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[patch] Do not generate useless branches for multi-word comparison
- From: Eric Botcazou <ebotcazou at adacore dot com>
- To: gcc-patches at gcc dot gnu dot org
- Date: Thu, 24 Mar 2011 15:38:30 +0100
- Subject: [patch] Do not generate useless branches for multi-word comparison
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 <ebotcazou@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);