guile/doc/ref/compiler.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  2008, 2009, 2010, 2011, 2012, 2013, 2014
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Compiling to the Virtual Machine
@section Compiling to the Virtual Machine

Compilers!  The word itself inspires excitement and awe, even among
experienced practitioners.  But a compiler is just a program: an
eminently hackable thing.  This section aims to to describe Guile's
compiler in such a way that interested Scheme hackers can feel
comfortable reading and extending it.

@xref{Read/Load/Eval/Compile}, if you're lost and you just wanted to
know how to compile your @code{.scm} file.

@menu
* Compiler Tower::                   
* The Scheme Compiler::                   
* Tree-IL::                 
* Continuation-Passing Style::                 
* Bytecode::                
* Writing New High-Level Languages::
* Extending the Compiler::
@end menu

@node Compiler Tower
@subsection Compiler Tower

Guile's compiler is quite simple -- its @emph{compilers}, to put it more
accurately.  Guile defines a tower of languages, starting at Scheme and
progressively simplifying down to languages that resemble the VM
instruction set (@pxref{Instruction Set}).

Each language knows how to compile to the next, so each step is simple
and understandable.  Furthermore, this set of languages is not hardcoded
into Guile, so it is possible for the user to add new high-level
languages, new passes, or even different compilation targets.

Languages are registered in the module, @code{(system base language)}:

@example
(use-modules (system base language))
@end example

They are registered with the @code{define-language} form.

@deffn {Scheme Syntax} define-language @
                       [#:name] [#:title] [#:reader] [#:printer] @
                       [#:parser=#f] [#:compilers='()] @
                       [#:decompilers='()] [#:evaluator=#f] @
                       [#:joiner=#f] [#:for-humans?=#t] @
                       [#:make-default-environment=make-fresh-user-module]
Define a language.

This syntax defines a @code{<language>} object, bound to @var{name} in
the current environment.  In addition, the language will be added to the
global language set.  For example, this is the language definition for
Scheme:

@example
(define-language scheme
  #:title	"Scheme"
  #:reader      (lambda (port env) ...)
  #:compilers   `((tree-il . ,compile-tree-il))
  #:decompilers `((tree-il . ,decompile-tree-il))
  #:evaluator	(lambda (x module) (primitive-eval x))
  #:printer	write
  #:make-default-environment (lambda () ...))
@end example
@end deffn

The interesting thing about having languages defined this way is that
they present a uniform interface to the read-eval-print loop.  This
allows the user to change the current language of the REPL:

@example
scheme@@(guile-user)> ,language tree-il
Happy hacking with Tree Intermediate Language!  To switch back, type `,L scheme'.
tree-il@@(guile-user)> ,L scheme
Happy hacking with Scheme!  To switch back, type `,L tree-il'.
scheme@@(guile-user)> 
@end example

Languages can be looked up by name, as they were above.

@deffn {Scheme Procedure} lookup-language name
Looks up a language named @var{name}, autoloading it if necessary.

Languages are autoloaded by looking for a variable named @var{name} in
a module named @code{(language @var{name} spec)}.

The language object will be returned, or @code{#f} if there does not
exist a language with that name.
@end deffn

Defining languages this way allows us to programmatically determine
the necessary steps for compiling code from one language to another.

@deffn {Scheme Procedure} lookup-compilation-order from to
Recursively traverses the set of languages to which @var{from} can
compile, depth-first, and return the first path that can transform
@var{from} to @var{to}. Returns @code{#f} if no path is found.

This function memoizes its results in a cache that is invalidated by
subsequent calls to @code{define-language}, so it should be quite
fast.
@end deffn

There is a notion of a ``current language'', which is maintained in the
@code{current-language} parameter, defined in the core @code{(guile)}
module.  This language is normally Scheme, and may be rebound by the
user.  The run-time compilation interfaces
(@pxref{Read/Load/Eval/Compile}) also allow you to choose other source
and target languages.

The normal tower of languages when compiling Scheme goes like this:

@itemize
@item Scheme
@item Tree Intermediate Language (Tree-IL)
@item Continuation-Passing Style (CPS)
@item Bytecode
@end itemize

As discussed before (@pxref{Object File Format}), bytecode is in ELF
format, ready to be serialized to disk.  But when compiling Scheme at
run time, you want a Scheme value: for example, a compiled procedure.
For this reason, so as not to break the abstraction, Guile defines a
fake language at the bottom of the tower:

@itemize
@item Value
@end itemize

Compiling to @code{value} loads the bytecode into a procedure, turning
cold bytes into warm code.

Perhaps this strangeness can be explained by example:
@code{compile-file} defaults to compiling to bytecode, because it
produces object code that has to live in the barren world outside the
Guile runtime; but @code{compile} defaults to compiling to @code{value},
as its product re-enters the Guile world.

@c FIXME: This doesn't work anymore :(  Should we add some kind of
@c special GC pass, or disclaim this kind of code, or what?

Indeed, the process of compilation can circulate through these
different worlds indefinitely, as shown by the following quine:

@example
((lambda (x) ((compile x) x)) '(lambda (x) ((compile x) x)))
@end example

@node The Scheme Compiler
@subsection The Scheme Compiler

The job of the Scheme compiler is to expand all macros and all of Scheme
to its most primitive expressions.  The definition of ``primitive
expression'' is given by the inventory of constructs provided by
Tree-IL, the target language of the Scheme compiler: procedure calls,
conditionals, lexical references, and so on.  This is described more
fully in the next section.

The tricky and amusing thing about the Scheme-to-Tree-IL compiler is
that it is completely implemented by the macro expander.  Since the
macro expander has to run over all of the source code already in order
to expand macros, it might as well do the analysis at the same time,
producing Tree-IL expressions directly.

Because this compiler is actually the macro expander, it is extensible.
Any macro which the user writes becomes part of the compiler.

The Scheme-to-Tree-IL expander may be invoked using the generic
@code{compile} procedure:

@lisp
(compile '(+ 1 2) #:from 'scheme #:to 'tree-il)
@result{}
#<tree-il (call (toplevel +) (const 1) (const 2))>
@end lisp

@code{(compile @var{foo} #:from 'scheme #:to 'tree-il)} is entirely
equivalent to calling the macro expander as @code{(macroexpand @var{foo}
'c '(compile load eval))}.  @xref{Macro Expansion}.
@code{compile-tree-il}, the procedure dispatched by @code{compile} to
@code{'tree-il}, is a small wrapper around @code{macroexpand}, to make
it conform to the general form of compiler procedures in Guile's
language tower.

Compiler procedures take three arguments: an expression, an
environment, and a keyword list of options. They return three values:
the compiled expression, the corresponding environment for the target
language, and a ``continuation environment''. The compiled expression
and environment will serve as input to the next language's compiler.
The ``continuation environment'' can be used to compile another
expression from the same source language within the same module.

For example, you might compile the expression, @code{(define-module
(foo))}. This will result in a Tree-IL expression and environment. But
if you compiled a second expression, you would want to take into
account the compile-time effect of compiling the previous expression,
which puts the user in the @code{(foo)} module. That is purpose of the
``continuation environment''; you would pass it as the environment
when compiling the subsequent expression.

For Scheme, an environment is a module. By default, the @code{compile}
and @code{compile-file} procedures compile in a fresh module, such
that bindings and macros introduced by the expression being compiled
are isolated:

@example
(eq? (current-module) (compile '(current-module)))
@result{} #f

(compile '(define hello 'world))
(defined? 'hello)
@result{} #f

(define / *)
(eq? (compile '/) /)
@result{} #f
@end example

Similarly, changes to the @code{current-reader} fluid (@pxref{Loading,
@code{current-reader}}) are isolated:

@example
(compile '(fluid-set! current-reader (lambda args 'fail)))
(fluid-ref current-reader)
@result{} #f
@end example

Nevertheless, having the compiler and @dfn{compilee} share the same name
space can be achieved by explicitly passing @code{(current-module)} as
the compilation environment:

@example
(define hello 'world)
(compile 'hello #:env (current-module))
@result{} world
@end example

@node Tree-IL
@subsection Tree-IL

Tree Intermediate Language (Tree-IL) is a structured intermediate
language that is close in expressive power to Scheme. It is an
expanded, pre-analyzed Scheme.

Tree-IL is ``structured'' in the sense that its representation is
based on records, not S-expressions. This gives a rigidity to the
language that ensures that compiling to a lower-level language only
requires a limited set of transformations. For example, the Tree-IL
type @code{<const>} is a record type with two fields, @code{src} and
@code{exp}. Instances of this type are created via @code{make-const}.
Fields of this type are accessed via the @code{const-src} and
@code{const-exp} procedures. There is also a predicate, @code{const?}.
@xref{Records}, for more information on records.

@c alpha renaming

All Tree-IL types have a @code{src} slot, which holds source location
information for the expression. This information, if present, will be
residualized into the compiled object code, allowing backtraces to
show source information. The format of @code{src} is the same as that
returned by Guile's @code{source-properties} function. @xref{Source
Properties}, for more information.

Although Tree-IL objects are represented internally using records,
there is also an equivalent S-expression external representation for
each kind of Tree-IL. For example, the S-expression representation
of @code{#<const src: #f exp: 3>} expression would be:

@example
(const 3)
@end example

Users may program with this format directly at the REPL:

@example
scheme@@(guile-user)> ,language tree-il
Happy hacking with Tree Intermediate Language!  To switch back, type `,L scheme'.
tree-il@@(guile-user)> (call (primitive +) (const 32) (const 10))
@result{} 42
@end example

The @code{src} fields are left out of the external representation.

One may create Tree-IL objects from their external representations via
calling @code{parse-tree-il}, the reader for Tree-IL. If any source
information is attached to the input S-expression, it will be
propagated to the resulting Tree-IL expressions. This is probably the
easiest way to compile to Tree-IL: just make the appropriate external
representations in S-expression format, and let @code{parse-tree-il}
take care of the rest.

@deftp {Scheme Variable} <void> src
@deftpx {External Representation} (void)
An empty expression.  In practice, equivalent to Scheme's @code{(if #f
#f)}.
@end deftp

@deftp {Scheme Variable} <const> src exp
@deftpx {External Representation} (const @var{exp})
A constant.
@end deftp

@deftp {Scheme Variable} <primitive-ref> src name
@deftpx {External Representation} (primitive @var{name})
A reference to a ``primitive''.  A primitive is a procedure that, when
compiled, may be open-coded.  For example, @code{cons} is usually
recognized as a primitive, so that it compiles down to a single
instruction.

Compilation of Tree-IL usually begins with a pass that resolves some
@code{<module-ref>} and @code{<toplevel-ref>} expressions to
@code{<primitive-ref>} expressions.  The actual compilation pass has
special cases for calls to certain primitives, like @code{apply} or
@code{cons}.
@end deftp

@deftp {Scheme Variable} <lexical-ref> src name gensym
@deftpx {External Representation} (lexical @var{name} @var{gensym})
A reference to a lexically-bound variable.  The @var{name} is the
original name of the variable in the source program. @var{gensym} is a
unique identifier for this variable.
@end deftp

@deftp {Scheme Variable} <lexical-set> src name gensym exp
@deftpx {External Representation} (set! (lexical @var{name} @var{gensym}) @var{exp})
Sets a lexically-bound variable.
@end deftp

@deftp {Scheme Variable} <module-ref> src mod name public?
@deftpx {External Representation} (@@ @var{mod} @var{name})
@deftpx {External Representation} (@@@@ @var{mod} @var{name})
A reference to a variable in a specific module. @var{mod} should be
the name of the module, e.g.@: @code{(guile-user)}.

If @var{public?} is true, the variable named @var{name} will be looked
up in @var{mod}'s public interface, and serialized with @code{@@};
otherwise it will be looked up among the module's private bindings,
and is serialized with @code{@@@@}.
@end deftp

@deftp {Scheme Variable} <module-set> src mod name public? exp
@deftpx {External Representation} (set! (@@ @var{mod} @var{name}) @var{exp})
@deftpx {External Representation} (set! (@@@@ @var{mod} @var{name}) @var{exp})
Sets a variable in a specific module.
@end deftp

@deftp {Scheme Variable} <toplevel-ref> src name
@deftpx {External Representation} (toplevel @var{name})
References a variable from the current procedure's module.
@end deftp

@deftp {Scheme Variable} <toplevel-set> src name exp
@deftpx {External Representation} (set! (toplevel @var{name}) @var{exp})
Sets a variable in the current procedure's module.
@end deftp

@deftp {Scheme Variable} <toplevel-define> src name exp
@deftpx {External Representation} (define (toplevel @var{name}) @var{exp})
Defines a new top-level variable in the current procedure's module.
@end deftp

@deftp {Scheme Variable} <conditional> src test then else
@deftpx {External Representation} (if @var{test} @var{then} @var{else})
A conditional. Note that @var{else} is not optional.
@end deftp

@deftp {Scheme Variable} <call> src proc args
@deftpx {External Representation} (call @var{proc} . @var{args})
A procedure call.
@end deftp

@deftp {Scheme Variable} <primcall> src name args
@deftpx {External Representation} (primcall @var{name} . @var{args})
A call to a primitive.  Equivalent to @code{(call (primitive @var{name})
. @var{args})}.  This construct is often more convenient to generate and
analyze than @code{<call>}.

As part of the compilation process, instances of @code{(call (primitive
@var{name}) . @var{args})} are transformed into primcalls.
@end deftp

@deftp {Scheme Variable} <seq> src head tail
@deftpx {External Representation} (seq @var{head} @var{tail})
A sequence.  The semantics is that @var{head} is evaluated first, and
any resulting values are ignored.  Then @var{tail} is evaluated, in tail
position.
@end deftp

@deftp {Scheme Variable} <lambda> src meta body
@deftpx {External Representation} (lambda @var{meta} @var{body})
A closure.  @var{meta} is an association list of properties for the
procedure.  @var{body} is a single Tree-IL expression of type
@code{<lambda-case>}.  As the @code{<lambda-case>} clause can chain to
an alternate clause, this makes Tree-IL's @code{<lambda>} have the
expressiveness of Scheme's @code{case-lambda}.
@end deftp

@deftp {Scheme Variable} <lambda-case> req opt rest kw inits gensyms body alternate
@deftpx {External Representation} @
  (lambda-case ((@var{req} @var{opt} @var{rest} @var{kw} @var{inits} @var{gensyms})@
                @var{body})@
               [@var{alternate}])
One clause of a @code{case-lambda}.  A @code{lambda} expression in
Scheme is treated as a @code{case-lambda} with one clause.

@var{req} is a list of the procedure's required arguments, as symbols.
@var{opt} is a list of the optional arguments, or @code{#f} if there
are no optional arguments. @var{rest} is the name of the rest
argument, or @code{#f}.

@var{kw} is a list of the form, @code{(@var{allow-other-keys?}
(@var{keyword} @var{name} @var{var}) ...)}, where @var{keyword} is the
keyword corresponding to the argument named @var{name}, and whose
corresponding gensym is @var{var}.  @var{inits} are tree-il expressions
corresponding to all of the optional and keyword arguments, evaluated to
bind variables whose value is not supplied by the procedure caller.
Each @var{init} expression is evaluated in the lexical context of
previously bound variables, from left to right.

@var{gensyms} is a list of gensyms corresponding to all arguments:
first all of the required arguments, then the optional arguments if
any, then the rest argument if any, then all of the keyword arguments.

@var{body} is the body of the clause.  If the procedure is called with
an appropriate number of arguments, @var{body} is evaluated in tail
position.  Otherwise, if there is an @var{alternate}, it should be a
@code{<lambda-case>} expression, representing the next clause to try.
If there is no @var{alternate}, a wrong-number-of-arguments error is
signaled.
@end deftp

@deftp {Scheme Variable} <let> src names gensyms vals exp
@deftpx {External Representation} (let @var{names} @var{gensyms} @var{vals} @var{exp})
Lexical binding, like Scheme's @code{let}.  @var{names} are the original
binding names, @var{gensyms} are gensyms corresponding to the
@var{names}, and @var{vals} are Tree-IL expressions for the values.
@var{exp} is a single Tree-IL expression.
@end deftp

@deftp {Scheme Variable} <letrec> in-order? src names gensyms vals exp
@deftpx {External Representation} (letrec @var{names} @var{gensyms} @var{vals} @var{exp})
@deftpx {External Representation} (letrec* @var{names} @var{gensyms} @var{vals} @var{exp})
A version of @code{<let>} that creates recursive bindings, like
Scheme's @code{letrec}, or @code{letrec*} if @var{in-order?} is true.
@end deftp

@deftp {Scheme Variable} <prompt> escape-only? tag body handler
@deftpx {External Representation} (prompt @var{escape-only?} @var{tag} @var{body} @var{handler})
A dynamic prompt.  Instates a prompt named @var{tag}, an expression,
during the dynamic extent of the execution of @var{body}, also an
expression.  If an abort occurs to this prompt, control will be passed
to @var{handler}, also an expression, which should be a procedure.  The
first argument to the handler procedure will be the captured
continuation, followed by all of the values passed to the abort.  If
@var{escape-only?} is true, the handler should be a @code{<lambda>} with
a single @code{<lambda-case>} body expression with no optional or
keyword arguments, and no alternate, and whose first argument is
unreferenced.  @xref{Prompts}, for more information.
@end deftp

@deftp {Scheme Variable} <abort> tag args tail
@deftpx {External Representation} (abort @var{tag} @var{args} @var{tail})
An abort to the nearest prompt with the name @var{tag}, an expression.
@var{args} should be a list of expressions to pass to the prompt's
handler, and @var{tail} should be an expression that will evaluate to
a list of additional arguments.  An abort will save the partial
continuation, which may later be reinstated, resulting in the
@code{<abort>} expression evaluating to some number of values.
@end deftp

There are two Tree-IL constructs that are not normally produced by
higher-level compilers, but instead are generated during the
source-to-source optimization and analysis passes that the Tree-IL
compiler does.  Users should not generate these expressions directly,
unless they feel very clever, as the default analysis pass will generate
them as necessary.

@deftp {Scheme Variable} <let-values> src names gensyms exp body
@deftpx {External Representation} (let-values @var{names} @var{gensyms} @var{exp} @var{body})
Like Scheme's @code{receive} -- binds the values returned by
evaluating @code{exp} to the @code{lambda}-like bindings described by
@var{gensyms}.  That is to say, @var{gensyms} may be an improper list.

@code{<let-values>} is an optimization of a @code{<call>} to the
primitive, @code{call-with-values}.
@end deftp

@deftp {Scheme Variable} <fix> src names gensyms vals body
@deftpx {External Representation} (fix @var{names} @var{gensyms} @var{vals} @var{body})
Like @code{<letrec>}, but only for @var{vals} that are unset
@code{lambda} expressions.

@code{fix} is an optimization of @code{letrec} (and @code{let}).
@end deftp

Tree-IL is a convenient compilation target from source languages.  It
can be convenient as a medium for optimization, though CPS is usually
better.  The strength of Tree-IL is that it does not fix order of
evaluation, so it makes some code motion a bit easier.

Optimization passes performed on Tree-IL currently include:

@itemize
@item Open-coding (turning toplevel-refs into primitive-refs,
and calls to primitives to primcalls)
@item Partial evaluation (comprising inlining, copy propagation, and
constant folding)
@item Common subexpression elimination (CSE)
@end itemize

In the future, we will move the CSE pass to operate over the lower-level
CPS language.

@node Continuation-Passing Style
@subsection Continuation-Passing Style

@cindex CPS
Continuation-passing style (CPS) is Guile's principal intermediate
language, bridging the gap between languages for people and languages
for machines.  CPS gives a name to every part of a program: every
control point, and every intermediate value.  This makes it an excellent
medium for reasoning about programs, which is the principal job of a
compiler.

@menu
* An Introduction to CPS::
* CPS in Guile::
* Building CPS::
* Compiling CPS::
@end menu

@node An Introduction to CPS
@subsubsection An Introduction to CPS

Consider the following Scheme expression:

@lisp
(begin
  (display "The sum of 32 and 10 is: ")
  (display 42)
  (newline))
@end lisp

Let us identify all of the sub-expressions in this expression,
annotating them with unique labels:

@lisp
(begin
  (display "The sum of 32 and 10 is: ")
  |k1      k2
  k0
  (display 42)
  |k4      k5
  k3
  (newline))
  |k7
  k6
@end lisp

Each of these labels identifies a point in a program.  One label may be
the continuation of another label.  For example, the continuation of
@code{k7} is @code{k6}.  This is because after evaluating the value of
@code{newline}, performed by the expression labelled @code{k7}, we
continue to apply it in @code{k6}.

Which expression has @code{k0} as its continuation?  It is either the
expression labelled @code{k1} or the expression labelled @code{k2}.
Scheme does not have a fixed order of evaluation of arguments, though it
does guarantee that they are evaluated in some order.  Unlike general
Scheme, continuation-passing style makes evaluation order explicit.  In
Guile, this choice is made by the higher-level language compilers.

Let us assume a left-to-right evaluation order.  In that case the
continuation of @code{k1} is @code{k2}, and the continuation of
@code{k2} is @code{k0}.

With this example established, we are ready to give an example of CPS in
Scheme:

@smalllisp
(lambda (ktail)
  (let ((k1 (lambda ()
              (let ((k2 (lambda (proc)
                          (let ((k0 (lambda (arg0)
                                      (proc k4 arg0))))
                            (k0 "The sum of 32 and 10 is: ")))))
                (k2 display))))
        (k4 (lambda _
              (let ((k5 (lambda (proc)
                          (let ((k3 (lambda (arg0)
                                      (proc k7 arg0))))
                            (k3 42)))))
                (k5 display))))
        (k7 (lambda _
              (let ((k6 (lambda (proc)
                          (proc ktail))))
                (k6 newline)))))
    (k1))
@end smalllisp

Holy code explosion, Batman!  What's with all the lambdas?  Indeed, CPS
is by nature much more verbose than ``direct-style'' intermediate
languages like Tree-IL.  At the same time, CPS is simpler than full
Scheme, because it makes things more explicit.

In the original program, the expression labelled @code{k0} is in effect
context.  Any values it returns are ignored.  In Scheme, this fact is
implicit.  In CPS, we can see it explicitly by noting that its
continuation, @code{k4}, takes any number of values and ignores them.
Compare this to @code{k2}, which takes a single value; in this way we
can say that @code{k1} is in a ``value'' context.  Likewise @code{k6} is
in tail context with respect to the expression as a whole, because its
continuation is the tail continuation, @code{ktail}.  CPS makes these
details manifest, and gives them names.

@node CPS in Guile
@subsubsection CPS in Guile

Guile's CPS language is composed of @dfn{terms}, @dfn{expressions},
and @dfn{continuations}.

A term can either evaluate an expression and pass the resulting values
to some continuation, or it can declare local continuations and contain
a sub-term in the scope of those continuations.

@deftp {CPS Term} $continue k src exp
Evaluate the expression @var{exp} and pass the resulting values (if any)
to the continuation labelled @var{k}.  The source information associated
with the expression may be found in @var{src}, which is either an alist
as in @code{source-properties} or is @code{#f} if there is no associated
source.
@end deftp

@deftp {CPS Term} $letk conts body
Bind @var{conts}, a list of continuations (@code{$cont} instances), in
the scope of the sub-term @var{body}.  The continuations are mutually
recursive.
@end deftp

Additionally, the early stages of CPS allow for a set of mutually
recursive functions to be declared as a term.  This @code{$letrec} type
is like Tree-IL's @code{<fix>}.  The contification pass will attempt to
transform the functions declared in a @code{$letrec} into local
continuations.  Any remaining functions are later lowered to @code{$fun}
expressions.

@deftp {CPS Term} $letrec names syms funs body
Declare the mutually recursive set of functions denoted by @var{names},
@var{syms}, and @var{funs} within the sub-term @var{body}.  @var{names}
and @var{syms} are lists of symbols, and @var{funs} is a list of
@code{$fun} values.  @var{syms} are globally unique.
@end deftp

Here is an inventory of the kinds of expressions in Guile's CPS
language.  Recall that all expressions are wrapped in a @code{$continue}
term which specifies their continuation.

@deftp {CPS Expression} $void
Continue with the unspecified value.
@end deftp

@deftp {CPS Expression} $const val
Continue with the constant value @var{val}.
@end deftp

@deftp {CPS Expression} $prim name
Continue with the procedure that implements the primitive operation
named by @var{name}.
@end deftp

@deftp {CPS Expression} $fun src meta free body
Continue with a procedure.  @var{src} identifies the source information
for the procedure declaration, and @var{meta} is the metadata alist as
described above in Tree-IL's @code{<lambda>}.  @var{free} is a list of
free variables accessed by the procedure.  Early CPS uses an empty list
for @var{free}; only after closure conversion is it correctly populated.
Finally, @var{body} is the @code{$kentry} @code{$cont} of the procedure
entry.
@end deftp

@deftp {CPS Expression} $call proc args
@deftpx {CPS Expression} $callk label proc args
Call @var{proc} with the arguments @var{args}, and pass all values to
the continuation.  @var{proc} and the elements of the @var{args} list
should all be variable names.  The continuation identified by the term's
@var{k} should be a @code{$kreceive} or a @code{$ktail} instance.

@code{$callk} is for the case where the call target is known to be in
the same compilation unit.  @var{label} should be some continuation
label, though it need not be in scope.  In this case the @var{proc} is
simply an additional argument, since it is not used to determine the
call target at run-time.
@end deftp

@deftp {CPS Expression} $primcall name args
Perform the primitive operation identified by @code{name}, a well-known
symbol, passing it the arguments @var{args}, and pass all resulting
values to the continuation.  The set of available primitives includes
all primitives known to Tree-IL and then some more; see the source code
for details.
@end deftp

@deftp {CPS Expression} $values args
Pass the values named by the list @var{args} to the continuation.
@end deftp

@deftp {CPS Expression} $prompt escape? tag handler
Push a prompt on the stack identified by the variable name @var{tag},
which may be escape-only if @var{escape?} is true, and continue with
zero values.  If the body aborts to this prompt, control will proceed at
the continuation labelled @var{handler}, which should be a
@code{$kreceive} continuation.  Prompts are later popped by
@code{pop-prompt} primcalls.
@end deftp

The remaining element of the CPS language in Guile is the continuation.
In CPS, all continuations have unique labels.  Since this aspect is
common to all continuation types, all continuations are contained in a
@code{$cont} instance:

@deftp {CPS Continuation Wrapper} $cont k cont
Declare a continuation labelled @var{k}.  All references to the
continuation will use this label.
@end deftp

The most common kind of continuation binds some number of values, and
then evaluates a sub-term.  @code{$kargs} is this kind of simple
@code{lambda}.

@deftp {CPS Continuation} $kargs names syms body
Bind the incoming values to the variables @var{syms}, with original
names @var{names}, and then evaluate the sub-term @var{body}.
@end deftp

Variable names (the names in the @var{syms} of a @code{$kargs}) should
be globally unique, and also disjoint from continuation labels.  To bind
a value to a variable and then evaluate some term, you would continue
with the value to a @code{$kargs} that declares one variable.  The bound
value would then be available for use within the body of the
@code{$kargs}.

@deftp {CPS Continuation} $kif kt kf
Receive one value.  If it is true for the purposes of Scheme, branch to
the continuation labelled @var{kt}, passing no values; otherwise, branch
to @var{kf}.
@end deftp

For internal reasons, only certain terms may continue to a @code{$kif}.
Compiling @code{$kif} avoids allocating space for the test variable, so
it needs to be preceded by expressions that can test-and-branch without
temporary values.  In practice this condition is true for
@code{$primcall}s to @code{null?}, @code{=}, and similar primitives that
have corresponding @code{br-if-@var{foo}} VM operations; see the source
code for full details.  When in doubt, bind the test expression to a
variable, and continue to the @code{$kif} with a @code{$values}
expression.  The optimizer should elide the @code{$values} if it is not
needed.

Calls out to other functions need to be wrapped in a @code{$kreceive}
continuation in order to adapt the returned values to their uses in the
calling function, if any.

@deftp {CPS Continuation} $kreceive arity k
Receive values on the stack.  Parse them according to @var{arity}, and
then proceed with the parsed values to the @code{$kargs} continuation
labelled @var{k}.  As a limitation specific to @code{$kreceive},
@var{arity} may only contain required and rest arguments.
@end deftp

@code{$arity} is a helper data structure used by @code{$kreceive} and
also by @code{$kclause}, described below.

@deftp {CPS Data} $arity req opt rest kw allow-other-keys?
A data type declaring an arity.  @var{req} and @var{opt} are lists of
source names of required and optional arguments, respectively.
@var{rest} is either the source name of the rest variable, or @code{#f}
if this arity does not accept additional values.  @var{kw} is a list of
the form @code{((@var{keyword} @var{name} @var{var}) ...)}, describing
the keyword arguments.  @var{allow-other-keys?} is true if other keyword
arguments are allowed and false otherwise.

Note that all of these names with the exception of the @var{var}s in the
@var{kw} list are source names, not unique variable names.
@end deftp

Additionally, there are three specific kinds of continuations that can
only be declared at function entries.

@deftp {CPS Continuation} $kentry self tail clauses
Declare a function entry.  @var{self} is a variable bound to the
procedure being called, and which may be used for self-references.
@var{tail} declares the @code{$cont} wrapping the @code{$ktail} for this
function, corresponding to the function's tail continuation.
@var{clauses} is a list of @code{$kclause} @code{$cont} instances.
@end deftp

@deftp {CPS Continuation} $ktail
A tail continuation.
@end deftp

@deftp {CPS Continuation} $kclause arity cont
A clause of a function with a given arity.  Applications of a function
with a compatible set of actual arguments will continue to @var{cont}, a
@code{$kargs} @code{$cont} instance representing the clause body.
@end deftp


@node Building CPS
@subsubsection Building CPS

Unlike Tree-IL, the CPS language is built to be constructed and
deconstructed with abstract macros instead of via procedural
constructors or accessors, or instead of S-expression matching.

Deconstruction and matching is handled adequately by the @code{match}
form from @code{(ice-9 match)}.  @xref{Pattern Matching}.  Construction
is handled by a set of mutually recursive builder macros:
@code{build-cps-term}, @code{build-cps-cont}, and @code{build-cps-exp}.

In the following interface definitions, consider variables containing
@code{cont} to be recursively build by @code{build-cps-cont}, and
likewise for @code{term} and @code{exp}.  Consider any other name to be
evaluated as a Scheme expression.  Many of these forms recognize
@code{unquote} in some contexts, to splice in a previously-built value;
see the specifications below for full details.

@deffn {Scheme Syntax} build-cps-term ,val
@deffnx {Scheme Syntax} build-cps-term ($letk (cont ...) term)
@deffnx {Scheme Syntax} build-cps-term ($letrec names syms funs term)
@deffnx {Scheme Syntax} build-cps-term ($continue k src exp)
@deffnx {Scheme Syntax} build-cps-exp ,val
@deffnx {Scheme Syntax} build-cps-exp ($void)
@deffnx {Scheme Syntax} build-cps-exp ($const val)
@deffnx {Scheme Syntax} build-cps-exp ($prim name)
@deffnx {Scheme Syntax} build-cps-exp ($fun src meta free body)
@deffnx {Scheme Syntax} build-cps-exp ($call proc (arg ...))
@deffnx {Scheme Syntax} build-cps-exp ($call proc args)
@deffnx {Scheme Syntax} build-cps-exp ($primcall name (arg ...))
@deffnx {Scheme Syntax} build-cps-exp ($primcall name args)
@deffnx {Scheme Syntax} build-cps-exp ($values (arg ...))
@deffnx {Scheme Syntax} build-cps-exp ($values args)
@deffnx {Scheme Syntax} build-cps-exp ($prompt escape? tag handler)
@deffnx {Scheme Syntax} build-cps-cont ,val
@deffnx {Scheme Syntax} build-cps-cont (k ($kargs (name ...) (sym ...) term))
@deffnx {Scheme Syntax} build-cps-cont (k ($kargs names syms term))
@deffnx {Scheme Syntax} build-cps-cont (k ($kif kt kf))
@deffnx {Scheme Syntax} build-cps-cont (k ($kreceive req rest kargs))
@deffnx {Scheme Syntax} build-cps-cont (k ($kentry self tail-cont ,clauses))
@deffnx {Scheme Syntax} build-cps-cont (k ($kentry self tail-cont (cont ...)))
@deffnx {Scheme Syntax} build-cps-cont (k ($kclause ,arity cont))
@deffnx {Scheme Syntax} build-cps-cont (k ($kclause (req opt rest kw aok?) cont))
Construct a CPS term, expression, or continuation.
@end deffn

There are a few more miscellaneous interfaces as well.

@deffn {Scheme Procedure} make-arity req opt rest kw allow-other-keywords?
A procedural constructor for @code{$arity} objects.
@end deffn

@deffn {Scheme Syntax} let-gensyms (sym ...) body ...
Bind @var{sym...} to fresh names, and evaluate @var{body...}.
@end deffn

@deffn {Scheme Syntax} rewrite-cps-term val (pat term) ...
@deffnx {Scheme Syntax} rewrite-cps-exp val (pat exp) ...
@deffnx {Scheme Syntax} rewrite-cps-cont val (pat cont) ...
Match @var{val} against the series of patterns @var{pat...}, using
@code{match}.  The body of the matching clause should be a template in
the syntax of @code{build-cps-term}, @code{build-cps-exp}, or
@code{build-cps-cont}, respectively.
@end deffn

@node Compiling CPS
@subsubsection Compiling CPS

Compiling CPS in Guile has three phases: conversion, optimization, and
code generation.

CPS conversion is the process of taking a higher-level language and
compiling it to CPS.  Source languages can do this directly, or they can
convert to Tree-IL (which is probably easier) and let Tree-IL convert to
CPS later.  Going through Tree-IL has the advantage of running Tree-IL
optimization passes, like partial evaluation.  Also, the compiler from
Tree-IL to CPS handles assignment conversion, in which assigned local
variables (in Tree-IL, locals that are @code{<lexical-set>}) are
converted to being boxed values on the heap.  @xref{Variables and the
VM}.

After CPS conversion, Guile runs some optimization passes.  The major
optimization performed on CPS is contification, in which functions that
are always called with the same continuation are incorporated directly
into a function's body.  This opens up space for more optimizations, and
turns procedure calls into @code{goto}.  It can also make loops out of
recursive function nests.

At the time of this writing (2014), most high-level optimization in
Guile is done on Tree-IL.  We would like to rewrite many of these passes
to operate on CPS instead, as it is easier to reason about CPS.

The rest of the optimization passes are really cleanups and
canonicalizations.  CPS spans the gap between high-level languages and
low-level bytecodes, which allows much of the compilation process to be
expressed as source-to-source transformations.  Such is the case for
closure conversion, in which references to variables that are free in a
function are converted to closure references, and in which functions are
converted to closures.  There are a few more passes to ensure that the
only primcalls left in the term are those that have a corresponding
instruction in the virtual machine, and that their continuations expect
the right number of values.

Finally, the backend of the CPS compiler emits bytecode for each
function, one by one.  To do so, it determines the set of live variables
at all points in the function.  Using this liveness information, it
allocates stack slots to each variable, such that a variable can live in
one slot for the duration of its lifetime, without shuffling.  (Of
course, variables with disjoint lifetimes can share a slot.)  Finally
the backend emits code, typically just one VM instruction, for each
continuation in the function.


@node Bytecode
@subsection Bytecode

As mentioned before, Guile compiles all code to bytecode, and that
bytecode is contained in ELF images.  @xref{Object File Format}, for
more on Guile's use of ELF.

To produce a bytecode image, Guile provides an assembler and a linker.

The assembler, defined in the @code{(system vm assembler)} module, has a
relatively straightforward imperative interface.  It provides a
@code{make-assembler} function to instantiate an assembler and a set of
@code{emit-@var{inst}} procedures to emit instructions of each kind.

The @code{emit-@var{inst}} procedures are actually generated at
compile-time from a machine-readable description of the VM.  With a few
exceptions for certain operand types, each operand of an emit procedure
corresponds to an operand of the corresponding instruction.

Consider @code{vector-length}, from @pxref{Miscellaneous Instructions}.
It is documented as:

@deftypefn Instruction {} vector-length u12:@var{dst} u12:@var{src}
@end deftypefn

Therefore the emit procedure has the form:

@deffn {Scheme Procedure} emit-vector-length asm dst src
@end deffn

All emit procedure take the assembler as their first argument, and
return no useful values.

The argument types depend on the operand types.  @xref{Instruction Set}.
Most are integers within a restricted range, though labels are generally
expressed as opaque symbols.

There are a few macro-instructions as well.

@deffn {Scheme Procedure} emit-label asm label
Define a label at the current program point.
@end deffn

@deffn {Scheme Procedure} emit-source asm source
Associate @var{source} with the current program point.
@end deffn

@deffn {Scheme Procedure} emit-cache-current-module! asm module scope
@deffnx {Scheme Procedure} emit-cached-toplevel-box asm dst scope sym bound?
@deffnx {Scheme Procedure} emit-cached-module-box asm dst module-name sym public? bound?
Macro-instructions to implement caching of top-level variables.  The
first takes the current module, in the slot @var{module}, and associates
it with a cache location identified by @var{scope}.  The second takes a
@var{scope}, and resolves the variable.  @xref{Top-Level Environment
Instructions}.  The last does not need a cached module, rather taking
the module name directly.
@end deffn

@deffn {Scheme Procedure} emit-load-constant asm dst constant
Load the Scheme datum @var{constant} into @var{dst}.
@end deffn

@deffn {Scheme Procedure} emit-begin-program asm label properties
@deffnx {Scheme Procedure} emit-end-program asm
Delimit the bounds of a procedure, with the given @var{label} and the
metadata @var{properties}.
@end deffn

@deffn {Scheme Procedure} emit-load-static-procedure asm dst label
Load a procedure with the given @var{label} into local @var{dst}.  This
macro-instruction should only be used with procedures without free
variables -- procedures that are not closures.
@end deffn

@deffn {Scheme Procedure} emit-begin-standard-arity asm req nlocals alternate
@deffnx {Scheme Procedure} emit-begin-opt-arity asm req opt rest nlocals alternate
@deffnx {Scheme Procedure} emit-begin-kw-arity asm req opt rest kw-indices allow-other-keys? nlocals alternate
@deffnx {Scheme Procedure} emit-end-arity asm
Delimit a clause of a procedure.
@end deffn

@deffn {Scheme Procedure} emit-br-if-symbol asm slot invert? label
@deffnx {Scheme Procedure} emit-br-if-variable asm slot invert? label
@deffnx {Scheme Procedure} emit-br-if-vector asm slot invert? label
@deffnx {Scheme Procedure} emit-br-if-string asm slot invert? label
@deffnx {Scheme Procedure} emit-br-if-bytevector asm slot invert? label
@deffnx {Scheme Procedure} emit-br-if-bitvector asm slot invert? label
TC7-specific test-and-branch instructions.  The TC7 is a 7-bit code that
is part of a heap object's type.  @xref{The SCM Type in Guile}.  Also,
@xref{Branch Instructions}.
@end deffn

The linker is a complicated beast.  Hackers interested in how it works
would do well do read Ian Lance Taylor's series of articles on linkers.
Searching the internet should find them easily.  From the user's
perspective, there is only one knob to control: whether the resulting
image will be written out to a file or not.  If the user passes
@code{#:to-file? #t} as part of the compiler options (@pxref{The Scheme
Compiler}), the linker will align the resulting segments on page
boundaries, and otherwise not.

@deffn {Scheme Procedure} link-assembly asm #:page-aligned?=#t
Link an ELF image, and return the bytevector.  If @var{page-aligned?} is
true, Guile will align the segments with different permissions on
page-sized boundaries, in order to maximize code sharing between
different processes.  Otherwise, padding is minimized, to minimize
address space consumption.
@end deffn

To write an image to disk, just use @code{put-bytevector} from
@code{(ice-9 binary-ports)}.

Compiling object code to the fake language, @code{value}, is performed
via loading objcode into a program, then executing that thunk with
respect to the compilation environment. Normally the environment
propagates through the compiler transparently, but users may specify the
compilation environment manually as well, as a module.  Procedures to
load images can be found in the @code{(system vm loader)} module:

@lisp
(use-modules (system vm loader))
@end lisp

@deffn {Scheme Variable} load-thunk-from-file file
@deffnx {C Function} scm_load_thunk_from_file (file)
Load object code from a file named @var{file}. The file will be mapped
into memory via @code{mmap}, so this is a very fast operation.
@end deffn

@deffn {Scheme Variable} load-thunk-from-memory bv
@deffnx {C Function} scm_load_thunk_from_memory (bv)
Load object code from a bytevector.  The data will be copied out of the
bytevector in order to ensure proper alignment of embedded Scheme
values.
@end deffn

Additionally there are procedures to find the ELF image for a given
pointer, or to list all mapped ELF images:

@deffn {Scheme Variable} find-mapped-elf-image ptr
Given the integer value @var{ptr}, find and return the ELF image that
contains that pointer, as a bytevector.  If no image is found, return
@code{#f}.  This routine is mostly used by debuggers and other
introspective tools.
@end deffn

@deffn {Scheme Variable} all-mapped-elf-images
Return all mapped ELF images, as a list of bytevectors.
@end deffn


@node Writing New High-Level Languages
@subsection Writing New High-Level Languages

In order to integrate a new language @var{lang} into Guile's compiler
system, one has to create the module @code{(language @var{lang} spec)}
containing the language definition and referencing the parser,
compiler and other routines processing it. The module hierarchy in
@code{(language brainfuck)} defines a very basic Brainfuck
implementation meant to serve as easy-to-understand example on how to
do this. See for instance @url{http://en.wikipedia.org/wiki/Brainfuck}
for more information about the Brainfuck language itself.


@node Extending the Compiler
@subsection Extending the Compiler

At this point we take a detour from the impersonal tone of the rest of
the manual.  Admit it: if you've read this far into the compiler
internals manual, you are a junkie.  Perhaps a course at your university
left you unsated, or perhaps you've always harbored a desire to hack the
holy of computer science holies: a compiler.  Well you're in good
company, and in a good position.  Guile's compiler needs your help.

There are many possible avenues for improving Guile's compiler.
Probably the most important improvement, speed-wise, will be some form
of native compilation, both just-in-time and ahead-of-time. This could
be done in many ways. Probably the easiest strategy would be to extend
the compiled procedure structure to include a pointer to a native code
vector, and compile from bytecode to native code at run-time after a
procedure is called a certain number of times.

The name of the game is a profiling-based harvest of the low-hanging
fruit, running programs of interest under a system-level profiler and
determining which improvements would give the most bang for the buck.
It's really getting to the point though that native compilation is the
next step.

The compiler also needs help at the top end, enhancing the Scheme that
it knows to also understand R6RS, and adding new high-level compilers.
We have JavaScript and Emacs Lisp mostly complete, but they could use
some love; Lua would be nice as well, but whatever language it is
that strikes your fancy would be welcome too.

Compilers are for hacking, not for admiring or for complaining about.
Get to it!