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[web] More efficient fallback font selection #44526
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LGTM with nits
Thanks so much for this contribution!
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Thank you so much for taking the time to do all this! This is going to be an amazing improvement.
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…133820) flutter/engine@10e2df6...b867c4d 2023-08-31 [email protected] [web] More efficient fallback font selection (flutter/engine#44526) 2023-08-31 [email protected] Roll Dart SDK from ac3bc9f6351a to 9f9bd8cddfb0 (1 revision) (flutter/engine#45352) 2023-08-31 [email protected] Roll Skia from bf6b239838d3 to e1b27dcecd0a (1 revision) (flutter/engine#45349) If this roll has caused a breakage, revert this CL and stop the roller using the controls here: https://autoroll.skia.org/r/flutter-engine-flutter-autoroll Please CC [email protected],[email protected],[email protected] on the revert to ensure that a human is aware of the problem. To file a bug in Flutter: https://github.com/flutter/flutter/issues/new/choose To report a problem with the AutoRoller itself, please file a bug: https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug Documentation for the AutoRoller is here: https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md
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The PR improves the code size and runtime performance of fallback font selection.
Performance improvements
Initialization of the data structures to support fallback font selection has been moved from creating the FallbackFontManager (first frame) to the first use, i.e. the first frame actually needing a fallback font.
The numbers reported below are for a lightly edited version of the counter demo that appends to the counter about ~300 missing code points that need ~25 fallback fonts to cover the missing code points. Timings taken from a few profiles on my performance workstation.
Size improvements
Algorithm notes
Startup
The old algorithm built an interval tree from the code point ranges of the ~140 fallback fonts and uses the interval tree to build a list of fonts that support each missing code point. The new algorithm uses a binary search map that directly produces the list of fonts. There are fewer binary search ranges (~22k) than the aggregate ranges for all the fonts (~26k).
Most of the startup time gain comes from using a data unpacks directly into a useful form rather than needing processing to build an interval tree (~12ms vs ~100ms).
Running
The runtime for font selection is greatly improved for several reasons
Batching, incremental update and avoiding hash tables are roughly multiplicative in effect.
Issues
If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.
Pre-launch Checklist
///).If you need help, consider asking for advice on the #hackers-new channel on Discord.