On Mon, 6 Feb 2023 22:03:17 GMT, Joe Darcy <da...@openjdk.org> wrote:
>> 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
> ...}
Let's discuss this C fragment.
`*( (((*(unsigned*)&one)>>29)) + (unsigned*)&x)`
On a big-endian platform, `*(unsigned*)&one` returns `__HI(one) = 0x3ff0_0000`.
It is then shifted right by 29, giving `1`. And `(unsigned*)&x` is the
_address_ of the high word of `x`. The sum with `1` thus gives the address of
the low word of `x`, which is dereferenced by the leftmost `*`, resulting in
`__LO(x)`.
On a little-endian platform, `*(unsigned*)&one` returns `__LO(one) = 0x0`. It
is then shifted right by 29, giving `0`. And `(unsigned*)&x` is the _address_
of the low word of `x`. The sum with `0` thus gives the address of the low word
of `x`, which is dereferenced by the leftmost `*`, resulting in `__LO(x)` as
well.
In other words, this is a complicated way to say `__LO(x)`, right? I wonder why
the author came up with this creative code...
Would it make sense to add this discussion as a block comment?
-------------
PR: https://git.openjdk.org/jdk/pull/12429
