Re:Addressing Criticism of RISC-V Microprocessors
by avianes on Sunday March 20, 2022 @02:08PM
(#30745035)
The encoding space is not the number of bits used to encode an instruction,
the encoding space is the ratio of values that encode a valid instructions over the total number of instructions that could be encoded.
As an example, in an ISA with 8-bits fixed instruction length and 8 registers (reg index encoded on 3 bit):
If the last 2 bits are the opcode and we define 2 instructions (eg. AND, XOR) that manipulate 2 registers (3 bits + 3 bits), then instruction word values 0b00_000_000 to 0b01_111_111 might encode these two instructions (ignore "_" they are separators).
Therefore, instruction word values from 0b10_000_000 to 0b11_111_111 remain free, which represents half of the encoding space. So half of the encoding space remains free.
This means we still have room to put new instructions.
Similarly, RISC-V valid instructions use almost all the available bits, but there is still room in the encoding space because some opcodes remain free.
If the compressed extension is great in practice it might be a win. If the early criticism of overfit to gcc -O0 proves sound and in practice compilers don't emit it then it was an expensive experiment.