Re: Re: NOP ATHROW hack defeated !

On 06/08/2011 02:36 AM, Charles Oliver Nutter wrote:
> On Tue, Jun 7, 2011 at 6:09 PM, Rémi Forax<[email protected]>  wrote:
>> After discussion with some friends from Cambridge,
>> I figure out why you generate unreachable code.
>> In your compiler, the instruction visitor should return if the next
>> instruction
>> will be called or not.
>> By example, after these Java instructions break, continue, return or throw
>> the next instruction is dead. So if you have an if/else (it's the same with
>> a loop),
>> if the block of the true branchends with one of these instruction,
>> you should not generate the goto to jump after the else block.
> Yes, I know that. In practice, though, it's more difficult to fix.
> early returns can be done as part of an expression:
>
> a = if b
>    c
> else
>    return
> end
>
> And because all code in Ruby is an expression, loops, conditionals,
> exception-handling blocks, and so on are all expressions. Then imagine
> these expressions are nested arbitrarily deep. That makes it difficult
> to just "not generate the goto" because I would need to analyze an
> arbitrary amount of downstream code to find which paths will continue
> executing and which ones will not.

It's easy if you have a specific pass on your AST that calculates
that information. And as a trick, you can reuse the typechecking pass
to implement this analysis if your visitors returns a couple type,alive.

> Regardless...I'm still convinced this is at its core a problem in ASM,
> since the same code without ASM doing frame calculation verifies
> correctly on pre-Java 6 verifiers. Therefore it ought to be possible
> to make that code with frame calculation verify on Java 6+. Agreed?

Yes :)

> I admit it's a weakness in JRuby's compiler, but since the code *does*
> verify when ASM isn't rewriting it, I think the ball is in ASM's
> court.
>
> - Charlie

Rémi

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