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

[Francisco Jerez]

Erik Faye-Lund <kusmabite at gmail.com> writes:

> 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.
>
No, you can't, you can only assign to the result of dereferencing the
result of a casted pointer.  See the footnote in section 6.5.4 of the
C99 spec:

| 89) A cast does not yield an lvalue. Thus, a cast to a qualified type
|     has the same effect as a cast to the unqualified version of the type.

>> 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.

OK, I've replied to this point on the other thread.
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