[backport from gcc-4.7.0 r183512, adjusted for 4.6 using
 rtx not 'struct insn_link *' for LOG_LINKS <mikpe@it.uu.se>]

gcc/

2012-01-25  Ramana Radhakrishnan  <ramana.radhakrishnan@linaro.org>

	PR rtl-optimization/48308
	(do_SUBST_LINK): New.
	(SUBST_LINK): New.
	(try_combine): Handle LOG_LINKS for the dummy i1 case.

--- gcc-4.6.3/gcc/combine.c.~1~	2012-02-09 18:29:38.000000000 +0100
+++ gcc-4.6.3/gcc/combine.c	2012-06-12 18:00:22.000000000 +0200
@@ -762,6 +762,33 @@ do_SUBST_MODE (rtx *into, enum machine_m
 }
 
 #define SUBST_MODE(INTO, NEWVAL)  do_SUBST_MODE(&(INTO), (NEWVAL))
+
+/* Similar to SUBST, but NEWVAL is a LOG_LINKS expression.  */
+
+static void
+do_SUBST_LINK (rtx *into, rtx newval)
+{
+  struct undo *buf;
+  rtx oldval = *into;
+
+  if (oldval == newval)
+    return;
+
+  if (undobuf.frees)
+    buf = undobuf.frees, undobuf.frees = buf->next;
+  else
+    buf = XNEW (struct undo);
+
+  buf->kind = UNDO_RTX;
+  buf->where.r = into;
+  buf->old_contents.r = oldval;
+  *into = newval;
+
+  buf->next = undobuf.undos, undobuf.undos = buf;
+}
+
+#define SUBST_LINK(oldval, newval) do_SUBST_LINK (&oldval, newval)
+
 
 /* Subroutine of try_combine.  Determine whether the combine replacement
    patterns NEWPAT, NEWI2PAT and NEWOTHERPAT are cheaper according to
@@ -2867,6 +2894,7 @@ try_combine (rtx i3, rtx i2, rtx i1, rtx
 	  SUBST (PATTERN (i2), XVECEXP (PATTERN (i2), 0, 0));
 	  SUBST (XEXP (SET_SRC (PATTERN (i2)), 0),
 		 SET_DEST (PATTERN (i1)));
+	  SUBST_LINK (LOG_LINKS (i2), alloc_INSN_LIST (i1, LOG_LINKS (i2)));
 	}
     }
 #endif