8370013: Refactor Double.toHexString to eliminate regex and StringBuilder #27811

wenshao · 2025-10-14T23:26:00Z

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.

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Issue

JDK-8370013: Refactor Double.toHexString to eliminate regex and StringBuilder (Enhancement - P4)

Reviewers

Raffaello Giulietti (@rgiulietti - Reviewer)
Joe Darcy (@jddarcy - Reviewer)

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$ git checkout pull/27811

Update a local copy of the PR:
$ git checkout pull/27811
$ git pull https://git.openjdk.org/jdk.git pull/27811/head

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$ git pr checkout 27811

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bridgekeeper · 2025-10-14T23:27:54Z

👋 Welcome back swen! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2025-10-14T23:28:25Z

@wenshao This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8370013: Refactor Double.toHexString to eliminate regex and StringBuilder

Reviewed-by: rgiulietti, darcy

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 40 new commits pushed to the master branch:

a925461: 8370442: Compilation error in jpackage EntitlementsTest test
afba636: 8369991: Thread blocking during JFR emergency dump must be in safepoint safe state
92e380c: 8361451: Test vmTestbase/metaspace/stressHierarchy/stressHierarchy012/TestDescription.java fails with OutOfMemoryError: Metaspace
... and 37 more: https://git.openjdk.org/jdk/compare/eee2908853342ae305c200f7ec37081ea939a4fa...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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openjdk · 2025-10-14T23:29:16Z

@wenshao The following label will be automatically applied to this pull request:

core-libs

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

wenshao · 2025-10-15T00:02:58Z

The test results run under Mac Book M1 Pro are as follows

# before 3f11c4307ee1ca918926c8c522d2fc496947980e
-Benchmark            Mode  Cnt    Score    Error  Units
-Doubles.toHexString  avgt   15  228.383 ± 17.172  ns/op

# after c18f7632f2f2cf0752b3f4651d4cadf95bb859c6
+Benchmark            Mode  Cnt   Score   Error  Units
+Doubles.toHexString  avgt   15  15.138 ± 1.070  ns/op

More than 10 times performance improvement

rgiulietti · 2025-10-16T15:24:03Z

@wenshao Seems like a nice improvement.
Will have a closer look in the next days.

mlbridge · 2025-10-16T23:40:39Z

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jddarcy · 2025-10-20T04:29:55Z

/reviewers 2 reviewer

openjdk · 2025-10-20T04:31:28Z

@jddarcy
The total number of required reviews for this PR (including the jcheck configuration and the last /reviewers command) is now set to 2 (with at least 2 Reviewers).

src/java.base/share/classes/java/lang/Double.java

jddarcy · 2025-10-20T04:38:07Z

src/java.base/share/classes/java/lang/Double.java

+                // Calculate the number of trailing zeros in the significand (in groups of 4 bits)
+                // This is used to remove trailing zeros from the hex representation
+                // We limit to 12 because we want to keep at least 1 hex digit (13 total - 12 = 1)
+                int trailingZeros = Math.min(12, ((Long.numberOfTrailingZeros(signifBits) & 0xFC) >> 2));


Some additional discussion of the computation here would be helpful to casual readers.

I would also be tempted to add

assert 0 <= trailingZeros && trailingZeros <= 12;

jddarcy · 2025-10-20T04:40:45Z

The existing regression tests may be adequate for this refactoring as the high-level algorithm and special cases are similar, but I'll want to take another look before approving this PR. Thanks.

theRealAph · 2025-10-20T08:58:07Z

src/java.base/share/classes/java/lang/Double.java

+                    // Extract 4 bits at a time from left to right
+                    // Shift right by (12 - i) * 4 positions and mask with 0xF
+                    // Integer.digits maps values 0-15 to '0'-'f' characters
+                    chars[index++] = Integer.digits[((int)(signifBits >> ((12 - i) << 2))) & 0xF];


Suggested change

chars[index++] = Integer.digits[((int)(signifBits >> ((12 - i) << 2))) & 0xF];

int digit = ((int)(signifBits >> ((12 - i) << 2))) & 0xF;

chars[index++] = digit + digit < 10 ? '0' : 'a';

Keeping the same as Integer.toHexString/Long.toHexString, I think it is okay to continue using table lookup here. If we want to eliminate the table lookup, we can use vectorization to eliminate it. I have submitted another PR before #22942

src/java.base/share/classes/java/lang/Double.java

jddarcy · 2025-10-20T15:24:03Z

The existing regression tests may be adequate for this refactoring as the high-level algorithm and special cases are similar, but I'll want to take another look before approving this PR. Thanks.

Looking over test/jdk/java/lang/ToHexString.java and the new code, I think test cases like the following should be added:

0x1.0p1, // Max trailing zeros
0x1.1p1,
0x1.01p1,
...
0x1.000000000001p1 // No trailing zeros

Specifically, numbers with all the possible numbers of trailing zeros should be explicitly tested.

wenshao · 2025-10-20T15:34:38Z

@wenshao What's wrong with my other suggestions? In particular, the abs() line can be moved after the check for 0.0, and I cannot see the need for the 0x1000_0000_0000_0000L bit in signifBits.

Your suggestion is fine, but I have to deal with them one by one. They should be done by now. Is there anything I missed?

rgiulietti · 2025-10-20T15:39:33Z

Ah OK, thanks.
I currently have nothing more to add.

src/java.base/share/classes/java/lang/Double.java

test/jdk/java/lang/Double/ToHexString.java

openjdk · 2025-10-21T04:10:43Z

@wenshao this pull request can not be integrated into master due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout float_to_hex_str_2510
git fetch https://git.openjdk.org/jdk.git master
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge master"
git push

…2510 # Conflicts: # test/jdk/java/lang/Double/ToHexString.java

src/java.base/share/classes/java/lang/Double.java

Co-authored-by: Raffaello Giulietti <raffaello.giulietti@oracle.com>

rgiulietti

@wenshao Thanks for your contribution.

wenshao · 2025-10-24T00:38:26Z

/integrate

openjdk · 2025-10-24T00:40:18Z

Going to push as commit 5862358.
Since your change was applied there have been 63 commits pushed to the master branch:

d720a84: 8343220: Add test cases to AppContentTest jpackage test
62f11cd: 8326609: New AES implementation with updates specified in FIPS 197
b2e431a: 8369068: GenShen: Generations still aren't reconciled assertion failure
... and 60 more: https://git.openjdk.org/jdk/compare/eee2908853342ae305c200f7ec37081ea939a4fa...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-10-24T00:40:26Z

@wenshao Pushed as commit 5862358.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

add toHexString benchmark

3f11c43

wenshao changed the title ~~Refactor Double::toHexString(~~ Refactor Double::toHexString Oct 14, 2025

openjdk bot added the core-libs core-libs-dev@openjdk.org label Oct 14, 2025

wenshao changed the title ~~Refactor Double::toHexString~~ Refactor Double.toHexString to eliminate regex and StringBuilder Oct 14, 2025

wenshao force-pushed the float_to_hex_str_2510 branch 5 times, most recently from 442bad2 to 8dfe482 Compare October 15, 2025 16:25

speed up Double.toHexString

e845b83

wenshao force-pushed the float_to_hex_str_2510 branch from 8dfe482 to e845b83 Compare October 15, 2025 16:40

wenshao changed the title ~~Refactor Double.toHexString to eliminate regex and StringBuilder~~ 8370013: Refactor Double.toHexString to eliminate regex and StringBuilder Oct 16, 2025

wenshao added 2 commits October 16, 2025 23:37

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wenshao marked this pull request as ready for review October 16, 2025 23:35

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add tests for removed trailing zeros, from @jddarcy

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extract doubleToLongBits, from @jddarcy

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…2510 # Conflicts: # test/jdk/java/lang/Double/ToHexString.java

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Update src/java.base/share/classes/java/lang/Double.java

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Co-authored-by: Raffaello Giulietti <raffaello.giulietti@oracle.com>

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	chars[index++] = Integer.digits[((int)(signifBits >> ((12 - i) << 2))) & 0xF];
	int digit = ((int)(signifBits >> ((12 - i) << 2))) & 0xF;
	chars[index++] = digit + digit < 10 ? '0' : 'a';

8370013: Refactor Double.toHexString to eliminate regex and StringBuilder #27811

8370013: Refactor Double.toHexString to eliminate regex and StringBuilder #27811

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