Re: [PATCH] Improve VRP range intersection for partly symbolic ranges

["Bin.Cheng" <amker dot cheng at gmail dot com>]

On Thu, Apr 27, 2017 at 2:49 PM, Richard Biener <rguenther@suse.de> wrote:
>
> The following makes intersecting [-INF, +10] and [a + -1, +INF]
> to [10, a + -1] possible with the chance that for a <= 10 the
> resulting range will be empty (but not trivially visible as so).
Hi,
I noticed operand_less_p is quite simple, so does
fold_binary_to_constant take range information into consideration?  In
this case, it is a's range information to be considered.  Otherwise it
can't tell between "a + -1" and 10.  It is good to have [10, a+-1] in
this case if we can do it (by using compare_values or similar
interface), but I remember there are quite lots of fallouts in
handling symbolic ranges, which could result in worse range
information overall in the end.  It is PR71437 when I found out this
in VRP.  I had some patches improving symbolic range handling, but
gave up last time because keep running into new cases.

Thanks,
bin
>
> Bootstrap / regtest running on x86_64-unknown-linux-gnu.
>
> I'll add a testcase later.
>
> Richard.
>
> 2017-04-27  Richard Biener  <rguenther@suse.de>
>
>         * tree-vrp.c (intersect_ranges): Better handle partly
>         symbolic ranges.
>
> Index: gcc/tree-vrp.c
> ===================================================================
> --- gcc/tree-vrp.c      (revision 247334)
> +++ gcc/tree-vrp.c      (working copy)
> @@ -8989,6 +8989,28 @@ intersect_ranges (enum value_range_type
>        else
>         gcc_unreachable ();
>      }
> +  else if (operand_less_p (*vr0min, vr1max) == 1
> +          && operand_less_p (*vr0min, vr1min) == 1
> +          && operand_less_p (*vr0max, vr1max) == 1
> +          && operand_less_p (vr1min, *vr0max) == -2)
> +    {
> +      /* [ (] )  with ] and ( being unordered as (partly) symbolic.
> +         This can result in ranges that are effectively empty.  */
> +      if (*vr0type == VR_RANGE
> +         && vr1type == VR_RANGE)
> +       *vr0min = vr1min;
> +    }
> +  else if (operand_less_p (vr1min, *vr0max) == 1
> +          && operand_less_p (vr1min, *vr0min) == 1
> +          && operand_less_p (vr1max, *vr0max) == 1
> +          && operand_less_p (*vr0min, vr1max) == -2)
> +    {
> +      /* ( [) ]  with ] and ( being unordered as (partly) symbolic.
> +         This can result in ranges that are effectively empty.  */
> +      if (*vr0type == VR_RANGE
> +         && vr1type == VR_RANGE)
> +       *vr0max = vr1max;
> +    }
>
>    /* As a fallback simply use { *VRTYPE, *VR0MIN, *VR0MAX } as
>       result for the intersection.  That's always a conservative