Optimisation work on Zero/ARM

[Gary Benson]

Hi Ed,

Edward Nevill wrote:
> I am not clear how Shark (or any JIT for that matter) replaces a
> threaded method (ie if you are executing say, SieveAtom, how does
> it hijack control from the interpreter?).

So each method is defined in HotSpot with a methodOop object (defined
in hotspot/src/share/vm/oops/methodOop.hpp).  Each methodOop has two
volatile pointers, _from_interpreted_entry and _from_compiled_entry,
which are the entry points from interpreted and compiled code.  When
a bytecode method is loaded these are initialized to point to the code
in the interpreter that executes methods (CppInterpreter::normal_entry
in Zero).  When methods are compiled these pointers are replaced.

> And how does it replace methods which might be lower down on the
> machine stack. I can see how you might do this non portably by
> grubbing around on the machine stack.

HotSpot can do this, but Shark doesn't yet -- ie, I haven't looked at
it so I don't know how it works :)  If you grep for OnStackReplacement
or OSR you should find somewhere to start.

Cheers,
Gary

-- 
http://gbenson.net/