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STATIC-DISPATCH

Static dispatch is a library, inspired by inlined-generic-function, which allows standard Common Lisp generic function dispatch to be performed statically (at compile time) rather than dynamically (at runtime). This is similar to what is known as "overloading" in languages such as C++ and Java.

The purpose of static dispatch is to provide an optimization in cases where the usual dynamic dispatch is too slow, and the dynamic features of generic functions, such as adding/removing methods at runtime are not required. An example of such a case is a generic equality comparison function. Currently generic functions are considered far too slow to implement generic arithmetic and comparison operations when used heavily in numeric code.

Usage

Generic functions and their methods are defined as usual, however using DEFGENERIC and DEFMETHOD from the STATIC-DISPATCH-CL package rather than the CL package. By default, generic functions are dispatched dynamically, and are identical to standard Common Lisp generic functions.

A generic function call is statically dispatched when an OPTIMIZE declaration with a SPEED level of 3 is in-place, in the environment of the call.

Example:

(locally (declare (optimize (speed 3) (safety 2)))
    (declare (type integer x y))
    (foo x y)) ; Call to FOO statically dispatched if type of X and Y known

In order for the applicable method to be chosen at compile-time, the types of the arguments to the generic function call must be known. On SBCL this is dependent on whether the compiler can determine the types of the arguments. On the remaining implementations cl-form-types is used to determine the argument types, and in order for this to be successful, the types of variables and return types of functions have to be declared.

NOTE: EQL specializers are supported however EQL methods are only chosen if the argument is either a constant value EQL to the EQL specializer value, or is a variable declared of type (EQL VALUE) where VALUE is eql to the specializer value.

If the types of the arguments cannot be determined, no method is chosen and the generic function call form is left as is, which falls back to the standard dynamic dispatch.

NOTE: All standard method combinations are supported as well as user-defined method combinations, as long as they are defined with DEFINE-METHOD-COMBINATION from the STATIC-DISPATCH-CL package. If a generic function uses a method-combination, which is defined using DEFINE-METHOD-COMBINATION from the COMMON-LISP package, then calls to the generic function will not be statically dispatched, and will be dynamically dispatched instead.

NOTE: The package STATIC-DISPATCH-CL exports all symbols from the CL package as well as the symbols in the STATIC-DISPATCH package, including the shadowed DEFMETHOD and DEFGENERIC macros. Use this package instead of CL.

NOTE: Static dispatching is inhibited when an OPTIMIZE declaration with either a SAFETY, DEBUG or COMPILATION-SPEED level of 3, is in place.

Overloading

Static-dispatch also supports replacing a generic function call with a call to an ordinary function that implements the method, rather than inlining the method body.

This is done when there is an OPTIMIZE declaration with a SPACE level of 3.

(locally (declare (optimize (speed 3) (space 3)))
  (foo 1) ;; FOO statically dispatch and method function called

Alternatively the STATIC-DISPATCH-TYPE declaration can be used to configure how a single function is statically dispatched.

STATIC-DISPATCH-TYPE

Declaration: STATIC-DISPATCH-TYPE TYPE . FNS

TYPE is the dispatch type, which can either be one of the following:

  • INLINE - The generic function is always inlined, regardless of SPACE optimizations.

  • FUNCTION - The generic function call is always replaced with a method function call, regardless of SPACE optimizations.

FNS is the list of functions to which this dispatch type applies.

(locally (declare (optimize (speed 3)) (static-dispatch-type function foo bar))
  ;; The following generic-function calls are replaced with method
  ;; function calls
  
  (foo 1)
  (bar 2))

NOTE: The FUNCTION dispatch type is most useful, when you have large methods and you want true overloading, as is found in languages such as Java and C++. The INLINE type will likely result in faster, however also larger code.

Prevent Static Dispatching

Static dispatching can be inhibited for a generic function, even when the speed level is 3, by declaring that function NOTINLINE.

Example:

(locally (declare (optimize speed)
                  (notinline foo))
    (declare (type integer x y))
    (FOO X Y)) ; FOO not statically dispatched

To inhibit static dispatching entirely for all generic functions, use a SAFETY, or DEBUG level of 3.

Example:

(locally (declare (optimize (speed 3) (safety 3)))
    (declare (type integer x y))
    (FOO X Y)) ; FOO not statically dispatched

Static dispatching can also be explicitly disabled with the STATIC-DISPATCH-INHIBIT declaration, which accepts one argument a Boolean flag. If the flag is true (T) static dispatching is disabled entirely for all generic function calls in the environment of the declaration regardless of the OPTIMIZE levels or NOTINLINE declarations. If the flag is NIL, static dispatching is reenabled in the environment, though will still only be performed if the appropriate OPTIMIZE declarations are in place.

Example:

(locally (declare (optimize (speed 3)))
    (declare (static-dispatch-inhibit t)) ; Disable static dispatch in LOCALLY form
    (declare (type integer x y))
    (FOO X Y)) ; FOO not statically dispatched

Optimize Declarations

The OPTIMIZE levels not only control whether static dispatching is performed but also have an effect on the code that is inserted in place of the generic function call expression. Specifically, the definition of the lexical CALL-NEXT-METHOD function.

The types of the arguments which may be passed to CALL-NEXT-METHOD are unknown. By default type checks, using CHECK-TYPE, for the arguments are added. A condition is raised if the types of the arguments passed to CALL-NEXT-METHOD are not compatible with the types in the next applicable method's specializer list.

When a SAFETY level of 0 is given, the type checks are omitted and it is the programmer's responsibility to ensure that the arguments passed to the next method are compatible with the types in the method's specializer list. When CALL-NEXT-METHOD is called with no arguments, which is equivalent to being called with the same arguments as the current method, the argument types are guaranteed to be compatible.

Examples

Given a generic function with the following methods:

(defmethod foo ((a number))
  (list :number a))

(defmethod foo ((a integer))
  (list :integer (call-next-method (1+ a))))

An optimize declaration such as the following will result in type checks being inserted:

(locally
  (declare (optimize (speed 3) (safety 1)))
  (declare (type integer x))

  (foo x))

This results in the expression (foo x) being replaced with something similar to the following (simplified to remove the case of CALL-NEXT-METHOD being called with no arguments):

(flet
  ((call-next-method (x)
     (check-type x number)
     (list :number x)))

 (list :integer (call-next-method (1+ x))))

If the optimize declaration is changed to (optimize (speed 3) (safety 0)), the type checks are omitted which results in the following:

(flet
  ((call-next-method (x)
     (list :number x)))

 (list :integer (call-next-method (1+ x))))

Interaction with other Compiler Macros

By default static-dispatch does not add a compiler macro to a generic function which already has a compiler macro function. To statically dispatch such functions the STATIC-DISPATCH function has to be called manually from the compiler macro.

Function: STATIC-DISPATCH FORM &OPTIONAL ENV

Determines whether to statically dispatch the call to the generic function FORM. If so returns the body of the selected method otherwise returns FORM as is.

ENV is the lexical environment of the generic function call.

NOTE: On SBCL this is a no-op since IR1 transforms, using SB-C:DEFTRANSFORM, specified directly on the argument types, are used rather than compiler macros.

Static Dispatch Failure Warnings

By default a style-warning is emitted whenever static-dispatch fails for any reason. Static dispatching can fail if the types of the arguments cannot be determined, there are no applicable methods, or due to other error conditions such as encountering a malformed form, or missing method information due to it not being defined with DEFMETHOD from the STATIC-DISPATCH package.

The declaration STATIC-DISPATCH-WARN controls for which static dispatch failures, a warning is emitted, for generic function calls in the environment of the declaration.

STATIC-DISPATCH-WARN

Declaration: STATIC-DISPATCH-WARN LEVEL

Control for which static dispatch failures a warning is emitted.

LEVEL is the warning level, which currently can be one of the following symbols:

ALL - Emit a warning for every static dispatch failure.

NONE - Do not emit a warning for any static dispatch failure.

The ALL level is set by default when there is no declaration in place.

NOTE: Any symbol, in any package, with symbol-name equal to ALL or NONE can be used.

Example:

(let ((x 1) (y 2))
  (declare (optimize speed))
  (declare (static-dispatch-warn none)) ; Do not emit any warnings

  ;; No warnings emitted, for static dispatch failures,
  ;; in environment of LET form.

  ;; Generic function add. Static-dispatch will fail on most implementations
  ;; Since the types of X and Y have not been declared
  (add x y))

Common Pitfalls

The semantics of generic functions are changed when statically dispatched.

Statically dispatched generic functions are dispatched based on the declared types of the arguments at compile-time. The declared type may differ from the actual runtime type of the argument, which may result in a different method being called when statically dispatched from when the generic function is dynamically dispatched. An intuitive analogy is that dynamically dispatched generic functions have the semantics of C++ virtual methods whereas statically dispatched functions have the semantics of overloaded functions/methods in C++ and Java.

Consider the following methods:

(defmethod foo ((a number))
  (list :number a))

(defmethod foo ((a integer))
  (list :integer a))

And consider the following code:

(let ((x 1))
  (declare (type number x)
           (optimize speed))

  (foo x))

Since FOO is statically dispatched, the method specialized on NUMBER will be called, due to the (TYPE NUMBER X) declaration, and hence (FOO X) will evaluate to (:NUMBER 1). However since X is actually bound to an integer value, with dynamic dispatch, when the (INLINE FOO) declaration is removed, the method specialized on INTEGER will be called and hence (FOO X) will evaluate to (:INTEGER 1).

It's also possible that a particular implementation may change the declared type of a variable to a subtype if it can be deduced to be of that subtype, so in this example the declaration (TYPE NUMBER X) may be changed to (TYPE INTEGER X) by the implementation. As a result it's not even possible to rely on the declaration forcing the method specialized on NUMBER to be called. Therefore you should only use statically dispatched functions for optimization where each method has the same behaviour just implemented for different types.

NOTE: If the type of an argument cannot be determined, or is declared T, the generic function will not be statically dispatched even if when the appropriate OPTIMIZE declaration is in place.

Another aspect in which the semantics of generic functions are changed is that when dynamically dispatched the list of methods can be changed at runtime, that is new methods can be added while the program is running and existing methods can be removed. Obviously, static-dispatch cannot predict what methods will be added or removed, therefore adding or removing methods will not have the desired effect on statically dispatched generic function calls.

Dependencies

Main Dependencies

  • closer-mop - Required to obtain information about generic-function methods, namely the argument precedence order and class precedence list.

  • cl-environments - Used to extract declaration information from environments, on any implementation.

  • cl-form-types - Used to determine the types of the arguments to generic function calls.

Other Dependencies

Status

Tested on: CCL, SBCL, Clisp, ECL, CMUCL and ABCL.

Tests pass (static dispatch is successful) on: SBCL, CCL, Clisp and ECL.

Tests fail (falls back to dynamic dispatch) on: CMUCL and ABCL.

Whether static-dispatching is successful depends on whether the cl-environments library is able to extract the type information from the environment. Where the CLTL2 API is natively supported (SBCL, CCL, CMUCL, LispWorks and Allegro) the type information can be accessed without any workarounds. On the remaining implementations the type information can be extracted in most cases however workarounds are required for some edge cases, see CL-ENVIRONMENTS: Ensuring Code Walking.

Known Issues:

Planned Future Additions

  • Removal of CALL-NEXT-METHOD and NEXT-METHOD-P if not used by the method.
  • Optimization of CALL-NEXT-METHOD, where it is inlined in some cases.
  • Method return type declarations.
  • Enhance generic functions to allow for specialization on all types rather than just classes.

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Static generic function dispatch for Common Lisp

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