8368825: Use switch expression for DateTimeFormatterBuilder pattern character lookup

[wenshao]

The DateTimeFormatterBuilder::FIELD_MAP previously used a Map<Character, TemporalField> for mapping pattern characters to TemporalField
instances. This PR refactors that implementation to use a switch expression instead, which eliminates the need to hold a Map in
memory.

The switch expression approach offers these advantages:

• No memory overhead for maintaining a HashMap structure
• More direct character-to-field mapping without hash computation
• Better code readability and maintainability

This change maintains the same functionality while improving the memory efficiency of pattern character lookup in
DateTimeFormatterBuilder by eliminating the static Map that was previously used for character-to-field mapping.

• before

---

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Issue

• JDK-8368825: Use switch expression for DateTimeFormatterBuilder pattern character lookup (Enhancement - P4)

Reviewers

• Roger Riggs (@RogerRiggs - Reviewer)
• Naoto Sato (@naotoj - Reviewer)
• Stephen Colebourne (@jodastephen - Author)

Reviewing

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[openjdk]

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

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8368825: Use switch expression for DateTimeFormatterBuilder pattern character lookup

Reviewed-by: rriggs, naoto, scolebourne

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At the time when this comment was updated there had been 835 new commits pushed to the master branch:

• 5251405: 8368848: JShell's code completion not always working for multi-snippet inputs
• fa3af82: 8338197: [ubsan] ad_x86.hpp:6417:11: runtime error: shift exponent 100 is too large for 32-bit type 'unsigned int'
• 4df41d2: 8368698: runtime/cds/appcds/aotCache/OldClassSupport.java assert(can_add()) failed: Cannot add TrainingData objects
• ... and 832 more: https://git.openjdk.org/jdk/compare/57553ca1dbc63e329116bc11764816a4c5ccb297...master

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[openjdk]

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[liach]

with key values in the range 'A-Z' and 'a-z'.

You can use a table of size 64, and access the table with & 63.

[wenshao]

with key values in the range 'A-Z' and 'a-z'.

You can use a table of size 64, and access the table with & 63.

This will save memory space, but it will also increase the complexity a little bit.

    @Stable
    private static final TemporalField[] FIELD_MAP = new TemporalField[64];
    static {
        // SDF = SimpleDateFormat
        FIELD_MAP['G' & 0x3f] = ChronoField.ERA;                       // SDF, LDML (different to both for 1/2 chars)
        FIELD_MAP['y' & 0x3f] = ChronoField.YEAR_OF_ERA;               // SDF, LDML
        FIELD_MAP['u' & 0x3f] = ChronoField.YEAR;                      // LDML (different in SDF)
        FIELD_MAP['Q' & 0x3f] = IsoFields.QUARTER_OF_YEAR;             // LDML (removed quarter from 310)
        FIELD_MAP['q' & 0x3f] = IsoFields.QUARTER_OF_YEAR;             // LDML (stand-alone)
        FIELD_MAP['M' & 0x3f] = ChronoField.MONTH_OF_YEAR;             // SDF, LDML
        FIELD_MAP['L' & 0x3f] = ChronoField.MONTH_OF_YEAR;             // SDF, LDML (stand-alone)
        FIELD_MAP['D' & 0x3f] = ChronoField.DAY_OF_YEAR;               // SDF, LDML
        FIELD_MAP['d' & 0x3f] = ChronoField.DAY_OF_MONTH;              // SDF, LDML
        FIELD_MAP['F' & 0x3f] = ChronoField.ALIGNED_WEEK_OF_MONTH;     // SDF, LDML
        FIELD_MAP['E' & 0x3f] = ChronoField.DAY_OF_WEEK;               // SDF, LDML (different to both for 1/2 chars)
        FIELD_MAP['c' & 0x3f] = ChronoField.DAY_OF_WEEK;               // LDML (stand-alone)
        FIELD_MAP['e' & 0x3f] = ChronoField.DAY_OF_WEEK;               // LDML (needs localized week number)
        FIELD_MAP['a' & 0x3f] = ChronoField.AMPM_OF_DAY;               // SDF, LDML
        FIELD_MAP['H' & 0x3f] = ChronoField.HOUR_OF_DAY;               // SDF, LDML
        FIELD_MAP['k' & 0x3f] = ChronoField.CLOCK_HOUR_OF_DAY;         // SDF, LDML
        FIELD_MAP['K' & 0x3f] = ChronoField.HOUR_OF_AMPM;              // SDF, LDML
        FIELD_MAP['h' & 0x3f] = ChronoField.CLOCK_HOUR_OF_AMPM;        // SDF, LDML
        FIELD_MAP['m' & 0x3f] = ChronoField.MINUTE_OF_HOUR;            // SDF, LDML
        FIELD_MAP['s' & 0x3f] = ChronoField.SECOND_OF_MINUTE;          // SDF, LDML
        FIELD_MAP['S' & 0x3f] = ChronoField.NANO_OF_SECOND;            // LDML (SDF uses milli-of-second number)
        FIELD_MAP['A' & 0x3f] = ChronoField.MILLI_OF_DAY;              // LDML
        FIELD_MAP['n' & 0x3f] = ChronoField.NANO_OF_SECOND;            // 310 (proposed for LDML)
        FIELD_MAP['N' & 0x3f] = ChronoField.NANO_OF_DAY;               // 310 (proposed for LDML)
        FIELD_MAP['g' & 0x3f] = JulianFields.MODIFIED_JULIAN_DAY;

    private void parsePattern(String pattern) {
        // ...
        TemporalField field = FIELD_MAP[cur & 0x3f];
        // ...
    }

[mlbridge]

Webrevs

• 03: Full - Incremental (d3aa1333)
• 02: Full - Incremental (c6ce33c5)
• 01: Full - Incremental (abb2123e)
• 00: Full (aeab63da)

[liach]

A sensible minor optimization. Looking at this code, I think to accomplish real performance gains, we might need some redesign to simplify the branchiness and speed up lookup.

[RogerRiggs]

The suggestion, from Claes, to use and expression switch would work well too.

[wenshao]

j.t.f.DateTimeFormatterBuilder#parsePattern This is a large method with codeSize 1300+. I think optimizing it should be a separate PR.

[liach]

We might just remove this map when we optimize that huge method, which is this map's only user.

[RogerRiggs]

looks good.

[wenshao]

/integrate

[openjdk]

Going to push as commit 2c7f738.
Since your change was applied there have been 839 commits pushed to the master branch:

• 56baf64: 8368520: TLS 1.3 KeyUpdate fails with SunPKCS11 provider
• cc563c8: 8368866: compiler/codecache/stress/UnexpectedDeoptimizationTest.java intermittent timed out
• 8be1616: 8367609: serviceability/sa/ClhsdbPmap.java fails when built with Clang
• ... and 836 more: https://git.openjdk.org/jdk/compare/57553ca1dbc63e329116bc11764816a4c5ccb297...master

Your commit was automatically rebased without conflicts.

[openjdk]

@wenshao Pushed as commit 2c7f738.

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