Why IL?

One of the earliest and most frequently-asked questions we got when we announced the Roslyn project was "is this like LLVM for .NET?"

No, Roslyn is not anything like LLVM for .NET. LLVM stands for Low-Level Virtual Machine; as I understand it (admittedly never having used it), compiler "front ends" take in code written in some language -- say C++ -- and spit out equivalent code written in the LLVM language. Another compiler then takes the code written in the LLVM language and writes that into optimized machine code.

We already have such a system for .NET; in fact, .NET is entirely built upon it and always has been, so Roslyn isn't it. The C#, VB and other compilers take in programs written in those languages and spit out code written in the Common Intermediate Language (CIL, or also commonly MSIL or just IL). Then another compiler -- either the jitter, which runs "just in time" at runtime, or the NGEN tool which runs before runtime -- translates the IL into optimized machine code that can actually run on the target platform.

I'm occasionally asked why we use this strategy; why not just have the C# compiler write out optimized machine code directly, and skip the middleman? Why have two compilers to go from C# to machine code when you could have one?

There are a number of reasons, but they pretty much all boil down to one good reason: the two-compilers-with-an-intermediate-language system is much less expensive in our scenario.

That might seem counterintuitive; after all, now we have two languages to specify, two languages to analyze, and so on. To understand why this is such a big win you have to look at the larger picture.

Suppose you have n languages: C#, VB, F#, JScript .NET, and so on. Suppose you have m different runtime environments: Windows machines running on x86 or x64, XBOX 360, phones, Silverlight running on the Mac... and suppose you go with the one-compiler strategy for each. How many compiler back-end code generators do you end up writing? For each language you need a code generator for each target environment, so you end up writing n x m code generators.

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