Scalable builds

[davidmorgan]

The benchmark I'm adding in #3802 shows the problem pretty clearly,

json_serializable
libraries	clean/ms	no changes/ms	incremental/ms
1	21573	2840	4278
100	23040	2974	6327
250	28098	3285	12578
500	42940	4288	35061
750	69483	6561	67670
1000	115941	9178	119308

notice the incremental build time for 500 libraries and 1000 libraries: it increases from 35s to 119s, an increase of x3.4. For double the number of libraries the time increase should be x2 :)

The benchmark also runs for built_value, freezed and mockito, numbers on the PR, the story is pretty similar, except:

• built_value and json_serializable are about 2x as slow as freezed on the big build, it turns out this is purely because they use shared parts, filed Faster shared parts #3803 for that sub-problem
• mockito is about 2x as fast as freezed, I believe that is because mockito adds a part file to the test where it does not become a dep of all the other generators; conversely built_value, json_serializable and freezed all add part files in the app code, doubling the number of deps of all the generators

Apart from these multipliers the numbers are very similar, I think it does not matter much what the generators are actually doing.

[rrousselGit]

• mockito is about 2x as fast as freezed, I believe that is because mockito adds a part file to the test where it does not become a dep of all the other generators; conversely built_value, json_serializable and freezed all add part files in the app code, doubling the number of deps of all the generators

Going back to this:

This could be because freezed starts by asking for the AST of the generated file.
I need the AST, and build_runner doesn't give it to me. So I have to ask for it again. And I don't think it's cached, right?

If build_runner could expose us the AST, that'd be cool. Most of my generators use it.

[davidmorgan]

• mockito is about 2x as fast as freezed, I believe that is because mockito adds a part file to the test where it does not become a dep of all the other generators; conversely built_value, json_serializable and freezed all add part files in the app code, doubling the number of deps of all the generators

Going back to this:

This could be because freezed starts by asking for the AST of the generated file. I need the AST, and build_runner doesn't give it to me. So I have to ask for it again. And I don't think it's cached, right?

If build_runner could expose us the AST, that'd be cool. Most of my generators use it.

The performance issue I'm looking at is just due to the deps graph, it's nothing to do with what the generator does. When you look at the CPU flame charts of the large benchmarks, the generator barely shows up.

Once that's solved--yes, I think there's a possibility of handling ASTs better/faster, we'd have to compare :) thanks.