You've got class foo, with methods "DoBar" which calls "DoBazStep1(), DoBazStep2(), SomeHelper.CalcPosition()".
You use a Java bytecode toolkit and identify leaf functions, small functions, whatever the criteria needs to be. Then you check every callsite for those functions, and if found, remove the call, and inline the code, doing fixup on locals and parameters. Now, "DoBar" has a lot more bytecode in it, as the bodies of DoBazStep1 and the others are now contained in it. The small functions are completely removed.
This process can be done totally offline, on the compiled Java code. The runtime just sees that you wrote one big function, versus a bunch of small ones.
The only real work is making sure you got everything, and perhaps being clever with stack local allocations. As the FB team says they moved to a new style with lots of small functions, I'm assuming they aren't doing all sorts of metaprogramming and inheritance, but have just split things up at the function level.
You've got class foo, with methods "DoBar" which calls "DoBazStep1(), DoBazStep2(), SomeHelper.CalcPosition()".
You use a Java bytecode toolkit and identify leaf functions, small functions, whatever the criteria needs to be. Then you check every callsite for those functions, and if found, remove the call, and inline the code, doing fixup on locals and parameters. Now, "DoBar" has a lot more bytecode in it, as the bodies of DoBazStep1 and the others are now contained in it. The small functions are completely removed.
This process can be done totally offline, on the compiled Java code. The runtime just sees that you wrote one big function, versus a bunch of small ones.
The only real work is making sure you got everything, and perhaps being clever with stack local allocations. As the FB team says they moved to a new style with lots of small functions, I'm assuming they aren't doing all sorts of metaprogramming and inheritance, but have just split things up at the function level.