Why is llvm considered unsuitable for implementing a JIT?

Question

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

Answer 1

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.