On Mon, Nov 24, 2014 at 04:49:25PM +0100, Jakub Jelinek wrote:
> 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.
Oh well. I ran the testsuite with an assert checking that I always have
INTEGER_CSTs and didn't see any ICEs.
> 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.
Ok, I'll add that. Thank for pointing that out.
> > +
> > + /* 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.
ISTR int_const_binop calls wide_int routines wi::rshift/wi::lshift and these
return 0 and do not have any overflow flag, so that might not help (?).
Marek
