On 12/5/19 6:45 PM, David Holmes wrote:
Hi Serguei,
On 6/12/2019 11:31 am, serguei.spit...@oracle.com wrote:
Hi Chris and Alex,
(I've also included Dan, David and Dean to the mailing list)
We have to reach a consensus about this.
This is just part of a much broader issue with JVM TI that I tried to
have a discussion started based on Richard Reingruber's proposals
around Escape Analysis:
http://mail.openjdk.java.net/pipermail/serviceability-dev/2019-September/029285.html
Unfortunately that discussion did not get much traction.
Hmm. I have the emails that precede yours above, but not that one. Not
sure how what happened. Just read through it and it did give me one
thought. Consider a model where the program is designed drive behavior
of the agent, triggering the agent to do certain things by having the
program do certain things. Normally an agent monitors the application,
but in this case the application is purposefully controlling actions
performed by the agent. If code is elided from the program, then the
agent no longer performs as expected. It's a kind of backwards jvmti
programming model, and you may ask why would anyone do this. I'm not
sure if there's a good reason for it, but should it be expected to work
given how the spec is written?
We have 3 options:
Option #1:
The JIT optimization to delete a code which "looks useless"
has to be disabled if can_pop_frame capability is enabled.
Than this problem becomes a JIT compiler bug.
Option #2:
Consider to relax the JVMTI PopFrame spec by changing it to
something like:
"Note however, that the original argument values are not
preserved and can be changed by the called method;"
Than this problem becomes a JVM TI spec bug.
Option #3:
Consider it is Okay for compiler to eliminate useless code,
so the argument values can be reinitialized by the PopFrame.
Than this problem becomes just a test bug.
My preference is option #3.
The point is that if the arguments are not really used in
a method then restoring them to any values is a no-op.
It is really meaningless use case, so why should we care about it.
Is "restoring" the proper term here? I thought they were just left on
the stack and reused on the subsequent invoke. In fact I figured the
reason for the language in the spec in the first place is to alleviate
JVMTI from having to restore them to their original values, which is
probably not even possible.
Thanks for setting that out clearly.
I'd like to agree this is particular case is a test bug. If we have a
method:
int incr(int val) {
val++;
popFrameHere();
return val;
}
then the change to the argument is necessary and must be preserved. In
contrast:
void incr(int val) {
val++;
popFrameHere();
}
the change to the argument is meaningless and I would hope any decent
JIT would simply elide it.
So, this goes back to my example above where the program is trying to
elicit behavior from the agent. It's not meaningless in that case, but
that doesn't mean I think we need to support it.
But we must have a consistent approach to such things. What would
happen if a breakpoint were to be placed on the instruction that
uselessly modified the argument - would we still see the modification
or would it be elided?
Breakpoints force interpreted mode for the method, although I suppose
that's a hotspot implementation detail and not something a VM would be
required to do. A VM that allows breakpoints in compiled methods has the
potential to miss the breakpoint if code is elided.
Also, what if you put a breakpoint in a method, the call to it is
elided. You would never hit the breakpoint. That could cause some
serious head scratching for a debugger user if they know the code doing
the method call is "executed".
And how do C1 and C2 avoid this issue? Do they simply not optimise
away the useless assignment? Or do they actively disable that
optimization in this context?
We need, IMO, to establish the basic philosophy of how to manage JVM
TI / JIT interactions, so we know what things must remain visible and
which can be optimised away.
That said, changing the test allows us to defer having to reach that
consensus.
Agreed. I think it's ok to work around the test issue as long as we keep
this overall issue on the radar. Do we have a bug field for that?
thanks,
Chris
David
-----
Thanks,
Serguei
On 11/11/19 3:17 AM, serguei.spit...@oracle.com wrote:
Hi Alex,
The fix itself looks Okay.
Minor: replace in the comment: "compiler don't drop" => "compiler
doesn't drop".
However, we still have to reach a consensus on how we treat this
issue (as Chris already commented).
Thanks,
Serguei
On 11/8/19 15:22, Alex Menkov wrote:
Hi all,
Please review the fix for
https://bugs.openjdk.java.net/browse/JDK-8215196
webrev:
http://cr.openjdk.java.net/~amenkov/jdk14/popframe_args/webrev/
Currently PopFrame is disabled with JVMCI by [1], so for testing I
reverted [1] changes.
[1] https://bugs.openjdk.java.net/browse/JDK-8218025
--alex
