Re: RFC - VRP1 default mode

[Andrew MacLeod <amacleod@redhat.com>]

On 10/28/22 09:46, Richard Biener wrote:
> On Fri, Oct 28, 2022 at 3:43 PM Andrew MacLeod <amacleod@redhat.com> wrote:
>>
>> On 10/28/22 03:17, Richard Biener wrote:
>>> On Wed, Oct 26, 2022 at 4:24 PM Andrew MacLeod <amacleod@redhat.com> wrote:
>>>> Figured I would ask what you guys think of making ranger the default for
>>>> the VRP1 pass now.
>>>>
>>>> With partial equivalences and the other bits I checked in the past few
>>>> weeks I'm not aware of much that the legacy VRP pass gets that ranger
>>>> doesn't.  The only exception to that which I am aware of is the trick
>>>> played with the unreachable edges to set global ranges, but that is done
>>>> in the DOM passes now anyway... so it just happens slightly later in the
>>>> optimization cycle.
>>> Note DOM should go away at some point.  Why can this not happen during
>>> ranger driven VRP?
>> I have been working on that for the last 2 days.  Turns out VRP1 can
>> remove builtin_unreachable from the
>>     if (X)
>>       __builtin_unreachable ()
>>
>> idiom and set the appropriate global ranges, but it has to leave those
>> with 2 ssa-names:
>>
>>     if (a_1 != b_2)
>>       __builtin_unreachable()
>>
>> until the second pass of VRP or we lose the relationship between a_1 and
>> b_2.  That triggers some failures.  Specifically a vectorizor fail
>> because it cant be sure that the start and end point are not the same
>> without the condition in the IL. Trying to store global relations over
>> multiple passes would be problematic at this stage of development, so I
>> don't see a problem with leaving it that way.
> Hmm, I don't remember VRP1 doing anything special with the above though?
> Did it somehow propagate the (un!)conditional equivalence?

So as I looked at builtin_unreachable(), it was very adhoc.  That one of 
the roots of that artificial testcase in the PR I opened. Cascading 
calls were not being handled in a consistent way. VRP1 removed some, dom 
removed some..  they just kind of disappeared at some point, but not 
consistently.  The PR that Uli opened that Aldy fixed, I could make fail 
again with minor adjustments to the conditions.  So I worked on a 
consistent approach.

My guess is the old range stored globally for that case for a_1 was 
probably ~[b_2, b_2]  meaning it was carried in the range. Until we have 
an overall global relation tracker, we can't represent that across passes.

It appears that leaving those until VRP2 works fine...  testsuite 
currently running tho ;-)

Andrew