Provide a GrContextOptions::PersistentCache to Skia

[chinmaygarde]

Implement GrContextOptions::PersistentCache and provide it to Skia GrContextOptions.fPersistentCache. This allows Skia to cache shaders compilations to persistent storage. Even if platforms cache shader bytecode, Skia still has to do some non-trivial amount of work to generate the shader source in the first place. Using the persistent cache allows Skia to avoid repeating this work.

[liyuqian]

@chinmaygarde : are you still actively working on this? One developer is having this issue and posted a very simple reproduction: #813 (comment)

[Hixie]

This won't help first-launch performance, though, right?

[liyuqian]

Yes, it would only help if the app is reopened. We have to compile the shaders at some time.

[Hixie]

Can we compile them at the same time as we compile the Dart code?

[chinmaygarde]

We don't know what shaders to compile because the Dart code will generate the picture for the rasterizer to render. The shaders to create depend on the complexity of the picture.

[Hixie]

Can we use some slower but more general shaders that are precompiled ahead of time to render the first few frames, while we compile the more specialised shaders we actually need, then swap to the faster ones once we have them?

[liyuqian]

There are probably no such general shaders (or uber shaders as I heard from other Skia engineers). However, we can use software backend to render it if no shaders are compiled.

On the other hand, maybe we can improve flutter tools to capture some shaders? The developers would have to run their apps before shipping and flutter tools may be able to find and package those shaders.

[mklim]

Reopened by flutter/engine#6835

[liyuqian]

I'll mark this as a duplicate of #813 so we'll put link all our future work to that single issue.

[liyuqian]

Duplicate of #813

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