Default shader compilation takes unreasonably long on iOS

[BerstanioDev]

Hello,
I'm maintaining an ANGLE build with a GLKit replacement for iOS: https://github.com/Berstanio/MetalANGLEKit

I noticed that on iOS, especially on weaker devices, the compiling of the default shaders takes a very long time (`DisplayMtl::initializeShaderLibrary` to be specific).
On an iPad Mini 5th generation I measured a 7sec startup delay because of the shader compilation.
While the delay often only occurs first startup, it's still too long for a small mobile app to startup.
Looking at the source it seems theoretically possible to precompile the shaders, but it is noted that `Building internal shaders for iOS is not supported.`

Is there a way to somehow still prebuild the shaders for iOS? Or any other way to cut down on startup time? 

[Geoff Lang]

We disable prebuilt Metal shaders on iOS because Chrome's hermetic MacOS SDK doesn't include the compiler for iOS. It could probably be made to work on iOS if you're building with a local full SDK.

Prebuilt shaders are disabled on iOS in metal_backend.gni. The actual compilation happens with the build steps here which appears to handle iOS already. You could try enabling metal_internal_shader_compilation_supported locally and with some print statements in the compile actions to verify it finds the compilers.

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[BerstanioDev]

Thank you for the help! As I'm building with a full SDK, I managed to prebuild the shaders for iOS and got the fast startup time back.

I was wondering, whether this is something that could be configured automatically? Whether the SDK contains an iOS compiler or not.

Because when using ANGLE for an iOS App, the delay is just too long and users might just close the app.