RFR: 8370013: Refactor Double.toHexString to eliminate regex and StringBuilder [v9]

[Joe Darcy]

On Tue, 21 Oct 2025 01:28:50 GMT, Shaojin Wen <swen at openjdk.org> 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:
> 
>   fix test comment

test/jdk/java/lang/Double/ToHexString.java line 182:

> 180:             {"0x1.1p1",                                 "4001000000000000"}, // Maximum trailing zeros (all zeros after '1')
> 181:             {"0x1.0p1",                                 "4000000000000000"}, // Maximum trailing zeros (all zeros after '0')
> 182:             {"0x1.01p1",                                "4000100000000000"}, // 1  trailing zero removed

Shouldn't the number of trailing zeros be counting down at this point?

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PR Review Comment: https://git.openjdk.org/jdk/pull/27811#discussion_r2446630172