(* *********************************************************************)
(*                                                                     *)
(*              The Compcert verified compiler                         *)
(*                                                                     *)
(*          Xavier Leroy, INRIA Paris-Rocquencourt                     *)
(*                                                                     *)
(*  Copyright Institut National de Recherche en Informatique et en     *)
(*  Automatique.  All rights reserved.  This file is distributed       *)
(*  under the terms of the INRIA Non-Commercial License Agreement.     *)
(*                                                                     *)
(* *********************************************************************)
(** Function calling conventions and other conventions regarding the use of machine registers and stack slots. *)
Require Archi.
(** * 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 proj_sig_res s with
| Tint | Tany32 => One R0
| Tfloat | Tsingle | Tany64 => One F0
| Tlong => if Archi.big_endian
then Twolong R0 R1
else Twolong R1 R0
end.
(** The result registers have types compatible with that given in the signature. *)
(** The result locations are caller-save registers *)
(** If the result is in a pair of registers, those registers are distinct and have type [Tint] at least. *)
(** The location of the result depends only on the result part of the signature *)

Lemma loc_result_exten:
forall s1 s2, s1.(sig_res) = s2.(sig_res) -> loc_result s1 = loc_result s2.