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Re: [C++ patch] Set attributes for C++ runtime library calls


On Fri, Aug 23, 2013 at 7:32 PM, Mike Stump <mikestump@comcast.net> wrote:
> On Aug 22, 2013, at 7:16 PM, Gabriel Dos Reis <gdr@integrable-solutions.net> wrote:

>> But even so, in light of this, I don't think you original assertion is definitive.
>
> Nothing is ever definitive.  Now, if you want to say I quoted something wrong, or that I am reading the standard wrong, or that it doesn't apply as I think it does, feel free to point this out.  I do view this as as a two way street, despite the certainty I might profess.

Ha!
If you quoted the standard to back up your assertions, I would have been
able to "feel free to point this out" :-)

The thing is I am still trying to figure out what (1) what you would have liked;
(2) what you believe the standards mandate, with appropriate quote; and
(3) what you consider QoI.

(3) would be a matter of GCC design choice discussion: for example,
what we would like to guarantee under certain flags, etc.

You need to separate those three things so we can make progress.

>>
>> Really?
>
>>> statement, or, is it merely changing the value?
>>
>> That is an assignment to an existing int storage.
>
>>> And what if we do a memcpy (ip, &j, sizeof (int));
>>
>> Well, the case of 'memcpy' isn't defined by the standard
>
> Odd, in the c++98 standard memcpy was defined by the standard, as well as the open code that memcpy would be.  I find memcpy in [diff.library].  I can quote all the requirements that makes the open code work as well.  It is defined.

When I say "the case of 'memcpy' isn't defined", I was not saying that
the function "memcpy" itself isn't defined by the standard.  I was discussing
its effect in the scope of this discussion, e.g. lifetime vs. assignment.
That isn't defined in the standard.

>
> You can see evidence that we meant for it to work here:
>
> 2 For any complete POD object type T, whether or not the object holds  a
>   valid value of type T, the underlying bytes (_intro.memory_) making up
>   the object can be copied into an array of char or unsigned char.36) If
>   the content of the array of char or unsigned char is copied back  into
>   the  object,  the  object  shall subsequently hold its original value.
>   [Example:
>   #define N sizeof(T)
>   char buf[N];
>   T obj;                          // obj initialized to its original value
>   memcpy(buf, &obj, N);           // between these two calls to memcpy,
>                                   // obj might be modified
>   memcpy(&obj, buf, N);           // at this point, each subobject of obj of scalar type
>                                   // holds its original value
>
> thought, I view this as completely redundant with the standard and should just be a note.  From the base standard, in C99 is it defined in 7.24.4.2.3 and I suspect remains largely unchanged from previous standards.
>

This does not say whether the effect of memcpy is assigment or copy
construction.
Which was the point I was making.

>>> Is that reused, or merely changing the value.
>>
>> The current proposal is that it constructs an int value, therefore
>> is moral equivalent of copy-constructor call.
>
> I'd be interested in the final resolution.  You have a DR number for the issue or a paper where they talk about the issues?

This came out of a discussion of a larger issue on the SG12 mailing list.
I do not have the paper number yet since it is to be part of the Chicago
mailing list.

>>> I think the most logical line of reasoning is that when the requirements of [basic.lval] are met, the, this is a change of value of an object, not a modification to it's lifetime.
>>
>> Why?
>
> Because if you end the lifetime of the original int, you destroy the semantic that everyone knows for C.  This cannot be done.

But:
  (1) we are not talking about C
  (2) C does not have a notion of lifetime -- at least not in the sense of C++.

So, whatever notion of semantics you think everyone knows of C
is largely irrelevant.

>>> So, in the case quoted, since the type of the accesses are both int, we don't reuse the storage, since the requirements of [basic.lval] are met.
>>
>> Consider:
>>
>>    struct S {
>>        S(int);
>>        ~S();
>>        // …
>>    };
>>
>>    int main() {
>>        S s(8);
>>
>>         new (&s) S(9); // #1
>>    }
>>
>> Is #1 a reuse of storage to create a new object or assignment?
>
> Behaves as assignment, s exists post the new, til the closing }.

What does not mean concretely?   Note that the "// …" could
have declared S::operator= to behave differently or be entirely deleted.

>>> Indeed, the programmer expects that they can access i after *ip = j; and that the _value_ that object, while changed from the original 1, will be 2 just after the *ip = j; statement.
>>>
>>> Since we know that i must be 3 at the end, we then know what the wording, reused, must mean, cause other meanings that could possibly make it work for you in the case you are considering, would destroy this property of pointers, and everyone knows the semantics of pointers, they are undisputed.  Or put another way, you cannot misread reused in this way.
>>
>> And why do you assert that I misread 'reused' in this way?
>
> See your email that I originally replied to.
>
> Let me quote it here:
>
>> If the user-supplied operator new returns &a, then it must
>> also ensure that 'a' is not used anywhere else -- e.g. I you can't
>> do lvalue-to-value conversion on 'a' to see what is written there.
>> Because its storage has been reused.
>
> You said it was reused, this is wrong.

You keep saying it is wrong without quoting the standards that is the
basis for that.

> You use that as backing to say that the lifetime of the original object ends, this is wrong.

You *assert* it is wrong.

-- Gaby


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