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fmaf128: fix exponent calculation for subnormals

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When `fmaf128` was introduced in [1], it included a bug where `self`
gets returned rather than the expected minimum positive value. Resolve
this and add a regression test.

[1]: https://github.com/rust-lang/libm/pull/494
This commit is contained in:
Trevor Gross 2025-02-07 00:52:56 +00:00
parent f028611faf
commit d35a443527
2 changed files with 26 additions and 12 deletions
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25
library/compiler-builtins/libm/crates/libm-test/src/gen/case_list.rs
View file

@ -269,15 +269,26 @@ fn fmaf128_cases() -> Vec<TestCase<op::fmaf128::Routine>> {
let mut v = vec![];
TestCase::append_pairs(
&mut v,
&[(
// Tricky rounding case that previously failed in extensive tests
&[
(
hf128!("-0x1.1966cc01966cc01966cc01966f06p-25"),
hf128!("-0x1.669933fe69933fe69933fe6997c9p-16358"),
hf128!("-0x0.000000000000000000000000048ap-16382"),
// Tricky rounding case that previously failed in extensive tests
(
hf128!("-0x1.1966cc01966cc01966cc01966f06p-25"),
hf128!("-0x1.669933fe69933fe69933fe6997c9p-16358"),
hf128!("-0x0.000000000000000000000000048ap-16382"),
),
Some(hf128!("0x0.c5171470a3ff5e0f68d751491b18p-16382")),
),
Some(hf128!("0x0.c5171470a3ff5e0f68d751491b18p-16382")),
)],
(
// Subnormal edge case that caused a failure
(
hf128!("0x0.7ffffffffffffffffffffffffff7p-16382"),
hf128!("0x1.ffffffffffffffffffffffffffffp-1"),
hf128!("0x0.8000000000000000000000000009p-16382"),
),
Some(hf128!("0x1.0000000000000000000000000000p-16382")),
),
],
);
v
}

13
library/compiler-builtins/libm/src/math/generic/fma.rs
View file

@ -146,6 +146,7 @@ where
// exact +/- 0.0
return x * y + z;
}
e -= d;
// Use int->float conversion to populate the significand.
@ -174,7 +175,7 @@ where
if r == c {
// Min normal after rounding,
return r.raise_underflow_ret_self();
return r.raise_underflow_as_min_positive();
}
if (rhi << (F::SIG_BITS + 1)) != zero {
@ -275,12 +276,14 @@ impl<F: Float> Norm<F> {
/// Type-specific helpers that are not needed outside of fma.
pub trait FmaHelper {
fn raise_underflow_ret_self(self) -> Self;
/// Raise underflow and return the minimum positive normal value with the sign of `self`.
fn raise_underflow_as_min_positive(self) -> Self;
/// Raise underflow and return zero.
fn raise_underflow_ret_zero(self) -> Self;
}
impl FmaHelper for f64 {
fn raise_underflow_ret_self(self) -> Self {
fn raise_underflow_as_min_positive(self) -> Self {
/* min normal after rounding, underflow depends
* on arch behaviour which can be imitated by
* a double to float conversion */
@ -298,8 +301,8 @@ impl FmaHelper for f64 {
#[cfg(f128_enabled)]
impl FmaHelper for f128 {
fn raise_underflow_ret_self(self) -> Self {
self
fn raise_underflow_as_min_positive(self) -> Self {
f128::MIN_POSITIVE.copysign(self)
}
fn raise_underflow_ret_zero(self) -> Self {