Re: C provenance semantics proposal

[Jeff Law <law at redhat dot com>]

On 4/24/19 4:19 AM, Richard Biener wrote:
> On Thu, Apr 18, 2019 at 3:42 PM Jeff Law <law@redhat.com> wrote:
>>
>> On 4/18/19 6:20 AM, Uecker, Martin wrote:
>>> Am Donnerstag, den 18.04.2019, 11:45 +0100 schrieb Peter Sewell:
>>>> On Thu, 18 Apr 2019 at 10:32, Richard Biener <richard.guenther@gmail.com> wrote:
>>>
>>>
>>>> An equality test of two pointers, on the other hand, doesn't necessarily
>>>> mean that they are interchangeable.  I don't see any good way to
>>>> avoid that in a provenance semantics, where a one-past
>>>> pointer might sometimes compare equal to a pointer to an
>>>> adjacent object but be illegal for accessing it.
>>>
>>> As I see it, there are essentially four options:
>>>
>>> 1.) Compilers do not use conditional equivalences for
>>> optimizations of pointers (or only when additional
>>> conditions apply which make it safe)
>> I know this will hit DOM and CSE.  I wouldn't be surprised if it touches
>> VRP as well, maybe PTA.  It seems simple enough though :-)
> 
> Also touches fundamental PHI-OPT transforms like
> 
>  if (a == b)
> ...
> 
>  # c = PHI <a, b>
> 
> where we'd lose eliding such a conditional.  IMHO that's bad
> and very undesirable.
But if we only suppress this optimization for pointers is it that terrible?



>>>
>>> 3.) We make comparison have the side effect that
>>> afterwards any of the two pointers could have any
>>> of the two provenances. (with disambiguitation
>>> similar to what we have for casts).
>> This could have some interesting effects on PTA.  Richi?
> 
> I played with this and doing this in an incomplete way like
> just handling
> 
>   if (a == b)
> 
> as two-way assignment during constraint building is possible.
> But that's not enough of course since every call is implicitely
> producing equivalences between everything [escaped] ...
> which makes points-to degrade to a point where it is useless.
But the calls aren't generating conditional equivalences.  I must be
missing something here.  You're the expert in this space, so if you say
it totally degrades PTA, then it's a non-starter.

> 
> So I think we need a working scheme where points-to doesn't
> degrade from equivalencies being computed and the compiler
> being free to introduce equivalences as well as copy-propagate
> those.
> 
> Honestly I can't come up with a working solution to this
> problem.
> 
>>
>>>
>>> 4.) Compilers make sure that exposed objects never
>>> are allocated next to each other (as Jens proposed).
>> Ugh.  Not sure how you enforce that.  Consider that the compiler may
>> ultimately have no control over layout of data in static storage.
> 
> Make everything 1 byte larger.
Not a bad idea.  I suspect the embedded folks would go bananas though.

jeff