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Merge pull request rust-lang/libm#267 from Amanieu/fmaf

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Amanieu d'Antras 2022-08-10 02:17:52 +02:00 • committed by GitHub
commit d0c0eb9c23
2 changed files with 23 additions and 18 deletions
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6
library/compiler-builtins/libm/src/math/fenv.rs
View file

@ -5,7 +5,6 @@ pub(crate) const FE_UNDERFLOW: i32 = 0;
pub(crate) const FE_INEXACT: i32 = 0;
pub(crate) const FE_TONEAREST: i32 = 0;
pub(crate) const FE_TOWARDZERO: i32 = 0;
#[inline]
pub(crate) fn feclearexcept(_mask: i32) -> i32 {
@ -26,8 +25,3 @@ pub(crate) fn fetestexcept(_mask: i32) -> i32 {
pub(crate) fn fegetround() -> i32 {
FE_TONEAREST
}
#[inline]
pub(crate) fn fesetround(_r: i32) -> i32 {
0
}

35
library/compiler-builtins/libm/src/math/fmaf.rs
View file

@ -29,8 +29,7 @@ use core::f32;
use core::ptr::read_volatile;
use super::fenv::{
feclearexcept, fegetround, feraiseexcept, fesetround, fetestexcept, FE_INEXACT, FE_TONEAREST,
FE_TOWARDZERO, FE_UNDERFLOW,
feclearexcept, fegetround, feraiseexcept, fetestexcept, FE_INEXACT, FE_TONEAREST, FE_UNDERFLOW,
};
/*
@ -91,16 +90,28 @@ pub fn fmaf(x: f32, y: f32, mut z: f32) -> f32 {
* If result is inexact, and exactly halfway between two float values,
* we need to adjust the low-order bit in the direction of the error.
*/
fesetround(FE_TOWARDZERO);
// prevent `vxy + z` from being CSE'd with `xy + z` above
let vxy: f64 = unsafe { read_volatile(&xy) };
let mut adjusted_result: f64 = vxy + z as f64;
fesetround(FE_TONEAREST);
if result == adjusted_result {
ui = adjusted_result.to_bits();
let neg = ui >> 63 != 0;
let err = if neg == (z as f64 > xy) {
xy - result + z as f64
} else {
z as f64 - result + xy
};
if neg == (err < 0.0) {
ui += 1;
adjusted_result = f64::from_bits(ui);
} else {
ui -= 1;
}
f64::from_bits(ui) as f32
}
#[cfg(test)]
mod tests {
#[test]
fn issue_263() {
let a = f32::from_bits(1266679807);
let b = f32::from_bits(1300234242);
let c = f32::from_bits(1115553792);
let expected = f32::from_bits(1501560833);
assert_eq!(super::fmaf(a, b, c), expected);
}
z = adjusted_result as f32;
z
}