Hi Andrew,
We need to know what it's used for to know which solution is right. I'm guessing the primary use case is asynchronous runtime stack probes, used by debugging tools.
Yes, we also have os::stack_shadow_pages_available() which uses it to calculate how much space there is between the current SP and the end of the stack. In this case I think it's ok as long as we don't return a value that's *above* the actual stack pointer. And os::current_frame(), which constructs a `frame' object using the FP of the caller, but the SP will point into the frame of os::current_frame(), so it seems it's already inaccurate.
There's also a comment in os_linux.cpp that says "Don't use os::current_stack_pointer(), as its result can be slightly below current stack pointer". So I'm wondering if we can simplify this a lot and use __builtin_frame_address(0) which will give us the FP in os::current_stack_pointer (ought to be the caller's SP - 16). Zero does this currently. And maybe we can replace _get_previous_fp() with __builtin_frame_address(1)?
Thanks, Nick
