How to add a new custom instruction?

[Nado15]

Hello,

is there a current guide how to add a new RISC-V instruction to riscv-binutils-gdb?

[Nelson1225]

No, we don't have any standardized guidance to introduce it. Perhaps there are, but at least I don't know. But the following discussion maybe help,
riscv-collab/riscv-gnu-toolchain#782

[Nado15]

Ok thanks I checked the source and I have a question. Lets say I wanne add a instruction that increments the value in the rd register by 1. And I decide to create a U-Type similar instruction. Now I take for example custom-0 (from the table 24.1 in the riscv spec) with the opcode 0001011. Now I have to modify the riscv-opc.c

{"addone", 32, INSN_CLASS_I, "d", MATCH_ADDONE, MASK_ADDONE, match_opcode, 0 }

So as far as I understand, now I am saying that I have added a new instruction to the I set and with the "d" I am telling the risc-v assembler that yyyyyyyyyyyyyyyyyyxxxxx0001011 the register is encoded in the x bits and he ignores the rest of the y bits? Would that be a valid instruction?

[jim-wilson]

MASK_ADDONE is all of the non operand bits. MASK_ADDONE plus the "d" bits should be 32 one bits. MATCH_ADDONE is every non-operand bit that is non-zero. So the encoding is MATCH_ADDONE plus the "d" bits. On disassembly, if (insn & MASK_ADDONE) == MATCH_ADDONE then we print the addone opcode followed by its operands.

[Nado15]

MASK_ADDONE plus the "d" bits should be 32 one bits.

But d is not a fixed bitpattern right (like in my example with the "x" bits)? d can be one register so if in one instruction d has the encoding for x1 and in the next instruction d has the encoding for x2 how should that work out?

[jim-wilson]

MASK_ADDONE plus the "d" bits should be 32 one bits.

I meant all of the bits in the d field. It should be obvious that the operand bits plus non-operand bits should cover every bit. If they don't, then there is a bug either in the mask or the operands.

[Nado15]

I got it know, thank you!

Ok thanks I checked the source and I have a question. Lets say I wanne add a instruction that increments the value in the rd register by 1. And I decide to create a U-Type similar instruction. Now I take for example custom-0 (from the table 24.1 in the riscv spec) with the opcode 0001011. Now I have to modify the riscv-opc.c

{"addone", 32, INSN_CLASS_I, "d", MATCH_ADDONE, MASK_ADDONE, match_opcode, 0 }

So as far as I understand, now I am saying that I have added a new instruction to the I set and with the "d" I am telling the risc-v assembler that yyyyyyyyyyyyyyyyyyxxxxx0001011 the register is encoded in the x bits and he ignores the rest of the y bits? Would that be a valid instruction?

Btw. for the next person who reads this, this wont work as the y bits are not covered similar to #233
Add the "u" operand and you have a correct U-Type instruction

{"addone", 32, INSN_CLASS_I, "d,u", MATCH_ADDONE, MASK_ADDONE, match_opcode, 0 }

MATCH_ADDONE = 0x0000000b
MASK_ADDONE= 0x00000007f