RFR: 8367603: Optimize exact division in BigDecimal [v15]

[Raffaello Giulietti]

On Thu, 25 Sep 2025 12:26:27 GMT, fabioromano1 <duke at openjdk.org> wrote:

>> A formula to lower the overestimate for the precision of an exact quotient, without introducing too expensive operations of division on numerator and denominator.
>
> fabioromano1 has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Comments

Otherwise looks good.

src/java.base/share/classes/java/math/BigDecimal.java line 1807:

> 1805:              *   - if i > j, then a/b = (a/b') ⋅ 5^(i-j) / 10^i.
> 1806:              */
> 1807:             BigInteger den = divisor.unscaledValue();

Maybe "synchronize" the names between the comment and the code.
For example, either rename `powsOf2` to `i` or the other way round.
Alternatively, provide a "translation table", e.g., `i` <-> `powsOf2`, etc.

src/java.base/share/classes/java/math/BigDecimal.java line 1815:

> 1813:             BigInteger[] qr;
> 1814:             int i;
> 1815:             for (i = 0; ; i++) {

What about a comment explaining the idea of this and the following fast loops using 5^(2^i) powers to find the correct `powsOf5` (the `j` in the comment)?

test/micro/org/openjdk/bench/java/math/BigDecimalDivide.java line 2:

> 1: /*
> 2:  * Copyright (c) 2024, Oracle and/or its affiliates. All rights reserved.

Suggestion:

 * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.

test/micro/org/openjdk/bench/java/math/BigDecimalDivide.java line 153:

> 151:         }
> 152:     }
> 153: }

It would be useful to publish the "before" and "after" results of running these benchmarks.

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PR Review: https://git.openjdk.org/jdk/pull/27271#pullrequestreview-3268234720
PR Review Comment: https://git.openjdk.org/jdk/pull/27271#discussion_r2379473964
PR Review Comment: https://git.openjdk.org/jdk/pull/27271#discussion_r2379476985
PR Review Comment: https://git.openjdk.org/jdk/pull/27271#discussion_r2379478040
PR Review Comment: https://git.openjdk.org/jdk/pull/27271#discussion_r2379479325