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From "Mikhail Loenko" <mloe...@gmail.com>
Subject Re: [drlvm][verifier] Dead-code and Java 6 verification issues
Date Thu, 08 Nov 2007 07:29:29 GMT
2007/11/7, Mikhail Loenko <mloenko@gmail.com>:
> 2007/11/7, Asaf Yaffe <asaf_yaffe@yahoo.com>:
> > Hi Mikhail,
> >
> > You wrote:
> >
> > I like the idea of simplification which you pointed, but I think this
> > approach is not quite correct in general: the dead code might be in
> > the try block. In this case stackmap locals at the dead block should
> > be assignable to stackmap at the handler_pc instruction.
> >
> > In other words, if catch block accesses locals defined before the try
> >  block and
> > this try block has a dead code, then stackmap for the dead code can't
> > have empty locals: otherwise Java6 verification would fail
> >
> >
> >
> > I am not sure I understand the problem. Assume you "nop" all instructions in the
dead block and finalize it with an "athrow". According to the Java 6 verification rules, the
only thing you need is a [][java/lang/Throwable] StackMap. This is because (from the verification
algorithm):
> >
> > 1. A nop instruction is always type safe. The nop instruction does not affect the
type state
> > 2. An athrow instruction is type safe iff the top of the operand stack matches Throwable
> >
> > The verifier cannot (and will not) check the consistency of this block with other
blocks because this block in unreachable.
>
>
> > It will also not check the consistency of this block with "applicable" exception
handlers because it does not perform this check for "nop" and "athrow" instructions.
>
> That's what I missed! Could you please provide a link to the quote in
> the spec that nop and athrow are not checked against "applicable"
> exception handlers?

seems like the spec says that it must perform the check...

instructionIsTypeSafe(nop, _Environment, _Offset, StackFrame,
StackFrame, ExceptionStackFrame) :-
exceptionStackFrame(StackFrame, ExceptionStackFrame).

Thanks,
Mikhail

>
> Thanks,
> Mikhail
>
>
> >
> > Thanks,
> > Asaf
> >
> >
> >
> >
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