = drop_through_label;
 +  drop_through_if_false = true;
 +}

Missing drop_through_if_true = false.

Otherwise ok.


r~