RISC-V Atomics

This repository contains a standalone implementation of the C/C++ Atomic operations library; one implementation using GCC compiler intrinsics, and another implementation using in-line assembly.

The assembly macros follow guidelines for mapping C/C++ Atomic primitives to the RISC-V atomic primitives, as outlined in The RISC-V Instruction Set Manual Volume I: Unprivileged ISA, Appendix A — RVWMO Explanatory Material, Version 0.1, Section A.5 Code Porting and Mapping Guidelines, Table A.6: Mappings from C/C++ primitives to RISC-V primitives.

The following versions of the C/C++ atomics are implemented in bare-metal inline assembly based on those guidelines:

• atomic_compare_exchange
• atomic_fetch_add
• atomic_fetch_sub
• atomic_fetch_or
• atomic_fetch_xor
• atomic_fetch_and
• atomic_exchange
• atomic_load
• atomic_store
• atomic_flag_test_and_set
• atomic_flag_clear
• atomic_thread_fence
• atomic_signal_fence

Dependencies

A recent version of riscv-gnu-toolchin with a multilib build of RISC-V GCC.

• riscv-gnu-toolchain

Suggest: sudo apt install gcc-riscv64-linux-gnu with Ubuntu 20.04.

Code Generation Tests

The primary purpose of the repository is to test the GCC atomic builtins and allow comparison of their output with hand-coded assembly versions that have been constructed to match the atomic mappings in the RISC-V Instruction Set Manual Memory Model Guidelines.

The test directory contains code generation test macros along with assembly language exemplars. The Makefile is set up to compile using the RISC-V GCC compiler intrinsics (-DATOMIC_ASM=0), or the in-line assembly versions (-DATOMIC_ASM=1). To build the tests, type make:

$ make

There currently are no automated tests, thus visually inspection must be used to compare against the exemplars in the RISC-V Instruction Set Manual Memory Model Guidelines, the test-case comments, the object code emitted by the GCC compiler intrinsics, and lastly, the object code generated by the hand-coded versions. The following files should be output:

atomic_compare_exchange_asm.txt
atomic_compare_exchange_gcc.txt
atomic_exchange_asm.txt
atomic_exchange_gcc.txt
atomic_fetch_add_asm.txt
atomic_fetch_add_gcc.txt
atomic_fetch_and_asm.txt
atomic_fetch_and_gcc.txt
atomic_fetch_or_asm.txt
atomic_fetch_or_gcc.txt
atomic_fetch_sub_asm.txt
atomic_fetch_sub_gcc.txt
atomic_fetch_xor_asm.txt
atomic_fetch_xor_gcc.txt
atomic_flag_clear_asm.txt
atomic_flag_clear_gcc.txt
atomic_flag_test_and_set_asm.txt
atomic_flag_test_and_set_gcc.txt
atomic_load_asm.txt
atomic_load_gcc.txt
atomic_signal_fence_asm.txt
atomic_signal_fence_gcc.txt
atomic_store_asm.txt
atomic_store_gcc.txt
atomic_thread_fence_asm.txt
atomic_thread_fence_gcc.txt