On Mon, 20 Oct 2025 06:34:39 GMT, Shaojin Wen <[email protected]> wrote:
>> This PR refactors the Double.toHexString method to improve its performance
>> by eliminating the use of StringBuilder and regex operations. The new
>> implementation uses a direct byte array approach to construct the hex string
>> representation, which avoids the overhead of StringBuilder operations and
>> regex pattern matching.
>>
>> Existing tests in `java/lang/Double/ToHexString.java`.
>
> Shaojin Wen has updated the pull request incrementally with one additional
> commit since the last revision:
>
> from @jddarcy
src/java.base/share/classes/java/lang/Double.java line 711:
> 709: // For infinity and NaN, use the decimal output.
> 710: return Double.toString(d);
> 711: else {
Remove the `else` (but of course, not the code)
src/java.base/share/classes/java/lang/Double.java line 713:
> 711: else {
> 712: boolean negative = Math.copySign(1.0, d) == -1.0;
> 713: d = Math.abs(d);
Move this down, after the check for `0.0`.
src/java.base/share/classes/java/lang/Double.java line 717:
> 715: if (d == 0.0) {
> 716: return negative ? "-0x0.0p0" : "0x0.0p0";
> 717: } else {
As above, remove the `else`.
src/java.base/share/classes/java/lang/Double.java line 724:
> 722: // so that the string representation has a known length.
> 723: // This ensures we always have 13 hex digits to work
> with (52 bits / 4 bits per hex digit)
> 724: long signifBits = (Double.doubleToLongBits(d)
Suggestion:
long signifBits = Double.doubleToLongBits(d) &
DoubleConsts.SIGNIF_BIT_MASK;
plus the change below.
src/java.base/share/classes/java/lang/Double.java line 732:
> 730: // We limit to 12 because we want to keep at least 1 hex
> digit (13 total - 12 = 1)
> 731: // assert 0 <= trailingZeros && trailingZeros <= 12
> 732: int trailingZeros = Math.min(12,
> ((Long.numberOfTrailingZeros(signifBits) & 0xFC) >> 2));
Suggestion:
int trailingZeros = Long.numberOfTrailingZeros(signifBits | 1L
<< 4 * 12) >> 2;
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/27811#discussion_r2445142978
PR Review Comment: https://git.openjdk.org/jdk/pull/27811#discussion_r2445143666
PR Review Comment: https://git.openjdk.org/jdk/pull/27811#discussion_r2445143853
PR Review Comment: https://git.openjdk.org/jdk/pull/27811#discussion_r2445144092
PR Review Comment: https://git.openjdk.org/jdk/pull/27811#discussion_r2445144567
