A lot of the barriers that Unladen Swallow / Reid and friends ran into were fixed as part of their efforts (e.g. MB limits on emitted code, no gdb support, etc.). Building a dynamic language runtime on top of LLVM is much much easier today than it was when Unladen Swallow tried, and I'd say it's partly thanks to their initial effort.
I'd bet they've also benefited considerably from the work done with the WebKit FTLJIT project, too:
Right, I should have mentioned that. Though as the WebKit-related patchpoint and other bits happened more recently, I had (still?) assumed that Pyston hasn't gotten to leverage those much yet. Turns out I'm wrong: https://github.com/dropbox/pyston/blob/master/src/codegen/pa...
I remember when Unladen Swallow project was active, they were encountering some issues with LLVM which slowed their progress down. They often had to stop to fix LLVM.
Wonder if Pyston team got a chance to look or learn from Unladen Swallow's idea or even use any of the code?