[backport gcc-4.8/trunk r194837 <mikpe@it.uu.se>]

gcc/

2013-01-03  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/55838
	* loop-iv.c (iv_number_of_iterations): Call lowpart_subreg on
	iv0.step, iv1.step and step.

gcc/testsuite/

2013-01-03  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/55838
	* gcc.dg/pr55838.c: New test.

--- gcc-4.6.3/gcc/loop-iv.c.~1~	2010-11-30 12:41:24.000000000 +0100
+++ gcc-4.6.3/gcc/loop-iv.c	2013-01-05 15:14:46.000000000 +0100
@@ -2361,6 +2361,9 @@ iv_number_of_iterations (struct loop *lo
       iv1.step = const0_rtx;
     }
 
+  iv0.step = lowpart_subreg (mode, iv0.step, comp_mode);
+  iv1.step = lowpart_subreg (mode, iv1.step, comp_mode);
+
   /* This is either infinite loop or the one that ends immediately, depending
      on initial values.  Unswitching should remove this kind of conditions.  */
   if (iv0.step == const0_rtx && iv1.step == const0_rtx)
@@ -2471,6 +2474,7 @@ iv_number_of_iterations (struct loop *lo
 	step = simplify_gen_unary (NEG, comp_mode, iv1.step, comp_mode);
       else
 	step = iv0.step;
+      step = lowpart_subreg (mode, step, comp_mode);
       delta = simplify_gen_binary (MINUS, comp_mode, iv1.base, iv0.base);
       delta = lowpart_subreg (mode, delta, comp_mode);
       delta = simplify_gen_binary (UMOD, mode, delta, step);
--- gcc-4.6.3/gcc/testsuite/gcc.dg/pr55838.c.~1~	1970-01-01 01:00:00.000000000 +0100
+++ gcc-4.6.3/gcc/testsuite/gcc.dg/pr55838.c	2013-01-05 15:14:46.000000000 +0100
@@ -0,0 +1,13 @@
+/* PR rtl-optimization/55838 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -funroll-loops" } */
+
+int a;
+unsigned char c;
+
+void
+f (void)
+{
+  while (c++ < 2)
+    c = a += 129;
+}