Erik Faye-Lund <kusmab...@gmail.com> writes: > 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. > 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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