SpliceMutator.swift

// Copyright 2022 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 splices programs together.
public class SpliceMutator: BaseInstructionMutator {
    private var deadCodeAnalyzer = DeadCodeAnalyzer()

    public init() {
        super.init(maxSimultaneousMutations: defaultMaxSimultaneousMutations)
    }

    public override func beginMutation(of program: Program) {
        deadCodeAnalyzer = DeadCodeAnalyzer()
    }

    public override func canMutate(_ instr: Instruction) -> Bool {
        deadCodeAnalyzer.analyze(instr)

        // Same as for the CodeGenMutator: We do not splice directly into the
        // TypeGroup, rather we splice onto the EndTypeGroup instruction, we
        // therefore don't return the instructions inside a TypeGroup as
        // candidates. This collapses the candidates that we can splice into
        // into the EndTypeGroup instruction, which then insures that we
        // actually update that instruction to make spliced type definitions
        // available.
        if (instr.op.requiredContext.contains(.wasmTypeGroup) && !(instr.op is WasmEndTypeGroup))
            || (instr.op is WasmBeginTypeGroup) {
            return false
        }
        // It only makes sense to copy code if we're not currently in dead code.
        return !deadCodeAnalyzer.currentlyInDeadCode
    }

    public override func mutate(_ instr: Instruction, _ b: ProgramBuilder) {
        switch instr.op.opcode {
        case .wasmEndTypeGroup:
            b.buildIntoTypeGroup(endTypeGroupInstr: instr, by: .splicing)
        default:
            b.adopt(instr)
            b.build(n: defaultCodeGenerationAmount, by: .splicing)
        }
    }
}