deadcode-eliminator/optimizer.js at main · dsdeshna/deadcode-eliminator · GitHub

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class Optimizer {
    constructor() {
        this.eliminatedIndices = [];
    }

    optimize(instructions) {
        this.eliminatedIndices = [];
        let current = [...instructions];
        let changed = true;
        let passes = 0;

        while (changed && passes < 10) {
            changed = false;
            passes++;
            const result = this.eliminateDeadCode(current);
            if (result.removed > 0) {
                changed = true;
                current = result.instructions;
            }
        }

        return current;
    }

    // returns { instructions, removed }
    eliminateDeadCode(instructions) {
        const hasControlFlow = instructions.some(i =>
            ['label','goto','if_false_goto','if_goto'].includes(i.type)
        );

        const keep = [];
        let removed = 0;

        for (let i = 0; i < instructions.length; i++) {
            const instr = instructions[i];

            if (this.isEliminable(instr) && instr.result) {
                const used = hasControlFlow
                    ? this.isUsedAnywhere(instr.result, i, instructions)
                    : this.isUsedAfterLinear(instr.result, i, instructions);

                if (!used) {
                    instr._dead = true;
                    this.eliminatedIndices.push(i);
                    removed++;
                    continue;
                }
            }

            // remove unused declarations
            if (instr.type === 'declare') {
                if (!this.isUsedAnywhere(instr.result, -1, instructions)) {
                    instr._dead = true;
                    this.eliminatedIndices.push(i);
                    removed++;
                    continue;
                }
            }

            keep.push(instr);
        }

        return { instructions: keep, removed };
    }

    // can this instruction be eliminated if its result is unused?
    isEliminable(instr) {
        // never eliminate these side-effect instructions
        const nonEliminable = [
            'label','goto','if_false_goto','if_goto',
            'call','param','return',
            'func_begin','func_end',
            'break','continue'
        ];
        if (nonEliminable.includes(instr.type)) return false;
        return instr.result != null;
    }

    // for code with control flow: check if var is used anywhere (conservative)
    isUsedAnywhere(varName, defIndex, instructions) {
        for (let i = 0; i < instructions.length; i++) {
            if (i === defIndex) continue;
            if (this.instrUsesVar(instructions[i], varName)) return true;
        }
        return false;
    }

    // for straight-line code: check forward until redefinition
    isUsedAfterLinear(varName, defIndex, instructions) {
        for (let i = defIndex + 1; i < instructions.length; i++) {
            const instr = instructions[i];
            if (this.instrUsesVar(instr, varName)) return true;
            // If redefined and not self-referencing, stop
            if (instr.result === varName && instr.arg1 !== varName && instr.arg2 !== varName) {
                return false;
            }
        }
        return false;
    }

    // does this instruction reference varName as an operand?
    instrUsesVar(instr, varName) {
        if (instr.arg1 === varName) return true;
        if (instr.arg2 === varName) return true;
        if (instr.type === 'return' && instr.arg1 === varName) return true;
        if ((instr.type === 'if_false_goto' || instr.type === 'if_goto') && instr.arg1 === varName) return true;
        return false;
    }

    // run a diff between original and optimized to find dead lines
    static diff(original, optimized) {
        const optimizedSet = new Set();
        for (const instr of optimized) {
            optimizedSet.add(JSON.stringify(instr));
        }
        const deadLines = [];
        for (let i = 0; i < original.length; i++) {
            const key = JSON.stringify(original[i]);
            if (!optimizedSet.has(key)) {
                deadLines.push(i);
            }
        }
        return deadLines;
    }
}