[Bug regression/40665] dereferencing type-punned pointer warnings cannot be disabled

[pinskia at gcc dot gnu dot org]

------- Comment #9 from pinskia at gcc dot gnu dot org  2009-07-07 16:54 -------
>So you say that converting the char * pointer to struct * pointer is understood
as "accessing the stored value" by the standard?

No.
Let's look at the code:
char buffer[512];

(void *)&((struct structure *)(void *)buffer)->x

You are accessing a character array via a "struct structure" and then taking
the address.  This why it is undefined.  Even though it does not look like an
access in the assembly as it is &a->b is an addition, it is an access according
to the C/C++ standard.  Also aliasing is not transitive, that is you can access
anything via a character type but you cannot access a character by everything
(only character types themselves).


>the code is valid and the function b() must return a fixed value depending on
the endianity of the machine. That __asm__ statement works as a barrier that
prevents the compiler from reordering two accesses to "c".

Actually even with the memory barrier is still undefined according to the C/C++
standards, it does not mean we won't do what you expect but it means we don't
have to do what you expect it to do.  This is why we warn about it.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40665