B plugin generator for VexRiscv

This repostory

This is a quick'n'dirty plugin generator to add a subset of the https://github.com/riscv/riscv-bitmanip extension to the https://github.com/SpinalHDL/VexRiscv core.

The generated plugin is for RV32 only. It doesn't yet support all instructions; missing instructions are:

• all instructions ending in 'W', as they are RV64-only
• BMAT*, as they are RV64-only
• CLMUL*
• BDEP/BEXT/BFP
• CRC32*
• Three-operands instrutioncs (CMIX, CMOV, FS[RL]*)

There is no support for 'partial' instruction - implementing only a subset of the functionality of one instruction. So grev is supported, but rev8 alone isn't. Subextension are defined without 'partial' instructions - so Zbb doesn't have rev8 or orc.B.

This has received limited testing in a https://github.com/litex-hub/linux-on-litex-vexriscv SoC. YMMV.

Also, the implementations of the instructions in SpinalHDL are written for tuncitonality, and not tuned or optimized in any way for performance/area/... (file usage.txt has some numbers).

How to use

There shouldn't be any dependency beyond gcc, g++, flex and bison. Instructions are defined in data_bitmanip.txt, look at the header of that file for the format - it should be fairly easy to add custom instructions if needed (as long as they are register-register, register-immediate, or unary and execute in one cycle).

The tool need an extension name, the data file and the list of instructions and/or sub-extension to support in the plugin:

./gen_plugin BitManipZbpZba data_bitmanip.txt Zba Zbb GORC GREV > BitManipZbbZba.scala

Will generate a plugin supporting Zbb (using the full version of grev and gorc) and Zba. You can use a star to say 'all supported instructions':

./gen_plugin BitManipAll data_bitmanip.txt '*' > BitManipAllPlugin.scala