util: fix codec for negative zero | tidb-test=pr/2427

[wshwsh12]

What problem does this PR solve?

Issue Number: close #41878
Negative zero is incorrectly converted to zero after being encoded and then decoded.

Problem Summary:
Correctly handle the encoding issue of negative zero.

What changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

tidb> CREATE TABLE t1 (c0 decimal(10,0));
Query OK, 0 rows affected (0.11 sec)

tidb> INSERT INTO t1 VALUES (0);
Query OK, 1 row affected (0.02 sec)

tidb> SELECT c0 FROM t1 WHERE CAST(ATAN2(((t1.c0) IS NULL), (- (''))) AS TIME); -- actual: {}, expected: {0}
+------+
| c0   |
+------+
|    0 |
+------+
1 row in set (0.00 sec)

• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.4596%. Comparing base (4fded5f) to head (cf74ab5).
Report is 11 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #57343        +/-   ##
================================================
+ Coverage   72.9236%   73.4596%   +0.5359%     
================================================
  Files          1672       1672                
  Lines        462267     462315        +48     
================================================
+ Hits         337102     339615      +2513     
+ Misses       104417     102008      -2409     
+ Partials      20748      20692        -56     

Flag	Coverage Δ
integration	43.3910% <100.0000%> (?)
unit	72.2869% <100.0000%> (-0.0160%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.7673% <ø> (ø)
parser	∅ <ø> (∅)
br	45.1133% <ø> (-0.0154%)	⬇️

[wshwsh12]

/retest

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[XuHuaiyu]

Add some comments to make it more readable

func encodeFloatToCmpUint64(f float64) uint64 {
    u := math.Float64bits(f)
    // Check the sign bit (the highest bit in the IEEE 754 representation).
    // If the sign bit is 0, the number is non-negative (+0 is considered non-negative).
    if u&signMask == 0 {
        // For non-negative numbers (+0 included), set the sign bit to 1.
        // This ensures non-negative numbers are ordered after negative numbers
        // when compared as unsigned integers.
        u |= signMask
    } else {
        // For negative numbers (-0 included), invert all bits.
        // This reorders negative numbers so that the smallest (closest to 0)
        // has the smallest encoded value, and the most negative has the largest.
        u = ^u
    }
    return u
}

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guo-shaoge, XuHuaiyu

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-11-15 09:29:54.367532151 +0000 UTC m=+607756.558401143: ☑️ agreed by XuHuaiyu.
• 2024-11-15 09:30:35.672407303 +0000 UTC m=+607797.863276300: ☑️ agreed by guo-shaoge.

[wshwsh12]

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In response to a cherrypick label: new pull request created to branch release-8.5: #57453.