To support variable length instructions, I think of them as fixed length instructions with the "maximum length". For example, if there're three instructions with 2, 6 and 9 bytes, we can fit them into the algorithm by treating them all as 9 bytes.
Also, since we can't know the length of the instruction in advance, there is a function object with type `void(APInt &, uint64_t)` added in the parameter list of `decodeInstruction` and `fieldFromInstruction`. We can use this to supply the additional bits the decoder needs after we know the opcode of the instruction.
Finally, `InstrLenTable` is added to let the decoder know the length of the instructions.
See D120960 for its usage.
Reviewed By: myhsu
Differential Revision: https://reviews.llvm.org/D120958
Full write up:
https://gist.github.com/mshockwave/66e98d099256deefc062633909bb7b5b
The existing CodeEmitterGen infrastructure is unable to generate encoder
function for ISAs with variable-length instructions. This patch
introduces a new infrastructure to support variable-length instruction
encoding, including a new TableGen syntax for writing instruction
encoding directives and a new TableGen backend component,
VarLenCodeEmitterGen, built on top of CodeEmitterGen.
Differential Revision: https://reviews.llvm.org/D115128
Sheng