The Cross-Compiler

The RISCV cross-compiler is a custom-built riscv32-boxlambda-elf toolchain. The compiler build is created using the excellent Crosstool-ng project. Crosstool-ng uses a menuconfig similar to the Linux kernel menuconfig. You just focus on the specifics of the toolchain you want to build (RISC-V, 32-bit, Static Toolchain...). The tool selects good defaults for all the rest.

I selected:

• Target Architecture: riscv
• Architecture level: rv32im_zicsr_za_zb_zbs
• ABI: ilp32
• Build Static Toolchain
• Tuple's vendor string: boxlambda
• Target OS: bare-metal
• Additional support languages: C++

The resulting crosstool-ng config file can be found here.

The toolchain tarball is checked into the BoxLambda repo under assets/. The boxlambda_setup.sh script unpacks the toolchain tarball in the tools/ directory, so the user no longer needs to provide the toolchain as a prerequisite.

RISC-V GCC Compile Flags

The following compile flags are used:

• march=rv32im_zba_zbb_zbs_zicsr:

  • rv32: 32-bit RISC-V base architecture.
  • i: Base integer instruction set (mandatory).
  • m: Integer multiplication and division extension.
  • zba: Bit-Manipulation instructions for address generation.
  • zbb: Bit-Manipulation Base sub-extension.
  • zbs: Single-bit instructions.
  • zicsr: Control and Status Register (csr) instructions.

• -mabi=ilp32:

  • i: Integer-based ABI (does not use floating-point registers for function arguments/returns).
  • l: Long and int types are 32-bit.
  • p: Pointers are 32-bit wide.
  • 32: Indicates a 32-bit ABI (used for RV32 architectures).

• Wl,--no-warn-rwx-segments: Avoid linker warning because executable segments are writable. This is intentional on BoxLambda.

Toolchain.cmake

RISC-V cross-compilation for C and C++ is set up by passing in a Toolchain File to CMake. The toolchain file specifies the names of the compiler executables and the compile flags. The file is located in scripts/toolchain.cmake.