Function calling conventions and other conventions regarding the use of machine registers and stack slots.

Require Import Coqlib.
Require Import AST.
Require Import Locations.
Require Export Conventions1.

The processor-dependent and EABI-dependent definitions are in arch/abi/Conventions1.v. This file adds various processor-independent definitions and lemmas.

Lemma loc_arguments_acceptable_2:
forall s l,
In l (regs_of_rpairs (loc_arguments s)) -> loc_argument_acceptable l.

Proof.

  intros until l. generalize (loc_arguments_acceptable s). generalize (loc_arguments s).
  induction l0 as [ | p pl]; simpl; intros.
- contradiction.
- rewrite in_app_iff in H0. destruct H0.
  exploit H; eauto. destruct p; simpl in *; intuition congruence.
  apply IHpl; auto.
Qed.

Location of function parameters

A function finds the values of its parameter in the same locations where its caller stored them, except that the stack-allocated arguments, viewed as Outgoing slots by the caller, are accessed via Incoming slots (at the same offsets and types) in the callee.

Definition parameter_of_argument (l: loc) : loc :=
  match l with
  | S Outgoing n ty => S Incoming n ty
  | _ => l
  end.

Definition loc_parameters (s: signature) : list (rpair loc) :=
  List.map (map_rpair parameter_of_argument) (loc_arguments s).

Lemma incoming_slot_in_parameters:
  forall ofs ty sg,
  In (S Incoming ofs ty) (regs_of_rpairs (loc_parameters sg)) ->
  In (S Outgoing ofs ty) (regs_of_rpairs (loc_arguments sg)).

Proof.

  intros.
  replace (regs_of_rpairs (loc_parameters sg)) with (List.map parameter_of_argument (regs_of_rpairs (loc_arguments sg))) in H.
  change (S Incoming ofs ty) with (parameter_of_argument (S Outgoing ofs ty)) in H.
  exploit list_in_map_inv. eexact H. intros [x [A B]]. simpl in A.
  exploit loc_arguments_acceptable_2; eauto. unfold loc_argument_acceptable; intros.
  destruct x; simpl in A; try discriminate.
  destruct sl; try contradiction.
  inv A. auto.
  unfold loc_parameters. generalize (loc_arguments sg). induction l as [ | p l]; simpl; intros.
  auto.
  rewrite map_app. f_equal; auto. destruct p; auto.
Qed.

Tail calls

A tail-call is possible for a signature if the corresponding arguments are all passed in registers.

Definition tailcall_possible (s: signature) : Prop :=
forall l, In l (regs_of_rpairs (loc_arguments s)) ->
match l with R _ => True | S _ _ _ => False end.

Decide whether a tailcall is possible.

Definition tailcall_is_possible (sg: signature) : bool :=
  List.forallb
    (fun l => match l with R _ => true | S _ _ _ => false end)
    (regs_of_rpairs (loc_arguments sg)).

Lemma tailcall_is_possible_correct:
  forall s, tailcall_is_possible s = true -> tailcall_possible s.

Proof.

unfold tailcall_is_possible; intros. rewrite forallb_forall in H.
red; intros. apply H in H0. destruct l; [auto|discriminate].
Qed.

Lemma zero_size_arguments_tailcall_possible:
forall sg, size_arguments sg = 0 -> tailcall_possible sg.

Proof.

  intros; red; intros. exploit loc_arguments_acceptable_2; eauto.
  unfold loc_argument_acceptable.
  destruct l; intros. auto. destruct sl; try contradiction. destruct H1.
  generalize (loc_arguments_bounded _ _ _ H0).
  generalize (typesize_pos ty). omega.
Qed.

Module Conventions

Location of function parameters

Tail calls