vendor/cranelift-isle/isle_examples/pass/test3.isle

(type Opcode extern (enum
  Iadd
  Isub
  Load
  Store))

(type Inst (primitive Inst))
(type InstInput (primitive InstInput))
(type Reg (primitive Reg))
(type u32 (primitive u32))

(decl Op (Opcode) Inst)
(extern extractor infallible Op get_opcode)

(decl InstInputs2 (InstInput InstInput) Inst)
(extern extractor infallible InstInputs2 get_inst_inputs_2)

(decl Producer (Inst) InstInput)
(extern extractor Producer get_input_producer)

(decl UseInput (InstInput) Reg)
(extern constructor UseInput put_input_in_reg)

(type MachInst (enum
  (Add (a Reg) (b Reg))
  (Add3 (a Reg) (b Reg) (c Reg))
  (Sub (a Reg) (b Reg))))

(decl Lower (Inst) MachInst)

;; Extractors that give syntax sugar for (Iadd ra rb), etc.
;;
;; Note that this is somewhat simplistic: it directly connects inputs to
;; MachInst regs; really we'd want to return a VReg or InstInput that we can use
;; another extractor to connect to another (producer) inst.
;;
;; Also, note that while it looks a little indirect, a verification effort could
;; define equivalences across the `rule` LHS/RHS pairs, and the types ensure that
;; we are dealing (at the semantic level) with pure value equivalences of
;; "terms", not arbitrary side-effecting calls.

(decl Iadd (InstInput InstInput) Inst)
(decl Isub (InstInput InstInput) Inst)
(extractor
  (Iadd a b)
  (and
   (Op (Opcode.Iadd))
   (InstInputs2 a b)))
(extractor
  (Isub a b)
  (and
   (Op (Opcode.Isub))
   (InstInputs2 a b)))

;; Now the nice syntax-sugar that "end-user" backend authors can write:
(rule
  (Lower (Iadd ra rb))
  (MachInst.Add (UseInput ra) (UseInput rb)))
(rule
  (Lower (Iadd (Producer (Iadd ra rb)) rc))
  (MachInst.Add3 (UseInput ra) (UseInput rb) (UseInput rc)))
(rule
  (Lower (Isub ra rb))
  (MachInst.Sub (UseInput ra) (UseInput rb)))