Re: C provenance semantics proposal

[Peter Sewell <Peter.Sewell@cl.cam.ac.uk>]

On 24/04/2019, Jeff Law <law@redhat.com> wrote:
> 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?

As far as I can see right now, there isn't a serious alternative.
Suppose x and y are adjacent, p=&x+1, and q=&y, so p==q might
be true (either in a semantics for the source-language == that just
compares the concrete representations or in one that's allowed
but not required to be provenance-sensitive).   It's not possible
to simultaneously have *p UB (which AIUI the compiler has to
have in the intermediate language, to make alias analysis sound),
*q not UB, and p interchangeable with q.    Am I missing something?

Peter


>
>>>>
>>>> 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
>
>