Persona: Senior Compiler Engineer & Systems Architect
Abstract:
This document details the architectural design and implementation strategies of "DeiMOS," a superoptimizer tailored for the MOS 6502 microprocessor. Unlike heuristic-based compilers, DeiMOS utilizes exhaustive search to identify optimal machine code sequences. Given the 6502’s 8-bit architecture and small instruction set, the search space is computationally tractable, provided that aggressive pruning and optimization techniques are employed. The report explores advanced methods including multi-process networked clusters, state-caching during emulation, warp-based parallel test-case execution for early branch pruning, and the utilization of "shadow instructions"—leveraging instruction operands as executable opcodes via overlapping memory offsets. The resulting tool is capable of synthesizing highly non-intuitive code sequences, including exploits of unofficial opcodes.
Summary:
Core Objective: To replace heuristic-driven code generation with exhaustive search to find the mathematically "shortest" or "fastest" instruction sequence for a given task on the 8-bit MOS 6502 CPU.
Search Space Management:
Pruning: The engine discards sequences that crash the CPU, perform operations on uninitialized data, or attempt to overwrite designated input parameters.
Instruction Filtering: Users can restrict memory access ranges, stack operations, and branch depth to exponentially reduce the search space.
Emulation & Verification:
State Caching: The emulator stores CPU/memory snapshots, allowing the engine to rewind and mutate individual bytes of an instruction sequence rather than re-emulating from the entry point.
Parallelization: The system offloads the search to networked clusters. A master process distributes "prefixes" (valid instruction starts) to workers, ensuring near-100% core utilization.
"Warp" Execution Model: To accelerate branch-heavy code synthesis, the system emulates multiple test cases in parallel using a "warp" (vectorized CPU state). If the warp encounters states that imply failure or redundancy (merged states with mismatched output requirements), the entire branch is pruned immediately.
Shadow Instructions: The synthesizer exploits 6502 branch behavior, where jumps can target the second or third byte of a multi-byte instruction, forcing the CPU to interpret those operands as new, valid opcodes.
Unofficial Opcodes: The engine supports "undocumented" 6502 instructions (e.g., RRA), which can be leveraged for high-density, non-standard arithmetic optimizations, albeit with platform-specific portability caveats.
Performance: Implemented in Zig for high-performance metaprogramming (customizing the emulator at compile-time for specific tasks). On a modern 8-core system, it can synthesize optimal sequences of approximately 11 bytes within one hour.
Output: The tool generates the Pareto frontier, providing the user with a collection of programs representing the best trade-offs between speed and size.
Suggested Reviewer Profile:
To critically evaluate the methodology, code synthesis, and emulator architecture described in this document, the following experts would be most qualified:
Compiler Backend Engineer (LLVM/GCC/MLIR focus): To assess the validity of the "superoptimization" approach versus traditional instruction selection and peephole optimization.
Retrocomputing/Microprocessor Architect: To verify the accuracy of the 6502 cycle counting, unofficial opcode behavior, and the feasibility of shadow instruction usage.
High-Performance Computing (HPC) Systems Engineer: To review the TCP-based distributed worker model and the "warp" parallelization strategies for task-parallel search.
Static Analysis & Formal Verification Researcher: To evaluate the logic behind the "early pruning" heuristics and the correctness of the equivalence classes derived during emulation.