[LLVMdev] Writing pass for llc

[shu]

Is there a beginner's tutorial/documentation on how to write passes
for the llc tool?  I've managed to write some simple analysis passes
for the opt tool but can't figure out how to do the same for llc.
What is the proper way to integrate a new MachineFunction pass with
llc?  Is there a way to register passes and selectively enable them on
the command line (as with opt) or do I need to hard code it into
llc.cpp?

I would like to be able to write a pass that looks at MachineFunctions
and inserts instructions right before register allocation (but after
all other optimization/instruction selection passes).  Any direction/
advice would be of great help.  Thanks!