GAC vs JIT

Question

Is everything under the GAC precompiled (ngened)? If so, then all of .NET is precompiled, so it's not possible for the CLR to optimize them at runtime?

Like if you use List in your application then CLR will not be able to optimize the List itself, but only how it's used in your app? Doesn't this defeat the purpose of JIT, to get lots of optimizations at runtime? So effectively losing all the potential optimizations for the BCL?

Answer 1

No, the GAC is not automatically pre-JITted; however, GAC is a pre-requisite to pre-JIT. Actually, only a small minority of things are pre-JITted. Besides which - if the BCL was pre-JITted, then those optimisations would have already been done by NGEN, so the " losing all the potential optimizations" is a non-issue.