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

Francisco Jerez currojerez at riseup.net
Fri Jun 26 08:25:19 PDT 2015

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

> On Fri, Jun 26, 2015 at 4:01 PM, Francisco Jerez <currojerez at riseup.net> wrote:
>> Davin McCall <davmac at davmac.org> writes:
>>> On 26/06/15 14:31, Eirik Byrkjeflot Anonsen wrote:
>>>> Erik Faye-Lund <kusmabite at gmail.com> writes:
>>>>> 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.
>>>>> You *are* accessing the underlying memory of 'n' through a different
>>>>> type, and this is what strict aliasing is all about. But it takes two
>>>>> steps, a single step isn't enough to do so.
>>>>> Those other spec-quotes doesn't undo the strict-aliasing definitions;
>>>>> knowing how things are laid out in memory doesn't mean the compiler
>>>>> cannot assume two differently typed variables doesn't overlap.
>>>> So basically, you're saying that e.g.:
>>>>     p->next = a;
>>>>     q = exec_node_get_next_const(p);
>>>> is equivalent to:
>>>>     exec_node * p1 = p;
>>>>     exec_node ** p2 = (exec_node**)p;
>>>>     p1->next = a;
>>>>     q = p2[0];
>>> It is, once the patch is applied (or if strict aliasing is disabled).
>>>> And at this point p1 and p2 are different types, so the compiler can
>>>> freely assume that p1 and p2 are non-overlapping.
>>> p1 and p2 are two separate variables and of course they are
>>> non-overlapping, but *p1 and **p2 are the same type and so may overlap.
>> Also note that even *p1 and *p2 are allowed to overlap even though they
>> are of different types because of section 6.5 of C99:
>> | 7 An object shall have its stored value accessed only by an lvalue
>> |   expression that has one of the following types:
>> |[...]
>> |    - an aggregate or union type that includes one of the aforementioned
>> |      types among its members (including, recursively, a member of a
>> |      subaggregate or contained union)[...]
> I don't see how this wording legitimates the code above. There's no
> unions involved, so that part is irrelevant.

Yeah, only the "aggregate" part is relevant.

> And "n" isn't an aggregate type, it's a pointer type that happens to
> point to an aggregate type, no? But even if it were, it needs to
> include one of the "aforementioned" types among its members, which I
> cannot see that it does either.
> Care to explain?

In the example this was replying to, *p1 was an lvalue of aggregate type
(struct exec_node), and *p2 (or equivalently p2[0]) was an lvalue of
type "struct exec_node *".  The latter is "a type compatible with the
effective type of the object", the object being the "next" member of an
aggregate object of type "struct exec_node", hence the text quoted above
applies and *p1 and *p2 may legitimately alias the same object.
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