Ivan Popov wrote:
> I'd like to discuss the problem with Eclipse TPTP profiler working
> with DRLVM, which is described in HARMONY-2905 [1]. The problem is
> that verifier in DRLVM rejects class instrumented by TPTP profiler.
>
> TPTP profiler instruments class bytecodes by enclosing each method
> call into try-finally block, in order to report correctly entry/
return
> of this method. It instruments calls of all methods including
> invocation of super() in constructors of class instance. Here is
> fragment of instrumented bytecodes:
>
> <…>
> try {
> 19: aload_0 #0=this
> 20: invokespecial #8=<Method java.lang.Object.<init> ()void>
> 23: goto_w 28
> 28: nop
> 29: ldc_w #62=<Integer 70056>
> } finally {
> <…>
>
> This leads to the value of 'this' variable, which is considered
> uninitialized before call to super(), is used inside try-finally
> block. This contradicts to the JVMS spec 2nd edition [2], which
states
> in the last paragraph of section 4.9.4:
>
> A valid instruction sequence must not have an uninitialized
object
> on the operand stack or in a local variable during a
backwards branch,
> or in a local variable in code protected by an exception handler
> or a finally clause. Otherwise, a devious piece of code might
fool
> the verifier into thinking it had initialized a class
instance when
> it had,
> in fact, initialized a class instance created in a previous pass
> through a loop.
>
> And another statement in proposed changes for JDK 1.5 in "Class
file
> format" [3] in section 4.11.1 (which is new item in this proposal):
>
> -- There is never an uninitialized class instance in a local
> variable in code protected
> by an exception handler. However, an uninitialized class
instance may
> be on the operand stack in code protected by an exception
handler.
> When an
> exception is thrown, the contents of the operand stack are
discarded.
>
> Verifier in DRLVM follows these statements and rejects class
bytecodes
> instrumented in such a way. However, both Sun JDK version 1.5
and 1.6
> and JRockit JDK 1.5 accept such instrumented class. Either they
just
> ignore these statements or interpret 'uninitialized' status of
'this'
> variable in a different way.
>
> I've submitted bug against TPTP profiler [4], which produces
incorrect
> instrumentation, and this bug was accepted by TPTP developers to be
> fixed in the next release. However, this is a general approach for
> instrumenting method calls and any other Java profiler may use
it and
> work fine with RI, but and face this problem with DRLVM. I think it
> makes sense to adjust DRLVM verifier to follow RI behavior. I'd
like
> to see comment from DRLVM gurus.
There is always a workaround to verifier exceptions. You can run the
program with -Xverify:none to disable verifier completely. Turning
this
particular check is simple too. The question is whether this
should be a
default mode in VM or whether it should be enabled by some special
option which doesn't disable all other verifications.
> [1] https://issues.apache.org/jira/browse/HARMONY-2905
> [2]
> http://java.sun.com/docs/books/vmspec/2nd-edition/html/
ClassFile.doc.html#9839
>
> [3]
> http://java.sun.com/docs/books/vmspec/2nd-edition/
ClassFileFormat-Java5.pdf
> [4] https://bugs.eclipse.org/bugs/show_bug.cgi?id=170075
>
--
Gregory
