Fix optimization bug.

GCC-PR rtl-optimization/34628
	* combine.c (try_combine): Stop and undo after the first combination
	if an autoincrement side-effect on the first insn has effectively
	been lost.

The issue was detected in OpenBSD but their fix was not very good. Huge
thanks to the upstream author, Eric Botcazou, for permitting the use of
this patch under GPLv2.

MFC after:	5 days

1 files changed, 11 insertions, 6 deletions

diff --git a/contrib/gcc/combine.c b/contrib/gcc/combine.c
index f44b246..2053381 100644
--- a/contrib/gcc/combine.c
+++ b/contrib/gcc/combine.c
@@ -2370,12 +2370,17 @@ try_combine (rtx i3, rtx i2, rtx i1, int *new_direct_jump_p)
 
   if (i1 && GET_CODE (newpat) != CLOBBER)
     {
-      /* Before we can do this substitution, we must redo the test done
-	 above (see detailed comments there) that ensures  that I1DEST
-	 isn't mentioned in any SETs in NEWPAT that are field assignments.  */
-
-      if (! combinable_i3pat (NULL_RTX, &newpat, i1dest, NULL_RTX,
-			      0, (rtx*) 0))
+      /* Check that an autoincrement side-effect on I1 has not been lost.
+	 This happens if I1DEST is mentioned in I2 and dies there, and
+	 has disappeared from the new pattern.  */
+      if ((FIND_REG_INC_NOTE (i1, NULL_RTX) != 0
+	   && !i1_feeds_i3
+	   && dead_or_set_p (i2, i1dest)
+	   && !reg_overlap_mentioned_p (i1dest, newpat))
+	  /* Before we can do this substitution, we must redo the test done
+	     above (see detailed comments there) that ensures  that I1DEST
+	     isn't mentioned in any SETs in NEWPAT that are field assignments.  */
+          || !combinable_i3pat (NULL_RTX, &newpat, i1dest, NULL_RTX, 0, 0))
 	{
 	  undo_all ();
 	  return 0;