On Fri, Jun 6, 2014 at 12:52 PM, Eric Botcazou <ebotca...@adacore.com> wrote:
>> So when computing a range for z in
>>
>> z = y - x;
>>
>> with x = [-INF, y - 1] and y = [x + 1, +INF] (deduced from !(x >= y)) we
>> fail to do sth sensible with [y, y] - [-INF, y - 1] or [x + 1, +INF] - [x,
>> x] but we do sth with [x + 1, +INF] - [-INF, x]? That seems odd to me.
>>
>> With the patch we compute z to [1, +INF(OVF)]
>
> Right, and note the overflow.
>
>> Going the [x + 1, +INF] - [x,x] path first we obtain
>>
>> [1, -x + INF]
>>
>> which fails the sanity checking
>>
>> cmp = compare_values (min, max);
>> if (cmp == -2 || cmp == 1)
>> {
>> /* If the new range has its limits swapped around (MIN > MAX),
>> then the operation caused one of them to wrap around, mark
>> the new range VARYING. */
>> set_value_range_to_varying (vr);
>> }
>> else
>> set_value_range (vr, type, min, max, NULL);
>>
>> but clearly the same reasoning you can apply that makes trying
>> with [-INF, x] valid (it's just enlarging the range) can be applied
>> here, too, when computing +INF - x for the upper bound. We can
>> safely increase that to +INF making the range valid for the above
>> test.
>
> I don't think we can enlarge to +INF because -x + INF can overflow, we can
> only enlarge to +INF(OVF).
>
>> But I wonder what code path in the routine still relies on that sanity
>> checking to produce a valid result (so I'd rather try removing it, or
>> taking uncomparable bounds as a valid range).
>>
>> Simplest would be to simply do
>>
>> set_value_range (vr, type, min, max, NULL);
>> return;
>>
>> and be done with that in the plus/minus handling. With that the
>> testcase optimizes ok for me.
>
> With [1, -x + INF] as the resulting range? But it can be bogus if x is itself
> equal to +INF (unlike the input range [x + 1, +INF] which is always correct)
Hmm, indeed.
> so this doesn't look valid to me. I don't see how we can get away without a
> +INF(OVF) here, but I can compute it in extract_range_from_binary_expr_1 if
> you prefer and try only [op0,op0] and [op1,op1].
Yeah, I'd prefer that.
Thanks,
Richard.
> --
> Eric Botcazou
