Writing a simple inliner isn't too bad, writing a good one is hard. Let's pretend you just "copy/paste" the code into place, there's the question of deciding w

Writing a simple inliner isn't too bad, writing a good one is hard. Let's pretend you just "copy/paste" the code into place, there's the question of deciding when/if to copy/paste the code in -- what heuristic do you use. Now there's constant propagation, possibly more doing dead code (from constant propagation), dependency reanalysis (because you've changed the basic block, possibly adding more basic blocks). There are reasons the resultant code could even be less efficient (register pressure for locals for the inline code comes to mind), and there's i-cache complexities that come from blowing up your code size. Inlining is really hard.