_Region_Get and strict aliasing

Till Straumann strauman at slac.stanford.edu
Wed Dec 20 04:01:46 UTC 2006

Welcome on board. It seems you got it.

Steven Johnson wrote:
> Lets see if I follow your logic,
> You say, because the pointer to the object returned by _Region_Get may 
> have been at its creation a Region_Control structure, then the casting 
> doesn't change that effective type, and so the pointer to the object can 
> be cast to anything (provided that the cast pointer is not accessed) and 
> when it is cast back then it can still be accessed as a Region_Control 
> structure.  It would just be an invalid alias to access it through one 
> of it's type cast intermediaries.
> Is this the basis of your argument that this is not an illegal alias?
> So the following applies:
> int a;
> long *b = (long*)&a;
> ...
> short *c = (shortc*)b;
> ...
> void *d = (void*)c;
> ...
> int *e = (int*)d;
> long* f = (long*)d;
> In the above ... means a lot of code, function calls, indirection, etc, 
> that mean the true effective type of the object previously cast can not 
> be determined by the compiler.
> One can access *e to get at 'a', but any use of *c, *d or *f to get at 
> any part of the object 'a' would be an invalid alias.  Even if their was 
> no possible way for the compiler to determine that the object *c, *d and 
> *f points to has an effective type of int.
> Steven J
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