Enable underflow check in canonicalize_comparison. (PR86995)

gcc/
2018-08-30  Vlad Lazar  <vlad.lazar@arm.com>

	PR middle-end/86995
	* expmed.c (canonicalize_comparison): Use wi::sub instead of wi::add
	if to_add is negative.

From-SVN: r263973

gcc/ChangeLog

+2018-08-30  Vlad Lazar  <vlad.lazar@arm.com>
+
+	PR middle-end/86995
+	* expmed.c (canonicalize_comparison): Use wi::sub instead of wi::add
+	if to_add is negative.
+
 2018-08-29  Bernd Edlinger  <bernd.edlinger@hotmail.de>
 
 	PR middle-end/87053

gcc/expmed.c

@@ -6239,7 +6239,13 @@ canonicalize_comparison (machine_mode mode, enum rtx_code *code, rtx *imm)
      wrapping around in the case of unsigned values.  If any occur
      cancel the optimization.  */
   wi::overflow_type overflow = wi::OVF_NONE;
-  wide_int imm_modif = wi::add (imm_val, to_add, sgn, &overflow);
+  wide_int imm_modif;
+
+  if (to_add == 1)
+    imm_modif = wi::add (imm_val, 1, sgn, &overflow);
+  else
+    imm_modif = wi::sub (imm_val, 1, sgn, &overflow);
+
   if (overflow)
     return;