RFR: 8341470: BigDecimal.stripTrailingZeros() optimization [v42]

[fabioromano1]

On Sun, 13 Oct 2024 13:13:30 GMT, Raffaello Giulietti <rgiulietti at openjdk.org> wrote:

>> fabioromano1 has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Minor change
>
> src/java.base/share/classes/java/math/BigDecimal.java line 5270:
> 
>> 5268: 
>> 5269:         intVal = intVal.shiftRight(powsOf2); // remove powers of 2
>> 5270:         // maxPowsOf5 >= floor(log5(intVal)) >= max{n : (intVal % 5^n) == 0}
> 
> Suggestion:
> 
>         // Let k = max{n : (intVal % 5^n) == 0}, m = max{n : 5^n <= intVal}, so m >= k.
>         // Let b = intVal.bitLength(). It can be shown that
>         // | b * LOG_5_OF_2 - b log5(2) | < 2^(-21) (fp viz. real arithmetic),
>         // which entails m <= maxPowsOf5 <= m + 1, where maxPowsOf5 is as below.
>         // Hence, maxPowsOf5 >= k and is never off by more than 1 from the theoretical m.

@rgiulietti Good, but I would not put the inequality `maxPowsOf5 <= m + 1` and say that `maxPowsOf5` is never off by more than 1 from the theoretical `m`, because it is not crucial as `maxPowsOf5 >= k`, and because `m` is in function of `intVal`, while `maxPowsOf5` is in function of `2^b`, so it is not obvious that there's no power of five `pow` such that `intVal < pow < 2^b`...

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

PR Review Comment: https://git.openjdk.org/jdk/pull/21323#discussion_r1798415994