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use sign variable in abs and wrapping_abs methods #64386

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Merged
merged 1 commit into from
Sep 25, 2019
Merged

use sign variable in abs and wrapping_abs methods #64386

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29 changes: 26 additions & 3 deletions src/libcore/num/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1402,7 +1402,16 @@ $EndFeature, "
#[stable(feature = "no_panic_abs", since = "1.13.0")]
#[inline]
pub const fn wrapping_abs(self) -> Self {
(self ^ (self >> ($BITS - 1))).wrapping_sub(self >> ($BITS - 1))
// sign is -1 (all ones) for negative numbers, 0 otherwise.
let sign = self >> ($BITS - 1);
// For positive self, sign == 0 so the expression is simply
// (self ^ 0).wrapping_sub(0) == self == abs(self).
//
// For negative self, self ^ sign == self ^ all_ones.
// But all_ones ^ self == all_ones - self == -1 - self.
// So for negative numbers, (self ^ sign).wrapping_sub(sign) is
// (-1 - self).wrapping_sub(-1) == -self == abs(self).
(self ^ sign).wrapping_sub(sign)
}
}

Expand Down Expand Up @@ -1761,7 +1770,7 @@ $EndFeature, "
#[stable(feature = "no_panic_abs", since = "1.13.0")]
#[inline]
pub const fn overflowing_abs(self) -> (Self, bool) {
(self ^ (self >> ($BITS - 1))).overflowing_sub(self >> ($BITS - 1))
(self.wrapping_abs(), self == Self::min_value())
}
}

Expand Down Expand Up @@ -1969,7 +1978,21 @@ $EndFeature, "
// Note that the #[inline] above means that the overflow
// semantics of the subtraction depend on the crate we're being
// inlined into.
(self ^ (self >> ($BITS - 1))) - (self >> ($BITS - 1))

// sign is -1 (all ones) for negative numbers, 0 otherwise.
let sign = self >> ($BITS - 1);
// For positive self, sign == 0 so the expression is simply
// (self ^ 0) - 0 == self == abs(self).
//
// For negative self, self ^ sign == self ^ all_ones.
// But all_ones ^ self == all_ones - self == -1 - self.
// So for negative numbers, (self ^ sign) - sign is
// (-1 - self) - -1 == -self == abs(self).
//
// The subtraction overflows when self is min_value(), because
// (-1 - min_value()) - -1 is max_value() - -1 which overflows.
// This is exactly when we want self.abs() to overflow.
(self ^ sign) - sign
}
}

Expand Down