Null value opts for shm #1241
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Null value opts for shm #1241
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…to equalize some data structure sizes in zenoh-c)
Mallets
reviewed
Jul 16, 2024
| 1usize << self.pow | ||
| pub fn get_alignment_value(&self) -> NonZeroUsize { | ||
| // SAFETY: this is safe | ||
| unsafe { NonZeroUsize::new_unchecked(1usize << self.pow) } |
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AllocAlignment::new() takes pow: u8 that returns Ok if pow < 64.
1usize << self.pow may overflow in case usize is 32bit.
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Fixed through usize::BITS as @wyfo suggested
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By the way, you could maybe remove this unsafe according to https://godbolt.org/z/c66j343Eq
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@wyfo my understanding is:
- Fist of all,
1usize << powis evaluated, and it's result isusize. An overflow might happen here, which will panic on Debug and be UB on Release (UB because it is processor-defined) - In
NonZeroUsize::newcompiler knows that FOR TARGET ARCHITECTURE any shift operation applied to1usizeconstant at step 1 will always give nonzero result, even in case of overflow in Release. It means that for TARGET ARCHITECTURE a check may be elided, but for other architectures this might be wrong and check assembly will be generated.
As long as the whole AllocAlignment structure guarantees that pow will not cause overflow, we'd better use unsafe unchecked version
Mallets
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Jul 16, 2024
wyfo
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Jul 16, 2024
Mallets
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Jul 16, 2024
Mallets
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Jul 16, 2024
wyfo's addition Co-authored-by: Joseph Perez <[email protected]>
Mallets
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Jul 17, 2024
Mallets
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Jul 18, 2024
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Some null value optimizations to equalize SHM data structures for zenoh-c