Add splitting caveats to {read,write}_volatile#156931
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RalfJung
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| /// | ||
| /// # Load splitting | ||
| /// | ||
| /// Exactly which hardware loads are performed by this function are, in general, highly target-dependent. |
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| /// Exactly which hardware loads are performed by this function are, in general, highly target-dependent. | |
| /// Exactly which hardware loads are performed by this function is, in general, highly target-dependent. |
| /// Exactly which hardware loads are performed by this function are, in general, highly target-dependent. | ||
| /// | ||
| /// For a simple scalar, such as when `T` is a thin pointer, this will typically be one load assuming | ||
| /// your target supports a load of exactly that size. |
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| /// your target supports a load of exactly that size. | |
| /// your target supports a load of exactly that size and alignment. |
| /// | ||
| /// For anything else, it will be split into multiple loads in some unspecified way. This can happen | ||
| /// even for scalars: notably, on many targets loading a `u128` will still need to be split, despite | ||
| /// being "one" scalar. A load over, say, 64 bytes will almost certainly need to be split. Any load |
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| /// being "one" scalar. A load over, say, 64 bytes will almost certainly need to be split. Any load | |
| /// being "one" scalar. A load larger than, say, 64 bytes will almost certainly need to be split. Any load |
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| /// | ||
| /// For anything else, it will be split into multiple loads in some unspecified way. This can happen | ||
| /// even for scalars: notably, on many targets loading a `u128` will still need to be split, despite | ||
| /// being "one" scalar. A load over, say, 64 bytes will almost certainly need to be split. Any load |
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is this really intended to say 64 bytes (not bits)? as far as i am aware, on most supported platforms, splits tend to happen for anything larger than usize, and as the previous sentence says, even u128 is split on x86_64.
Maybe this should say "larger than size_of::<usize>()" or similar?
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It sounds like we don't actually have guarantees right now (#t-opsem > Defining volatile splitting @ 💬) so let's document that. I think this doesn't need an FCP since it's not actually guaranteeing anything.