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RISC-V Bitmanip Extension

1 Introduction

This is the RISC-V Bitmanip Extension draft spec.

1.1 ISA Extension Proposal Design Criteria

Any proposed changes to the ISA should be evaluated according to the following criteria.

  • Architecture Consistency: Decisions must be consistent with RISC-V philosophy. ISA changes should deviate as little as possible from existing RISC-V standards (such as instruction encodings), and should not re-implement features that are already found in the base specification or other extensions.

  • Threshold Metric: The proposal should provide significant savings in terms of clocks or instructions. As a heuristic, any proposal should replace at least three instructions. An instruction that only replaces two may be considered, but only if the frequency of use is very high and/or the implementation very cheap.

  • Data-Driven Value: Usage in real world applications, and corresponding benchmarks showing a performance increase, will contribute to the score of a proposal. A proposal will not be accepted on the merits of its theoretical value alone, unless it is used in the real world.

  • Hardware Simplicity: Though instructions saved is the primary benefit, proposals that dramatically increase the hardware complexity and area, or are difficult to implement, should be penalized and given extra scrutiny. The final proposals should only be made if a test implementation can be produced.

  • Compiler Support: ISA changes that can be natively detected by the compiler, or are already used as intrinsics, will score higher than instructions which do not fit that criteria.

1.2 B Extension Adoption Strategy

The overall goal of this extension is pervasive adoption by minimizing potential barriers and ensuring the instructions can be mapped to the largest number of ops, either direct or pseudo, that are supported by the most popular processors and compilers. By adding generic instructions and taking advantage of the RISC-V base instructions that already operate on bits, the minimal set of instructions need to be added while at the same time enabling a rich of operations.

The instructions cover the four major categories of bit manipulation: Count, Extract, Insert, Swap. The spec supports RV32, RV64, and RV128. “Clever” obscure and/or overly specific instructions are avoided in favor of more straightforward, fast, generic ones. Coordination with other emerging RISC-V ISA extensions groups is required to ensure our instruction sets are architecturally consistent.

1.3 Next steps

  • Assign concrete instruction encodings so that we can start implementing the extension in processor cores and compilers.

  • Add support for this extension to processor cores and compilers so we can run quantitative evaluations on the instructions.

  • Create assembler snippets for common operations that do not map 1:1 to any instruction in this spec, but can be implemented easily using clever combinations of the instructions. Add support for those snippets to compilers.