RFR: 8315585: Optimization for decimal to string

[Johannes Graham]

On Sat, 25 Jan 2025 07:25:40 GMT, Shaojin Wen <swen at openjdk.org> wrote:

> Continue to complete PR #16006 and PR #21593 to improve BigDecimal::toString and BigDecimal::toPlainString performance and reduce duplicate code

src/java.base/share/classes/java/math/BigDecimal.java line 3538:

> 3536:             return (signum < 0 ? "-0." : "0.").concat(intString);
> 3537:         } else if (insertionPoint > 0) { /* Point goes inside intVal */
> 3538:             buf = new StringBuilder();

Could calculate the precise size for the StringBuilder

src/java.base/share/classes/java/math/BigDecimal.java line 3542:

> 3540:                 buf.append('-');
> 3541:             buf.append(intString)
> 3542:                     .insert(insertionPoint + (signum < 0 ? 1 : 0), '.');

Instead of the insert, could do an append of the  prefix, then dot, then the suffix.

src/java.base/share/classes/java/math/BigDecimal.java line 4209:

> 4207: 
> 4208:     private String layoutCharsE(boolean sci, String coeff, int coeffLen, long adjusted) {
> 4209:         StringBuilder buf = new StringBuilder(32);

The comment above about constructing a buffer probably belongs here. Also could calculate a better approximation of the size - possibly coeffLen+14 if that comment is accurate.

src/java.base/share/classes/jdk/internal/util/DecimalDigits.java line 347:

> 345:      * @return index of the most significant digit or minus sign, if present
> 346:      */
> 347:     public static int getChars(long i, int index, char[] buf) {

Before dropping this method, there is another candidate to use  it here : https://github.com/openjdk/jdk/blob/1d2eb2fbaea700fc77b644b5eb5a8a7c40ede108/src/java.base/share/classes/jdk/internal/math/FloatingDecimal.java#L367

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PR Review Comment: https://git.openjdk.org/jdk/pull/23310#discussion_r1929857010
PR Review Comment: https://git.openjdk.org/jdk/pull/23310#discussion_r1929857288
PR Review Comment: https://git.openjdk.org/jdk/pull/23310#discussion_r1929860496
PR Review Comment: https://git.openjdk.org/jdk/pull/23310#discussion_r1929495730