AnalyzerDriver should not allocate in proportion to the number of symbols declared

[agocke]

The analyzer driver currently allocates a new closure and delegate for every symbol declared:

roslyn/src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerExecutor.cs

Line 1578 in 32149eb

private static Action<Diagnostic> GetAddDiagnostic(

This is one example of a couple of places where we allocate memory in proportion of the number of symbols declared in the compilation. Symbols are extremely common in the compiler and this can represent a very heavy allocation burden. We should try to make all analyzer APIs allocate a constant amount when possible, or re-use as many allocations as possible.

[agocke]

cc @mavasani

[mavasani]

Tag @sharwell, who has a handle on bunch of few low hanging perf optimizations on the driver that can help improve performance. Pretty sure this was one of them in his list.

[sharwell]

This is high impact, but not trivial. Changes to this space are constrained by the fact that it's called from public APIs.