RFR: JDK-8301444: Port fdlibm hyperbolic transcendental functions to Java [v3]

[Joe Darcy]

On Mon, 6 Feb 2023 21:55:26 GMT, Joe Darcy <darcy at openjdk.org> wrote:

>> test/jdk/java/lang/StrictMath/FdlibmTranslit.java line 874:
>> 
>>> 872:             // lx = *( (((*(unsigned*)&one)>>29)) + (unsigned*)&x);
>>> 873:             // lx =  (((*(unsigned*)&one)>>29)) + (unsigned*)&x ;
>>> 874:             lx = __LO(x);
>> 
>> This aspect of the transliteration is unclear.
>> The C code seems to ignore endianness of `double` values, and there's a shift by 29 that seems intentional for some reason.
>> Are we safe with the `__LO(x)` transliteration?
>
> Yes, it gave me pause when doing the transliteration.
> 
> Unpacking the code a bit,
> 
> 
>             /* |x| in [log(maxdouble), overflowthresold] */
>             // lx = *( (((*(unsigned*)&one)>>29)) + (unsigned*)&x);
>             lx = __LO(x);
>             if (ix<0x408633CE || 
>             ((ix==0x408633ce)&&(Long.compareUnsigned(lx, 0x8fb9f87d) <= 0 ))) {...}
> 
> 
> Assuming the comment accurate describe the intention of the code, identifying values between log(maxdouble) and the overflow threshold. Those particular values are as decimal fp, hex hp, and long bits:
> 
> 709.782712893384    0x1.62e42fefa39efp9    40862e42__fefa39ef
> 710.4758600739439    0x1.633ce8fb9f87dp9    408633ce_8fb9f87d
> 
> so the conditional is checking if the high word (ix) is for something less than the low end of the range OR if the high word matches the high word for the high end of the range AND low bits are for less than the high end of the range. Therefore, I think taking __LO(x) is the correct semantics here.
> 
> FWIW, the ExhaustingTest of running all the float values against sinh/cosh/tanh in the transliteration port passes when using JDK 20 baseline as a reference.

PS One possible future refactoring to the code in

src/java.base/share/classes/java/lang/FdLibm.java

would be to replace such integer-based range checks with floating-point based range checks, in this case something like:


if (absX <= 0x1.633ce8fb9f87dp9) { // 710.4758600739439
    ...}

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

PR: https://git.openjdk.org/jdk/pull/12429