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Fix expm1f overflow threshold

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Due to an erroneous overflow threshold, `expm1f` was incorrectly
returning `inf` for inputs in the range `[88.72169, 88.72283]`. This
additionally caused `sinhf` to return `NaN` for inputs in that range.

The bug was ported from the original in musl, which has since been fixed
in [1].

[1]: https://git.musl-libc.org/cgit/musl/commit/?id=964104f9f0e056cf58d9defa0b716d7756f040f6
This commit is contained in:
Juho Kahala 2025-12-17 07:56:45 +02:00 • committed by GitHub
parent 5faeb43a5c
commit a9fa80ce91
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GPG key ID: B5690EEEBB952194
2 changed files with 2 additions and 23 deletions
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21
library/compiler-builtins/libm-test/src/precision.rs
View file

@ -218,27 +218,6 @@ impl MaybeOverride<(f16,)> for SpecialCase {}
impl MaybeOverride<(f32,)> for SpecialCase { impl MaybeOverride<(f32,)> for SpecialCase {
fn check_float<F: Float>(input: (f32,), actual: F, expected: F, ctx: &CheckCtx) -> CheckAction { fn check_float<F: Float>(input: (f32,), actual: F, expected: F, ctx: &CheckCtx) -> CheckAction {
if ctx.base_name == BaseName::Expm1
&& !input.0.is_infinite()
&& input.0 > 80.0
&& actual.is_infinite()
&& !expected.is_infinite()
{
// we return infinity but the number is representable
if ctx.basis == CheckBasis::Musl {
return XFAIL_NOCHECK;
}
return XFAIL("expm1 representable numbers");
}
if ctx.base_name == BaseName::Sinh && input.0.abs() > 80.0 && actual.is_nan() {
// we return some NaN that should be real values or infinite
if ctx.basis == CheckBasis::Musl {
return XFAIL_NOCHECK;
}
return XFAIL("sinh unexpected NaN");
}
if (ctx.base_name == BaseName::Lgamma || ctx.base_name == BaseName::LgammaR) if (ctx.base_name == BaseName::Lgamma || ctx.base_name == BaseName::LgammaR)
&& input.0 > 4e36 && input.0 > 4e36
&& expected.is_infinite() && expected.is_infinite()

4
library/compiler-builtins/libm/src/math/expm1f.rs
View file

@ -13,7 +13,6 @@
* ==================================================== * ====================================================
*/ */
const O_THRESHOLD: f32 = 8.8721679688e+01; /* 0x42b17180 */
const LN2_HI: f32 = 6.9313812256e-01; /* 0x3f317180 */ const LN2_HI: f32 = 6.9313812256e-01; /* 0x3f317180 */
const LN2_LO: f32 = 9.0580006145e-06; /* 0x3717f7d1 */ const LN2_LO: f32 = 9.0580006145e-06; /* 0x3717f7d1 */
const INV_LN2: f32 = 1.4426950216e+00; /* 0x3fb8aa3b */ const INV_LN2: f32 = 1.4426950216e+00; /* 0x3fb8aa3b */
@ -50,7 +49,8 @@ pub fn expm1f(mut x: f32) -> f32 {
if sign { if sign {
return -1.; return -1.;
} }
if x > O_THRESHOLD { if hx > 0x42b17217 {
/* x > log(FLT_MAX) */
x *= x1p127; x *= x1p127;
return x; return x;
} }