Replace one of the examples of the aliasing rule #2715
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djmitche
reviewed
Apr 18, 2025
| changes the result. | ||
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| - The equivalent code in C exhibits undefined behavior, which may result in | ||
| mis-compilation and unexpected behavior, even if it doesn't cause a crash. |
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What is the basis for this assertion? I was under the impression that C assumed everything aliased and therefore could not re-order operations very aggressively.
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Pointers would need to be tagged with restrict to allow this reordering in C.
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The existing example is too similar to the first one, since it also is demonstrating a dangling reference to a heap allocation after modifying the
Vec. This new example demonstrates a different angle to the aliasing rule: That it prevents incorrect optimizations. This gives us a chance to bring up undefined behavior, since that can cause problems in much more subtle ways.I'm not sure if this example is quite correct, though. I'm still trying to figure out why exactly this might get mis-compiled in C, but that subtlety may not matter for the purpose of showing the point of the aliasing rule.