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

[Erik Faye-Lund]

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.

> 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.

> 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.

"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.

> 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.
>
>
> I'm sorry, but your understanding is incorrect. Most pointer casts would be
> illegal otherwise. And in fact most casts would be illegal. For instance:
>
> int a;
> long b = (long) a;
>
> You reasoning says that the second line is a strict-aliasing violation,
> because it access the object in 'a' which is of type 'int' via a glvalue of
> type 'long'.

No, that is not in violation, because it's accessed through, and *then* casted.