[Bug c/22278] gcc -O2 discards cast to volatile

["gdr at integrable-solutions dot net" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From gdr at integrable-solutions dot net  2005-07-03 01:42 -------
Subject: Re:  gcc -O2 discards cast to volatile

"gcc2eran at tromer dot org" <gcc-bugzilla@gcc.gnu.org> writes:

| > but that is a fundamental departure from the language semantics.
| > Replace "volatile" with "const" -- both are qualifiers -- and observe
| > the disaster that would ensue.
| 
| I must be showing my ignorance again, but what disaster would that be?

If when the compiler sees "const T* p", it assumes that *p is
effectively const, then it would miscompile

   int foo(int* p, const int* q) {
       int r = *q;
       *p = r * r;
       return *q;
   }

If the compiler assumed that *q is effectively const, therefore cannot
have its value changed through *p, then the following assertion will fail

    int i = 4;
    assert(foo(&i, &i) == 16);

because the compiler could just return the cached value "r". 

There is more a pointer than just its type. 

| BTW, according to your interpretation, what is the standard-compliant behavior
| of the following?
| 
|   void quux(int *bar) {
|     *bar = 42;
|   }
| 
|   volatile int foo;
|   quux((int*)&foo);

       [#5] If an attempt is made to modify an object defined  with
       a  const-qualified  type  through use of an lvalue with non-
       const-qualified type, the  behavior  is  undefined.   If  an
       attempt  is  made  to  refer  to  an  object  defined with a
       volatile-qualified type through use of an lvalue  with  non-
       volatile-qualified type, the behavior is undefined.113)

The footnote says

       113This applies to those objects that behave as if they were
          defined with qualified types,  even  if  they  are  never
          actually  defined  as  objects in the program (such as an
          object at a memory-mapped input/output address).


-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22278