RFR: 8341470: BigDecimal.stripTrailingZeros() optimization [v44]

[Raffaello Giulietti]

On Sun, 13 Oct 2024 22:45:25 GMT, fabioromano1 <duke at openjdk.org> wrote:

>> An optimized algorithm for `BigDecimal.stripTrailingZeros()` that uses repeated squares trick.
>
> fabioromano1 has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Added mathematical comments for maxPowsOf5

Will approve in a couple of days if by then there are no other substantial changes.

src/java.base/share/classes/java/math/BigDecimal.java line 5280:

> 5278:         BigInteger[] qr; // quotient-remainder pair
> 5279:         // Remove 5^(2^i) from the factors of intVal, until 5^remainingZeros < 5^(2^i)
> 5280:         // (i.e., there are too few powers of five left to remove).

It's a bit unclear what's meant here with "_too few_ powers of five left to remove".
Can we have something more explicit?

src/java.base/share/classes/java/math/BigDecimal.java line 5297:

> 5295:         // bitLength(remainingZeros) == min{n >= 0 : 5^(2^n) > 5^remainingZeros}
> 5296:         // so, while the loop condition is true,
> 5297:         // the invariant i == max{n : 5^(2^n) <= 5^remainingZeros}

Suggestion:

        // the invariant i == max{n : 5^(2^n) <= 5^remainingZeros},
        // which is equivalent to i = bitLengthForLong(remainingZeros) - 1,

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

PR Review: https://git.openjdk.org/jdk/pull/21323#pullrequestreview-2366952196
PR Review Comment: https://git.openjdk.org/jdk/pull/21323#discussion_r1799690935
PR Review Comment: https://git.openjdk.org/jdk/pull/21323#discussion_r1799691009