Implement format-specific encoding mechanism #546

The current instruction encoding mechanism for RISC-V in Sail uses a simple mapping, from “ast” to a 32-bit opcode value (ignoring compressed instructions for now). For example: ``` union clause ast = RISCV_JALR : (bits(12), regidx, regidx) mapping clause encdec = RISCV_JALR(imm, rs1, rd) <-> imm @ rs1 @ 0b000 @ rd @ 0b1100111 ``` This is certainly functional. However, what’s missing from this representation is: - Any notion of the opcode format - Any enforcement mechanism for the order of the respective bit fields with respect to the opcode format - Any enforcement mechanism for the sizes of the respective bit fields A *wrong* encoding representation would readily compile: ``` mapping clause encdec = RISCV_JALR(imm, rs1, rd) <-> 0b1100111 @ rd @ rs1 @ 0b000 @ imm /* WRONG */ ``` This would (hopefully) be caught in runtime testing, of course. This (draft) PR proposes an encoding mechanism with enhancements: - Each opcode is clearly associated with an encoding format - Each instruction only provides a structured list of encoding-agnostic opcode inputs - A format-specific, instruction-agnostic encoding mechanism is provided This forces instructions in the Sail code to clearly identify their formats, and allows them to provide the associated inputs for the encoding without concern for the specifics of the encoding. It also isolates the encoding scheme to a small set of format-specific encodings separate from the instruction definitions. An example for format-specific encoding: ``` enum Format = { R_Format, U_Format, I_Format, J_Format, S_Format, B_Format, Unknown_Format } val opcode2format : bits(7) -> Format scattered function opcode2format union instruction_input = { IFormat: { imm: bits(12), rs1: regidx, funct3: bits(3), rd: regidx, opcode: bits(7) }, [...] } val encdec : ast <-> instruction_input scattered mapping encdec mapping fmt2bits : instruction_input <-> bits(32) = { IFormat(struct { imm = imm, rs1 = rs1, funct3 = funct3, rd = rd, opcode = opcode }) if opcode2format(opcode) == I_Format <-> imm @ rs1 @ funct3 @ rd @ opcode if opcode2format(opcode) == I_Format, [...] } ``` An example of a instruction’s encoding information: ``` function clause opcode2format 0b1100111 = I_Format mapping clause encdec = RISCV_JALR(imm, rs1, rd) <-> IFormat(struct { imm = imm, rs1 = rs1, funct3 = 0b000, rd = rd, opcode = 0b1100111 }) ```

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Implement format-specific encoding mechanism #546

Implement format-specific encoding mechanism #546

Commits on Sep 10, 2024

Implement format-specific encoding mechanism #546

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Implement format-specific encoding mechanism #546

Commits on Sep 10, 2024