On 11/27/2011 07:16 AM, Mark Roos wrote:
One of the key parts of Smalltalk is the 'live' debugger. Unlike the
general dynamic language features which
are well supported by the additions from JSR292 the debugger requires
support which may not have been considered
as necessary to support dynamic languages. So we were not sure we
would be able to provide that portion
of the Smalltalk experience on the jvm.
The good news is that we were able to implement almost all of the
Smalltalk debugger features using only the
services provided in the released jdk7. I thought I would take a
moment to describe how we did it to both
demonstrate the approach and to solicit suggestions for improvements.
The Smalltalk debugger is 'live' in that it exists as a separate
thread within the same process/memory space as the
thread being debugged. This allows one to manipulate and inspect all
objects from the same viewpoint as the
debugged thread. Smalltalk offers the ability to inspect all
instances of a class(type), all references to a specific
object, the variables on all levels of the stack, senders and
implementers of methods, and the ability to single step
through method sends. There is also the ability to restart a thread
from any level of the stack but we opted to
wait on the coro patch before implementing this ( I also don't use it
as it can have quite a few side effects ).
The approach we took has two facets, we ( mainly oscar ) coded a C++
jvmti agent with a JNI interface which allowed
us to call some JVMTI apis from within the jvm being debugged and we
added some logic to the callsite to handle the
stepping.
Implementers and senders of methods is handled via reflection on the
classes and methods present so that was easy.
To support all instances and all references requires heap inspection
which we get from using the jvmti heap functions.
This had some issues with some of the support classes for invoke
dynamic but we were able to use a two pass tagging
approach to make sure we found all of the references to our objects.
This has to find objects both in arrays and in
instance vars.
Inspecting the stack was straight forward once we filtered the stack
trace to only have our method sends present. As
an option one can also inspect the full jvm trace. Using the jvmti
variable access api allows the locating the variable
which is then placed into a static field of out debugger support
class. This field is then access by Smallltalk via a
primitive (in Smalltalk a primitive is the way we share with the
underlying environment). Once we have this object
we can manipulate its instance vars from the Smalltalk side as well.
When an error is thrown the thread is suspended ( we added some jvmti
thread management apis just to get away
from the deprecated methods) and a new thread is launched with an
instance of the debugger and a pointer to the
thread to debug. At this point one can only inspect the thread locals
and anything else in the object memory. The
thread is not restartable so we kill it ( by sending ThreadDeath ).
But it the error is a halt or breakpoint we can then step the thread
along. We tried this with jvmti but is was broken
and seems to add quite a bit of delay to everything. Plus its a
callback approach which looked like a lot of work. So
instead we tweaked the call site logic to add a debug check. I liked
the way this worked a lot.
For a dynamic look up we already have a callsite with a target of one
or more GWTs to select the implementation
which matches the receiver class. What we have to do to implement a
debugger is to place before the first GWT
a test which determines if this is the time to suspend. Unfortunately
GWTs are added to the end so we need a
way to keep the test at the beginning (thanks to John and Réme
suggestion) we can simply have a callsite have a target which
is another callsite. The first callsite points at the test logic and
the second gets the GWT chain. One nice thing
is that we can revert to the single site version using a debug flag.
The code to get the initial target for the bootstrap callsite looks
like:
*private**void*setBootstrapTarget(MethodHandle mh){
// get the appropriate initial call site sequence
*if*( RtDebugger./_debugEnable/){
// for debugging we need to have a sequence of methodHandles that is
always the first
// code executed when a _callsite_ is invoked. This checks to see if
we should
// hold here for a debug step or continue.
_realSite= *new*MutableCallSite(mh); // this is the extra call site
to hold the gwts
MethodHandle invoker = _realSite.dynamicInvoker();
MethodHandles.Lookup lookup=MethodHandles./lookup/();
MethodHandle debugEntry= *null*;
MethodType mt=MethodType./methodType/(*void*.*class*,
RtObject.*class*);
*try*{
debugEntry = lookup.findStatic(RtDebugger.*class*,
"debugEntry", mt);
}
*catch*(Throwable e) {
e.printStackTrace();
}
invoker = MethodHandles./foldArguments/(invoker , debugEntry);
// does nothing except suspend if necessary
_realSite.setTarget(mh);
*this*.setTarget(invoker);
}*else*{
// normal behavior is to just use the RtCallSite to hold the GWT chain
as its target
*this*.setTarget(mh);
}
}
The debug test code tests for the correct thread and for the stack
depth ( jvmti api) and looks like:
*public**static**void*debugEntry(RtObject arg){
*if*(/_debugThread/== Thread./currentThread/()){
*if*(/getStackSize/(/_debugThread/) <= /_debugDepth/){
// send debugger update notice
/notifyDebugStep/();
/suspendThreadPrim/(/_debugThread/);
}
}
}
As you can see it did not take much code ( lots of thinking though )
to implement a stepping debugger.
A small remark, you can do the findStatic to find debugEntry once
and store the resulting method handle,in a static final field.
Also, here you choose to launch your application or in debugging mode or
in non-debugging mode,
there is a way to choose if you want to debug at runtime without paying
a cost
(in fact you pay something if the code is run by the interpreter but not
in the JITed code)
if the debugger is never started.
You can install a SwitchPoint guard in front all your targets and no
call to debugEntry by default.
When the debugger is launched, you invalidate the SwitchPoint and
re-install your debugEntry
call in front of all target with a new SwitchPoint in front of that.
When the debugging session is done, you can switch off the new SwitchPoint
to remove all call to debugEntry i.e. invalidate all targets that do the
debugEntry check and
re-install a new SwitchPoint with no call to debugEntry until the
debugger is started again.
So you will only pay the price of the debugger only during the debugging
session.
regards
mark
cheers,
Rémi
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