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add exact bitshifts #144342

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Sep 5, 2025
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add exact bitshifts #144342

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117 changes: 117 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 @@ -1413,6 +1413,66 @@ macro_rules! int_impl {
}
}

/// Exact shift left. Computes `self << rhs` as long as it can be reversed losslessly.
///
/// Returns `None` if any bits that would be shifted out differ from the resulting sign bit
/// or if `rhs` >=
#[doc = concat!("`", stringify!($SelfT), "::BITS`.")]
/// Otherwise, returns `Some(self << rhs)`.
///
/// # Examples
///
/// ```
/// #![feature(exact_bitshifts)]
///
#[doc = concat!("assert_eq!(0x1", stringify!($SelfT), ".exact_shl(4), Some(0x10));")]
#[doc = concat!("assert_eq!(0x1", stringify!($SelfT), ".exact_shl(", stringify!($SelfT), "::BITS - 2), Some(1 << ", stringify!($SelfT), "::BITS - 2));")]
#[doc = concat!("assert_eq!(0x1", stringify!($SelfT), ".exact_shl(", stringify!($SelfT), "::BITS - 1), None);")]
#[doc = concat!("assert_eq!((-0x2", stringify!($SelfT), ").exact_shl(", stringify!($SelfT), "::BITS - 2), Some(-0x2 << ", stringify!($SelfT), "::BITS - 2));")]
#[doc = concat!("assert_eq!((-0x2", stringify!($SelfT), ").exact_shl(", stringify!($SelfT), "::BITS - 1), None);")]
/// ```
#[unstable(feature = "exact_bitshifts", issue = "144336")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const fn exact_shl(self, rhs: u32) -> Option<$SelfT> {
if rhs < self.leading_zeros() || rhs < self.leading_ones() {
// SAFETY: rhs is checked above
Some(unsafe { self.unchecked_shl(rhs) })
} else {
None
}
}

/// Unchecked exact shift left. Computes `self << rhs`, assuming the operation can be
/// losslessly reversed and `rhs` cannot be larger than
#[doc = concat!("`", stringify!($SelfT), "::BITS`.")]
///
/// # Safety
///
/// This results in undefined behavior when `rhs >= self.leading_zeros() && rhs >=
/// self.leading_ones()` i.e. when
#[doc = concat!("[`", stringify!($SelfT), "::exact_shl`]")]
/// would return `None`.
#[unstable(feature = "exact_bitshifts", issue = "144336")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const unsafe fn unchecked_exact_shl(self, rhs: u32) -> $SelfT {
assert_unsafe_precondition!(
check_language_ub,
concat!(stringify!($SelfT), "::unchecked_exact_shl cannot shift out non-zero bits"),
(
zeros: u32 = self.leading_zeros(),
ones: u32 = self.leading_ones(),
rhs: u32 = rhs,
) => rhs < zeros || rhs < ones,
);

// SAFETY: this is guaranteed to be safe by the caller
unsafe { self.unchecked_shl(rhs) }
Comment on lines +1462 to +1473
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@RalfJung RalfJung Sep 12, 2025

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The use of check_language_ub here seems wrong. The following code has language UB if rhs >= BITS, but the condition you are checking is stronger than that. check_language_ub should only be used if a violation of the precondition will always lead to language UB, and that is not the case here.

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@tgross35 tgross35 Sep 20, 2025

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@Qelxiros are you interested in putting up a PR adding the intrinsics mentioned at #144342 (comment)? The comment here could be fixed at the same time.

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@Qelxiros Qelxiros Sep 23, 2025

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I am interested, but I'm not sure exactly how to do it. Do I need to add implementations to all the rustc_codegen_* crates, or just llvm? How exactly are intrinsics added? It looks like I need a combination of LLVMBuildShl/LLVMBuildShr and LLVMSetNUW/LLVMSetNSW/LLVMSetExact (although LLVMSetExact currently only exists in C++ code). However, I'm not sure where that goes or how to associate it with the appropriate symbol.

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@tgross35 tgross35 Sep 23, 2025 •
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Ralf got these already in #146878, thanks for the interest though

}

/// Checked shift right. Computes `self >> rhs`, returning `None` if `rhs` is
/// larger than or equal to the number of bits in `self`.
///
Expand Down Expand Up @@ -1534,6 +1594,63 @@ macro_rules! int_impl {
}
}

/// Exact shift right. Computes `self >> rhs` as long as it can be reversed losslessly.
///
/// Returns `None` if any non-zero bits would be shifted out or if `rhs` >=
#[doc = concat!("`", stringify!($SelfT), "::BITS`.")]
/// Otherwise, returns `Some(self >> rhs)`.
///
/// # Examples
///
/// ```
/// #![feature(exact_bitshifts)]
///
#[doc = concat!("assert_eq!(0x10", stringify!($SelfT), ".exact_shr(4), Some(0x1));")]
#[doc = concat!("assert_eq!(0x10", stringify!($SelfT), ".exact_shr(5), None);")]
/// ```
#[unstable(feature = "exact_bitshifts", issue = "144336")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const fn exact_shr(self, rhs: u32) -> Option<$SelfT> {
if rhs <= self.trailing_zeros() && rhs < <$SelfT>::BITS {
// SAFETY: rhs is checked above
Some(unsafe { self.unchecked_shr(rhs) })
} else {
None
}
}

/// Unchecked exact shift right. Computes `self >> rhs`, assuming the operation can be
/// losslessly reversed and `rhs` cannot be larger than
#[doc = concat!("`", stringify!($SelfT), "::BITS`.")]
///
/// # Safety
///
/// This results in undefined behavior when `rhs > self.trailing_zeros() || rhs >=
#[doc = concat!(stringify!($SelfT), "::BITS`")]
/// i.e. when
#[doc = concat!("[`", stringify!($SelfT), "::exact_shr`]")]
/// would return `None`.
#[unstable(feature = "exact_bitshifts", issue = "144336")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const unsafe fn unchecked_exact_shr(self, rhs: u32) -> $SelfT {
assert_unsafe_precondition!(
check_language_ub,
concat!(stringify!($SelfT), "::unchecked_exact_shr cannot shift out non-zero bits"),
(
zeros: u32 = self.trailing_zeros(),
bits: u32 = <$SelfT>::BITS,
rhs: u32 = rhs,
) => rhs <= zeros && rhs < bits,
);

// SAFETY: this is guaranteed to be safe by the caller
unsafe { self.unchecked_shr(rhs) }
}

/// Checked absolute value. Computes `self.abs()`, returning `None` if
/// `self == MIN`.
///
Expand Down
114 changes: 114 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 @@ -1734,6 +1734,63 @@ macro_rules! uint_impl {
}
}

/// Exact shift left. Computes `self << rhs` as long as it can be reversed losslessly.
///
/// Returns `None` if any non-zero bits would be shifted out or if `rhs` >=
#[doc = concat!("`", stringify!($SelfT), "::BITS`.")]
/// Otherwise, returns `Some(self << rhs)`.
///
/// # Examples
///
/// ```
/// #![feature(exact_bitshifts)]
///
#[doc = concat!("assert_eq!(0x1", stringify!($SelfT), ".exact_shl(4), Some(0x10));")]
#[doc = concat!("assert_eq!(0x1", stringify!($SelfT), ".exact_shl(129), None);")]
/// ```
#[unstable(feature = "exact_bitshifts", issue = "144336")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const fn exact_shl(self, rhs: u32) -> Option<$SelfT> {
if rhs <= self.leading_zeros() && rhs < <$SelfT>::BITS {
// SAFETY: rhs is checked above
Some(unsafe { self.unchecked_shl(rhs) })
} else {
None
}
}

/// Unchecked exact shift left. Computes `self << rhs`, assuming the operation can be
/// losslessly reversed `rhs` cannot be larger than
#[doc = concat!("`", stringify!($SelfT), "::BITS`.")]
///
/// # Safety
///
/// This results in undefined behavior when `rhs > self.leading_zeros() || rhs >=
#[doc = concat!(stringify!($SelfT), "::BITS`")]
/// i.e. when
#[doc = concat!("[`", stringify!($SelfT), "::exact_shl`]")]
/// would return `None`.
#[unstable(feature = "exact_bitshifts", issue = "144336")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const unsafe fn unchecked_exact_shl(self, rhs: u32) -> $SelfT {
assert_unsafe_precondition!(
check_language_ub,
concat!(stringify!($SelfT), "::exact_shl_unchecked cannot shift out non-zero bits"),
(
zeros: u32 = self.leading_zeros(),
bits: u32 = <$SelfT>::BITS,
rhs: u32 = rhs,
) => rhs <= zeros && rhs < bits,
);

// SAFETY: this is guaranteed to be safe by the caller
unsafe { self.unchecked_shl(rhs) }
}

/// Checked shift right. Computes `self >> rhs`, returning `None`
/// if `rhs` is larger than or equal to the number of bits in `self`.
///
Expand Down Expand Up @@ -1849,6 +1906,63 @@ macro_rules! uint_impl {
}
}

/// Exact shift right. Computes `self >> rhs` as long as it can be reversed losslessly.
///
/// Returns `None` if any non-zero bits would be shifted out or if `rhs` >=
#[doc = concat!("`", stringify!($SelfT), "::BITS`.")]
/// Otherwise, returns `Some(self >> rhs)`.
///
/// # Examples
///
/// ```
/// #![feature(exact_bitshifts)]
///
#[doc = concat!("assert_eq!(0x10", stringify!($SelfT), ".exact_shr(4), Some(0x1));")]
#[doc = concat!("assert_eq!(0x10", stringify!($SelfT), ".exact_shr(5), None);")]
/// ```
#[unstable(feature = "exact_bitshifts", issue = "144336")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const fn exact_shr(self, rhs: u32) -> Option<$SelfT> {
if rhs <= self.trailing_zeros() && rhs < <$SelfT>::BITS {
// SAFETY: rhs is checked above
Some(unsafe { self.unchecked_shr(rhs) })
} else {
None
}
}

/// Unchecked exact shift right. Computes `self >> rhs`, assuming the operation can be
/// losslessly reversed and `rhs` cannot be larger than
#[doc = concat!("`", stringify!($SelfT), "::BITS`.")]
///
/// # Safety
///
/// This results in undefined behavior when `rhs > self.trailing_zeros() || rhs >=
#[doc = concat!(stringify!($SelfT), "::BITS`")]
/// i.e. when
#[doc = concat!("[`", stringify!($SelfT), "::exact_shr`]")]
/// would return `None`.
#[unstable(feature = "exact_bitshifts", issue = "144336")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const unsafe fn unchecked_exact_shr(self, rhs: u32) -> $SelfT {
assert_unsafe_precondition!(
check_language_ub,
concat!(stringify!($SelfT), "::exact_shr_unchecked cannot shift out non-zero bits"),
(
zeros: u32 = self.trailing_zeros(),
bits: u32 = <$SelfT>::BITS,
rhs: u32 = rhs,
) => rhs <= zeros && rhs < bits,
);

// SAFETY: this is guaranteed to be safe by the caller
unsafe { self.unchecked_shr(rhs) }
}

/// Checked exponentiation. Computes `self.pow(exp)`, returning `None` if
/// overflow occurred.
///
Expand Down
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