3-rd round RFR (S) 8008678: JSR 292: constant pool reconstitution must support pseudo strings
serguei.spitsyn at oracle.com
serguei.spitsyn at oracle.com
Fri Jan 16 23:35:19 UTC 2015
On 1/16/15 3:10 PM, serguei.spitsyn at oracle.com wrote:
> On 1/16/15 3:03 PM, Coleen Phillimore wrote:
>>
>> On 1/16/15, 6:01 PM, serguei.spitsyn at oracle.com wrote:
>>> John R. suggested to use the CPSlot(Symbol* ptr) to mark pseudo-strings.
>>
>> I was sort of wondering about this along the same lines. You're
>> setting the second bit, right? :)
>
> I'm not sure yet.
> I'll check if setting the first bit would not create an ambiguity.
> Otherwise, will set the second one.
In fact, the suggestion was to look at the PCSlot as an example.
I'm not sure this class is good to extend or reuse for pseudo-strings
as it was added for klass entries only.
>
> Thanks,
> Serguei
>
>> Coleen
>>
>>> The updated webrev is going to be close to the .3 webrev.
>>> I will send it soon.
>>>
>>> Thanks,
>>> Serguei
>>>
>>> On 1/16/15 2:53 PM, Coleen Phillimore wrote:
>>>>
>>>> This change looks good to me also.
>>>> Thanks,
>>>> Coleen
>>>>
>>>> On 1/16/15, 3:07 PM, serguei.spitsyn at oracle.com wrote:
>>>>> Please, review the fix for:
>>>>> https://bugs.openjdk.java.net/browse/JDK-8008678
>>>>>
>>>>>
>>>>> Open webrev:
>>>>> http://cr.openjdk.java.net/~sspitsyn/webrevs/2015/hotspot/8008678-JVMTI-pseudo.3/
>>>>>
>>>>>
>>>>> Summary:
>>>>> Currently, a JVM_CONSTANT_String CP entry having a NULL
>>>>> reference to Symbol*
>>>>> indicates that it is a pseudo-string (patched string).
>>>>> This creates nasty issues for the constant pool reconstitution.
>>>>>
>>>>> Current suggestion is to avoid having a NULL reference and
>>>>> retain the original
>>>>> Symbol* reference for pseudo-strings. The pseudo-string
>>>>> indication will be
>>>>> if the Utf8 representation starts from "CONSTANT_PLACEHOLDER_".
>>>>> This approach makes the fix much simpler.
>>>>>
>>>>> I need a confirmation from the Compiler team that this won't
>>>>> break any assumptions or invariants.
>>>>> Big thanks to Coleen for previous round reviews and good advices!
>>>>>
>>>>>
>>>>> Testing:
>>>>> Run:
>>>>> - java/lang/instrument tests
>>>>> - new jtreg test (see webrev) that was written by Filipp Zhinkin
>>>>>
>>>>>
>>>>> Thanks,
>>>>> Serguei
>>>>>
>>>>>
>>>>> On 12/18/14 2:00 PM, serguei.spitsyn at oracle.com wrote:
>>>>>> Hi Coleen,
>>>>>>
>>>>>> Thank you for reviewing!
>>>>>>
>>>>>>
>>>>>> On 12/18/14 11:23 AM, Coleen Phillimore wrote:
>>>>>>>
>>>>>>> Hi Serguei,
>>>>>>>
>>>>>>> Thank you for making the patches an optional field.
>>>>>>>
>>>>>>> http://cr.openjdk.java.net/~sspitsyn/webrevs/2014/hotspot/8008678-JVMTI-pseudo.2/src/share/vm/oops/constantPool.hpp.sdiff.html
>>>>>>> 198 if (!patched()) {
>>>>>>> 199 assert(false, "a pseudo-string map may exists for patched CP only");
>>>>>>> 200 return 0;
>>>>>>> 201 }
>>>>>>> Why not
>>>>>>> assert(patched(), "a pseud-string map must exist
>>>>>>> for patched CP only");
>>>>>>
>>>>>> Wanted it to be more reliable but it looks pointless.
>>>>>> Will make this change.
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Why is this? Is this really a ShouldNotReachHere? should it be
>>>>>>> false?
>>>>>>>
>>>>>>> 215 assert(true, "not found a matching entry in pseudo-string map");
>>>>>>
>>>>>>
>>>>>> A typo, must be false.
>>>>>> It is the last minute change.
>>>>>> Thanks for the catch!
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> http://cr.openjdk.java.net/~sspitsyn/webrevs/2014/hotspot/8008678-JVMTI-pseudo.2/src/share/vm/prims/jvmtiRedefineClasses.cpp.udiff.html
>>>>>>>
>>>>>>> Don't you have to move the value of the patched field from the
>>>>>>> old constant pool to the new one? I hate to ask but is there
>>>>>>> merging that needs to be done? I don't know how to write this
>>>>>>> test case though. Is it possible to redefine a class with a
>>>>>>> constant pool patches with another that has constant pool patches?
>>>>>>
>>>>>> Thank you for asking this question.
>>>>>> If I understand correctly, the patching comes from the compiler
>>>>>> side for anonymous classes only.
>>>>>> I saw it for LambdaForm's only.
>>>>>> I think, the patching can not be changed with a retransformation.
>>>>>> But I'm not sure if it can not be changed with a redefinition.
>>>>>>
>>>>>> But if it can - then it would be safe to merge the 'patched'
>>>>>> condition, i.e. make it patched
>>>>>> if either the_class or scratch class is patched.
>>>>>>
>>>>>>>
>>>>>>> Somehow I thought you'd have to save the value of the cp_patches
>>>>>>> oops passed in.
>>>>>>>
>>>>>>> So I was wondering why you can't change this instead:
>>>>>>>
>>>>>>> bool is_pseudo_string_at(int which) {
>>>>>>> // A pseudo string is a string that doesn't have a symbol in
>>>>>>> the cpSlot
>>>>>>> - return unresolved_string_at(which) == NULL;
>>>>>>> + return (strncmp(unresolved_string_at(which)->as_utf8(),
>>>>>>> "CONSTANT_PLACEHOLDER_" , strlen("CONSTANT_PLACEHOLDER")) == 0);
>>>>>>> }
>>>>>>
>>>>>> I was thinking about the same but was not sure if it would work
>>>>>> for the compiler team.
>>>>>> We have to ask John about this (added John and Christian to the
>>>>>> cc-list).
>>>>>> This question to John was in my plan! :)
>>>>>>
>>>>>> The beauty of the above approach is that there is no need to
>>>>>> create an intermediate
>>>>>> pseudo-string map and most of the code in from this webrev is not
>>>>>> needed.
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> Serguei
>>>>>>
>>>>>>>
>>>>>>> And the asserts in the other functions below it.
>>>>>>>
>>>>>>> Coleen
>>>>>>>
>>>>>>>
>>>>>>> On 12/17/14, 12:26 PM, serguei.spitsyn at oracle.com wrote:
>>>>>>>> Please, review the second round fix for:
>>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8008678
>>>>>>>>
>>>>>>>> Open webrev:
>>>>>>>> http://cr.openjdk.java.net/~sspitsyn/webrevs/2014/hotspot/8008678-JVMTI-pseudo.2/
>>>>>>>>
>>>>>>>>
>>>>>>>> Summary:
>>>>>>>>
>>>>>>>> This fix implements a footprint saving approach suggested by
>>>>>>>> Coleen.
>>>>>>>> To be able to reconstitute a class constant pool, an
>>>>>>>> intermediate pseudo-string map is used.
>>>>>>>> Now, this field is accounted optionally, only if the
>>>>>>>> 'cp_patches' is provided in the
>>>>>>>> ClassFileParser::parseClassFile() before ConstantPool is
>>>>>>>> allocated.
>>>>>>>> This fix is not elegant, even a little bit ugly, but it is
>>>>>>>> the only way I see so far.
>>>>>>>>
>>>>>>>> Unfortunately, this approach did not help much to make some
>>>>>>>> other fields (eg., 'operands') optional.
>>>>>>>> The problem is that we have to account optional fields before
>>>>>>>> parsing, at the CP allocation time.
>>>>>>>> It is possible to re-allocate the ConstantPool when any
>>>>>>>> InvokeDynamic bytecode is discovered,
>>>>>>>> but it looks too complicated.
>>>>>>>>
>>>>>>>> Testing:
>>>>>>>> - the unit test from bug report
>>>>>>>> - nsk.jvmti,testlist, nsk.jdi.testlist, JTREG
>>>>>>>> java/lang/instrument
>>>>>>>> - vm.mlvm.testlist, vm.quick.testlist,
>>>>>>>> vm.parallel_class_loading.testlist (in progress)
>>>>>>>>
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Serguei
>>>>>>>>
>>>>>>>>
>>>>>>>> On 11/26/14 11:53 AM, serguei.spitsyn at oracle.com wrote:
>>>>>>>>> Coleen,
>>>>>>>>>
>>>>>>>>> Thank you for looking at this!
>>>>>>>>> I'll check how this can be improved.
>>>>>>>>> It is my concern too.
>>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>> Serguei
>>>>>>>>>
>>>>>>>>> On 11/26/14 9:17 AM, Coleen Phillimore wrote:
>>>>>>>>>>
>>>>>>>>>> Serguei,
>>>>>>>>>> I had a quick look at this. I was wondering if we could make
>>>>>>>>>> the pseudo_string_map conditional in ConstantPool and not
>>>>>>>>>> make all classes pay in footprint for this field? The same
>>>>>>>>>> thing probably could be done for operands too. There are
>>>>>>>>>> flags that you can set to conditionally add a pointer to
>>>>>>>>>> base() in this function.
>>>>>>>>>>
>>>>>>>>>> Typical C++ would subclass ConstantPool to add
>>>>>>>>>> InvokeDynamicConstantPool fields, but this is not typical C++
>>>>>>>>>> so the trick we use is like the one in ConstMethod. I think
>>>>>>>>>> it's worth doing in this case.
>>>>>>>>>>
>>>>>>>>>> Thanks,
>>>>>>>>>> Coleen
>>>>>>>>>>
>>>>>>>>>> On 11/26/14, 3:59 AM, serguei.spitsyn at oracle.com wrote:
>>>>>>>>>>> Please, review the fix for:
>>>>>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8008678
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Open webrev:
>>>>>>>>>>> http://cr.openjdk.java.net/~sspitsyn/webrevs/2014/hotspot/8008678-JVMTI-pseudo.1/
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Summary:
>>>>>>>>>>> The pseudo-strings are currently not supported in
>>>>>>>>>>> reconstitution of constant pool.
>>>>>>>>>>>
>>>>>>>>>>> This is an explanation from John Rose about what the
>>>>>>>>>>> pseudo-strings are:
>>>>>>>>>>>
>>>>>>>>>>> "We still need "live" oop constants pre-linked into the
>>>>>>>>>>> constant pool of bytecodes which
>>>>>>>>>>> implement some method handles. We use the anonymous
>>>>>>>>>>> class pseudo-string feature for that.
>>>>>>>>>>> The relevant code is here:
>>>>>>>>>>> http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/tip/src/share/classes/java/lang/invoke/InvokerBytecodeGenerator.java
>>>>>>>>>>>
>>>>>>>>>>> These oops are what "pseudo-strings" are.
>>>>>>>>>>> The odd name refers to the fact that, even though they
>>>>>>>>>>> are random oops, they appear in the constant pool
>>>>>>>>>>> where one would expect (because of class file syntax) to
>>>>>>>>>>> find a string."
>>>>>>>>>>> ...
>>>>>>>>>>> If you really wanted to reconstitute a class file for an
>>>>>>>>>>> anonymous class, and
>>>>>>>>>>> if that class has oop patching (pseudo-strings), you
>>>>>>>>>>> would need either to (a) reconstitute the patches array
>>>>>>>>>>> handed to Unsafe.defineAnonymousClass, or (b) accept
>>>>>>>>>>> whatever odd strings were there first, as an approximation.
>>>>>>>>>>> The "odd strings" are totally insignificant, and are
>>>>>>>>>>> typically something like "CONSTANT_PLACEHOLDER_42"
>>>>>>>>>>> (see java/lang/invoke/InvokerBytecodeGenerator.java)."
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Reconstitution of the ConstantPool is needed for both the
>>>>>>>>>>> JVMTI GetConstantPool() and RetransformClasses().
>>>>>>>>>>> Finally, it goes to the ConstantPool::copy_cpool_bytes().
>>>>>>>>>>>
>>>>>>>>>>> The problem is that a pseudo-string is a patched string
>>>>>>>>>>> that does not have
>>>>>>>>>>> a reference to the string symbol anymore:
>>>>>>>>>>> unresolved_string_at(idx) == NULL
>>>>>>>>>>>
>>>>>>>>>>> The fix is to create and fill in a map from
>>>>>>>>>>> JVM_CONSTANT_String cp index to the JVM_CONSTANT_Utf8 cp index
>>>>>>>>>>> to be able to restore this assotiation in the
>>>>>>>>>>> JvmtiConstantPoolReconstituter.
>>>>>>>>>>>
>>>>>>>>>>> Testing:
>>>>>>>>>>> Run:
>>>>>>>>>>> - java/lang/instrument tests
>>>>>>>>>>> - new jtreg test (see webrev) that was written by Filipp
>>>>>>>>>>> Zhinkin
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Thanks,
>>>>>>>>>>> Serguei
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.openjdk.java.net/pipermail/serviceability-dev/attachments/20150116/ec7c363a/attachment-0001.html>
More information about the serviceability-dev
mailing list