InputMutator.swift

// Copyright 2019 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

/// A mutator that changes the input variables of instructions in a program.
public class InputMutator: BaseInstructionMutator {
    /// Whether this instance is type aware or not.
    /// A type aware InputMutator will attempt to find "compatible" replacement
    /// variables, which have roughly the same type as the replaced variable.
    public let typeAwareness: TypeAwareness

    private let logger: Logger

    public enum TypeAwareness {
        case loose
        case aware
    }

    public init(typeAwareness: TypeAwareness) {
        self.typeAwareness = typeAwareness
        self.logger = Logger(withLabel: "InputMutator \(String(describing: typeAwareness))")
        var maxSimultaneousMutations = defaultMaxSimultaneousMutations
        // A type aware instance can be more aggressive. Based on simple experiments and
        // the mutator correctness rates, it can very roughly be twice as aggressive.
        switch self.typeAwareness {
        case .aware:
                maxSimultaneousMutations *= 2
        default:
            break
        }
        super.init(name: "InputMutator (\(String(describing: self.typeAwareness)))", maxSimultaneousMutations: maxSimultaneousMutations)
    }

    public override func canMutate(_ instr: Instruction) -> Bool {
        if instr.isNotInputMutable {
            // This is currently the case for some WasmInstructions that have to adhere to
            // more rules than just strict typing, e.g. WasmStoreGlobal/WasmLoadGlobal
            // Also the case for wasmReassign.
            return false
        }

        return instr.numInputs > 0
    }

    public override func mutate(_ instr: Instruction, _ b: ProgramBuilder) {
        var inouts = b.adopt(instr.inouts)

        // Replace one input
        let selectedInput = Int.random(in: 0..<instr.numInputs)
        // Inputs to block end instructions must be taken from the outer scope since the scope
        // closed by the instruction is currently still active.
        let replacement: Variable?

        // In wasm we need strict typing, so there is no notion of loose or aware.
        if b.context.contains(.wasm) || b.context.contains(.wasmFunction) {
            let type = b.type(of: inouts[selectedInput])
            replacement = b.randomVariable(ofType: type)
        } else {
            switch self.typeAwareness {
            case .loose:
                replacement = b.randomVariable()
            case .aware:
                let type = b.type(of: inouts[selectedInput])
                replacement = b.randomVariable(forUseAs: type)
            }
        }

        if let replacement = replacement {
            b.trace("Replacing input \(selectedInput) (\(inouts[selectedInput])) with \(replacement)")
            inouts[selectedInput] = replacement

            // This assert is here to prevent subtle bugs if we ever decide to add flags that are "alive" during program building / mutation.
            // If we add flags, remove this assert and change the code below.
            assert(instr.flags == .empty)
            b.append(Instruction(instr.op, inouts: inouts, flags: .empty))
        }
    }
}