On Fri, Jun 26, 2015 at 4:53 PM, Francisco Jerez <curroje...@riseup.net> wrote: > Erik Faye-Lund <kusmab...@gmail.com> writes: > >> On Fri, Jun 26, 2015 at 4:16 PM, Davin McCall <dav...@davmac.org> wrote: >>> On 26/06/15 14:53, Erik Faye-Lund wrote: >>>> >>>> On Fri, Jun 26, 2015 at 3:05 PM, Davin McCall <dav...@davmac.org> wrote: >>>>> >>>>> On 26/06/15 12:55, Erik Faye-Lund wrote: >>>>> >>>>> On Fri, Jun 26, 2015 at 1:23 PM, Davin McCall <dav...@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. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
