[Mesa-dev] [PATCH v2] glsls: Modify exec_list to avoid strict-aliasing violations

[Erik Faye-Lund]

On Fri, Jun 26, 2015 at 4:53 PM, Francisco Jerez <currojerez at riseup.net> wrote:
> Erik Faye-Lund <kusmabite at gmail.com> writes:
>
>> On Fri, Jun 26, 2015 at 4:16 PM, Davin McCall <davmac at davmac.org> wrote:
>>> On 26/06/15 14:53, Erik Faye-Lund wrote:
>>>>
>>>> On Fri, Jun 26, 2015 at 3:05 PM, Davin McCall <davmac at davmac.org> wrote:
>>>>>
>>>>> On 26/06/15 12:55, Erik Faye-Lund wrote:
>>>>>
>>>>> On Fri, Jun 26, 2015 at 1:23 PM, Davin McCall <davmac at davmac.org> wrote:
>>>>>
>>>>> On 26/06/15 12:03, Davin McCall wrote:
>>>>>
>>>>> ... The stored value of 'n' is not accessed by any other type than the
>>>>> type of n itself. This value is then cast to a different pointer type.
>>>>> You
>>>>> are mistaken if you think that the cast accesses the stored value of n.
>>>>> The
>>>>> other "stored value" access that it occurs in that expression is to the
>>>>> object pointed at by the result of the cast. [...]:
>>>>>
>>>>> I'm sorry, I think that was phrased somewhat abrasively, which I did not
>>>>> intend. Let me try this part again. If we by break up the expression in
>>>>> order of evaluation:
>>>>>
>>>>> From:
>>>>>     return ((const struct exec_node **)n)[0]
>>>>>
>>>>> In order of evaluation:
>>>>>
>>>>> n
>>>>> - which accesses the stored value of n, i.e. a value of type 'struct exec
>>>>> node *', via n, which is obviously of that type.
>>>>>
>>>>> (const struct exec_node **)n
>>>>>   - which casts that value, after it has been retrieved, to another type.
>>>>> If
>>>>> this were an aliasing violation, then casting any pointer variable to
>>>>> another type would be an aliasing violation; this is clearly not the
>>>>> case.
>>>>>
>>>>> ((const struct exec_node **)n)[0]
>>>>> - which de-references the result of the above cast, thereby accessing a
>>>>> stored value of type 'exec node *' using a glvalue of type 'exec node *'.
>>>>>
>>>>> I think breaking this up is a mistake, because the strict-aliasing
>>>>> rules is explicitly about the *combination* of these two things.
>>>>>
>>>>>
>>>>> It is not a mistake, and the strict aliasing rules are not about the
>>>>> combination of these two things.
>>>>
>>>> It is. In fact, it's not even possible to violate strict-aliasing
>>>> without doing at least two operations. You cannot validate operations
>>>> in a vacuum, because that's not how strict-aliasing is defined.
>>>
>>>
>>> Any pointer dereference can violate strict aliasing - that's one operation.
>>> If you mean that it's first necessary to construct a pointer value in such a
>>> way that de-referencing it will be an aliasing violation, then yes, I agree
>>> with this statement.
>>>
>>
>> Yes, I mean exactly the latter. You cannot look at one operation in
>> isolation, you need to look at the whole program.
>>
>>>>
>>>>> As I have pointed out, with your reading,
>>>>> pretty much any pointer cast constitutes an aliasing violation.
>>>>>
>>>> No, only those violating the strict aliasing rules I posted before.
>>>
>>>
>>> ... which would only allow changing const/volatile qualifiers, not the
>>> pointed-to type.
>>>
>>
>> You can change the pointed to type in terms of signedness, you can
>> cast it to a compatible type, you can cast a void-pointer or
>> char-pointer to any type. But you need to make sure you don't violate
>> the strict-aliasing rules in some other way while doing the latter.
>>
>> Aliasing *is* hard. But let's not go shopping for that reason.
>>
>>> Your reading also disallows casting an 'int' variable to type 'long',
>>> because that isn't on the list.
>>>
>>>>
>>>>> The strict aliasing rules specify what kind of reference you can use to
>>>>> access an object of a particular type. They say nothing about how that
>>>>> reference is obtained.
>>>>
>>>> Which means that it applies regardless of how you obtain it.
>>>
>>>
>>> Yes.
>>>
>>>> "If a program attempts to access the stored value of an object through
>>>> a glvalue of other than one of the following types the behavior is
>>>> undefined"
>>>>
>>>> It says "if a *program* attempts", not "if a *statement* attempts" or
>>>> "if an *opreation* attempts". This is a whole-program deal, not
>>>> limited to one operation in isolation.
>>>
>>>
>>> The key part of the wording is "through a glvalue":
>>>
>>> "If a program attempts to access the stored value of an object *through
>>> a glvalue* of other than one of the following types ..."
>>
>> This is exactly what makes this invalid AFAICT, see below.
>>
>>> Going back to the original example:
>>>
>>>    return ((const struct exec_node **)n)[0]
>>>
>>> The glvalue used to access the object in n is n itself. (I do not think that
>>> '(const struct exec_node **)n' is even a glvalue).
>>
>> Bur 'n' *is* an lvalue, which also makes it an glvalue (for reference,
>> a glvalue is a "generalized lvalue", which means that it's either an
>> lvalue or an xvalue). You can write stuff like:
>>
>
> "n" is indeed an lvalue (which in no way aliases the storage of any
> exec_node or exec_list object)

'(const struct exec_node **)n' is an lvalue who alias the storage to n.

> , the result of the cast expression is
> not,

The result of the cast is also an lvalue. You can assign to a casted pointer.

> and the result of the subscript expression is again an lvalue but
> of a type (exec_node *) which may legitimately alias an exec_node or
> exec_list object (because of the text from C99 6.5/7 I quoted earlier),

Again, as I just said in response to the other mail. I don't see how
that section is relevant in this case.