1/15 When compiling .NET ahead of time, the hardest question to answer is what code to compile.
2/15 For many non-.NET languages it's enough to compile the closure of Main(): start with Main() and compile all methods called from there transitively.
3/15 But .NET offers unconstrained reflection: one can `Type.GetType` a text string literally downloaded from the internet and call methods on that type
4/15 Thanks to the unconstrained reflection, determining a proper subset of methods a .NET program *can* call is pretty much the halting problem en.wikipedia.org/wiki/Halting_p…
5/15 AOT compiler has a strong incentive to only compile what's used. Native code tends to be bigger than the source IL: carrying dead code is more expensive
6/15 How does an AOT compiler decide what to compile? Turns out this problem also exists in non-AOT world - take IL Linker: hanselman.com/blog/MakingATi…
7/15 The easiest thing is to assume all the app code (including 3rd party NuGets) is used. FX/BCL is fair game: don't compile/keep if it's not visibly called.
8/15 In the rare case when this policy doesn't work, both IL Linker and AOT compilers have means (XML file) to specify other methods/types to compile/keep.
9/15 This policy makes a lot of existing code work, but comes with costs: we still keep/compile a lot of unreferenced code.
10/15 How expensive you ask? Last week I compiled a Blazor app with CoreRT in 5.5 MB, fully self-contained. I used the "compatible but big" setting.
11/15 This weekend I spent an hour debugging the Blazor app compiled without the compatiblity switch. ~20 lines of XML hints later, the Blazor app worked.
12/15 I call this mode the "hard mode" of CoreRT. One needs to do extra work, but gets rewarded with better perf numbers.
13/15 A fully self-contained x64 Blazor app compiled with CoreRT comes down to 3.5 MB (compressed). It also uses less memory as a bonus. 
14/15 This mode is not for everyone, but it's good to have it when you really need it. It beats having to rewrite the app in Go/C to meet size/memory quotas.
15/15 Check out my branch that has two commits: the first commit adds CoreRT in "easy mode", the second commit enables "hard mode" github.com/MichalStrehovs…
