uint: Fix overflowing_neg by implementing two's complement #611
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The operation `overflowing_neg` on the primitive integer types in the Rust standard library computes the negation of the integer value using two's complement, i.e., it returns `!self + 1`. The previous implementation of the uint library implemented `overflowing_neg` using `!self` for non-zero values which is bit-wise negation (NOT). This lead to behavior where 0 - 1 != -1 for U256 with the `overflowing_neg` and `overflow_sub` operations. This patch adapts the `uint_overflowing_binop` macro to implement the two's complement correctly: Starting from the least significant word we apply `u64::overflowing_neg` until we have seen the first one-bit in the original integer, i.e., until `overflowing_neg` reports an overflow. Then we use bit-wise NOT for the remaining words.
ordian
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Feb 4, 2022
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Good catch!
Thanks for adding the test as well.
Regarding the implementation, instead of duplicating the macro, I'd rather change the current implementation from !self to !self + 1 even if it's not as performant. We should prioritize simplicity and correctness here.
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ordian
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The operation
overflowing_negon the primitive integer types in theRust standard library computes the negation of the integer value using
two's complement, i.e., it returns
!self + 1.The current implementation of the uint library implemented
overflowing_negusing!selffor non-zero values which is bit-wisenegation (NOT). This lead to behavior where 0 - 1 != -1 for
U256 with the
overflowing_negandoverflow_suboperations.This patch adapts the
uint_overflowing_binopmacro to implement thetwo's complement correctly: Starting from the least significant word
we apply
u64::overflowing_neguntil we have seen the first one-bit inthe original integer, i.e., until
overflowing_negreports an overflow.Then we use bit-wise NOT for the remaining words
(cf. https://en.wikipedia.org/wiki/Two%27s_complement#Working_from_LSB_towards_MSB).