Fix checked i256 arithmetic (#3942) (#3941) #3943
Merged
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
tustvold
commented
Mar 25, 2023
| @@ -243,9 +243,8 @@ impl i256 { | |||
| /// Performs checked addition | |||
| #[inline] | |||
| pub fn checked_add(self, other: Self) -> Option<Self> { | |||
| let (low, carry) = self.low.overflowing_add(other.low); | |||
| let high = self.high.checked_add(other.high)?.checked_add(carry as _)?; | |||
There was a problem hiding this comment.
The mistake this makes is assuming that addition is transitive, in the case of overflow it is not.
In particular consider the case
i256::from_parts(u128::MAX, i128::MIN)
.checked_add(i256::from_parts(1, -1))
.unwrap();
The carry is enough to increase i128::MIN enough that adding -1 doesn't result in overflow
tustvold
commented
Mar 25, 2023
| @@ -259,9 +258,8 @@ impl i256 { | |||
| /// Performs checked subtraction | |||
| #[inline] | |||
| pub fn checked_sub(self, other: Self) -> Option<Self> { | |||
| let (low, carry) = self.low.overflowing_sub(other.low); | |||
| let high = self.high.checked_sub(other.high)?.checked_sub(carry as _)?; | |||
| Some(Self { low, high }) | |||
There was a problem hiding this comment.
Consider the case
i256::from_parts(0, i128::MAX).checked_sub(i256::MINUS_ONE).unwrap();
The carry is enough to reduce i128::MAX enough that subtracting -1 does not result in overflow
tustvold
commented
Mar 25, 2023
| @@ -303,13 +305,14 @@ impl i256 { | |||
| let hl = (l_abs.high as u128).checked_mul(r_abs.low)?; | |||
| let lh = l_abs.low.checked_mul(r_abs.high as u128)?; | |||
|
|
|||
| let high: i128 = high.checked_add(hl)?.checked_add(lh)?.try_into().ok()?; | |||
There was a problem hiding this comment.
If high == -i128::MIN this would fail, as it is performed before reversing the absolute value conversion
tustvold
commented
Mar 25, 2023
arrow-buffer/src/bigint.rs
Outdated
|
|
||
| /// Returns `true` if this [`i256`] is negative | ||
| #[inline] | ||
| pub fn is_negative(self) -> bool { |
There was a problem hiding this comment.
This matches the standard method - https://doc.rust-lang.org/std/primitive.i32.html#method.is_negative
viirya
approved these changes
Mar 26, 2023
| i256::ONE << 129, | ||
| i256::MINUS_ONE << 127, | ||
| i256::MINUS_ONE << 128, | ||
| i256::MINUS_ONE << 129, |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Which issue does this PR close?
Closes #3941
Closes #3942
Rationale for this change
The previous logic would report overflow erroneously
What changes are included in this PR?
Are there any user-facing changes?