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/* SPDX-License-Identifier: MIT */
/* origin: musl src/math/fma.c, fmaf.c Ported to generic Rust algorithm in 2025, TG. */
use super::generic;
use crate::support::Round;
// Placeholder so we can have `fmaf16` in the `Float` trait.
#[allow(unused)]
#[cfg(f16_enabled)]
#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
pub(crate) fn fmaf16(_x: f16, _y: f16, _z: f16) -> f16 {
unimplemented!()
}
/// Floating multiply add (f32)
///
/// Computes `(x*y)+z`, rounded as one ternary operation (i.e. calculated with infinite precision).
#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
pub fn fmaf(x: f32, y: f32, z: f32) -> f32 {
select_implementation! {
name: fmaf,
use_arch: any(
all(target_arch = "aarch64", target_feature = "neon"),
target_feature = "sse2",
),
args: x, y, z,
}
generic::fma_wide_round(x, y, z, Round::Nearest).val
}
/// Fused multiply add (f64)
///
/// Computes `(x*y)+z`, rounded as one ternary operation (i.e. calculated with infinite precision).
#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
pub fn fma(x: f64, y: f64, z: f64) -> f64 {
select_implementation! {
name: fma,
use_arch: any(
all(target_arch = "aarch64", target_feature = "neon"),
target_feature = "sse2",
),
args: x, y, z,
}
generic::fma_round(x, y, z, Round::Nearest).val
}
/// Fused multiply add (f128)
///
/// Computes `(x*y)+z`, rounded as one ternary operation (i.e. calculated with infinite precision).
#[cfg(f128_enabled)]
#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
pub fn fmaf128(x: f128, y: f128, z: f128) -> f128 {
generic::fma_round(x, y, z, Round::Nearest).val
}
#[cfg(test)]
mod tests {
use super::*;
use crate::support::{CastFrom, CastInto, Float, FpResult, HInt, MinInt, Round, Status};
/// Test the generic `fma_round` algorithm for a given float.
fn spec_test<F>(f: impl Fn(F, F, F) -> F)
where
F: Float,
F: CastFrom<F::SignedInt>,
F: CastFrom<i8>,
F::Int: HInt,
u32: CastInto<F::Int>,
{
let x = F::from_bits(F::Int::ONE);
let y = F::from_bits(F::Int::ONE);
let z = F::ZERO;
// 754-2020 says "When the exact result of (a × b) + c is non-zero yet the result of
// fusedMultiplyAdd is zero because of rounding, the zero result takes the sign of the
// exact result"
assert_biteq!(f(x, y, z), F::ZERO);
assert_biteq!(f(x, -y, z), F::NEG_ZERO);
assert_biteq!(f(-x, y, z), F::NEG_ZERO);
assert_biteq!(f(-x, -y, z), F::ZERO);
}
#[test]
fn spec_test_f32() {
spec_test::<f32>(fmaf);
// Also do a small check that the non-widening version works for f32 (this should ideally
// get tested some more).
spec_test::<f32>(|x, y, z| generic::fma_round(x, y, z, Round::Nearest).val);
}
#[test]
fn spec_test_f64() {
spec_test::<f64>(fma);
let expect_underflow = [
(
hf64!("0x1.0p-1070"),
hf64!("0x1.0p-1070"),
hf64!("0x1.ffffffffffffp-1023"),
hf64!("0x0.ffffffffffff8p-1022"),
),
(
// FIXME: we raise underflow but this should only be inexact (based on C and
// `rustc_apfloat`).
hf64!("0x1.0p-1070"),
hf64!("0x1.0p-1070"),
hf64!("-0x1.0p-1022"),
hf64!("-0x1.0p-1022"),
),
];
for (x, y, z, res) in expect_underflow {
let FpResult { val, status } = generic::fma_round(x, y, z, Round::Nearest);
assert_biteq!(val, res);
assert_eq!(status, Status::UNDERFLOW);
}
}
#[test]
#[cfg(f128_enabled)]
fn spec_test_f128() {
spec_test::<f128>(fmaf128);
}
#[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!(fmaf(a, b, c), expected);
}
#[test]
fn fma_segfault() {
// These two inputs cause fma to segfault on release due to overflow:
assert_eq!(
fma(
-0.0000000000000002220446049250313,
-0.0000000000000002220446049250313,
-0.0000000000000002220446049250313
),
-0.00000000000000022204460492503126,
);
let result = fma(-0.992, -0.992, -0.992);
//force rounding to storage format on x87 to prevent superious errors.
#[cfg(all(target_arch = "x86", not(target_feature = "sse2")))]
let result = force_eval!(result);
assert_eq!(result, -0.007936000000000007,);
}
#[test]
fn fma_sbb() {
assert_eq!(
fma(-(1.0 - f64::EPSILON), f64::MIN, f64::MIN),
-3991680619069439e277
);
}
#[test]
fn fma_underflow() {
assert_eq!(
fma(1.1102230246251565e-16, -9.812526705433188e-305, 1.0894e-320),
0.0,
);
}
}
|
