Microsoft made available this week for public testing a build of a new 64-bit .Net compiler built by the company's .Net runtime code-generation team.

The new just-in-time compiler is codenamed "RyuJIT."("Ryujin" is the deity of the sea in Japanese mythology.)
In a September 30 blog post on the .Net Framework blog, Microsoft execs noted there's already a 64-bit .Net JIT compiler. But the new, next-generation X64 RyuJIT "compiles code twice as fast," they claim.
"The 64-bit JIT currently in .NET isnít always fast to compile your code, meaning you have to rely on other technologies such as NGen or background JIT to achieve fast program startup," acknowledged the post author, Andrew Pardoe, PM Manager for the Common Language Runtime (CLR) Runtime PM team.
RyuJIT: Microsoft's new 64-bit .Net compiler | ZDNet