On Wed, 2011-07-20 at 13:32 -0400, Brandon Allbery wrote:
I think it's more correct to say that the compiler is free to do things that would lead to false positives if it knows that it's safe to do so (and purity means it can find more of those cases, and more of them *will* be safe) — but there is no way for it to crowbar pointer equality tests in that case.
I have looked up crowbar in a number of dictionaries of slang and informal usage... and still have no idea what you just said. Can you reword it? The point, I think, is that if pointer equality testing really does what it says, then there shouldn't *be* any correct implementation in which false positives are possible. It seems the claim is that the garbage collector might be moving things around, have just by chance happened to place the second value in the spot formerly occupied by the first, and have not updated the first pointer yet. But if that's the case, and it's executing arbitrary user code that may refer to that memory, then the garbage collector contains race conditions! Then this "false positives" issue is no different from any of the many other problems such a bug might trigger. -- Chris Smith