cmov: use black_box in portable impl
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The core predication function of the portable implementation is called `is_non_zero` and it returns a bitmask which is subsequently used to "select" between two values using bitwise masking. In some previous real-world cases in the Rust compiler, it has been able to deduce, when using this sort of pattern in a loop, that the entire loop can be skipped when e.g. the condition is zero. RUSTSEC-2024-0344 is one such example. Adding a `black_box` in this position should hopefully help hint to the compiler that we do not want such an "optimization" performed. Note that the only thing we're actually relying on from `black_box` for program correctness is that it behaves as an identity function, so this will not have deleterious effects. Whether it actually helps avoid problems like RUSTSEC-2024-0344 is something that can only be seen in practice, unfortunately, as we don't have any way to get actual guarantees out of the Rust compiler in cases where we can't use `asm!`.
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The core predication function of the portable implementation is called
is_non_zeroand it returns a 1-bit mask value which is subsequently used to "select" between two values using bitwise masking.In some previous real-world cases in the Rust compiler, it has been able to deduce, when using this sort of pattern in a loop, that the entire loop can be skipped when e.g. the condition is zero. RUSTSEC-2024-0344 is one such example.
Adding a
black_boxin this position should hopefully help hint to the compiler that we do not want such an "optimization" performed.Note that the only thing we're actually relying on from
black_boxfor program correctness is that it behaves as an identity function, so this will not have deleterious effects. Whether it actually helps avoid problems like RUSTSEC-2024-0344 is something that can only be seen in practice, unfortunately, as we don't have any way to get actual guarantees out of the Rust compiler in cases where we can't useasm!.