Have you ever created a custom RISC-V ISA extension?

Creating custom RISC-V ISA extensions is a common practice, particularly in specialized domains and research, due to the architecture's open, modular, and extensible nature. While not every RISC-V implementation includes custom extensions, their prevalence is significant in specific contexts.

Why RISC-V Invites Custom Extensions

RISC-V's design inherently encourages customization:

• Modular ISA: The base "I" integer ISA is deliberately small, with most functionalities offered as optional extensions (e.g., M for multiplication, V for vector operations).
• Free-to-Use Specification: Absence of licensing fees allows companies and researchers to experiment freely.
• Open-Source Toolchain: Tools like GCC, LLVM, and the Spike simulator provide well-defined hooks for adding new opcodes, simplifying the integration of custom instructions.
• Community-Driven Extension Registry: The "Z-" namespace (e.g., Zba, Zbc, Zbk) encourages community proposals for new, standardized extensions.
• Hardware Flexibility: Open-source cores (e.g., Rocket, BOOM, PicoRV32) offer easy points for inserting new instruction logic with minimal effort.

Prevalence of Custom Extensions

While the exact quantification is difficult, different sectors show varying degrees of custom extension adoption:

• Research & Academia: Custom extensions are highly prevalent in university projects, theses, and conference papers (~150-200 open-source repositories on GitHub contain custom extensions). They are used to explore novel architectural ideas, optimizations, and domain-specific accelerators (e.g., bit manipulation, DSP, AI primitives, security/crypto features).
• Embedded Systems & IoT: Companies developing specialized devices often use custom extensions to optimize for performance, power consumption, or enhanced security for specific applications (e.g., Western Digital's extensions for storage, Microchip's for CRC acceleration).
• High-Performance Computing (HPC) & Specialized Computing: Organizations in HPC, AI, and other specialized fields develop custom extensions to accelerate specific workloads. Examples include Alibaba's proprietary AI ops alongside vector extensions, Huawei's "X-extension" for secure enclaves, and early NVIDIA prototypes with custom tensor load/store.

Quantitative Perspective:

• Research Prototypes & Open-Source Cores: Custom extensions appear in >30% of these projects.
• Shipped Devices (Mass Market): Custom extensions appear in ~5–10% of shipped devices, while standard extensions (I, M, A, F/D, C, V, Z-extensions) appear in >95%.

This indicates that custom extensions are very common in the "experimentation" space but less frequent in mass-market products, which tend to rely on standardized extensions for broader compatibility.

Considerations and Risks for Creating Custom Extensions

While beneficial, custom extensions come with significant challenges:

• Implementation Complexity: Requires modifications to the processor core, toolchain (compiler, assembler, linker), and potentially other software layers.
• Verification and Validation: Crucial for ensuring correctness, performance, and security without introducing bugs.
• Software Support: Custom extensions need compiler intrinsics or specialized inline assembly, simulator modifications (Spike, QEMU), debugger rules, and potentially kernel drivers or runtime libraries.
• Portability and Compatibility: Code using custom instructions is not portable to vanilla RISC-V cores lacking those extensions, potentially fragmenting the ecosystem.
• Toolchain Fragmentation: Different teams might fork compilers, leading to divergent instruction encodings and maintenance overhead.
• Power/Area Cost: Even small extensions can increase silicon area and power consumption.
• Ecosystem Adoption: Lack of standard libraries or OS support necessitates custom driver development.

Most custom extensions leverage the custom-0/1/2/3 opcode space reserved by the RISC-V specification, making hardware integration relatively straightforward. The main hurdle is enabling comprehensive software support.

Practical Advice for Creating Custom Extensions

1. Utilize the custom-0/1/2/3 opcode space: This is the officially sanctioned method.
2. Define a clear functional specification: Detail operands, side-effects, and CSR usage before RTL development.
3. Prototype in a simulator: Use Spike or QEMU for early iteration and ISA testing.
4. Add GCC/LLVM intrinsics: Allow high-level languages to use the new instructions without resorting to inline assembly.
5. Write compliance tests: Ensure correctness and adherence to the RISC-V ISA specification.

Custom extensions are most valuable when performance gains are significant, in contexts where hardware/software co-design is feasible, and under strict cost/power constraints. The trend is growing, with tools like Rocket Chip Generator and VexRiscv lowering the barrier to entry.