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

[Daniel Gredler]

On Mon, 12 May 2025 20:35:52 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!

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PR Comment: https://git.openjdk.org/jdk/pull/25193#issuecomment-2890826777