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Rollup merge of #67845 - jumbatm:also-unconst-hack-abs, r=oli-obk

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Also remove const-hack for abs

Closes #67842.

r? @oli-obk
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Mazdak Farrokhzad 2020-01-04 02:19:55 +01:00 • committed by GitHub
commit 745f771484
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1 changed files with 6 additions and 18 deletions
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24
src/libcore/num/mod.rs
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@ -1997,27 +1997,15 @@ $EndFeature, "
```"),
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_stable(feature = "const_int_methods", since = "1.32.0")]
#[allow_internal_unstable(const_if_match)]
#[inline]
#[rustc_inherit_overflow_checks]
pub const fn abs(self) -> Self {
// Note that the #[inline] above means that the overflow
// semantics of the subtraction depend on the crate we're being
// inlined into.
// sign is -1 (all ones) for negative numbers, 0 otherwise.
let sign = self >> ($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
if self.is_negative() {
-self
} else {
self
}
}
}