re PR c++/83996 (ICE with zero-sized array)

	PR c++/83996
	* constexpr.c (cxx_fold_indirect_ref): Compute ((foo *)&fooarray)[1]
	=> fooarray[1] in offset_int.

	* g++.dg/ext/pr83996.C: New test.

From-SVN: r257160

gcc/cp/ChangeLog

+2018-01-29  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/83996
+	* constexpr.c (cxx_fold_indirect_ref): Compute ((foo *)&fooarray)[1]
+	=> fooarray[1] in offset_int.
+
 2018-01-29  Jason Merrill  <jason@redhat.com>
 
 	PR c++/83942 - wrong unused warning with static_cast.

gcc/cp/constexpr.c

@@ -3143,11 +3143,17 @@ cxx_fold_indirect_ref (location_t loc, tree type, tree op0, bool *empty_base)
 	      tree min_val = size_zero_node;
 	      if (type_domain && TYPE_MIN_VALUE (type_domain))
 		min_val = TYPE_MIN_VALUE (type_domain);
-	      op01 = size_binop_loc (loc, EXACT_DIV_EXPR, op01,
-				     TYPE_SIZE_UNIT (type));
-	      op01 = size_binop_loc (loc, PLUS_EXPR, op01, min_val);
-	      return build4_loc (loc, ARRAY_REF, type, op00, op01,
-				 NULL_TREE, NULL_TREE);
+	      offset_int off = wi::to_offset (op01);
+	      offset_int el_sz = wi::to_offset (TYPE_SIZE_UNIT (type));
+	      offset_int remainder;
+	      off = wi::divmod_trunc (off, el_sz, SIGNED, &remainder);
+	      if (remainder == 0 && TREE_CODE (min_val) == INTEGER_CST)
+		{
+		  off = off + wi::to_offset (min_val);
+		  op01 = wide_int_to_tree (sizetype, off);
+		  return build4_loc (loc, ARRAY_REF, type, op00, op01,
+				     NULL_TREE, NULL_TREE);
+		}
 	    }
 	  /* Also handle conversion to an empty base class, which
 	     is represented with a NOP_EXPR.  */

gcc/testsuite/ChangeLog

@@ -3,6 +3,9 @@
 	PR c/83966
 	* gcc.dg/format/Wsuggest-attribute-1.c: New test.
 
+	PR c++/83996
+	* g++.dg/ext/pr83996.C: New test.
+
 2018-01-29  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/84057

gcc/testsuite/g++.dg/ext/pr83996.C

+// PR c++/83996
+// { dg-do compile }
+// { dg-options "" }
+
+int z[] = { };
+
+int
+main (void)
+{
+  __builtin_printf ("%d\n", *(z + 1));
+}