This patch is part of a bigger one D50314. It adds a possibility to emit a
custom sequence of bytes attributed to an Instruction.
This feature can be used to simplify CISC machine code emission as it done in
M68K D50314 patch. You can check out M680x0InstrFormats.td, M680x0InstrArithmetic.td and
M680x0MCCodeEmitter.cpp to see how it is used. In short, the process is
- Define a set of parameterized "beads" of length 8. The composition and meaning of those bits is up to the concrete backend. In m68k case first 4 bits is the type, the last 4 bits is the payload.
- Every non-pseudo instruction receives a number of those beads and stores them into Beads Instruction variable.
- The beads are read by this tablegen generator to emit an array of those beads referenced by instruction opcode.
- The MCCodeEmitter interprets the beads of a concrete instruction to emit a machine command. In m68k case type is read to interpret the payload, which can be a reference to an operand, an immediate value or a complex pattern.