Module Bounds

Computation of resource bounds forr Linear code.

Require Import Coqlib.
Require Import Maps.
Require Import Ast.
Require Import Op.
Require Import Locations.
Require Import Linear.
Require Import Lineartyping.
Require Import Conventions.

Resource bounds for a function

The bounds record capture how many local and outgoing stack slots and callee-save registers are used by a function.

We demand that all bounds are positive or null. These properties are used later to reason about the layout of the activation record.

Record bounds : Type := mkbounds {
  bound_int_local: Z;
  bound_float_local: Z;
  bound_int_callee_save: Z;
  bound_float_callee_save: Z;
  bound_outgoing: Z;
  bound_int_local_pos: bound_int_local >= 0;
  bound_float_local_pos: bound_float_local >= 0;
  bound_int_callee_save_pos: bound_int_callee_save >= 0;
  bound_float_callee_save_pos: bound_float_callee_save >= 0;
  bound_outgoing_pos: bound_outgoing >= 0

The following predicates define the correctness of a set of bounds for the code of a function.

Section BELOW.

Variable funct: function.
Variable b: bounds.

Definition mreg_within_bounds (r: mreg) :=
  match mreg_type r with
  | Tint => index_int_callee_save r < bound_int_callee_save b
  | Tfloat => index_float_callee_save r < bound_float_callee_save b

Definition slot_within_bounds (s: slot) :=
  match s with
  | Local ofs Tint => 0 <= ofs < bound_int_local b
  | Local ofs Tfloat => 0 <= ofs < bound_float_local b
  | Outgoing ofs ty => 0 <= ofs /\ ofs + typesize ty <= bound_outgoing b
  | Incoming ofs ty => In (S s) (loc_parameters funct.(fn_sig))

Definition instr_within_bounds (i: instruction) :=
  match i with
  | Lgetstack s r => slot_within_bounds s /\ mreg_within_bounds r
  | Lsetstack r s => slot_within_bounds s
  | Lop op args res => mreg_within_bounds res
  | Lload chunk addr args dst => mreg_within_bounds dst
  | Lcall sig ros => size_arguments sig <= bound_outgoing b
  | Latomic aop args dst => mreg_within_bounds dst
  | Lthreadcreate => size_arguments thread_create_sig <= bound_outgoing b
  | _ => True


Definition function_within_bounds (f: function) (b: bounds) : Prop :=
  forall instr, In instr f.(fn_code) -> instr_within_bounds f b instr.

Inference of resource bounds for a function

The resource bounds for a function are computed by a linear scan of its instructions.

Section BOUNDS.

Variable f: function.

In the proof of the Stacking pass, we only need to bound the registers written by an instruction. Therefore, this function returns these registers, ignoring registers used only as arguments.

Definition regs_of_instr (i: instruction) : list mreg :=
  match i with
  | Lgetstack s r => r :: nil
  | Lsetstack r s => r :: nil
  | Lop op args res => res :: nil
  | Lload chunk addr args dst => dst :: nil
  | Lstore chunk addr args src => nil
  | Lcall sig ros => nil
  | Llabel lbl => nil
  | Lgoto lbl => nil
  | Lcond cond args lbl => nil
  | Lreturn => nil
  | Lthreadcreate => nil
  | Latomic aop args res => res :: nil
  | Lfence => nil

Definition slots_of_instr (i: instruction) : list slot :=
  match i with
  | Lgetstack s r => s :: nil
  | Lsetstack r s => s :: nil
  | _ => nil

Definition max_over_list (A: Type) (valu: A -> Z) (l: list A) : Z :=
  List.fold_left (fun m l => Zmax m (valu l)) l 0.

Definition max_over_instrs (valu: instruction -> Z) : Z :=
  max_over_list instruction valu f.(fn_code).

Definition max_over_regs_of_instr (valu: mreg -> Z) (i: instruction) : Z :=
  max_over_list mreg valu (regs_of_instr i).

Definition max_over_slots_of_instr (valu: slot -> Z) (i: instruction) : Z :=
  max_over_list slot valu (slots_of_instr i).

Definition max_over_regs_of_funct (valu: mreg -> Z) : Z :=
  max_over_instrs (max_over_regs_of_instr valu).

Definition max_over_slots_of_funct (valu: slot -> Z) : Z :=
  max_over_instrs (max_over_slots_of_instr valu).

Definition int_callee_save (r: mreg) := 1 + index_int_callee_save r.

Definition float_callee_save (r: mreg) := 1 + index_float_callee_save r.

Definition int_local (s: slot) :=
  match s with Local ofs Tint => 1 + ofs | _ => 0 end.

Definition float_local (s: slot) :=
  match s with Local ofs Tfloat => 1 + ofs | _ => 0 end.

Definition outgoing_slot (s: slot) :=
  match s with Outgoing ofs ty => ofs + typesize ty | _ => 0 end.

Definition outgoing_space (i: instruction) :=
  match i with Lcall sig _ => size_arguments sig
             | Lthreadcreate => size_arguments thread_create_sig
             | _ => 0 end.

Lemma max_over_list_pos:
  forall (A: Type) (valu: A -> Z) (l: list A),
  max_over_list A valu l >= 0.
  intros until valu. unfold max_over_list.
  assert (forall l z, fold_left (fun x y => Zmax x (valu y)) l z >= z).
  induction l; simpl; intros.
  omega. apply Zge_trans with (Zmax z (valu a)).
  auto. apply Zle_ge. apply Zmax1. auto.

Lemma max_over_slots_of_funct_pos:
  forall (valu: slot -> Z), max_over_slots_of_funct valu >= 0.
  intros. unfold max_over_slots_of_funct.
  unfold max_over_instrs. apply max_over_list_pos.

Lemma max_over_regs_of_funct_pos:
  forall (valu: mreg -> Z), max_over_regs_of_funct valu >= 0.
  intros. unfold max_over_regs_of_funct.
  unfold max_over_instrs. apply max_over_list_pos.
Program Definition function_bounds :=
    (max_over_slots_of_funct int_local)
    (max_over_slots_of_funct float_local)
    (max_over_regs_of_funct int_callee_save)
    (max_over_regs_of_funct float_callee_save)
    (Zmax (max_over_instrs outgoing_space)
          (max_over_slots_of_funct outgoing_slot))
    (max_over_slots_of_funct_pos int_local)
    (max_over_slots_of_funct_pos float_local)
    (max_over_regs_of_funct_pos int_callee_save)
    (max_over_regs_of_funct_pos float_callee_save)
Next Obligation.
  apply Zle_ge. eapply Zle_trans. 2: apply Zmax2.
  apply Zge_le. apply max_over_slots_of_funct_pos.

We now show the correctness of the inferred bounds.

Lemma max_over_list_bound:
  forall (A: Type) (valu: A -> Z) (l: list A) (x: A),
  In x l -> valu x <= max_over_list A valu l.
  intros until x. unfold max_over_list.
  assert (forall c z,
            let f := fold_left (fun x y => Zmax x (valu y)) c z in
            z <= f /\ (In x c -> valu x <= f)).
    induction c; simpl; intros.
    split. omega. tauto.
    elim (IHc (Zmax z (valu a))); intros.
    split. apply Zle_trans with (Zmax z (valu a)). apply Zmax1. auto.
    intro H1; elim H1; intro.
    subst a. apply Zle_trans with (Zmax z (valu x)).
    apply Zmax2. auto. auto.
  intro. elim (H l 0); intros. auto.

Lemma max_over_instrs_bound:
  forall (valu: instruction -> Z) i,
  In i f.(fn_code) -> valu i <= max_over_instrs valu.
  intros. unfold max_over_instrs. apply max_over_list_bound; auto.

Lemma max_over_regs_of_funct_bound:
  forall (valu: mreg -> Z) i r,
  In i f.(fn_code) -> In r (regs_of_instr i) ->
  valu r <= max_over_regs_of_funct valu.
  intros. unfold max_over_regs_of_funct.
  apply Zle_trans with (max_over_regs_of_instr valu i).
  unfold max_over_regs_of_instr. apply max_over_list_bound. auto.
  apply max_over_instrs_bound. auto.

Lemma max_over_slots_of_funct_bound:
  forall (valu: slot -> Z) i s,
  In i f.(fn_code) -> In s (slots_of_instr i) ->
  valu s <= max_over_slots_of_funct valu.
  intros. unfold max_over_slots_of_funct.
  apply Zle_trans with (max_over_slots_of_instr valu i).
  unfold max_over_slots_of_instr. apply max_over_list_bound. auto.
  apply max_over_instrs_bound. auto.

Lemma int_callee_save_bound:
  forall i r,
  In i f.(fn_code) -> In r (regs_of_instr i) ->
  index_int_callee_save r < bound_int_callee_save function_bounds.
  intros. apply Zlt_le_trans with (int_callee_save r).
  unfold int_callee_save. omega.
  unfold function_bounds, bound_int_callee_save.
  eapply max_over_regs_of_funct_bound; eauto.

Lemma float_callee_save_bound:
  forall i r,
  In i f.(fn_code) -> In r (regs_of_instr i) ->
  index_float_callee_save r < bound_float_callee_save function_bounds.
  intros. apply Zlt_le_trans with (float_callee_save r).
  unfold float_callee_save. omega.
  unfold function_bounds, bound_float_callee_save.
  eapply max_over_regs_of_funct_bound; eauto.

Lemma int_local_slot_bound:
  forall i ofs,
  In i f.(fn_code) -> In (Local ofs Tint) (slots_of_instr i) ->
  ofs < bound_int_local function_bounds.
  intros. apply Zlt_le_trans with (int_local (Local ofs Tint)).
  unfold int_local. omega.
  unfold function_bounds, bound_int_local.
  eapply max_over_slots_of_funct_bound; eauto.

Lemma float_local_slot_bound:
  forall i ofs,
  In i f.(fn_code) -> In (Local ofs Tfloat) (slots_of_instr i) ->
  ofs < bound_float_local function_bounds.
  intros. apply Zlt_le_trans with (float_local (Local ofs Tfloat)).
  unfold float_local. omega.
  unfold function_bounds, bound_float_local.
  eapply max_over_slots_of_funct_bound; eauto.

Lemma outgoing_slot_bound:
  forall i ofs ty,
  In i f.(fn_code) -> In (Outgoing ofs ty) (slots_of_instr i) ->
  ofs + typesize ty <= bound_outgoing function_bounds.
  intros. change (ofs + typesize ty) with (outgoing_slot (Outgoing ofs ty)).
  unfold function_bounds, bound_outgoing.
  apply Zmax_bound_r. eapply max_over_slots_of_funct_bound; eauto.

Lemma size_arguments_bound:
  forall sig ros,
  In (Lcall sig ros) f.(fn_code) ->
  size_arguments sig <= bound_outgoing function_bounds.
  intros. change (size_arguments sig) with (outgoing_space (Lcall sig ros)).
  unfold function_bounds, bound_outgoing.
  apply Zmax_bound_l. apply max_over_instrs_bound; auto.

Consequently, all machine registers or stack slots mentioned by one of the instructions of function f are within bounds.

Lemma mreg_is_within_bounds:
  forall i, In i f.(fn_code) ->
  forall r, In r (regs_of_instr i) ->
  mreg_within_bounds function_bounds r.
  intros. unfold mreg_within_bounds.
  case (mreg_type r).
  eapply int_callee_save_bound; eauto.
  eapply float_callee_save_bound; eauto.

Lemma slot_is_within_bounds:
  forall i, In i f.(fn_code) ->
  forall s, In s (slots_of_instr i) -> Lineartyping.slot_valid f s ->
  slot_within_bounds f function_bounds s.
  intros. unfold slot_within_bounds.
  destruct s.
  destruct t.
  split. exact H1. eapply int_local_slot_bound; eauto.
  split. exact H1. eapply float_local_slot_bound; eauto.
  exact H1.
  split. simpl in H1. exact H1. eapply outgoing_slot_bound; eauto.

It follows that every instruction in the function is within bounds, in the sense of the instr_within_bounds predicate.

Lemma instr_is_within_bounds:
  forall i,
  In i f.(fn_code) ->
  Lineartyping.wt_instr f i ->
  instr_within_bounds f function_bounds i.
  destruct i;
  generalize (mreg_is_within_bounds _ H); generalize (slot_is_within_bounds _ H);
  simpl; intros; auto.
  - by inv H0; auto.
  - by inv H0; auto.
  - by eapply size_arguments_bound; eauto.
  - change (size_arguments thread_create_sig) with (outgoing_space Lthreadcreate).
    by apply Zmax_bound_l, max_over_instrs_bound; auto.

Lemma function_is_within_bounds:
  Lineartyping.wt_code f f.(fn_code) ->
  function_within_bounds f function_bounds.
  intros; red; intros. apply instr_is_within_bounds; auto.