Hi David,
Thank you for writing this down.
Totally agree with you here.
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.
I've mentioned the general discussion you started about JIT compiler
optimizations in one of my previous replies to this review threads.
Sorry, I was busy with other things and was not able to participate in
it properly.
But I'm looking forward to continue this when there is a chance.
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.
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.
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?
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.
It is painful that we have not established it yet.
That said, changing the test allows us to defer having to reach that
consensus.
Right.
Thanks,
Serguei
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
