[backport from gcc-4.8/trunk r189013, needs r185226 backport to not ICE
 when compiling elfutils on arm <mikpe.it.uu.se>]

gcc/

2012-06-27  Richard Guenther  <rguenther@suse.de>

	PR middle-end/53676
	* tree-chrec.c (chrec_convert_1): Represent truncation to
	a type with undefined overflow as truncation to an unsigned
	type converted to the type with undefined overflow.
	* tree-scalar-evolution.c (interpret_rhs_expr): For computing
	the scalar evolution of a truncated widened operation avoid
	looking at the non-existing evolution of the widened operation
	result.

gcc/testsuite/

2012-06-27  Richard Guenther  <rguenther@suse.de>

	PR middle-end/53676
	* gcc.dg/tree-ssa/scev-6.c: New testcase.

--- gcc-4.7.1/gcc/testsuite/gcc.dg/tree-ssa/scev-6.c.~1~	1970-01-01 01:00:00.000000000 +0100
+++ gcc-4.7.1/gcc/testsuite/gcc.dg/tree-ssa/scev-6.c	2012-07-15 12:39:22.000000000 +0200
@@ -0,0 +1,23 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+int main()
+{
+  int i;
+  signed char result = 0;
+  for (i = 0; i != 8000; ++i)
+    {
+      int tem = result;
+      tem = tem + 2;
+      result = tem;
+    }
+  if (__builtin_abs ((int)(signed char)((unsigned char ) result + 128)) != 0)
+    __builtin_abort ();
+  return 0;
+}
+
+/* SCEV constant propagation should be able to compute the overall effect
+   of the loop.  */
+
+/* { dg-final { scan-tree-dump-not "abort" "optimized" } } */
+/* { dg-final { cleanup-tree-dump "optimized" } } */
--- gcc-4.7.1/gcc/tree-chrec.c.~1~	2011-08-17 14:00:35.000000000 +0200
+++ gcc-4.7.1/gcc/tree-chrec.c	2012-07-15 12:39:22.000000000 +0200
@@ -1359,6 +1359,23 @@ keep_cast:
     res = fold_build2 (TREE_CODE (chrec), type,
 		       fold_convert (type, TREE_OPERAND (chrec, 0)),
 		       fold_convert (type, TREE_OPERAND (chrec, 1)));
+  /* Similar perform the trick that (signed char)((int)x + 2) can be
+     narrowed to (signed char)((unsigned char)x + 2).  */
+  else if (use_overflow_semantics
+	   && TREE_CODE (chrec) == POLYNOMIAL_CHREC
+	   && TREE_CODE (ct) == INTEGER_TYPE
+	   && TREE_CODE (type) == INTEGER_TYPE
+	   && TYPE_OVERFLOW_UNDEFINED (type)
+	   && TYPE_PRECISION (type) < TYPE_PRECISION (ct))
+    {
+      tree utype = unsigned_type_for (type);
+      res = build_polynomial_chrec (CHREC_VARIABLE (chrec),
+				    fold_convert (utype,
+						  CHREC_LEFT (chrec)),
+				    fold_convert (utype,
+						  CHREC_RIGHT (chrec)));
+      res = chrec_convert_1 (type, res, at_stmt, use_overflow_semantics);
+    }
   else
     res = fold_convert (type, chrec);
 
--- gcc-4.7.1/gcc/tree-scalar-evolution.c.~1~	2012-04-12 14:50:22.000000000 +0200
+++ gcc-4.7.1/gcc/tree-scalar-evolution.c	2012-07-15 12:39:22.000000000 +0200
@@ -1632,6 +1632,7 @@ interpret_rhs_expr (struct loop *loop, g
 		    tree type, tree rhs1, enum tree_code code, tree rhs2)
 {
   tree res, chrec1, chrec2;
+  gimple def;
 
   if (get_gimple_rhs_class (code) == GIMPLE_SINGLE_RHS)
     {
@@ -1714,7 +1715,29 @@ interpret_rhs_expr (struct loop *loop, g
       break;
 
     CASE_CONVERT:
-      chrec1 = analyze_scalar_evolution (loop, rhs1);
+      /* In case we have a truncation of a widened operation that in
+         the truncated type has undefined overflow behavior analyze
+	 the operation done in an unsigned type of the same precision
+	 as the final truncation.  We cannot derive a scalar evolution
+	 for the widened operation but for the truncated result.  */
+      if (TREE_CODE (type) == INTEGER_TYPE
+	  && TREE_CODE (TREE_TYPE (rhs1)) == INTEGER_TYPE
+	  && TYPE_PRECISION (type) < TYPE_PRECISION (TREE_TYPE (rhs1))
+	  && TYPE_OVERFLOW_UNDEFINED (type)
+	  && TREE_CODE (rhs1) == SSA_NAME
+	  && (def = SSA_NAME_DEF_STMT (rhs1))
+	  && is_gimple_assign (def)
+	  && TREE_CODE_CLASS (gimple_assign_rhs_code (def)) == tcc_binary
+	  && TREE_CODE (gimple_assign_rhs2 (def)) == INTEGER_CST)
+	{
+	  tree utype = unsigned_type_for (type);
+	  chrec1 = interpret_rhs_expr (loop, at_stmt, utype,
+				       gimple_assign_rhs1 (def),
+				       gimple_assign_rhs_code (def),
+				       gimple_assign_rhs2 (def));
+	}
+      else
+	chrec1 = analyze_scalar_evolution (loop, rhs1);
       res = chrec_convert (type, chrec1, at_stmt);
       break;