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), the result of the cast expression is not, 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), so this code seems valid to me (though admittedly it's far from obvious and introduces another kind of UB). > "((const struct exec_node **)n)[0] = foo;" > > ...so it can appear on the left-hand side of an assignment, which > makes it an lvalue. > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > http://lists.freedesktop.org/mailman/listinfo/mesa-dev
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