How does C-- compare to LLVM?
They differ in how expressive the low level machine type system is.
The LLVM machine is pretty expressive. The C-- machine on the other hand, puts a lot of responsibility on the language front end. Quoting from the C-- FAQ: "simply, C-- has no high-level types---it does not even distinguish floating-point variables from integer variables. This model gives the front end total control of representation and type system"
Also visually they look a lot different. C-- looks a lot like C, LLVM looks a lot like assembler.
Pragmatically, LLVM has a lot more momentum right now. It has a JIT compiler, Apple is using it for 3D pipeline things and people are using it to connect to GCC and all sorts of weird and wonderful things. Someone called it "almost absurdly easy to work with".
On the other hand C-- is much smaller and probably easier to entirely comprehend. (I imagine a normal person with some dedication can fully understand all aspects of it.)