2-nd round RFR (S) 8008678: JSR 292: constant pool reconstitution must support pseudo strings

serguei.spitsyn at oracle.com serguei.spitsyn at oracle.com
Thu Dec 18 22:00:19 UTC 2014

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(), 
>   }

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 
pseudo-string map and most of the code in from this webrev is not needed.


> 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

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