[Sablevm-developer] adventures in not understanding the java stack

[Chris P. <chr...@ma...>]

Hello,

In order for speculation to work, I require the following to be true for 
INVOKE<X> instructions:

1) the method has fully exited and the instruction after the INVOKE<X> 
is the next instruction to be executed when the height of 
stack.current_frame returns to the height before the INVOKE<X> (this 
handles the problem of recursion)
2) the stack after the INVOKE<X> is identical to the stack before the 
INVOKE<X>, except for possibly popped parameters (including a possible 
objectref) and a possibly pushed return value.

I've found that the following can invalidate this:

1) An exception being thrown in the callee (or one of the callee's 
callees), or some other exception-related irregular control flow -- 
currently flagged at exception_handler: in interpreter.c, at ATHROW, and 
at INTERNAL_CALL_END
2) GC being called in the callee (or one of the callee's callees) -- 
flagged inside _svmf_stop_the_world()
3) The callee (or one of the callee's callees) being a native method.

I can understand (1) -- irregular control flow means that the stack 
returns to the same height without the instruction after the INVOKE<X> 
being next
I can understand (2) -- GC being called would change references on the 
stack, thus making requirement 2 fail

but ... I don't understand (3).

So ... can somebody explain (3) for me?  Can native code affect stack 
values or control flow without going through the exception_handler: in 
interpreter.c or going through _svmf_stop_the_world?  If so, where does 
it do it?

Also, is there anything else you can think of that might break 
requirements 1 and 2?

Cheers, and thanks for any suggestions,
Chris