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[vtab and trunk?] ifcvt: try 'then' only if 'else' is unsuitable


If we try to handle an if/then/else construct whose else is unsuitable
for ifcvt (in my case, because it contained debug notes, before I
arranged for them to be ignored), but whose then is, retrying the
conversion discarding the else may be useful.

The patch below actually detects early the case of a bad else so as to
avoid the first miss.

Bootstrapped on x86_64-linux-gnu.  Is this appropriate for trunk at
this point, or should I keep it in vtab only, if at all?

Index: gcc/ChangeLog
from  Alexandre Oliva  <aoliva@redhat.com>

	* ifcvt.c (noce_process_if_block): Try to handle only the then
	block if the else block exists but isn't suitable.

Index: gcc/ifcvt.c
===================================================================
--- gcc/ifcvt.c.orig	2007-09-08 20:07:17.000000000 -0300
+++ gcc/ifcvt.c	2007-09-08 20:07:22.000000000 -0300
@@ -2204,6 +2204,7 @@ noce_process_if_block (struct noce_if_in
 	  || !NONJUMP_INSN_P (insn_b)
 	  || (set_b = single_set (insn_b)) == NULL_RTX
 	  || ! rtx_equal_p (x, SET_DEST (set_b))
+	  || ! noce_operand_ok (SET_SRC (set_b))
 	  || reg_overlap_mentioned_p (x, SET_SRC (set_b))
 	  || modified_between_p (SET_SRC (set_b),
 				 PREV_INSN (if_info->cond_earliest), jump)
@@ -2249,6 +2250,7 @@ noce_process_if_block (struct noce_if_in
   if (! noce_operand_ok (a) || ! noce_operand_ok (b))
     return FALSE;
 
+ retry:
   /* Set up the info block for our subroutines.  */
   if_info->insn_a = insn_a;
   if_info->insn_b = insn_b;
@@ -2332,6 +2334,13 @@ noce_process_if_block (struct noce_if_in
 	goto success;
     }
 
+  if (!else_bb && set_b)
+    {
+      insn_b = set_b = NULL_RTX;
+      b = orig_x;
+      goto retry;
+    }
+
   return FALSE;
 
  success:
-- 
Alexandre Oliva         http://www.lsd.ic.unicamp.br/~oliva/
FSF Latin America Board Member         http://www.fsfla.org/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}

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