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Implement saturating_shl and saturating_shr on ints #103441

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Implement saturating_shl and saturating_shr on ints #103441

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a712539
Implement saturating_shl and saturating_shr on ints
kellerkindt Oct 23, 2022
e6b0590
Do sign extension for signed int shift left saturation
kellerkindt Oct 23, 2022
78c4b4f
Fix saturating_shl for zero on signed integers
kellerkindt Oct 23, 2022
3ad4b80
Add more doctests for saturating_shl
kellerkindt Oct 23, 2022
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72 changes: 72 additions & 0 deletions library/core/src/num/int_macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -989,6 +989,78 @@ macro_rules! int_impl {
intrinsics::saturating_sub(0, self)
}

/// Panic-free bitwise shift-left; saturates to `MIN` or `MAX` instead of wrapping around.
///
/// Note that this is *not* the same as a rotate-left; the RHS of a saturating shift-left is restricted to
/// the range of the type, rather than the bits shifted out of the LHS being returned to the other end.
/// The primitive integer types all implement a [`rotate_left`](Self::rotate_left) function,
/// which may be what you want instead.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(saturating_bit_shifts)]
#[doc = concat!("assert_eq!(1_", stringify!($SelfT), ".saturating_shl(", stringify!($SelfT), "::BITS - 2), 1_", stringify!($SelfT), " << ", stringify!($SelfT), "::BITS - 2);")]
#[doc = concat!("assert_eq!(1_", stringify!($SelfT), ".saturating_shl(", stringify!($SelfT), "::BITS - 1), ", stringify!($SelfT), "::MAX);")]
#[doc = concat!("assert_eq!(-1_", stringify!($SelfT), ".saturating_shl(", stringify!($SelfT), "::BITS - 2), -1_", stringify!($SelfT), " << ", stringify!($SelfT), "::BITS - 2);")]
#[doc = concat!("assert_eq!(-1_", stringify!($SelfT), ".saturating_shl(", stringify!($SelfT), "::BITS - 1), ", stringify!($SelfT), "::MIN);")]
#[doc = concat!("assert_eq!(-1_", stringify!($SelfT), ".saturating_shl(", stringify!($SelfT), "::BITS), ", stringify!($SelfT), "::MIN);")]
/// ```
#[unstable(feature = "saturating_bit_shifts", issue = "103440")]
#[rustc_const_unstable(feature = "saturating_bit_shifts", issue = "103440")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline(always)]
pub const fn saturating_shl(self, rhs: u32) -> Self {
if rhs == 0 {
self
} else {
// leading zeros ignoring first bit (which indicates negative values)
let leading_zeros = (self << 1).leading_zeros();
let leading_ones = (self << 1).leading_ones();

// would overflow => MIN / MAX depending on whether the value is negative or not
if self >= 0 && leading_zeros < rhs {
<$SelfT>::MAX
} else if self < 0 && leading_ones < rhs {
<$SelfT>::MIN
} else {
// normal shift left
self << rhs
}
}
}

/// Panic-free bitwise shift-right; yields `0` where the shift exceeds the bitwidth of the type.
///
/// Note that this is *not* the same as a rotate-right; the RHS of a saturating shift-right is restricted
/// to the range of the type, rather than the bits shifted out of the LHS being returned to the other
/// end. The primitive integer types all implement a [`rotate_right`](Self::rotate_right) function,
/// which may be what you want instead.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(saturating_bit_shifts)]
#[doc = concat!("assert_eq!(0b1000_0000_u8.saturating_shr(u8::BITS), 0b0000_0000_u8);")]
/// ```
#[unstable(feature = "saturating_bit_shifts", issue = "103440")]
#[rustc_const_unstable(feature = "saturating_bit_shifts", issue = "103440")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline(always)]
pub const fn saturating_shr(self, rhs: u32) -> Self {
if rhs >= <$SelfT>::BITS {
0
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Is 0 the correct value here? Shouldn't it do a sign extend?

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Thanks, I am already working on an alternative solution with that in mind, but ./x.py test takes its time this time.

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I believe this can just be self >> rhs.min(<$SelfT>::BITS) as the whole implementation, for both signed and unsigned even.

} else {
self >> rhs
}
}

/// Saturating absolute value. Computes `self.abs()`, returning `MAX` if `self ==
/// MIN` instead of overflowing.
///
Expand Down
57 changes: 57 additions & 0 deletions library/core/src/num/uint_macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1164,6 +1164,63 @@ macro_rules! uint_impl {
}
}

/// Panic-free bitwise shift-left; saturates `MAX` instead of wrapping around.
///
/// Note that this is *not* the same as a rotate-left; the RHS of a saturating shift-left is restricted to
/// the range of the type, rather than the bits shifted out of the LHS being returned to the other end.
/// The primitive integer types all implement a [`rotate_left`](Self::rotate_left) function,
/// which may be what you want instead.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(saturating_bit_shifts)]
#[doc = concat!("assert_eq!(1_", stringify!($SelfT), ".saturating_shl(", stringify!($SelfT), "::BITS - 1), 1_", stringify!($SelfT), " << ", stringify!($SelfT), "::BITS - 1);")]
#[doc = concat!("assert_eq!(1_", stringify!($SelfT), ".saturating_shl(", stringify!($SelfT), "::BITS), ", stringify!($SelfT), "::MAX);")]
/// ```
#[unstable(feature = "saturating_bit_shifts", issue = "103440")]
#[rustc_const_unstable(feature = "saturating_bit_shifts", issue = "103440")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline(always)]
pub const fn saturating_shl(self, rhs: u32) -> Self {
if rhs > self.leading_zeros() {
<$SelfT>::MAX
} else {
self << rhs
}
}

/// Panic-free bitwise shift-right; yields `0` where the shift exceeds the bitwidth of the type.
///
/// Note that this is *not* the same as a rotate-right; the RHS of a saturating shift-right is restricted
/// to the range of the type, rather than the bits shifted out of the LHS being returned to the other
/// end. The primitive integer types all implement a [`rotate_right`](Self::rotate_right) function,
/// which may be what you want instead.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(saturating_bit_shifts)]
#[doc = concat!("assert_eq!(0b1000_0000_u8.saturating_shr(u8::BITS), 0b0000_0000_u8);")]
/// ```
#[unstable(feature = "saturating_bit_shifts", issue = "103440")]
#[rustc_const_unstable(feature = "saturating_bit_shifts", issue = "103440")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline(always)]
pub const fn saturating_shr(self, rhs: u32) -> Self {
if rhs >= <$SelfT>::BITS {
0
} else {
self >> rhs
}
}

/// Wrapping (modular) addition. Computes `self + rhs`,
/// wrapping around at the boundary of the type.
///
Expand Down