RFR: 8375285: Port fdlibm asinh to Java

[Anton Artemov]

On Fri, 23 Jan 2026 02:36:15 GMT, Joe Darcy <darcy at openjdk.org> wrote:

>> Hi, please consider the following changes:
>> 
>> This is a port of FDLIBM asinh method.
>> 
>> Original C vs transliteration port:
>> 
>> 
>> $ diff -w fdlib_asinh.c Asinh.translit.java
>> 1c1,3
>> < /* asinh(x)
>> ---
>>> /**
>>> * Return the Inverse Hyperbolic Sine of x
>>> *
>> 2a5
>>> *
>> 10a14,17
>>> private static final class Asinh {
>>>       private static final double one = 1.0;
>>>       private static final double ln2 = 6.93147180559945286227e-01;
>>>       private static final double huge = 1.0e300;
>> 12,29c19
>> < #include "fdlibm.h"
>> <
>> < #ifdef __STDC__
>> < static const double
>> < #else
>> < static double
>> < #endif
>> < one =  1.00000000000000000000e+00, /* 0x3FF00000, 0x00000000 */
>> < ln2 =  6.93147180559945286227e-01, /* 0x3FE62E42, 0xFEFA39EF */
>> < huge=  1.00000000000000000000e+300;
>> <
>> < #ifdef __STDC__
>> <       double asinh(double x)
>> < #else
>> <       double asinh(x)
>> <       double x;
>> < #endif
>> < {
>> ---
>>>       static double compute(double x) {
>> 39c29
>> <           w = __ieee754_log(fabs(x))+ln2;
>> ---
>>>                       w = log(Math.abs(x))+ln2;
>> 41,42c31,32
>> <           t = fabs(x);
>> <           w = __ieee754_log(2.0*t+one/(sqrt(x*x+one)+t));
>> ---
>>>                       t = Math.abs(x);
>>>                       w = log(2.0*t+one/(sqrt(x*x+one)+t));
>> 45c35
>> <           w =log1p(fabs(x)+t/(one+sqrt(one+t)));
>> ---
>>>                       w =log1p(Math.abs(x)+t/(one+sqrt(one+t)));
>> 47a38
>>>       }
>> 
>> Transliteration port vs more idiomatic port:
>> 
>> 
>> $ diff -w Asinh.translit.java Asinh.fdlibm.java
>> 6,9c6,12
>> < *  Based on
>> < *    asinh(x) = sign(x) * log [ |x| + sqrt(x*x+1) ]
>> < *   we have
>> < *   asinh(x) := x  if  1+x*x=1,
>> ---
>>>  *
>>>  *      asinh(x) is defined so that asinh(sinh(alpha)) = alpha -INF < alpha < < INF
>>>  *      and sinh(asinh(x)) = x, -INF < x  < INF.
>>>  *      It can be written as asinh(x) = ln(x + sqrt(x^2 + 1)), -INF < x  < INF.
>>>  *      1. Replace x by |x| as the function is odd.
>>>  *      2.
>>>  *          asinh(x) := x, if 1+x^2 = 1,
>> 12a16,22
>>>  *
>>>  *
>>>  *
>>>  * Special cases:
>>>  *      only asinh(0)=0 is exact for finite x.
>>>  *      asinh(NaN) is NaN
>>>  *      asinh(INF) is INF
>> 14,15c24
>> < private static final class Asinh {
>> <       private static final double one = 1.0;
>> ---
>>> static final class Asinh {
>> 24c33,35
>> <               if(ix>=0x7ff00000) return x+x;  /* x is inf or NaN */
>> ---
>>>               if(ix >= 0x7ff00000) {
>>>                       return x...
>
> src/java.base/share/classes/java/lang/FdLibm.java line 3543:
> 
>> 3541:             int hx,ix;
>> 3542:             hx = __HI(x);
>> 3543:             ix = hx&0x7fffffff;
> 
> As part of making the code more idiomatic, I would include adding space characters around `&` and switching one-line `/* ... */` over to `//` comments where possible.

Addressed.

> src/java.base/share/classes/java/lang/Math.java line 2762:
> 
>> 2760: 
>> 2761:     /**
>> 2762:      * Returns the inverse hyperbolic sine of a {@code double} value.
> 
> The method specification should provide some general statements in term of the accuracy of the method in terms of ulps.
> 
> Explicitly specifying asinh(NaN) is NaN is probably worthwhile too.

Addressed in the latest commit.

> src/java.base/share/classes/java/lang/StrictMath.java line 2173:
> 
>> 2171:     }
>> 2172: 
>> 2173:     /**
> 
> The list of "these methods must use FDLIBM algorithms..." up at the class-level specs of StrictMath should be updated to include asinh.

Addressed.

> test/jdk/java/lang/StrictMath/HyperbolicTests.java line 497:
> 
>> 495:     private static int testAsinh() {
>> 496:         int failures = 0;
>> 497:         double [][] testCases = {
> 
> How were these values generated?

These results were generated by running a small C++ program linked to a IEEE-standard built fdlibm.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/29273#discussion_r2727300965
PR Review Comment: https://git.openjdk.org/jdk/pull/29273#discussion_r2727300757
PR Review Comment: https://git.openjdk.org/jdk/pull/29273#discussion_r2727301096
PR Review Comment: https://git.openjdk.org/jdk/pull/29273#discussion_r2727301340