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document rounding behavior of rint/nearbyint for ties #119656

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Jan 7, 2024
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document rounding behavior of rint/nearbyint for ties #119656

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18 changes: 12 additions & 6 deletions library/core/src/intrinsics.rs
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Expand Up @@ -1783,27 +1783,33 @@ extern "rust-intrinsic" {
#[rustc_nounwind]
pub fn truncf64(x: f64) -> f64;

/// Returns the nearest integer to an `f32`. May raise an inexact floating-point exception
/// if the argument is not an integer.
/// Returns the nearest integer to an `f32`. Changing the rounding mode is not possible in Rust,
/// so this rounds half-way cases to the number with an even least significant digit.
///
/// May raise an inexact floating-point exception if the argument is not an integer.
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This exception isn't mentioned in the round_ties_even docs -- is this something we should be guarding against somehow? Is raising an exception UB, or does that just (guaranteed) abort the program? Feels... unhelpful to have a rounding function only allow exactly integer inputs.

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This is an FP exception. All it does is set a bit in some register that Rust code is not allowed to read.

One can set up the FP environment to turn these exceptions into traps, but Rust doesn't permit that.

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Sounds good, thanks.

///
/// The stabilized version of this intrinsic is
/// [`f32::round_ties_even`](../../std/primitive.f32.html#method.round_ties_even)
#[rustc_nounwind]
pub fn rintf32(x: f32) -> f32;
/// Returns the nearest integer to an `f64`. May raise an inexact floating-point exception
/// if the argument is not an integer.
/// Returns the nearest integer to an `f64`. Changing the rounding mode is not possible in Rust,
/// so this rounds half-way cases to the number with an even least significant digit.
///
/// May raise an inexact floating-point exception if the argument is not an integer.
///
/// The stabilized version of this intrinsic is
/// [`f64::round_ties_even`](../../std/primitive.f64.html#method.round_ties_even)
#[rustc_nounwind]
pub fn rintf64(x: f64) -> f64;

/// Returns the nearest integer to an `f32`.
/// Returns the nearest integer to an `f32`. Changing the rounding mode is not possible in Rust,
/// so this rounds half-way cases to the number with an even least significant digit.
///
/// This intrinsic does not have a stable counterpart.
#[rustc_nounwind]
pub fn nearbyintf32(x: f32) -> f32;
/// Returns the nearest integer to an `f64`.
/// Returns the nearest integer to an `f64`. Changing the rounding mode is not possible in Rust,
/// so this rounds half-way cases to the number with an even least significant digit.
///
/// This intrinsic does not have a stable counterpart.
#[rustc_nounwind]
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