RFR: 8356814: LineBreakMeasurer.nextLayout() slower than necessary when no break needed

[Harshitha Onkar]

On Mon, 19 May 2025 12:26:54 GMT, Daniel Gredler <dgredler at openjdk.org> wrote:

>> `LineBreakMeasurer.nextLayout()` calls `nextOffset()` internally to calculate the layout limit. When this happens, a `GlyphVector` is created and the layout engine is invoked to shape the text. The `GlyphVector` is cached in the relevant `ExtendedTextSourceLabel` component.
>> 
>> `LineBreakMeasurer.nextLayout()` then calls `TextMeasurer.getLayout()` which eventually asks that same `ExtendedTextSourceLabel` component for a subset component. This triggers the creation of a fresh `ExtendedTextSourceLabel` without the cached `GlyphVector`.
>> 
>> However, this fresh `ExtendedTextSourceLabel` is not necessary if the subset requested perfectly matches the already-existing `ExtendedTextSourceLabel` component. This happens when the text is short enough that no line break is needed.
>> 
>> I think we should change `ExtendedTextSourceLabel.getSubset()` to return `this` if the requested subset is identical to the existing instance. This will allow us to use the existing cached `GlyphVector`, and the call to `LineBreakMeasurer.nextLayout()` will trigger text shaping once, rather than twice.
>> 
>> In local testing, the test program below ran in ~1250 ms before this optimization, and ran in ~960 ms after the change (a 23% reduction in run time).
>> 
>> The following three existing test classes provide good regression test coverage for this change:
>> - test/jdk/java/awt/font/LineBreakMeasurer/LineBreakWithTrackingAuto
>> - test/jdk/java/awt/font/LineBreakMeasurer/TestLineBreakWithFontSub
>> - test/jdk/java/awt/font/LineBreakMeasurer/FRCTest
>> 
>> 
>> public class LineBreakMeasurerPerfTest {
>> 
>>     public static void main(String[] args) {
>> 
>>         float advance = 0;
>>         long start = System.currentTimeMillis();
>>         AttributedString string = new AttributedString("This is a test.");
>>         FontRenderContext frc = new FontRenderContext(new AffineTransform(), true, true);
>> 
>>         for (int i = 0; i < 100_000; i++) {
>>             LineBreakMeasurer measurer = new LineBreakMeasurer(string.getIterator(), frc);
>>             TextLayout layout = measurer.nextLayout(999); // large enough to not require break
>>             advance = Math.max(advance, layout.getAdvance());
>>         }
>> 
>>         long end = System.currentTimeMillis();
>>         System.out.println((end - start) + " ms elapsed (advance: " + advance + ")");
>>     }
>> 
>> }
>
> If any reviewers have some extra cycles, I'd appreciate a second review on this one. Thanks!

@gredler I'm in the process of reviewing it. I can sponsor the PR once done with testing.

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PR Comment: https://git.openjdk.org/jdk/pull/25193#issuecomment-2902500432