RFR: 8315585: Optimization for decimal to string [v2]

[Claes Redestad]

On Mon, 9 Oct 2023 19:20:29 GMT, Shaojin Wen <duke at openjdk.org> wrote:

>> I submitted PR #15555 before, and there were too many changes. I split it into multiple PRs with small changes. This one is one of them.
>> 
>> this PR removed the duplicate code for getChars in BigDecimal#StringBuilderHelper, i also make performance faster.
>> Please review and don't hesitate to critique my approach and patch.
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   refactor based on @liach 's review

I'm not really qualified to review the floating point code. Simplifying away offset and getting rid of the StringBuilderHelper all seem like good improvements, though I think it'd be good if we could either avoid or split out the `JLA` changes.

src/java.base/share/classes/java/lang/System.java line 2548:

> 2546: 
> 2547:             public int stringSize(int i) {
> 2548:                 return Long.stringSize(i);

Shouldn't this be `return Integer.stringSize(i);`?

src/java.base/share/classes/java/math/BigDecimal.java line 4168:

> 4166:             int lowInt = (int)intCompact % 100;
> 4167:             int highInt = (int)intCompact / 100;
> 4168:             int highIntSize = JLA.stringSize(highInt);

Which micros cover performance of this branch?

What performance would you get if you simplified this to, say, `return highInt + (lowInt < 10 ? ".0" : ".") + lowInt;`?

`lowInt` is currently unused since you use `(int) intCompact - highInt * 100` instead below. (While a clever optimization in theory, I believe the JIT should handle a pair of integer modulo and division operations (`a = intValue % 100; b = intValue / 100`) so that it only has to do one division, so please measure that whatever you do here has a significant benefit).

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

Changes requested by redestad (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16006#pullrequestreview-1668452019
PR Review Comment: https://git.openjdk.org/jdk/pull/16006#discussion_r1353035743
PR Review Comment: https://git.openjdk.org/jdk/pull/16006#discussion_r1354592018