[Soot-list] question about Soot optimization

[Eric Bodden]

Hi Mu.

The optimization you propose is not sound in multi-threaded programs.
The field's value could have been modified between both assignments,
which would make the second assignment necessary.
My guess is that, since usually Soot has no knowledge about whether
your program is multi-threaded or not, or which parts of it may be, it
assumes the worst and hence does not apply the optimization.

Eric

On 25 April 2012 01:29, Mu Zhang <muzhang82 at gmail.com> wrote:
> Hi,
>
> I have a question about Soot optimization. I'm writing a program to
> instrument Java bytecode and using Soot to remove redundant code. However, I
> found Soot was not able to optimize the instrumented code in some cases.
> Here is an example and the redundant instructions are in black:
>
> public int cryptWrapper(int, tester.BoolWrapper, tester.BoolWrapper,
> tester.BoolWrapper)
>     {
>         tester.B r0;
>         int i0, i1, $i2;
>         tester.BoolWrapper r1, r2, r3;
>         boolean z0, z1, z3, z4;
>
>         r0 := @this: tester.B;
>         i0 := @parameter0: int;
>         r1 := @parameter1: tester.BoolWrapper;
>         r2 := @parameter2: tester.BoolWrapper;
>         r3 := @parameter3: tester.BoolWrapper;
>         z0 = r2.<tester.BoolWrapper: boolean b>;
>         z1 = r1.<tester.BoolWrapper: boolean b>;
>         z3 = r3.<tester.BoolWrapper: boolean b>;
>         $i2 = r0.<tester.B: int key>;
>         i1 = i0 ^ $i2;
>         z4 = z1 | 0;
>         r2.<tester.BoolWrapper: boolean b> = z0;
>         r1.<tester.BoolWrapper: boolean b> = z1;
>         r3.<tester.BoolWrapper: boolean b> = z4;
>         return i1;
>     }
>
> Basically, z0 is assigned with r2.b and later is assigned to r2.b without
> any modification in between. I was wondering if it is because Soot considers
> r2.b to be an object field with side-effects and thus doesn't optimize it.
> If that is true, I'm curious to know how Soot determine whether a local has
> side-effects or not. Does it simply consider all objects have side-effects?
> Also, is there a way to trick Soot and bypass the check, so that it can
> optimize some certain instructions that may have side-effects?
>
> Thanks,
>
> Mu
>
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-- 
Eric Bodden, Ph.D., http://bodden.de/
Head of Secure Software Engineering Group at EC SPRIDE
Principal Investigator in Secure Services at CASED
Tel: +49 6151 16-75422    Fax: +49 6151 16-72051
Room 3.2.14, Mornewegstr. 30, 64293 Darmstadt