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Re: [PATCH]: Fix PR tree-optimization/21407

There's no casting between pointers and integers. There are reinterpet_casts between pointers. The mapping is implementation-defined, but, in practice, all implementations on, say, IA32, are going to leave the bit-pattern unchanged.

That's not my point:

A pointer to an object can be explicitly converted to a pointer to an object of different type. Except that converting an rvalue of type "pointer to T1" to the type "pointer to T2" (where T1 and T2 are object types and where the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified.

I just don't see anything that says conclusively that this code is invalid.

Anything but casting back the pointer to the original type is implementation-defined. That's also what Microsoft's MSDN documentation says ( "The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. Other uses are, at best, nonportable. [...] One practical use of reinterpret_cast is in a hash function, which maps a value to an index in such a way that two distinct values rarely end up with the same index, [where] the reinterpret_cast allows the pointer to be treated as an integral type."

So MSDN says that arithmetic is only useful after pointer-to-integer casts and the MSDN example does *not* cast anything back to a pointer. Regarding pointer-to-integer casts, the standard says the same: "[the result of reinterpret_cast of a pointer to integer] is intended to be unsurprising to those who know the addressing structure of the underlying machine." But it very clear in saying that "A pointer converted to an integer of sufficient size (if any such exists on the implementation) and back to the same pointer type will have its original value; mappings between pointers and integers are otherwise implementation-defined."


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