On Mon, Nov 24, 2014 at 02:51:14PM +0100, Marek Polacek wrote:
> --- gcc/cp/constexpr.c
> +++ gcc/cp/constexpr.c
> @@ -1451,6 +1451,43 @@ verify_constant (tree t, bool allow_non_constant, bool
> *non_constant_p,
> return *non_constant_p;
> }
>
> +/* Return true if the shift operation on LHS and RHS is undefined. */
> +
> +static bool
> +cxx_eval_check_shift_p (enum tree_code code, tree lhs, tree rhs)
> +{
> + if (code != LSHIFT_EXPR && code != RSHIFT_EXPR)
> + return false;
> +
> + tree lhstype = TREE_TYPE (lhs);
> + unsigned HOST_WIDE_INT uprec = TYPE_PRECISION (TREE_TYPE (lhs));
> +
> + /* [expr.shift] The behavior is undefined if the right operand
> + is negative, or greater than or equal to the length in bits
> + of the promoted left operand. */
> + if (tree_int_cst_sgn (rhs) == -1 || compare_tree_int (rhs, uprec) >= 0)
> + return true;
I think VERIFY_CONSTANT doesn't guarantee both operands are INTEGER_CSTs.
Consider say:
constexpr int p = 1;
constexpr int foo (int a)
{
return a << (int) &p;
}
constexpr int bar (int a)
{
return ((int) &p) << a;
}
constexpr int q = foo (5);
constexpr int r = bar (2);
constexpr int s = bar (0);
Now, for foo (5) and bar (2) fold_binary_loc returns NULL and thus
your cxx_eval_check_shift_p is not called, for bar (0) it returns
non-NULL and while the result still is not a constant expression and
right now is diagnosed, with your patch it would ICE.
So, I'd just return false if either lhs or rhs are not INTEGER_CSTs.
> +
> + /* The value of E1 << E2 is E1 left-shifted E2 bit positions; [...]
> + if E1 has a signed type and non-negative value, and E1x2^E2 is
> + representable in the corresponding unsigned type of the result type,
> + then that value, converted to the result type, is the resulting value;
> + otherwise, the behavior is undefined. */
> + if (code == LSHIFT_EXPR && !TYPE_UNSIGNED (lhstype))
> + {
> + if (tree_int_cst_sgn (lhs) == -1)
> + return true;
> + tree t = build_int_cst (unsigned_type_node, uprec - 1);
> + t = fold_build2 (MINUS_EXPR, unsigned_type_node, t, rhs);
> + tree ulhs = fold_convert (unsigned_type_for (lhstype), lhs);
> + t = fold_build2 (RSHIFT_EXPR, TREE_TYPE (ulhs), ulhs, t);
> + if (tree_int_cst_lt (integer_one_node, t))
> + return true;
I'll leave to Jason whether this shouldn't be using the various
cxx_eval_*_expression calls instead, or perhaps int_const_binop or wide_int
stuff directly.
Jakub
