[cfe-dev] Can indirect class parameters be noalias?

[John McCall via cfe-dev]

n 31 Jul 2020, at 7:35, Hal Finkel wrote:
> On 7/29/20 9:00 PM, John McCall via cfe-dev wrote:
>>
>> On 29 Jul 2020, at 17:42, Richard Smith wrote:
>>
>>     On Wed, 29 Jul 2020 at 12:52, John McCall <rjmccall at apple.com> 
>> wrote:
>>
>>     ...
>>
>>     I think concretely, the escape hatch doesn't stop things from
>>     going wrong,
>>     because -- as you note -- even though we *could* have made a 
>> copy,
>>     it's
>>     observable whether or not we *did* make a copy. For example:
>>
>> I would say that it’s observable whether the parameter variable has
>> the same address as the argument. That doesn’t /have/ to be the 
>> same
>> question as whether a copy was performed: we could consider there to 
>> be
>> a formal copy (or series of copies) that ultimately creates /an/ 
>> object
>> at the same address, but it’s not the /same/ object and so pointers
>> to the old object no longer validly pointer to it. But I guess that
>> would probably violate the lifetime rules, because it would make 
>> accesses
>> through old pointers UB when in fact they should at worst access a 
>> valid
>> object that’s just unrelated to the parameter object.
>>
>
> I think that it would be great to be able to do this, but 
> unfortunately, I think that the point that you raise here is a key 
> issue. Whether or not the copy is performed is visible in the model, 
> and so we can't simply act as though there was a copy when optimizing. 
> Someone could easily have code that looks like:
>
> Foo DefaultX;
>
> ...
>
> void something(Foo &A, Foo &B) {
>
>   if (&A == &B) { ... }
>
> }
>
> void bar(Foo X) { something(X, DefaultX); }

This example isn’t really on point; a call like `bar(DefaultX)` 
obviously cannot just pass the address of `DefaultX` as a by-value 
argument without first proving a lot of stuff about how `foo` uses both 
its parameter and `DefaultX`.  I think `noalias` is actually a subset of 
what would have to be proven there.

In general, the standard is clear that you cannot rely on escaping a 
pointer to/into a trivially-copyable pr-value argument prior to the call 
and then rely on that pointer pointing into the corresponding parameter 
object.  Implementations are *allowed* to introduce copies.  But it does 
seem like the current wording would allow you to rely on that pointer 
pointing into *some* valid object, at least until the end of the 
caller’s full-expression.  That means that, if we don’t guarantee to 
do an actual copy of the argument, we cannot make it UB to access the 
parameter variable through pointers to the argument temporary, which is 
what marking the parameter as `noalias` would do.

So I guess the remaining questions are:

- Is this something we can reasonably change in the standard?
- Are we comfortable setting `noalias` in C if the only place that would 
break is with a C++ caller?

John.

>
> As Richard's example shows, the code doesn't need to explicitly 
> compare the addresses to detect the copy either. Any code that 
> reads/writes to the objects can do it. A perhaps-more-realistic 
> example might be:
>
>   int Cnt = A.RefCnt; ++A.RefCnt; ++B.RefCnt; if (Cnt + 1 != 
> A.RefCnt) { /* same object case */ }
>
> The best suggestion that I have so far is that we could add an 
> attribute like 'can_copy' indicating that the optimizer can make a 
> formal copy of the argument in the callee and use that instead of the 
> original pointer if that seems useful. I can certainly imagine a 
> transformation such as LICM making use of such a thing (although the 
> cost modeling would probably need to be fairly conservative).
>
>  -Hal
>
>
>> ...
>>
>> John.
>>
>>
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>
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory


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