Re: C++ PATCH for c++/51489 (pointer subtraction in constant expression)

[Jason Merrill]

On 12/19/2011 12:56 AM, Jason Merrill wrote:
DR 1313 removes the blanket prohibition on pointer subtraction in
constant expressions and replaces it with a prohibition on operations
with undefined behavior, so this testcase ought to work.  It wasn't
working because our internal representation of pointer subtraction
involves converting the pointers to ptrdiff_t before subtracting, and
such a conversion is prohibited (previously specifically, now as a
reinterpret_cast).  There's currently no good way to distinguish between
a user-written cast and the compiler-generated one, so I've moved the
check to cxx_eval_outermost_constant_expr for now.

This fix worked for pointer arithmetic based on a static array, but not 
for a local array in a constexpr function.  The fix is a simple matter 
of not insisting that the ptrdiff_t operands be constant themselves.

Tested x86_64-pc-linux-gnu, applying to trunk and 5.


commit 9537133530fe9a6f6a9c0e61d9bbe7332a106c3c
Author: Jason Merrill <ja...@redhat.com>
Date:   Tue Mar 1 02:01:46 2016 -0500

    	PR c++/51489
    
    	* constexpr.c (cxx_eval_binary_expression): Don't VERIFY_CONSTANT
    	the operands.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index a21997a..bcb129f 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1612,15 +1612,14 @@ cxx_eval_binary_expression (const constexpr_ctx *ctx, tree t,
   tree lhs, rhs;
   lhs = cxx_eval_constant_expression (ctx, orig_lhs, /*lval*/false,
 				      non_constant_p, overflow_p);
-  /* Don't VERIFY_CONSTANT if this might be dealing with a pointer to
-     a local array in a constexpr function.  */
-  bool ptr = POINTER_TYPE_P (TREE_TYPE (lhs));
-  if (!ptr)
-    VERIFY_CONSTANT (lhs);
+  /* Don't VERIFY_CONSTANT here, it's unnecessary and will break pointer
+     subtraction.  */
+  if (*non_constant_p)
+    return t;
   rhs = cxx_eval_constant_expression (ctx, orig_rhs, /*lval*/false,
 				      non_constant_p, overflow_p);
-  if (!ptr)
-    VERIFY_CONSTANT (rhs);
+  if (*non_constant_p)
+    return t;
 
   location_t loc = EXPR_LOCATION (t);
   enum tree_code code = TREE_CODE (t);
@@ -1653,6 +1652,9 @@ cxx_eval_binary_expression (const constexpr_ctx *ctx, tree t,
     }
   else if (cxx_eval_check_shift_p (loc, ctx, code, type, lhs, rhs))
     *non_constant_p = true;
+  /* Don't VERIFY_CONSTANT if this might be dealing with a pointer to
+     a local array in a constexpr function.  */
+  bool ptr = POINTER_TYPE_P (TREE_TYPE (lhs));
   if (!ptr)
     VERIFY_CONSTANT (r);
   return r;
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-array4.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-array4.C
new file mode 100644
index 0000000..fc01047
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-array4.C
@@ -0,0 +1,12 @@
+// { dg-do compile { target c++14 } }
+
+constexpr bool g()
+{
+  int ar[4] = { 1, 2, 3, 4 };
+  auto e1 = ar;
+  auto e4 = ar+3;
+  return (e4-e1) == 3;
+}
+
+#define SA(X) static_assert((X),#X)
+SA(g());