[Soot-list] Disable Jimple unused variable elimination process
Eric Bodden
eric.bodden at uni-paderborn.de
Fri Oct 5 03:28:47 EDT 2018
Thanks for following up.
Yes it may be the case that Soot needs to do some cleaning up during the Jimple construction. I am still a bit surprised, though, that the variable does show in Shimple. But like I wrote, maybe that’s just an effect of the SSA transformation.
Cheers
Eric
> On 4. Oct 2018, at 20:16, Ben Holland <benjholla at gmail.com> wrote:
>
> I've noticed that SOOT does dead code elimination regardless of any configurations (its harded coded in the default packs). We had experimented with disabling it at one point, but found it was causing problems in type inference. If I remember right everything goes through the Jimple intermediate stage so even just loading bytecode and outputting bytecode directly went through the default Jimple packs that did dead code elimination/constant propagation sort of steps (sorry its been a little while since I looked at this so going on memory). So anyway even "a null transformation" did not output original bytecode. This was somewhat of a headache for me for a project I worked on because I wanted to create an alignment between a program graph we created from Jimple and each Jimple instruction, but each time SOOT loaded the jimple code into memory it went through another iteration of the jimple packs and messed up my existing alignment. I saw I could eventually reach a fixed point around give or take 3 round trips of null transformations., however in the end I opted for a simpler approach of just loading the original bytecode again with the exact same configurations and letting SOOT do its thing again in an identical way to get back to the state of how things looked when the jimple was originally outputted.
>
> Here was a test my colleague ran. For this example, he used Oracle Java 8 and it did not seem to remove that local variable.
> Note that foo1 and foo2 differ by whether the local is used. Note that there is still an istore_1 instruction. That suggests SOOT/Jimple is removing it somehow.
>
> Source:
> public class Test {
>
> public void foo1() {
> int a = method1();
> }
>
> public int foo2() {
> int a = method1();
> return a;
> }
>
> public int method1() {
> return 0;
> }
> }
>
> Decompiled:
> public void foo1();
> descriptor: ()V
> flags: ACC_PUBLIC
> Code:
> stack=1, locals=2, args_size=1
> 0: aload_0
> 1: invokevirtual #2 // Method method1:()I
> 4: istore_1
> 5: return
> LineNumberTable:
> line 4: 0
> line 5: 5
>
> public int foo2();
> descriptor: ()I
> flags: ACC_PUBLIC
> Code:
> stack=1, locals=2, args_size=1
> 0: aload_0
> 1: invokevirtual #2 // Method method1:()I
> 4: istore_1
> 5: iload_1
> 6: ireturn
> LineNumberTable:
> line 8: 0
> line 9: 5
>
>
>
> On Thu, Oct 4, 2018 at 1:04 PM Chris Shiflet <cshiflet at pjrcorp.com> wrote:
> I hope I'm correct in saying this, but I believe that Eric probably meant "javap".
>
> Running "javap <class file>" will disassemble the class file, and you can see if the Java compiler preserved your variable, "a".
>
> Best,
> ---Chris
> From: Soot-list <soot-list-bounces at CS.McGill.CA> on behalf of Eric Bodden <eric.bodden at uni-paderborn.de>
> Sent: Thursday, October 4, 2018 8:55:38 AM
> To: 陳日揚
> Cc: soot-list at cs.mcgill.ca
> Subject: Re: [Soot-list] Disable Jimple unused variable elimination process
>
> Hi again.
>
> This is strange, especially because Soot normally generates Shimple from the Jimple representation. Hence, if the variable is not present in Jimple, how can it show up in Shimple? I actually wonder whether it’s somehow reintroduced by the value numbering that occurs during the SSA generation.
>
> To be sure about the bytecode, would you mind using “javac” to inspect the .class file to see whether “a” is present there?
>
> Cheers
> Eric
>
> > On 2. Oct 2018, at 17:07, 陳日揚 <t106598014 at ntut.org.tw> wrote:
> >
> > Hi Eric,
> >
> > Thanks for your reply!
> > I've tried to output Shimple format instead of Jimple, and I found that the unused local variable's information was still retained in Shimple format("i0" for the following example).
> > So I think those variables are not thrown away by the compiler.
> >
> > Here are the bytecode and the Shimple code of the previous example:
> >
> > Bytecode:
> > public void foo();
> > Code:
> > 0: aload_0
> > 1: invokespecial #2 // Method method1:()I
> > 4: istore_1
> > 5: return
> >
> > Shimple code:
> > public void foo()
> > {
> > X r0;
> > int i0;
> >
> > r0 := @this;
> >
> > i0 = specialinvoke r0.method1();
> >
> > return;
> > }
> >
> > Thank you for your help!
> >
> > Best regard,
> > Andy
> >
> > Eric Bodden <eric.bodden at uni-paderborn.de> 於 2018年10月2日 週二 下午10:16寫道：
> > Hi Andy.
> >
> > Are you sure that those variables are actually not already thrown away by the compiler? I would think so, actually. If so, then obviously Soot has no way to recover them, except if you analyze the source code.
> >
> > Cheers
> > Eric
> >
> > > On 26. Sep 2018, at 15:42, 陳日揚 <t106598014 at ntut.org.tw> wrote:
> > >
> > > Hi,
> > >
> > > I want to do some analyses in BodyTransformer for Jimple code which was transformed from .class files. It seems that Soot've done some optimizations like eliminating unused variables in Jimple Body Creation phase. I'm wondering can I disable such elimination process so that I can get unused variables' informations in Jimple code.
> > >
> > > I've tried disable "Unused Local Eliminator" and "Dead Assignment Eliminator" commandline options("-p jb.ulp enabled:false -p jb.dae enabled:false"), but the input program below still output Jimple code without variable "a"'s information.
> > >
> > > Java source code(compile to .class file before running Soot):
> > > public void foo() {
> > > int a = method1();
> > > }
> > >
> > > Output Jimple code:
> > > public void foo() {
> > > X r0;
> > > r0 := @this: X;
> > > specialinvoke r0.<X: int method1()>();
> > > return;
> > > }
> > >
> > > Thanks a lot !
> > >
> > > Best regard,
> > > Andy
> > > _______________________________________________
> > > Soot-list mailing list
> > > Soot-list at CS.McGill.CA
> > > https://mailman.CS.McGill.CA/mailman/listinfo/soot-list
> >
>
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