Linker performance problems

[vargaz]

Linking a hello world with:
monolinker -a hello.exe -c link
Takes about 4.7 seconds with 8a82325. This is a problem since the linker is on a critical path of the build and its ran serially. This used to be about 1-2 seconds. Link time should be proportional to the size of the final app.

Notes:

• Adding --disable-opt ipconstprop reduces the run time to 3.3 secs.
• The TypeMapStep loads and processes the whole unlinked assembly.
• Multiple passes are made over the IL, i.e. in MethodBodyScanner.

[jeromelaban]

Note that this specific issue is impacting .NET 6 based build significantly, and impacts other projects such as http://github.com/unoplatform/uno.

The default Uno configuration is to use the linker (so the projects are the smallest possible), but we're also hinting users that want to improve build performance to disable it to get a faster inner dev loop.

[jeromelaban]

I've done some profiling of the linker, here's what stands out:

And a large portion of the time is spent only resolving types and modules:

[MichalStrehovsky]

Oh, interesting - we spend a surprising amount of time in the MethodBodyScanner. Last time I was looking at that, it seemed like we could delete the entire MethodBodyScanner without replacement, delete the tests that are testing stuff it keeps, and still have correct outputs (#1428). Do I read it right that it would be potentially a big speedup?

[jeromelaban]

In this particular profiling session, for a small application, the linking time is around 00:01:15 on an i9 machine. Optimizing this part would definitely make a difference.