(21w13a) net.minecraft.world.level.levelgen.SimpleRandomSource.nextGaussian fails to decompile

[zml2008]

Method nextGaussian ()D in net/minecraft/world/level/levelgen/SimpleRandomSource couldn't be decompiled.
java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
        at java.util.ArrayList.rangeCheck(ArrayList.java:659)
        at java.util.ArrayList.get(ArrayList.java:435)
        at org.jetbrains.java.decompiler.modules.decompiler.IfHelper.buildGraph(IfHelper.java:366)
        at org.jetbrains.java.decompiler.modules.decompiler.IfHelper.mergeIfs(IfHelper.java:72)
        at org.jetbrains.java.decompiler.modules.decompiler.SimplifyExprentsHelper.simplifyStackVarsStatement(SimplifyExprentsHelper.java:67)
        at org.jetbrains.java.decompiler.modules.decompiler.SimplifyExprentsHelper.simplifyStackVarsStatement(SimplifyExprentsHelper.java:65)
        at org.jetbrains.java.decompiler.modules.decompiler.SimplifyExprentsHelper.simplifyStackVarsStatement(SimplifyExprentsHelper.java:65)
        at org.jetbrains.java.decompiler.modules.decompiler.EliminateLoopsHelper.eliminateLoops(EliminateLoopsHelper.java:27)
        at org.jetbrains.java.decompiler.main.rels.MethodProcessorRunnable.codeToJava(MethodProcessorRunnable.java:157)
        at org.jetbrains.java.decompiler.main.rels.ClassWrapper.init(ClassWrapper.java:61)
        at org.jetbrains.java.decompiler.main.ClassesProcessor.initWrappers(ClassesProcessor.java:470)
        at org.jetbrains.java.decompiler.main.ClassesProcessor.processClass(ClassesProcessor.java:386)
        at org.jetbrains.java.decompiler.main.Fernflower.processClass(Fernflower.java:149)
        at org.jetbrains.java.decompiler.struct.ContextUnit.save(ContextUnit.java:217)
        at org.jetbrains.java.decompiler.struct.StructContext.saveContext(StructContext.java:63)
        at org.jetbrains.java.decompiler.main.Fernflower.decompileContext(Fernflower.java:120)
        at org.spongepowered.gradle.vanilla.worker.JarDecompileWorker.execute(JarDecompileWorker.java:90)
        at org.gradle.workers.internal.DefaultWorkerServer.execute(DefaultWorkerServer.java:63)
        at org.gradle.workers.internal.AbstractClassLoaderWorker$1.create(AbstractClassLoaderWorker.java:49)
        at org.gradle.workers.internal.AbstractClassLoaderWorker$1.create(AbstractClassLoaderWorker.java:43)
        at org.gradle.internal.classloader.ClassLoaderUtils.executeInClassloader(ClassLoaderUtils.java:97)
        at org.gradle.workers.internal.AbstractClassLoaderWorker.executeInClassLoader(AbstractClassLoaderWorker.java:43)
        at org.gradle.workers.internal.IsolatedClassloaderWorker.run(IsolatedClassloaderWorker.java:49)
        at org.gradle.workers.internal.IsolatedClassloaderWorker.run(IsolatedClassloaderWorker.java:30)
        at org.gradle.workers.internal.WorkerDaemonServer.run(WorkerDaemonServer.java:85)
        at org.gradle.workers.internal.WorkerDaemonServer.run(WorkerDaemonServer.java:55)
        at org.gradle.process.internal.worker.request.WorkerAction$1.call(WorkerAction.java:138)
        at org.gradle.process.internal.worker.child.WorkerLogEventListener.withWorkerLoggingProtocol(WorkerLogEventListener.java:41)
        at org.gradle.process.internal.worker.request.WorkerAction.run(WorkerAction.java:135)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:36)
        at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)
        at org.gradle.internal.remote.internal.hub.MessageHubBackedObjectConnection$DispatchWrapper.dispatch(MessageHubBackedObjectConnection.java:182)
        at org.gradle.internal.remote.internal.hub.MessageHubBackedObjectConnection$DispatchWrapper.dispatch(MessageHubBackedObjectConnection.java:164)
        at org.gradle.internal.remote.internal.hub.MessageHub$Handler.run(MessageHub.java:414)
        at org.gradle.internal.concurrent.ExecutorPolicy$CatchAndRecordFailures.onExecute(ExecutorPolicy.java:64)
        at org.gradle.internal.concurrent.ManagedExecutorImpl$1.run(ManagedExecutorImpl.java:48)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
        at java.lang.Thread.run(Thread.java:748)

this came up when decompiling this week's snapshot. haven't had a chance to look into it further, can do so later.

[Geolykt]

I probably fixed it within Geolykt@a9c2f7d a while back, however I haven't PR'd that commit yet since I have yet to test it out in a minecraft environment (even if most of the changes should be safe to handle).
However if that has been solved upstream by not going into the exception states in the first place, this would be better

[zml2008]

geo, did you look into any root causes of those exceptions, or just include stronger guards?

the exception still occurs after the recent rebase, so unfortunately upstream hasn't fixed it for us.

i'll take your commit for a spin, but it would be good to understand the root causes of the issues -- since clearly an assumption somewhere was wrong

[zml2008]

alright, tried it out -- my updated version is at master...zml2008:fix-exceptions.

On MCPConfig 1.16.5, there were no changes to the output, and the :1.16.5:testJdks task passed.

In a 21w13a workspace, SimpleRandomSource.nextGaussian() now decompiles properly, with no other changes to output.

The resulting source is:

   public double nextGaussian() {
      if (this.haveNextNextGaussian) {
         this.haveNextNextGaussian = false;
         return this.nextNextGaussian;
      } else {
         while(true) {
            double var0 = 2.0D * this.nextDouble() - 1.0D;
            double var2 = 2.0D * this.nextDouble() - 1.0D;
            double var4 = Mth.square(var0) + Mth.square(var2);
            if (!(var4 >= 1.0D)) {
               if (var4 != 0.0D) {
                  double var6 = Math.sqrt(-2.0D * Math.log(var4) / var4);
                  this.nextNextGaussian = var2 * var6;
                  this.haveNextNextGaussian = true;
                  return var0 * var6;
               }
            }
         }
      }
   }

For comparison, this is the same method decompiled in various other decompilers

CFR (0.151):

    @Override
    public double nextGaussian() {
        double var2;
        double var0;
        double var4;
        if (this.haveNextNextGaussian) {
            this.haveNextNextGaussian = false;
            return this.nextNextGaussian;
        }
        do {
            var0 = 2.0 * this.nextDouble() - 1.0;
            var2 = 2.0 * this.nextDouble() - 1.0;
        } while ((var4 = Mth.square(var0) + Mth.square(var2)) >= 1.0 || var4 == 0.0);
        double var6 = Math.sqrt(-2.0 * Math.log(var4) / var4);
        this.nextNextGaussian = var2 * var6;
        this.haveNextNextGaussian = true;
        return var0 * var6;
    }

Procyon (0.5.36):

    @Override
    public double nextGaussian() {
        if (this.haveNextNextGaussian) {
            this.haveNextNextGaussian = false;
            return this.nextNextGaussian;
        }
        double var4;
        double var5;
        double var6;
        do {
            var5 = 2.0 * this.nextDouble() - 1.0;
            var6 = 2.0 * this.nextDouble() - 1.0;
            var4 = Mth.square(var5) + Mth.square(var6);
        } while (var4 >= 1.0 || var4 == 0.0);
        final double var7 = Math.sqrt(-2.0 * Math.log(var4) / var4);
        this.nextNextGaussian = var6 * var7;
        this.haveNextNextGaussian = true;
        return var5 * var7;
    }

Fernflower (upstream, forge's build 403):

    public double nextGaussian() {
        if (this.haveNextNextGaussian) {
            this.haveNextNextGaussian = false;
            return this.nextNextGaussian;
        } else {
            double var0;
            double var2;
            double var4;
            do {
                do {
                    var0 = 2.0D * this.nextDouble() - 1.0D;
                    var2 = 2.0D * this.nextDouble() - 1.0D;
                    var4 = Mth.square(var0) + Mth.square(var2);
                } while(var4 >= 1.0D);
            } while(var4 == 0.0D);

            double var6 = Math.sqrt(-2.0D * Math.log(var4) / var4);
            this.nextNextGaussian = var2 * var6;
            this.haveNextNextGaussian = true;
            return var0 * var6;
        }
    }

ForgeFlower treats the loop a bit strangely, but it appears to be correct.

[Geolykt]

I was unable to find any root causes (except for one NPE that I commented how it happens, but that is unrelated), however I haven't found any larger side effects to the modifications (compared to the code produced by other decompilers like Krakatau).
The good thing about that patch is that the only side effect is that it will only make broken code "working" by preventing a fatal exception to be thrown. So technically we could use that patch as a bandaid now (and creating log statements beforehand to make sure the future does not have any larger issues) and find the issues later.
The patch probably also belongs upstream, but I have no idea how jetbrain's monorepos behave, so I won't do that