reduce end-to-end AOT compilation time for large applications

[mraleph]

This is an umbrella issue to track efforts around reduction of critical path when building large Flutter applications in AOT mode

[alexmarkov]

This looks duplicate to #42442.

[mraleph]

@alexmarkov this is more of an umbrella issue, not limited to just TFA but to general end-to-end performance.

[dnfield]

One way to do this would be to support breaking things up into smaller increments of compilation.

This would allow for fanning out compilation steps to many different worker machines, and would also potentially help with incrementally compiling smaller changes between builds.

I'm more interested in faster incremental builds for the sake of performance tuning/debugging - right now, changing a single line of code requires repeating the entire process of kernel compiling/tfa/snapshotting, which can be very expensive on large codebases.

[mraleph]

We could parallelize gen_snapshot step in a relatively straightforward fashion though it would potentially require us to forgo some of the late stage tree shaking (might not be that relevant for code quality after all).

Parallelizing TFA is an open problem - it is intrisically non-modular global step. You should compare it to similar global optimizations steps like LTO in LLVM or ProGuard et al

[xster]

Google concerns: b/203690870

[dnfield]

Perhaps something like what LLVM did with ThinLTO would be applicable for TFA? http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html