combine: Fix 79910

If the dest of an I0 or I1 is used in an insn before I2, as can happen
in various uncommon cases, and we manage to do the combination, the set
is moved to I2, which is wrong.  Don't allow combining the insns in this
case.


	PR rtl-optimization/79910
	* combine.c (can_combine_p): Do not allow combining an I0 or I1
	if its dest is used by an insn before I2 (other than the combined
	insns themselves, which are properly handled already).

From-SVN: r246297

gcc/ChangeLog

 2017-03-20  Segher Boessenkool  <segher@kernel.crashing.org>
 
+	PR rtl-optimization/79910
+	* combine.c (can_combine_p): Do not allow combining an I0 or I1
+	if its dest is used by an insn before I2 (other than the combined
+	insns themselves, which are properly handled already).
+
+2017-03-20  Segher Boessenkool  <segher@kernel.crashing.org>
+
 	Revert:
 	2017-03-17  Bernd Schmidt  <bschmidt@redhat.com>
 

gcc/combine.c

@@ -1953,6 +1953,10 @@ can_combine_p (rtx_insn *insn, rtx_insn *i3, rtx_insn *pred ATTRIBUTE_UNUSED,
       || (succ2 && FIND_REG_INC_NOTE (succ2, dest))
       /* Don't substitute into a non-local goto, this confuses CFG.  */
       || (JUMP_P (i3) && find_reg_note (i3, REG_NON_LOCAL_GOTO, NULL_RTX))
+      /* Make sure that DEST is not used after INSN but before SUCC, or
+	 between SUCC and SUCC2.  */
+      || (succ && reg_used_between_p (dest, insn, succ))
+      || (succ2 && reg_used_between_p (dest, succ, succ2))
       /* Make sure that DEST is not used after SUCC but before I3.  */
       || (!all_adjacent
 	  && ((succ2