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

[Raffaello Giulietti]

On Sun, 13 Oct 2024 15:40:27 GMT, fabioromano1 <duke at openjdk.org> wrote:

>> 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 `m <= maxPowsOf5`, 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`...

What's really crucial for _correctness_ is to ensure maxPowsOf5 >= k.

But for performance you also want maxPowsOf5 to be as small as possible. So, the fact that it turns out that maxPowsOf5 <= m + 1 guarantees that maxPowsOf5 is the best value that can be computed very efficiently. It's more a "quality of service" guarantee than anything fundamental.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21323#discussion_r1798429260