adding new opode extensions

[vanjoe]

at the bottom of riscv-opc.c there is comment

/* Removed const from the following to allow for dynamic extensions to the
   built-in instruction set.  */
struct riscv_opcode *riscv_opcodes =
  (struct riscv_opcode *) riscv_builtin_opcodes;
int bfd_riscv_num_opcodes = RISCV_NUM_OPCODES;

This seems to indicate that there is a [planned] way to easily add instructions. Is there currently a clean way of doing this?
I've currently hacked the riscv_buildin_opcodes data structure to add my extensions, but I would love to know if there is a better way of doing this.

[palmer-dabbelt]

There's no way to do this. I'm closing this in favor of riscvarchive/riscv-binutils-gdb#38

[TommyMurphyTM1234]

This is a list of all issues logged that I can find that relate to adding new instructions to the toolchain:

• How to add support for a new instruction? #1095
• Adding custom instruction to vector compiler #1055
• Adding custom instruction to the elf tool-chain #837
• How to Add custom instructions to riscv-elf-toolchain #782
• Adding rvv intrinsic to already installed toolchain #1003
• How to add a custom non-standard instruction type format #795
• how can I modify functionality of an instruction? #660
• question about adding "mod" instruction #543
• Searching for explainations to define a new instruction in the "struct riscv-opcode " #341
• adding new opode extensions #115

At this stage in the evolution of RISC-V,the majority of instruction addition will be done under the auspices of the relevant groups working on standardising, prototyping, and implementing/enhancing RISC-V extensions and their corresponding instructions. As such, there should be less of a demand for others to do so.

However, where somebody still wants to implement a non stadard custom extension/instruction here are some useful resources:

• GCC internals documentation: https://gcc.gnu.org/onlinedocs/gccint/
• https://hsandid.github.io/posts/risc-v-custom-instruction/
• https://ossg.bcs.org/blog/2020/10/29/adding-an-instruction-to-the-gnu-assembler/
• https://embdev.net/topic/228773
• https://groups.google.com/u/0/a/groups.riscv.org/g/sw-dev/c/sL_OHXYj3LY/m/NOm5Fwa3BAAJ
• https://forums.sifive.com/t/adding-custom-instruction-to-riscv-toolchain/4044/2