Re: [3/7] Optimize ZEXT_EXPR with tree-vrp

[Richard Biener <richard dot guenther at gmail dot com>]

On Thu, Oct 8, 2015 at 1:40 AM, Kugan <kugan.vivekanandarajah@linaro.org> wrote:
>
>
> On 07/10/15 19:20, Richard Biener wrote:
>> On Wed, Oct 7, 2015 at 1:12 AM, kugan <kugan.vivekanandarajah@linaro.org> wrote:
>>>
>>> Hi Richard,
>>>
>>> Thanks for the review.
>>>
>>> On 15/09/15 23:08, Richard Biener wrote:
>>>>
>>>> On Mon, Sep 7, 2015 at 4:58 AM, Kugan <kugan.vivekanandarajah@linaro.org>
>>>> wrote:
>>>>>
>>>>> This patch tree-vrp handling and optimization for ZEXT_EXPR.
>>>>
>>>>
>>>> +  else if (code == SEXT_EXPR)
>>>> +    {
>>>> +      gcc_assert (range_int_cst_p (&vr1));
>>>> +      unsigned int prec = tree_to_uhwi (vr1.min);
>>>> +      type = vr0.type;
>>>> +      wide_int tmin, tmax;
>>>> +      wide_int type_min, type_max;
>>>> +      wide_int may_be_nonzero, must_be_nonzero;
>>>> +
>>>> +      gcc_assert (!TYPE_UNSIGNED (expr_type));
>>>>
>>>> hmm, I don't think we should restrict SEXT_EXPR this way.  SEXT_EXPR
>>>> should operate on both signed and unsigned types and the result type
>>>> should be the same as the type of operand 0.
>>>>
>>>> +      type_min = wi::shwi (1 << (prec - 1),
>>>> +                          TYPE_PRECISION (TREE_TYPE (vr0.min)));
>>>> +      type_max = wi::shwi (((1 << (prec - 1)) - 1),
>>>> +                          TYPE_PRECISION (TREE_TYPE (vr0.max)));
>>>>
>>>> there is wi::min_value and max_value for this.
>>>
>>>
>>> As of now, SEXT_EXPR in gimple is of the form: x = y sext 8 and types of all
>>> the operand and results are of the wider type. Therefore we cant use the
>>> wi::min_value. Or do you want to convert this precision (in this case 8) to
>>> a type and use wi::min_value?
>>
>> I don't understand - wi::min/max_value get a precision and sign, not a type.
>> your 1 << (prec - 1) is even wrong for prec > 32 (it's an integer type
>> expression).
>> Thus
>>
>>   type_min = wi::min_value (prec, SIGNED);
>>   type_max = wi::max_value (prec, SIGNED);
>>
>
> Thanks for the comments. Is the attached patch looks better. It is based
> on the above. I am still assuming the position of sign-bit in SEXT_EXPR
> will be less than 64bit (for calculating sign_bit in wide_int format). I
> think this will always be the case but please let me know if this is not OK.

+      unsigned int prec = tree_to_uhwi (vr1.min);

this should use unsigned HOST_WIDE_INT

+      wide_int sign_bit = wi::shwi (1ULL << (prec - 1),
+                                   TYPE_PRECISION (TREE_TYPE (vr0.min)));

use wi::one (TYPE_PRECISION (TREE_TYPE (vr0.min))) << (prec - 1);

That is, you really need to handle precisions bigger than HOST_WIDE_INT.

But I suppose wide_int really misses a test_bit function (it has a set_bit
one already).

+         if (wi::bit_and (must_be_nonzero, sign_bit) == sign_bit)
+           {
+             /* If to-be-extended sign bit is one.  */
+             tmin = type_min;
+             tmax = may_be_nonzero;

I think tmax should be zero-extended may_be_nonzero from prec.

+         else if (wi::bit_and (may_be_nonzero, sign_bit)
+                  != sign_bit)
+           {
+             /* If to-be-extended sign bit is zero.  */
+             tmin = must_be_nonzero;
+             tmax = may_be_nonzero;

likewise here tmin/tmax should be zero-extended may/must_be_nonzero from prec.

+    case SEXT_EXPR:
+       {
+         unsigned int prec = tree_to_uhwi (op1);
+         wide_int sign_bit = wi::shwi (1ULL << (prec - 1),
+                                       TYPE_PRECISION (TREE_TYPE (vr0.min)));
+         wide_int mask = wi::shwi (((1ULL << (prec - 1)) - 1),
+                                   TYPE_PRECISION (TREE_TYPE (vr0.max)));

this has the same host precision issues of 1ULL (HOST_WIDE_INT).
There is wi::mask, eventually you can use wi::set_bit_in_zero to
produce the sign-bit wide_int (also above).

The rest of the patch looks ok.

Richard.


> Thanks,
> Kugan
>
>
>>
>>> Please find the patch that addresses the other comments.