[Bug c/17665] wrong code with -O2

[jsm at polyomino dot org dot uk]

------- Additional Comments From jsm at polyomino dot org dot uk  2004-09-25 18:30 -------
Subject: Re:  wrong code with -O2

On Sat, 25 Sep 2004, davids at webmaster dot com wrote:

> I'll check the standard again, but my recollection is that it's supposed to be 
> safe to cast any pointer to and from a 'void *'. That's why 'malloc' 
> returns 'void *'.

The problematic dereference is not of a void * pointer, but of void ** and 
char ** pointers pointing to the same place.  You can use void * and 
double * pointers (for example) pointing to the same place (though 
dereferencing the void * pointer must involve casting it to char * or 
double *).  You can use int * and unsigned int * pointers pointing to the 
same place.  But you cannot use void ** and char ** pointers pointing to 
the same place.

The *value* of a void * pointer can safely be converted to another pointer 
type.  What you are doing is treating the *representation* of that pointer 
as being another type, via void ** and char ** pointers, and this is not 
safe.

Read 6.5#7 again.  Your object has one of void * and char * as effective 
type, but its value is being accessed as the other, which is not 
permitted: it is not within any of the listed cases.  You need to access 
the value as its effective type, then convert, rather than type punning 
the representation.



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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17665