package ocaml-base-compiler

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package ocaml-base-compiler
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Intermediate language used for tree-based analysis and optimization.

type call_kind =
  1. | Indirect
  2. | Direct of Closure_id.t

Whether the callee in a function application is known at compile time.

type const =
  1. | Int of int
  2. | Char of char
    (*

    Char is kept separate from Int to improve printing

    *)

Simple constants. ("Structured constants" are rewritten to invocations of Pmakeblock so that they easily take part in optimizations.)

type apply = {
  1. func : Variable.t;
  2. args : Variable.t list;
  3. kind : call_kind;
  4. dbg : Debuginfo.t;
  5. inline : Lambda.inline_attribute;
    (*

    Instructions from the source code as to whether the callee should be inlined.

    *)
  6. specialise : Lambda.specialise_attribute;
    (*

    Instructions from the source code as to whether the callee should be specialised.

    *)
}

The application of a function to a list of arguments.

type assign = {
  1. being_assigned : Mutable_variable.t;
  2. new_value : Variable.t;
}

The update of a mutable variable. Mutable variables are distinct from immutable variables in Flambda.

type send = {
  1. kind : Lambda.meth_kind;
  2. meth : Variable.t;
  3. obj : Variable.t;
  4. args : Variable.t list;
  5. dbg : Debuginfo.t;
}

The invocation of a method.

type project_closure = Projection.project_closure

For details on these types, see projection.mli.

type move_within_set_of_closures = Projection.move_within_set_of_closures
type project_var = Projection.project_var
type specialised_to = {
  1. var : Variable.t;
    (*

    The "outer variable".

    *)
  2. projection : Projection.t option;
    (*

    The projecting_from value (see projection.mli) of any projection must be another free variable or specialised argument (depending on whether this record type is involved in free_vars or specialised_args respectively) in the same set of closures. As such, this field describes a relation of projections between either the free_vars or the specialised_args.

    *)
}

See free_vars and specialised_args, below.

type t =
  1. | Var of Variable.t
  2. | Let of let_expr
  3. | Let_mutable of let_mutable
  4. | Let_rec of (Variable.t * named) list * t
    (*

    CR-someday lwhite: give Let_rec the same fields as Let.

    *)
  5. | Apply of apply
  6. | Send of send
  7. | Assign of assign
  8. | If_then_else of Variable.t * t * t
  9. | Switch of Variable.t * switch
  10. | String_switch of Variable.t * (string * t) list * t option
    (*

    Restrictions on Lambda.Lstringswitch also apply to String_switch.

    *)
  11. | Static_raise of Static_exception.t * Variable.t list
  12. | Static_catch of Static_exception.t * Variable.t list * t * t
  13. | Try_with of t * Variable.t * t
  14. | While of t * t
  15. | For of for_loop
  16. | Proved_unreachable

Flambda terms are partitioned in a pseudo-ANF manner; many terms are required to be let-bound. This in particular ensures there is always a variable name for an expression that may be lifted out (for example if it is found to be constant). Note: All bound variables in Flambda terms must be distinct. Flambda_invariants verifies this.

and named =
  1. | Symbol of Symbol.t
  2. | Const of const
  3. | Allocated_const of Allocated_const.t
  4. | Read_mutable of Mutable_variable.t
  5. | Read_symbol_field of Symbol.t * int
    (*

    During the lifting of let bindings to program constructions after closure conversion, we generate symbols and their corresponding definitions (which may or may not be constant), together with field accesses to such symbols. We would like it to be the case that such field accesses are simplified to the relevant component of the symbol concerned. (The rationale is to generate efficient code and share constants as expected: see e.g. tests/asmcomp/staticalloc.ml.) The components of the symbol would be identified by other symbols. This sort of access pattern is feasible because the top-level structure of symbols is statically allocated and fixed at compile time. It may seem that Prim (Pfield, ...) expressions could be used to perform the field accesses. However for simplicity, to avoid having to keep track of properties of individual fields of blocks, Inconstant_idents never deems a Prim (Pfield, ...) expression to be constant. This would in general prevent field accesses to symbols from being simplified in the way we would like, since Lift_constants would not assign new symbols (i.e. the things we would like to simplify to) to the various projections from the symbols in question. To circumvent this problem we use Read_symbol_field when generating projections from the top level of symbols. Owing to the properties of symbols described above, such expressions may be eligible for declaration as constant by Inconstant_idents (and thus themselves lifted to another symbol), without any further complication. Read_symbol_field may only be used when the definition of the symbol is in scope in the program. For external unresolved symbols, Pfield may still be used; it will be changed to Read_symbol_field by Inline_and_simplify when (and if) the symbol is imported.

    *)
  6. | Set_of_closures of set_of_closures
  7. | Project_closure of project_closure
  8. | Move_within_set_of_closures of move_within_set_of_closures
  9. | Project_var of project_var
  10. | Prim of Clambda_primitives.primitive * Variable.t list * Debuginfo.t
  11. | Expr of t
    (*

    ANF escape hatch.

    *)

Values of type named will always be let-bound to a Variable.t.

and let_expr = private {
  1. var : Variable.t;
  2. defining_expr : named;
  3. body : t;
  4. free_vars_of_defining_expr : Variable.Set.t;
    (*

    A cache of the free variables in the defining expression of the let.

    *)
  5. free_vars_of_body : Variable.Set.t;
    (*

    A cache of the free variables of the body of the let. This is an important optimization.

    *)
}
and let_mutable = {
  1. var : Mutable_variable.t;
  2. initial_value : Variable.t;
  3. contents_kind : Lambda.value_kind;
  4. body : t;
}
and set_of_closures = private {
  1. function_decls : function_declarations;
  2. free_vars : specialised_to Variable.Map.t;
    (*

    Mapping from all variables free in the body of the function_decls to variables in scope at the definition point of the set_of_closures. The domain of this map is sometimes known as the "variables bound by the closure".

    *)
  3. specialised_args : specialised_to Variable.Map.t;
    (*

    Parameters whose corresponding arguments are known to always alias a particular value. These are the only parameters that may, during Inline_and_simplify, have non-unknown approximations.

    An argument may only be specialised to a variable in the scope of the corresponding set of closures declaration. Usually, that variable itself also appears in the position of the specialised argument at all call sites of the function. However it may also be the case (for example in code generated as a result of Augment_specialised_args) that the various call sites of such a function have differing variables in the position of the specialised argument. This is permissible *so long as it is certain they all alias the same value*. Great care must be taken in transformations that result in this situation since there are no invariant checks for correctness.

    As an example, supposing all call sites of f are represented here: let x = ... in let f a b c = ... in let y = ... in f x y 1; f x y 1 the specialised arguments of f can (but does not necessarily) contain the association a -> x, but cannot contain b -> y because f is not in the scope of y. If f were the recursive function let rec f a b c = f a 1 2 in, a -> x would still be a valid specialised argument because all recursive calls maintain the invariant.

    This information is used for optimization purposes, if such a binding is known, it is possible to specialise the body of the function according to its parameter. This is usually introduced when specialising a recursive function, for instance. let rec map f = function | [] -> [] | h :: t -> f h :: map f t let map_succ l = let succ x = x + 1 in map succ l map can be duplicated in map_succ to be specialised for the argument f. This will result in let map_succ l = let succ x = x + 1 in let rec map f = function | [] -> [] | h :: t -> f h :: map f t in map succ l with map having f -> succ in its specialised_args field.

    Specialised argument information for arguments that are used must never be erased. This ensures that specialised arguments whose approximations describe closures maintain those approximations, which is essential to transport the closure freshening information to the point of use (e.g. a Project_var from such an argument).

    *)
  4. direct_call_surrogates : Variable.t Variable.Map.t;
    (*

    If direct_call_surrogates maps fun_var1 to fun_var2 then direct calls to fun_var1 should be redirected to fun_var2. This is used to reduce the overhead of transformations that introduce wrapper functions (which will be inlined at direct call sites, but will penalise indirect call sites). direct_call_surrogates may not be transitively closed.

    *)
}

The representation of a set of function declarations (possibly mutually recursive). Such a set encapsulates the declarations themselves, information about their defining environment, and information used specifically for optimization. Before a function can be applied it must be "projected" from a set of closures to yield a "closure". This is done using Project_closure (see above). Given a closure, not only can it be applied, but information about its defining environment can be retrieved (using Project_var, see above). At runtime, a set_of_closures corresponds to an OCaml value with tag Closure_tag (possibly with inline Infix_tag(s)). As an optimization, an operation (Move_within_set_of_closures) is provided (see above) which enables one closure within a set to be located given another closure in the same set. This avoids keeping a pointer to the whole set of closures alive when compiling, for example, mutually-recursive functions.

and function_declarations = private {
  1. is_classic_mode : bool;
    (*

    Indicates whether this function_declarations was compiled with -Oclassic.

    *)
  2. set_of_closures_id : Set_of_closures_id.t;
    (*

    An identifier (unique across all Flambda trees currently in memory) of the set of closures associated with this set of function declarations.

    *)
  3. set_of_closures_origin : Set_of_closures_origin.t;
    (*

    An identifier of the original set of closures on which this set of function declarations is based. Used to prevent different specialisations of the same functions from being inlined/specialised within each other.

    *)
  4. funs : function_declaration Variable.Map.t;
    (*

    The function(s) defined by the set of function declarations. The keys of this map are often referred to in the code as "fun_var"s.

    *)
}
and function_declaration = private {
  1. closure_origin : Closure_origin.t;
  2. params : Parameter.t list;
  3. body : t;
  4. free_variables : Variable.Set.t;
    (*

    All variables free in the *body* of the function. For example, a variable that is bound as one of the function's parameters will still be included in this set. This field is present as an optimization.

    *)
  5. free_symbols : Symbol.Set.t;
    (*

    All symbols that occur in the function's body. (Symbols can never be bound in a function's body; the only thing that binds symbols is the program constructions below.)

    *)
  6. stub : bool;
    (*

    A stub function is a generated function used to prepare arguments or return values to allow indirect calls to functions with a special calling convention. For instance indirect calls to tuplified functions must go through a stub. Stubs will be unconditionally inlined.

    *)
  7. dbg : Debuginfo.t;
    (*

    Debug info for the function declaration.

    *)
  8. inline : Lambda.inline_attribute;
    (*

    Inlining requirements from the source code.

    *)
  9. specialise : Lambda.specialise_attribute;
    (*

    Specialising requirements from the source code.

    *)
  10. is_a_functor : bool;
    (*

    Whether the function is known definitively to be a functor.

    *)
  11. poll : Lambda.poll_attribute;
    (*

    Behaviour for polls

    *)
}
and switch = {
  1. numconsts : Numbers.Int.Set.t;
    (*

    Integer cases

    *)
  2. consts : (int * t) list;
    (*

    Integer cases

    *)
  3. numblocks : Numbers.Int.Set.t;
    (*

    Number of tag block cases

    *)