On Sat, Feb 4, 2017 at 3:52 PM, Jeff Law <[email protected]> wrote:
> This is the first of a 4 part series to address the issues around 79095.
>
> This patch addresses improvements in determining ranges of binary
> expressions in three ways.
>
> First if we are otherwise unable to find a range for the result of a
> MINUS_EXPR, if we know the arguments are not equal, then we know the
> resultant range is ~[0,0].
>
> Second, for EXACT_DIV_EXPR, if the numerator has the range ~[0,0], then
> resultant range is currently [TYPE_MIN/DENOM,TYPE_MAX/DENOM]. That is
> rarely a useful range. A resultant range of ~[0,0] is actually more useful
> since it often tells us something important about the difference of two
> pointers.
>
> Finally, when vrp2 discovers an updated range for an object that had a range
> discovered by vrp1, if the new range is ~[0,0], prefer that new range in
> some cases. This is needed to avoid losing the newly discovered ~[0,0]
> range for EXACT_DIV_EXPR.
>
> Bootstrapped and regression tested with the other patches in this series.
> OK for the trunk?
>
> Jeff
>
> * tree-vrp.c (extract_range_from_binary_expr): For EXACT_DIV_EXPR,
> if the numerator has the range ~[0,0] make the resultant range
> ~[0,0]. For MINUS_EXPR with no derived range, if the operands are
> known to be not equal, then the resulting range is ~[0,0].
> (intersect_ranges): In some cases prefer ~[0,0].
>
> commit b7baf46ab62e28d2dbc22e9dcd4404926d59df18
> Author: Jeff Law <[email protected]>
> Date: Fri Feb 3 15:45:58 2017 -0500
>
> Improved ranges
>
> diff --git a/gcc/tree-vrp.c b/gcc/tree-vrp.c
> index b429217..3338d8b 100644
> --- a/gcc/tree-vrp.c
> +++ b/gcc/tree-vrp.c
> @@ -3298,6 +3298,37 @@ extract_range_from_binary_expr (value_range *vr,
>
> extract_range_from_binary_expr_1 (vr, code, expr_type, &n_vr0, &vr1);
> }
> +
> + /* EXACT_DIV_EXPR is typically used for pointer subtraction;
> + as a result a ~[0,0] may be better than what has already
> + been computed.
> +
> + In particular if numerator has the range ~[0,0], then the
> + result range is going to be something like
> + [MININT/DIVISOR,MAXINT/DIVISOR], which is rarely useful.
> +
> + So instead make the result range ~[0,0]. */
> + if (code == EXACT_DIV_EXPR
> + && TREE_CODE (op0) == SSA_NAME
> + && vr0.type == VR_ANTI_RANGE
> + && vr0.min == vr0.max
> + && integer_zerop (vr0.min))
> + set_value_range_to_nonnull (vr, TREE_TYPE (op0));
The above belongs in extract_range_from_binary_expr_1, in principle the
cases below as well (though there's pre-existing VARYING result handling).
The _1 ones are supposed to be the actual range computations while
the routine you patched is responsible for interfacing with a lattice. The
_1 routines can be used from code outside of VRP.
> + /* If we didn't derive a range for MINUS_EXPR, and
> + op1's range is ~[op0,op0] or vice-versa, then we
> + can derive a non-null range. This happens often for
> + pointer subtraction. */
> + if (vr->type == VR_VARYING
> + && code == MINUS_EXPR
> + && TREE_CODE (op0) == SSA_NAME
> + && ((vr0.type == VR_ANTI_RANGE
> + && symbolic_range_based_on_p (&vr0, op1)
> + && vr0.min == vr0.max)
> + || (vr1.type == VR_ANTI_RANGE
> + && symbolic_range_based_on_p (&vr1, op0)
> + && vr1.min == vr1.max)))
> + set_value_range_to_nonnull (vr, TREE_TYPE (op0));
> }
>
> /* Extract range information from a unary operation CODE based on
> @@ -8620,6 +8651,12 @@ intersect_ranges (enum value_range_type *vr0type,
> else if (vrp_val_is_min (vr1min)
> && vrp_val_is_max (vr1max))
> ;
> + /* Choose the anti-range if it is ~[0,0], that range is special
> + enough to special case. */
> + else if (*vr0type == VR_ANTI_RANGE
> + && *vr0min == *vr0max
> + && integer_zerop (*vr0min))
> + ;
Huh. If I spotted the place of the change correctly then we cannot arrive
here with vr0 == ~[0,0] as *vr0type is VR_RANGE. In the case covered
we'd have the only case intersecting [-1, 1] and ~[0,0] that you'd change
to ~[0,0] instead of [-1,1] which generally would be a bad choice (apart
from your implementation error as vr1 is the anti-range here).
Richard.
> /* Else choose the range. */
> else
> {
>
