Module Cminorgen

Translation from Csharpminor to Cminor.

Require Import FSets FSetAVL Orders Mergesort.
Require Import Coqlib Maps Ordered Errors Integers Floats.
Require Import AST Linking.
Require Import Csharpminor Cminor.

Local Open Scope string_scope.
Local Open Scope error_monad_scope.

The main task in translating Csharpminor to Cminor is to explicitly stack-allocate local variables whose address is taken: these local variables become sub-blocks of the Cminor stack data block for the current function. Taking the address of such a variable becomes a Oaddrstack operation with the corresponding offset. Accessing or assigning such a variable becomes a load or store operation at that address. Only scalar local variables whose address is never taken in the Csharpminor code can be mapped to Cminor local variable, since the latter do not reside in memory. Another task performed during the translation to Cminor is to transform the Clight-like switch construct of Csharpminor into the simpler, lower-level switch constructs of Cminor.

Handling of variables

To every Csharpminor variable, the compiler associates its offset in the Cminor stack data block.

Definition compilenv := PTree.t Z.

Generation of a Cminor expression for taking the address of a Csharpminor variable.

Definition var_addr (cenv: compilenv) (id: ident): expr :=
  match PTree.get id cenv with
  | Some ofs => Econst (Oaddrstack (Ptrofs.repr ofs))
  | None => Econst (Oaddrsymbol id

Translation of expressions and statements.

Translation of constants.

Definition transl_constant (cst: Csharpminor.constant): constant :=
  match cst with
  | Csharpminor.Ointconst n =>
      Ointconst n
  | Csharpminor.Ofloatconst n =>
      Ofloatconst n
  | Csharpminor.Osingleconst n =>
      Osingleconst n
  | Csharpminor.Olongconst n =>
      Olongconst n

Translation of expressions.

Fixpoint transl_expr (cenv: compilenv) (e: Csharpminor.expr)
                     {struct e}: res expr :=
  match e with
  | Csharpminor.Evar id =>
      OK (Evar id)
  | Csharpminor.Eaddrof id =>
      OK (var_addr cenv id)
  | Csharpminor.Econst cst =>
      OK (Econst (transl_constant cst))
  | Csharpminor.Eunop op e1 =>
      do te1 <- transl_expr cenv e1;
      OK (Eunop op te1)
  | Csharpminor.Ebinop op e1 e2 =>
      do te1 <- transl_expr cenv e1;
      do te2 <- transl_expr cenv e2;
      OK (Ebinop op te1 te2)
  | Csharpminor.Eload chunk e =>
      do te <- transl_expr cenv e;
      OK (Eload chunk te)

Fixpoint transl_exprlist (cenv: compilenv) (el: list Csharpminor.expr)
                     {struct el}: res (list expr) :=
  match el with
  | nil =>
      OK nil
  | e1 :: e2 =>
      do te1 <- transl_expr cenv e1;
      do te2 <- transl_exprlist cenv e2;
      OK (te1 :: te2)

To translate switch statements, we wrap the statements associated with the various cases in a cascade of nested Cminor blocks. The multi-way branch is performed by a Cminor switch statement that exits to the appropriate case. For instance:
switch (e) {        --->    block { block { block {
  case N1: s1;                switch (e) { N1: exit 0; N2: exit 1; default: exit 2; }
  case N2: s2;                } ; s1  // with exits shifted by 2
  default: s;                 } ; s2  // with exits shifted by 1
}                             } ; s   // with exits unchanged
To shift exit statements appropriately, we use a second compile-time environment, of type exit_env, which records the blocks inserted during the translation. A true element means the block was present in the original code; a false element, that it was inserted during translation.

Definition exit_env := list bool.

Fixpoint shift_exit (e: exit_env) (n: nat) {struct e} : nat :=
  match e, n with
  | nil, _ => n
  | false :: e', _ => S (shift_exit e' n)
  | true :: e', O => O
  | true :: e', S m => S (shift_exit e' m)

Fixpoint switch_table (ls: lbl_stmt) (k: nat) {struct ls} : list (Z * nat) * nat :=
  match ls with
  | LSnil =>
      (nil, k)
  | LScons None _ rem =>
      let (tbl, dfl) := switch_table rem (S k) in (tbl, k)
  | LScons (Some ni) _ rem =>
      let (tbl, dfl) := switch_table rem (S k) in ((ni, k) :: tbl, dfl)

Fixpoint switch_env (ls: lbl_stmt) (e: exit_env) {struct ls}: exit_env :=
  match ls with
  | LSnil => e
  | LScons _ _ ls' => false :: switch_env ls' e

Translation of statements. The nonobvious part is the translation of switch statements, outlined above.

Fixpoint transl_stmt (cenv: compilenv) (xenv: exit_env) (s: Csharpminor.stmt)
                     {struct s}: res stmt :=
  match s with
  | Csharpminor.Sskip =>
      OK Sskip
  | Csharpminor.Sset id e =>
      do te <- transl_expr cenv e;
      OK (Sassign id te)
  | Csharpminor.Sstore chunk e1 e2 =>
      do te1 <- transl_expr cenv e1;
      do te2 <- transl_expr cenv e2;
      OK (Sstore chunk te1 te2)
  | Csharpminor.Scall optid sig e el =>
      do te <- transl_expr cenv e;
      do tel <- transl_exprlist cenv el;
      OK (Scall optid sig te tel)
  | Csharpminor.Sbuiltin optid ef el =>
      do tel <- transl_exprlist cenv el;
      OK (Sbuiltin optid ef tel)
  | Csharpminor.Sseq s1 s2 =>
      do ts1 <- transl_stmt cenv xenv s1;
      do ts2 <- transl_stmt cenv xenv s2;
      OK (Sseq ts1 ts2)
  | Csharpminor.Sifthenelse e s1 s2 =>
      do te <- transl_expr cenv e;
      do ts1 <- transl_stmt cenv xenv s1;
      do ts2 <- transl_stmt cenv xenv s2;
      OK (Sifthenelse te ts1 ts2)
  | Csharpminor.Sloop s =>
      do ts <- transl_stmt cenv xenv s;
      OK (Sloop ts)
  | Csharpminor.Sblock s =>
      do ts <- transl_stmt cenv (true :: xenv) s;
      OK (Sblock ts)
  | Csharpminor.Sexit n =>
      OK (Sexit (shift_exit xenv n))
  | Csharpminor.Sswitch long e ls =>
      let (tbl, dfl) := switch_table ls O in
      do te <- transl_expr cenv e;
      transl_lblstmt cenv (switch_env ls xenv) ls (Sswitch long te tbl dfl)
  | Csharpminor.Sreturn None =>
      OK (Sreturn None)
  | Csharpminor.Sreturn (Some e) =>
      do te <- transl_expr cenv e;
      OK (Sreturn (Some te))
  | Csharpminor.Slabel lbl s =>
      do ts <- transl_stmt cenv xenv s; OK (Slabel lbl ts)
  | Csharpminor.Sgoto lbl =>
      OK (Sgoto lbl)

with transl_lblstmt (cenv: compilenv) (xenv: exit_env) (ls: Csharpminor.lbl_stmt) (body: stmt)
                    {struct ls}: res stmt :=
  match ls with
  | Csharpminor.LSnil =>
      OK (Sseq (Sblock body) Sskip)
  | Csharpminor.LScons _ s ls' =>
      do ts <- transl_stmt cenv xenv s;
      transl_lblstmt cenv (List.tail xenv) ls' (Sseq (Sblock body) ts)

Stack layout

Layout of the Cminor stack data block and construction of the compilation environment. Every Csharpminor local variable is allocated a slot in the Cminor stack data. Sufficient padding is inserted to ensure adequate alignment of addresses.

Definition block_alignment (sz: Z) : Z :=
  if zlt sz 2 then 1
  else if zlt sz 4 then 2
  else if zlt sz 8 then 4 else 8.

Definition assign_variable
    (cenv_stacksize: compilenv * Z) (id_sz: ident * Z) : compilenv * Z :=
  let (id, sz) := id_sz in
  let (cenv, stacksize) := cenv_stacksize in
  let ofs := align stacksize (block_alignment sz) in
  (PTree.set id ofs cenv, ofs + Zmax 0 sz).

Definition assign_variables (cenv_stacksize: compilenv * Z) (vars: list (ident * Z)) : compilenv * Z :=
  List.fold_left assign_variable vars cenv_stacksize.

Before allocating stack slots, we sort variables by increasing size so as to minimize padding.

Module VarOrder <: TotalLeBool.
  Definition t := (ident * Z)%type.
  Definition leb (v1 v2: t) : bool := zle (snd v1) (snd v2).
  Theorem leb_total: forall v1 v2, leb v1 v2 = true \/ leb v2 v1 = true.
    unfold leb; intros.
    assert (snd v1 <= snd v2 \/ snd v2 <= snd v1) by omega.
    unfold proj_sumbool. destruct H; [left|right]; apply zle_true; auto.
End VarOrder.

Module VarSort := Mergesort.Sort(VarOrder).

Definition build_compilenv (f: Csharpminor.function) : compilenv * Z :=
  assign_variables (PTree.empty Z, 0) (VarSort.sort (Csharpminor.fn_vars f)).

Translation of functions

Translation of a Csharpminor function. We must check that the required Cminor stack block is no bigger than Int.max_signed, otherwise address computations within the stack block could overflow machine arithmetic and lead to incorrect code.

Definition transl_funbody
      (cenv: compilenv) (stacksize: Z) (f: Csharpminor.function): res function :=
   do tbody <- transl_stmt cenv nil f.(Csharpminor.fn_body);
       OK (mkfunction
              (Csharpminor.fn_sig f)
              (Csharpminor.fn_params f)
              (Csharpminor.fn_temps f)

Definition transl_function (f: Csharpminor.function): res function :=
  let (cenv, stacksize) := build_compilenv f in
  if zle stacksize Ptrofs.max_unsigned
  then transl_funbody cenv stacksize f
  else Error(msg "Cminorgen: too many local variables, stack size exceeded").

Definition transl_fundef (f: Csharpminor.fundef): res fundef :=
  transf_partial_fundef transl_function f.

Definition transl_program (p: Csharpminor.program) : res program :=
  transform_partial_program transl_fundef p.