Many dynamic languages implement (or want to implement) a JIT Compiler in order to speed up their execution times. Inevitably, someone from the peanut gallery asks why they don't use LLVM. The answer is often, "LLVM is unsuitable for building a JIT." (For Example, Armin Rigo's comment here.)
Why is LLVM Unsuitable for building a JIT?
Note: I know LLVM has its own JIT. If LLVM used to be unsuitable, but now is suitable, please say what changed. I'm not talking about running LLVM Bytecode on the LLVM JIT, I'm talking about using the LLVM libraries to implement a JIT for a dynamic language.
There are some notes about LLVM in the Unladen Swallow post-mortem blog post: http://qinsb.blogspot.com/2011/03/unladen-swallow-retrospective.html .
Unfortunately, LLVM in its current state is really designed as a static compiler optimizer and back end. LLVM code generation and optimization is good but expensive. The optimizations are all designed to work on IR generated by static C-like languages. Most of the important optimizations for optimizing Python require high-level knowledge of how the program executed on previous iterations, and LLVM didn't help us do that.