RISC-V: Immediate Encoding Variants

Question

In the RISC-V Instruction Set Manual, User-Level ISA, I couldn't understand section 2.3 Immediate Encoding Variants page 11.

There is four types of instruction formats R, I, S, and U, then there is a variants of S and U types which are SB and UJ which I suppose mean Branch and Jump as shown in figure 2.3. Then there is the types of Immediate produced by RISC-V instructions shown in figure 2.4.

So my questions are, why the SB and UJ are needed? and why shuffle the Immediate bits in that way? what does it mean to say "the Immediate produced by RISC-V instructions"? and how are they produced in this manner?

Answer 1

To speed up decoding, the base RISC-V ISA puts the most important fields in the same place in every instruction. As you can see in the instruction formats table,

• The major opcode is always in bits 0-6.
• The destination register, when present, is always in bits 7-11.
• The first source register, when present, is always in bits 15-19.
• The second source register, when present, is always in bits 20-24.

The other bits are used for the minor opcode or other data for the instruction (funct3 in bits 12-14 and funct7 in bits 25-31), and for the immediate. How many bits can be used for the immediate depends on how many register numbers are present in the instruction:

• Instructions with one destination and two source registers (R-type) have no immediate, for instance adding two registers (ADD);
• Instructions with one destination and one source register (I-type) have 12 bits for the immediate, for instance adding one register with an immediate (ADDI);
• Instructions with two source registers and no destination register (S-type), for instance the store instructions, have also 12 bits for the immediate, but they have to be in a different place since the register numbers are also in a different place;
• Finally, instructions with only a destination register and no minor opcode (U-type), for instance LUI, can use 20 bits for the immediate (the major opcode and the destination register number together need 12 bits).

Now think from the other point of view, of the instructions which will use these immediate values. The simplest users, I-immediate and S-immediate, need only a sign-extended 12-bit value. The U-immediate instructions need the immediate in the upper 20 bits of a 32-bit value. Finally, the branch/jump instructions need the sign-extended immediate in the lower bits of the value, except for the lowest bit which will always be zero, since RISC-V instructions are always aligned to even addresses.

But why are the immediate bits shuffled? Think this time about the physical circuit which decodes the immediate field. Since it's a hardware implementation, the bits will be decoded in parallel; each bit in the output immediate will have a multiplexer to select which input bit it comes from. The bigger the multiplexer, the costlier and slower it is.

The "shuffling" of the immediate bits in the instruction encoding, therefore, is to make each output immediate bit have as little input instruction bit options as possible. For instance, immediate bit 1 can only come from instruction bits 8 (S-immediate or B-immediate), 21 (I-immediate or J-immediate), or constant zero (U-immediate or R-type instruction which has no immediate). Immediate bit 0 can come from instruction bits 7 (S-immediate), 20 (I-immediate), or constant zero. Immediate bit 5 can only come from instruction bit 25 or constant zero. And so on.

Instruction bit 31 is a special case: for RV-64, bits 32-63 of the immediate are always copies of instruction bit 31. This high fan-out adds a delay, which would be even bigger if it also needed a multiplexer, so it only has one option (other than constant zero, which can be treated later in the pipeline by ignoring the whole immediate).

It's also interesting to note that only the major opcode (bits 0-6) is needed to know how to decode the immediate, so immediate decoding can be done in parallel with decoding the rest of the instruction.

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So, answering the questions:

• SB-type doubles the range of branches, since instructions are always aligned to even addresses;
• UJ-type has the same overall instruction format as U-type, but the immediate value is in the lower bits instead of the upper bits;
• The immediate bits are shuffled to reduce the cost of decoding the immediate value, by reducing the number of choices for each output immediate bit;
• The "immediate produced by RISC-V instructions" table shows the different kinds of immediate values which can be decoded from a RISC-V instruction, and from where in the instruction each bit comes from;
• They are produced by, for each output immediate bit, using the major opcode (bits 0-6) to chose an input instruction bit.

Answer 2

The encoding is done to try and make the actual hardware implementation as simple as possible, rather than make it easy for the reader to understand at a glance.

In practice the compiler will generate the output and so it does not matter if it is not easy for the user to understand.

When possible the SB type tries to use the same bits for the same immediate bit positions as type S, that minimizes the hardware design complexity. So imm[4:1] and imm[10:5] are in the same place for both. The top most bit of the immediate values is always at position 31 so that you can use that bit to decide if a sign extension is needed. Again, this makes the hardware easier because for multiple types of instruction the top bit is used to decide on sign extension.

Answer 3

The RISC-V instruction encoding is chosen to simplify the decoder

2.2 Base Instruction Formats

The RISC-V ISA keeps the source (rs1 and rs2) and destination (rd) registers at the same position in all formats to simplify decoding. Except for the 5-bit immediates used in CSR instructions(Chapter 9), immediates are always sign-extended, and are generally packed towards the left most available bits in the instruction and have been allocated to reduce hardware complexity. In particular, the sign bit for all immediates is always in bit 31 of the instruction to speed sign-extension circuitry.

2.3 Immediate Encoding Variants

The only difference between the S and B formats is that the 12-bit immediate field is used to encode branch offsets in multiples of 2 in the B format. Instead of shifting all bits in the instruction-encoded immediate left by one in hardware as is conventionally done, the middle bits (imm[10:1]) and sign bit stay in fixed positions, while the lowest bit in S format (inst[7]) encodes a high-order bit in B format.

Similarly, the only difference between the U and J formats is that the 20-bit immediate is shiftedleft by 12 bits to form U immediates and by 1 bit to form J immediates. The location of instructionbits in the U and J format immediates is chosen to maximize overlap with the other formats andwith each other.

https://riscv.org/technical/specifications/

The reason for the shuffling of the immediate in SB/UL formats has also been explained in the RISC-V spec

Although more complex implementations might have separate adders for branch and jump calculations and so would not benefit from keeping the location of immediate bits constant across types of instruction, we wanted to reduce the hardware cost of the simplest implementations. By rotating bits in the instruction encoding of B and J immediates instead of using dynamic hard-ware muxes to multiply the immediate by 2, we reduce instruction signal fanout and immediate mux costs by around a factor of 2. The scrambled immediate encoding will add negligible timeto static or ahead-of-time compilation. For dynamic generation of instructions, there is some small additional overhead, but the most common short forward branches have straight forward immediate encodings.