(** * Classification of machine registers *)

(** Machine registers (type [mreg] in module [Locations]) are divided in
  the following groups:
- Temporaries used for spilling, reloading, and parallel move operations.
- Allocatable registers, that can be assigned to RTL pseudo-registers.
  These are further divided into:
-- Callee-save registers, whose value is preserved across a function call.
-- Caller-save registers that can be modified during a function call.

  We follow the ARM application binary interface (EABI) in our choice
  of callee- and caller-save registers.
*)

Definition is_callee_save (r: mreg): bool :=
  match r with
  | R0  | R1  | R2  | R3  | R12 => false
  | R4  | R5  | R6  | R7  | R8  | R9  | R10 | R11 => true
  | F0  | F1  | F2  | F3  | F4  | F5  | F6  | F7 => false
  | F8  | F9  | F10  | F11 | F12  | F13  | F14  | F15 => true
  end.Definition int_caller_save_regs :=
  R0 :: R1 :: R2 :: R3 :: R12 :: nil.Definition float_caller_save_regs :=
  F0 :: F1 :: F2 :: F3 :: F4 :: F5 :: F6 :: F7 :: nil.Definition int_callee_save_regs :=
  R4 :: R5 :: R6 :: R7 :: R8 :: R9 :: R10 :: R11 :: nil.Definition float_callee_save_regs :=
  F8 :: F9 :: F10 :: F11 :: F12 :: F13 :: F14 :: F15 :: nil.Definition destroyed_at_call :=
  List.filter (fun r => negb (is_callee_save r)) all_mregs.Definition dummy_int_reg := R0.(**r Used in [Coloring]. *)
Definition dummy_float_reg := F0.(**r Used in [Coloring]. *)

Definition callee_save_type := mreg_type.Definition is_float_reg (r: mreg): bool :=
  match r with
  | R0  | R1  | R2  | R3
  | R4  | R5  | R6  | R7
  | R8  | R9  | R10 | R11  | R12 => false
  | F0  | F1  | F2  | F3  | F4  | F5  | F6  | F7
  | F8  | F9  | F10  | F11 | F12  | F13  | F14  | F15 => true
  end.(** * Function calling conventions *)

(** The functions in this section determine the locations (machine registers
  and stack slots) used to communicate arguments and results between the
  caller and the callee during function calls.  These locations are functions
  of the signature of the function and of the call instruction.
  Agreement between the caller and the callee on the locations to use
  is guaranteed by our dynamic semantics for Cminor and RTL, which demand
  that the signature of the call instruction is identical to that of the
  called function.

  Calling conventions are largely arbitrary: they must respect the properties
  proved in this section (such as no overlapping between the locations
  of function arguments), but this leaves much liberty in choosing actual
  locations.  *)

(** ** Location of function result *)

(** The result value of a function is passed back to the caller in
  registers [R0] or [F0] or [R0,R1], depending on the type of the
  returned value.  We treat a function without result as a function
  with one integer result.

  For the "softfloat" convention, results of FP types should be passed
  in [R0] or [R0,R1].  This doesn't fit the CompCert register model,
  so we have code in [arm/TargetPrinter.ml] that inserts additional moves
  to/from [F0].

  Concerning endianness for 64bit values in register pairs, the contents
  of the registers is as if the value had been loaded from memory
  representation with a single LDM instruction. *)

Definition loc_result (s: signature) : rpair mreg :=
  match s.(sig_res) with
  | None => One R0
  | Some (Tint | Tany32) => One R0
  | Some (Tfloat | Tsingle | Tany64) => One F0
  | Some Tlong => if Archi.big_endian
                  then Twolong R0 R1
                  else Twolong R1 R0
  end.

Lemma loc_result_caller_save: forall (s: signature), forall_rpair (fun r => is_callee_save r = false) (loc_result s).