(* *********************************************************************)
(*                                                                     *)
(*              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 *)
(** ** Location of function arguments *)
(** For the "hardfloat" configuration, we use the following calling conventions, adapted from the ARM EABI-HF: - The first 4 integer arguments are passed in registers [R0] to [R3]. - The first 2 long integer arguments are passed in an aligned pair of two integer registers. - The first 8 single- and double-precision float arguments are passed in registers [F0...F7] - Extra arguments are passed on the stack, in [Outgoing] slots, consecutively assigned (1 word for an integer or single argument, 2 words for a float or a long), starting at word offset 0.  This convention is not quite that of the ARM EABI-HF, whereas single float arguments are passed in 32-bit float registers.  Unfortunately, this does not fit the data model of CompCert.  In [PrintAsm.ml] we insert additional code around function calls that moves data appropriately. *)
Definition int_param_regs :=
R0 :: R1 :: R2 :: R3 :: nil.
Definition float_param_regs :=
F0 :: F1 :: F2 :: F3 :: F4 :: F5 :: F6 :: F7 :: nil.
Definition ireg_param (n: Z) : mreg :=
match list_nth_z int_param_regs n with Some r => r | None => R0 end.
Definition freg_param (n: Z) : mreg :=
match list_nth_z float_param_regs n with Some r => r | None => F0 end.
Fixpoint loc_arguments_hf
(tyl: list typ) (ir fr ofs: Z) {struct tyl} : list (rpair loc) :=
match tyl with
| nil => nil
| (Tint | Tany32) as ty :: tys =>
if zlt ir 4
then One (R (ireg_param ir)) :: loc_arguments_hf tys (ir + 1) fr ofs
else One (S Outgoing ofs ty) :: loc_arguments_hf tys ir fr (ofs + 1)
| (Tfloat | Tany64) as ty :: tys =>
if zlt fr 8
then One (R (freg_param fr)) :: loc_arguments_hf tys ir (fr + 1) ofs
else let ofs := align ofs 2 in
One (S Outgoing ofs ty) :: loc_arguments_hf tys ir fr (ofs + 2)
| Tsingle :: tys =>
if zlt fr 8
then One (R (freg_param fr)) :: loc_arguments_hf tys ir (fr + 1) ofs
else One (S Outgoing ofs Tsingle) :: loc_arguments_hf tys ir fr (ofs + 1)
| Tlong :: tys =>
let ohi := if Archi.big_endian then 0 else 1 in
let olo := if Archi.big_endian then 1 else 0 in
let ir := align ir 2 in
if zlt ir 4
then Twolong (R (ireg_param (ir + ohi))) (R (ireg_param (ir + olo))) :: loc_arguments_hf tys (ir + 2) fr ofs
else let ofs := align ofs 2 in
Twolong (S Outgoing (ofs + ohi) Tint) (S Outgoing (ofs + olo) Tint) :: loc_arguments_hf tys ir fr (ofs + 2)
end.
(** For the "softfloat" configuration, as well as for variable-argument functions in the "hardfloat" configuration, we use the default ARM EABI (not HF) calling conventions: - The first 4 integer arguments are passed in registers [R0] to [R3]. - The first 2 long integer arguments are passed in an aligned pair of two integer registers. - The first 2 double-precision float arguments are passed in [F0] or [F2] - The first 4 single-precision float arguments are passed in [F0...F3] - Integer arguments and float arguments are kept in sync so that they can all be mapped back to [R0...R3] in [PrintAsm.ml]. - Extra arguments are passed on the stack, in [Outgoing] slots, consecutively assigned (1 word for an integer or single argument, 2 words for a float or a long), starting at word offset 0.  This convention is not quite that of the ARM EABI, whereas every float argument are passed in one or two integer registers.  Unfortunately, this does not fit the data model of CompCert.  In [PrintAsm.ml] we insert additional code around function calls and returns that moves data appropriately. *)
Fixpoint loc_arguments_sf
(tyl: list typ) (ofs: Z) {struct tyl} : list (rpair loc) :=
match tyl with
| nil => nil
| (Tint|Tany32) as ty :: tys =>
One (if zlt ofs 0 then R (ireg_param (ofs + 4)) else S Outgoing ofs ty)
:: loc_arguments_sf tys (ofs + 1)
| (Tfloat|Tany64) as ty :: tys =>
let ofs := align ofs 2 in
One (if zlt ofs 0 then R (freg_param (ofs + 4)) else S Outgoing ofs ty)
:: loc_arguments_sf tys (ofs + 2)
| Tsingle :: tys =>
One (if zlt ofs 0 then R (freg_param (ofs + 4)) else S Outgoing ofs Tsingle)
:: loc_arguments_sf tys (ofs + 1)
| Tlong :: tys =>
let ohi := if Archi.big_endian then 0 else 1 in
let olo := if Archi.big_endian then 1 else 0 in
let ofs := align ofs 2 in
Twolong (if zlt ofs 0 then R (ireg_param (ofs+ohi+4)) else S Outgoing (ofs+ohi) Tint)
(if zlt ofs 0 then R (ireg_param (ofs+olo+4)) else S Outgoing (ofs+olo) Tint)
:: loc_arguments_sf tys (ofs + 2)
end.
(** [loc_arguments s] returns the list of locations where to store arguments when calling a function with signature [s].  *)
Definition loc_arguments (s: signature) : list (rpair loc) :=
match Archi.abi with
| Archi.Softfloat =>
loc_arguments_sf s.(sig_args) (-4)
| Archi.Hardfloat =>
if s.(sig_cc).(cc_vararg)
then loc_arguments_sf s.(sig_args) (-4)
else loc_arguments_hf s.(sig_args) 0 0 0
end.
(** Argument locations are either non-temporary registers or [Outgoing] stack slots at nonnegative offsets. *)
Definition loc_argument_acceptable (l: loc) : Prop :=
match l with
| R r => is_callee_save r = false
| S Outgoing ofs ty => ofs >= 0 /\ (typealign ty | ofs)
| _ => False
end.
Definition loc_argument_charact (ofs: Z) (l: loc) : Prop :=
match l with
| R r => is_callee_save r = false
| S Outgoing ofs' ty => ofs' >= ofs /\ typealign ty = 1
| _ => False
end.
Remark ireg_param_caller_save: forall n, is_callee_save (ireg_param n) = false.
Remark freg_param_caller_save: forall n, is_callee_save (freg_param n) = false.
Remark loc_arguments_hf_charact:
forall tyl ir fr ofs p,
In p (loc_arguments_hf tyl ir fr ofs) -> forall_rpair (loc_argument_charact ofs) p.
Remark loc_arguments_sf_charact:
forall tyl ofs p,
In p (loc_arguments_sf tyl ofs) -> forall_rpair (loc_argument_charact (Z.max 0 ofs)) p.

Lemma loc_arguments_acceptable:
forall (s: signature) (p: rpair loc),
In p (loc_arguments s) -> forall_rpair loc_argument_acceptable p.