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aarch64: Add a note saying why we use frintx rather than frintn #924

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May 28, 2025
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aarch64: Add a note saying why we use frintx rather than frintn #924

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6 changes: 6 additions & 0 deletions libm/src/math/arch/aarch64.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,12 @@ pub fn fmaf(mut x: f32, y: f32, z: f32) -> f32 {
x
}

// NB: `frintx` is technically the correct instruction for C's `rint`. However, in Rust (and LLVM
// by default), `rint` is identical to `roundeven` (no fpenv interaction) so we use the
// side-effect-free `frintn`.
//
// In general, C code that calls Rust's libm should assume that fpenv is ignored.

pub fn rint(mut x: f64) -> f64 {
// SAFETY: `frintn` is available with neon and has no side effects.
//
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