Module Csyntax

Abstract syntax for the Compcert C language

Require Import Coqlib Maps Integers Floats Errors.
Require Import AST Linking Values.
Require Import Ctypes Cop.

Expressions

Compcert C expressions are almost identical to those of C. The only omission is string literals. Some operators are treated as derived forms: array indexing, pre-increment, pre-decrement, and the && and || operators. All expressions are annotated with their types.

Inductive expr : Type :=
  | Eval (v: val) (ty: type) (* constant *)
  | Evar (x: ident) (ty: type) (* variable *)
  | Efield (l: expr) (f: ident) (ty: type)
  | Evalof (l: expr) (ty: type) (* l-value used as a r-value *)
  | Ederef (r: expr) (ty: type) (* pointer dereference (unary *) *)
  | Eaddrof (l: expr) (ty: type) (* address-of operators (&) *)
  | Eunop (op: unary_operation) (r: expr) (ty: type)
  | Ebinop (op: binary_operation) (r1 r2: expr) (ty: type)
  | Ecast (r: expr) (ty: type) (* type cast (ty)r *)
  | Eseqand (r1 r2: expr) (ty: type) (* sequential "and" r1 && r2 *)
  | Eseqor (r1 r2: expr) (ty: type) (* sequential "or" r1 || r2 *)
  | Econdition (r1 r2 r3: expr) (ty: type) (* conditional r1 ? r2 : r3 *)
  | Esizeof (ty': type) (ty: type) (* size of a type *)
  | Ealignof (ty': type) (ty: type) (* natural alignment of a type *)
  | Eassign (l: expr) (r: expr) (ty: type) (* assignment l = r *)
  | Eassignop (op: binary_operation) (l: expr) (r: expr) (tyres ty: type)
  | Epostincr (id: incr_or_decr) (l: expr) (ty: type)
  | Ecomma (r1 r2: expr) (ty: type) (* sequence expression r1, r2 *)
  | Ecall (r1: expr) (rargs: exprlist) (ty: type)
  | Ebuiltin (ef: external_function) (tyargs: typelist) (rargs: exprlist) (ty: type)
  | Eloc (b: block) (ofs: ptrofs) (bf: bitfield) (ty: type)
  | Eparen (r: expr) (tycast: type) (ty: type) (* marked subexpression *)

with exprlist : Type :=
  | Enil
  | Econs (r1: expr) (rl: exprlist).

Expressions are implicitly classified into l-values and r-values, ranged over by l and r, respectively, in the grammar above. L-values are those expressions that can occur to the left of an assignment. They denote memory locations. (Indeed, the reduction semantics for expression reduces them to Eloc b ofs expressions.) L-values are variables (Evar), pointer dereferences (Ederef), field accesses (Efield). R-values are all other expressions. They denote values, and the reduction semantics reduces them to Eval v expressions. A l-value can be used in a r-value context, but this use must be marked explicitly with the Evalof operator, which is not materialized in the concrete syntax of C but denotes a read from the location corresponding to the l-value l argument of Evalof l. The grammar above contains some forms that cannot appear in source terms but appear during reduction. These forms are:

Eval v where v is a pointer or Vundef. (Eval of an integer or float value can occur in a source term and represents a numeric literal.)
Eloc b ofs, which appears during reduction of l-values.
Eparen r tycast ty, which appears during reduction of conditionals r1 ? r2 : r3 as well as sequential `and' / `or'.

Some C expressions are derived forms. Array access r1[r2] is expressed as *(r1 + r2).

Definition Eindex (r1 r2: expr) (ty: type) :=
Ederef (Ebinop Oadd r1 r2 (Tpointer ty noattr)) ty.

Pre-increment ++l and pre-decrement --l are expressed as l += 1 and l -= 1, respectively.

Definition Epreincr (id: incr_or_decr) (l: expr) (ty: type) :=
Eassignop (match id with Incr => Oadd | Decr => Osub end)
l (Eval (Vint Int.one) type_int32s) (typeconv ty) ty.

Selection is a conditional expression that always evaluates both arms and can be implemented by "conditional move" instructions. It is expressed as an invocation of a builtin function.

Definition Eselection (r1 r2 r3: expr) (ty: type) :=
  let t := typ_of_type ty in
  let sg := mksignature (AST.Tint :: t :: t :: nil) t cc_default in
  Ebuiltin (EF_builtin "__builtin_sel"%string sg)
           (Tcons type_bool (Tcons ty (Tcons ty Tnil)))
           (Econs r1 (Econs r2 (Econs r3 Enil)))
           ty.

Extract the type part of a type-annotated expression.

Statements

Compcert C statements are very much like those of C and include:

empty statement Sskip
evaluation of an expression for its side-effects Sdo r
conditional if (...) { ... } else { ... }
the three loops while(...) { ... } and do { ... } while (...) and for(..., ..., ...) { ... }
the switch construct
break, continue, return (with and without argument)
goto and labeled statements.

Only structured forms of switch are supported; moreover, the default case must occur last. Blocks and block-scoped declarations are not supported.

Definition label := ident.

Inductive statement : Type :=
  | Sskip : statement (* do nothing *)
  | Sdo : expr -> statement (* evaluate expression for side effects *)
  | Ssequence : statement -> statement -> statement (* sequence *)
  | Sifthenelse : expr -> statement -> statement -> statement (* conditional *)
  | Swhile : expr -> statement -> statement (* while loop *)
  | Sdowhile : expr -> statement -> statement (* do loop *)
  | Sfor: statement -> expr -> statement -> statement -> statement (* for loop *)
  | Sbreak : statement (* break statement *)
  | Scontinue : statement (* continue statement *)
  | Sreturn : option expr -> statement (* return statement *)
  | Sswitch : expr -> labeled_statements -> statement (* switch statement *)
  | Slabel : label -> statement -> statement
  | Sgoto : label -> statement

with labeled_statements : Type := (* cases of a switch *)
  | LSnil: labeled_statements
  | LScons: option Z -> statement -> labeled_statements -> labeled_statements.

Functions

A function definition is composed of its return type (fn_return), the names and types of its parameters (fn_params), the names and types of its local variables (fn_vars), and the body of the function (a statement, fn_body).

Record function : Type := mkfunction {
  fn_return: type;
  fn_callconv: calling_convention;
  fn_params: list (ident * type);
  fn_vars: list (ident * type);
  fn_body: statement
}.

Definition var_names (vars: list(ident * type)) : list ident :=
  List.map (@fst ident type) vars.

Functions can either be defined (Internal) or declared as external functions (External).

Definition fundef := Ctypes.fundef function.

The type of a function definition.

Definition type_of_function (f: function) : type :=
  Tfunction (type_of_params (fn_params f)) (fn_return f) (fn_callconv f).

Definition type_of_fundef (f: fundef) : type :=
  match f with
  | Internal fd => type_of_function fd
  | External id args res cc => Tfunction args res cc
  end.

Programs and compilation units

As defined in module Ctypes, a program, or compilation unit, is composed of:

a list of definitions of functions and global variables;
the names of functions and global variables that are public (not static);
the name of the function that acts as entry point ("main" function).
a list of definitions for structure and union names
the corresponding composite environment
a proof that this environment is consistent with the definitions.

Definition program := Ctypes.program function.