Revert "Do ValueGeneration in the Splice- and CodeGenMutator."

This reverts commit db9aa6e.
We now allow splicing without visible variables as that is not a strict
requirement and also add some more comments as to why we have the
visibleVariables requirement for code generation.

Sources/Fuzzilli/Base/ProgramBuilder.swift

@@ -1252,7 +1252,6 @@ public class ProgramBuilder {
     }
 
     private func buildInternal(initialBuildingBudget: Int, mode: BuildingMode) {
-        assert(hasVisibleVariables, "CodeGenerators and our splicing implementation assume that there are visible variables to use. Use buildPrefix() to generate some initial variables in a new program")
         assert(initialBuildingBudget > 0)
 
         // Both splicing and code generation can sometimes fail, for example if no other program with the necessary features exists.
@@ -1292,7 +1291,9 @@ public class ProgramBuilder {
             let codeSizeBefore = code.count
             switch mode {
             case .generating:
-                assert(hasVisibleVariables)
+                // This requirement might seem somewhat arbitrary but our JavaScript code generators make use of `b.randomVariable` and as such rely on the availability of
+                // visible Variables. Therefore we should always have some Variables visible if we want to use them.
+                assert(hasVisibleVariables, "CodeGenerators assume that there are visible variables to use. Use buildPrefix() to generate some initial variables in a new program")
 
                 // Reset the code generator specific part of the state.
                 state.nextRecursiveBlockOfCurrentGenerator = 1

Sources/Fuzzilli/Mutators/CodeGenMutator.swift

@@ -15,6 +15,7 @@
 /// A mutator that generates new code at random positions in a program.
 public class CodeGenMutator: BaseInstructionMutator {
     private var deadCodeAnalyzer = DeadCodeAnalyzer()
+    private var variableAnalyzer = VariableAnalyzer()
     private let minVisibleVariables = 3
 
     public init() {
@@ -24,19 +25,20 @@ public class CodeGenMutator: BaseInstructionMutator {
 
     public override func beginMutation(of program: Program) {
         deadCodeAnalyzer = DeadCodeAnalyzer()
+        variableAnalyzer = VariableAnalyzer()
     }
 
     public override func canMutate(_ instr: Instruction) -> Bool {
         deadCodeAnalyzer.analyze(instr)
-        // It only makes sense to generate code if we're not currently in dead code.
-        return !deadCodeAnalyzer.currentlyInDeadCode
+        variableAnalyzer.analyze(instr)
+        // We can only generate code if there are some visible variables to use, and it only
+        // makes sense to generate code if we're not currently in dead code.
+        return variableAnalyzer.visibleVariables.count >= minVisibleVariables && !deadCodeAnalyzer.currentlyInDeadCode
     }
 
     public override func mutate(_ instr: Instruction, _ b: ProgramBuilder) {
         b.adopt(instr)
-        if b.numberOfVisibleVariables < minVisibleVariables {
-            b.buildPrefix()
-        }
+        assert(b.numberOfVisibleVariables >= minVisibleVariables)
         b.build(n: defaultCodeGenerationAmount, by: .generating)
     }
 }

Sources/Fuzzilli/Mutators/SpliceMutator.swift

@@ -15,7 +15,6 @@
 /// A mutator that splices programs together.
 public class SpliceMutator: BaseInstructionMutator {
     private var deadCodeAnalyzer = DeadCodeAnalyzer()
-    private let minVisibleVariables = 3
 
     public init() {
         super.init(maxSimultaneousMutations: defaultMaxSimultaneousMutations)
@@ -33,12 +32,6 @@ public class SpliceMutator: BaseInstructionMutator {
 
     public override func mutate(_ instr: Instruction, _ b: ProgramBuilder) {
         b.adopt(instr)
-        // If we currently don't have enough visible variables, we will do some value generation.
-        // This is necessary because our splice implementation requires some visible variables although this is
-        // not strictly necessary for splicing itself (it helps for rewiring inputs).
-        if b.numberOfVisibleVariables < minVisibleVariables {
-            b.buildPrefix()
-        }
         b.build(n: defaultCodeGenerationAmount, by: .splicing)
     }
 }