RFR: 8341470: BigDecimal.stripTrailingZeros() optimization [v33]

[Raffaello Giulietti]

On Fri, 11 Oct 2024 18:32:01 GMT, Raffaello Giulietti <rgiulietti at openjdk.org> wrote:

>> Actually, if we reason in terms of "ulp vs precision", the computation should be safe: `Math.log()`'s results are within 1 ulp of the exact result, and the floating point operations are a multiplication and a division. The division to compute `LOG_5_OF_2` costs 1/2 ulp plus the errors of the operands, so 2.5 ulps. Same for multiplication, but `intVal.bitLength()` has an exact `double` value, so the total roundoff error of `intVal.bitLength() * LOG_5_OF_2` is 3 ulps. Since the integer part of `intVal.bitLength() * LOG_5_OF_2` is representable with 31 bits, and double has 53 bits of precision, we can reasonably say that `Math.ceil()` can always guarantee `maxPowsOf5 >= log5(intVal)`.
>
> If the mathematical value v of the product and its floating-point value fp are separated by an integer i in such a way that fp < i < v, we are in trouble: the ceilings will be different, even if the values are very close to each other.

One might prove that this cannot happen for a specific approximation of log5(2), like `LOG_5_OF_2` and for all bit length, but I don't think it is worthwhile to put too much effort on this, given the performance figures.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21323#discussion_r1797310959