The implementation of Racket CS (Racket on Chez Scheme) in this
directory is organized into two layers:

 * The immediate directory contains Scheme sources to implement Racket
   functionality on top of Chez Scheme. It references sibling
   directories like "expander" and "io", which contain Racket code
   that is compiled to Chez Scheme to implement Racket.

 * The "c" subdirectory contains C sources and build scripts to create
   wrapper executables that combine Chez Scheme with the Racket
   functionality implemented in this immediate directory.


========================================================================
 Bootstrapping
========================================================================

The Racket source distribution includes already-bootstrapped files to
build Racket CS. Some of those files are checked into the Racket Git
rrepository, and the rest are in a sibling "pb" repository that that
Racket repository's top-level makefile checks out.

Building Racket CS from original sources requires an existing Racket
build:

 * Chez Scheme is typically used to compile Chez Scheme boot files,
   but any recent version of Racket (v7.1 and up) can generate boot
   files for Chez Scheme.

   When you use `configure` as described in "../README.txt", supply
   `--enable-racket=...` to select a Racket implementation to build
   Racket CS boot files. That implementation could be one that is
   built by first using `configure --enable-bc`.

   Alternatively, boot files for the pb (portable bytecode) Chez
   Scheme variant can be used to compile Chez Scheme on any supported
   platform. The pb boot files must correspond to practically the same
   version of Chez Scheme as being built, though. The should be
   installed in the "../ChezScheme/boot/pb" directory as described by
   "../ChezScheme/BUILDING".

 * Racket is needed to generate the files in the "schemified"
   directory from the sources in sibling directories like "../io". The
   Racket version must be practically the same as the current Racket
   verson, although it can be the Racket BC implementation (instead of
   the Racket CS implementation).

   Unlike Chez Scheme boot files, the files generated in "schemified"
   are human-readable and -editable Scheme code. That possibilities
   provides a way out of bootstrapping black holes, even without BC.


========================================================================
 Development versus Build
========================================================================

The Racket CS implementation can be built and run in two different
ways: development mode for running directly using a Chez Scheme
installation, and build mode for creating a `racket` executable that
combines Chez Scheme and Racket functionality into a single
executable.

Development Mode
----------------

The makefile in this directory is set up for modifying the
implementation of Racket functionality (including in the sibling
directories, like "io") and testing it out on a Chez Scheme
installation.

For this development mode, either Chez Scheme needs to be installed as
`scheme`, or you must use `make SCHEME=...` to set the command for
`scheme`.

Development mode also needs a Racket installation with at least the
"compiler-lib" package installed. By default, the makefile looks for
Racket installed as "../../bin/racket"; if this directory is in a
clone of the Git repository for Racket, you can get "../../bin/racket"
with

      make PKGS="compiler-lib"

in the clone's root directory. Alternatively, use `make RACKET=...`
to set the command for `racket`.

The use of development mode is described in more detail further below.

Development mode currently doesn't work on Windows, because the
makefile makes too many Unix-ish assumptions.

Build Mode
----------

To build a Racket CS executable, the `configure` script and makefile
in "c" subdirectory are normally used via `configure` and `make` in
the parent directory of this one, as described in "../README.txt".
However, you can use them directly with something like

   cd [build]
   mkdir cs
   cd cs
   [here]/c/configure
   make
   make install

where [here] is the directory containing this "README.txt" and [build]
is a build directory (usually "../build" relative to [here]).

The resulting Racket CS executable is named "racket" by default. To
generate an executable with a "cs" suffix, supply `--enable-bcdefault`
to `configure` (which means that the name "racket" should be reserved
for Racket BC). The option to select the presence or absence of "cs"
also affects the location of ".zo" files, where they are written to a
subdirectory of "compiled" if a "cs" suffix is used.

Compilation on Windows does not use the `configure` script in "c".
Instead, from the directory "[here]\..\worksp", run "csbuild.rkt"
using an installed (minimal) Racket --- perhaps one created by running
"[here]\..\build.bat". The "csbuild.rkt" script puts some intermediate
files in "[here]\..\build".


========================================================================
 Machine Code versus JIT
========================================================================

Racket CS currently supports three compilation modes:

 * Machine-code mode --- The compiled form of a module is machine code
   generated by compiling either whole linklets (for small enough
   linklets) or functions within linklets (with a "bytecode"
   interpreter around the compiled parts).

   Select this mode by setting the `PLT_CS_MACH` environment variable,
   but it's currently the default.

   When the "cs" suffix is used for build mode, compiled ".zo" files
   in this mode are written to a subdirectory of "compiled" using the
   Chez Scheme platform name (e.g., "ta6osx").

   Set `PLT_CS_COMPILE_LIMIT` to set the maximum size of forms to
   compile before falling back to interpreted "bytecode". The default
   is 10000. Setting `PLT_CS_COMPILE_LIMIT` to 0 effectively turns
   the implementation into a pure interpreter.

 * Interpreter mode --- The compiled form of a module is a "bytecode"
   tree (not unlike Racket BC's bytecode) that is interpreted.

   Select this mode by setting the `PLT_CS_INTERP` environment
   variable. Alternatively, set `PLT_LINKLET_COMPILE_QUICK` when
   otherwise using machine-code mode (where the difference has to do
   with where compiled file are read and written in development mode).
   At the linklet API level, this mode implements the 'quick option to
   `compile-linklet` and similar functions.

   When the "cs" suffix is used for build mode, compiled ".zo" files
   in this mode are written to a "cs" subdirectory of "compiled".

   Interpreter mode is used automatically for large modules in
   machine-code mode, as controlled by `PLT_CS_COMPILE_LIMIT`. It is
   also used by default for compile-time code within a module while
   that same module is being expanded.

 * JIT mode --- The compiled form of a module is an S-expression where
   individual `lambda`s are compiled on demand.

   Select this mode by setting the `PLT_CS_JIT` environment variable.

   When the "cs" suffix is used for build mode, compiled ".zo" files
   in this mode are written to a "cs" subdirectory of "compiled".

Set the `PLT_ZO_PATH` environment variable to override the path used
for ".zo" files. For example, you may want to preserve a normal build
while also building in machine-code mode with `PLT_CS_DEBUG` set, in
which case setting `PLT_ZO_PATH` to something like "compiled/debug"
could be a good idea.


========================================================================
 Development Mode
========================================================================

Development mode is driven by the makefile in this directory.

Building
--------

Running `make` will build the Racket CS implementation. Use `make
expander-demo` to run a demo that loads `racket/base` from source.

Use `make setup` (or `make setup-v` for a verbose version) to build
".zo" files for collection-based libraries.

If you want to control the `raco setup` that `make setup` runs, supply
an `ARGS` variable to make, such as

   make setup ARGS="-l typed/racket"  # only sets up TR
   make setup ARGS="--clean -Dd"      # clears ".zo" files
   make setup ARGS="--fail-fast"      # stop at the first error

Running
-------

Use `make run ARGS="..."` to run Racket on Chez Scheme analogous to
running plain `racket`, where command-line arguments are supplied in
`ARGS`.

Structure
---------

The Racket CS implementation is organized in layers. The immediate
layer over Chez Scheme is called "Rumble", and it implements delimited
continuations, structures, chaperones and impersonators, engines (for
threads), and similar base functionality. The Rumble layer is
implemented in Chez Scheme.

The rest of the layers are implemented in Racket:

   thread
   io
   regexp
   schemify
   expander

Each of those layers is implemented in a sibling directory of this
one. Each layer is expanded (using "expander", of course) and then
compiled to Chez Scheme (using "schemify") to implement Racket.

The fully expanded form of each layer must not refer to any
functionality of previous layers. For example, the expander "thread"
must not refer to functionality implemented by "io", "regexp", or
"expander", while the expanded form of "io" must not refer to "regexp"
or "expander" functionality. Each layer can use `racket/base`
functionality, but beware that code from `racket/base` will be
duplicated in each layer.

The "io" layer relies on a shared library, rktio, to provide a uniform
interface to OS resources. The rktio source is in a "rktio" sibling
directory to this one.

Files in this directory:

 *.sls - Chez Scheme libraries that provide implementations of Racket
         primitives, building up to the Racket expander. The
         "rumble.sls" library is implemented directly in Chez Scheme.
         For most other cases, a corresponding "compiled/*.scm" file
         contains the implementation extracted from from expanded and
         flattened Racket code. Each "*.sls" file is built to "*.so".

 rumble/*.ss - Parts of "rumble.sls" (via `include`) to implement data
         structures, immutable hash tables, structs, delimited
         continuations, engines, impersonators, etc.

 linklet/*.ss - Parts of "linklet.sls" (via `include`).

 compiled/*.rktl (generated) - A Racket library (e.g., to implement
         regexps) that has been fully macro expanded and flattened
         into a linklet from its source in "../*". A linklet's only
         free variables are primitives that will be implemented by
         various ".sls" libraries in lower layers.

         For example, "../thread" contains the implementation (in
         Racket) of the thread and event subsystem.

 schemified/*.scm (generated) - A conversion from a ".rktl" file to be
         `included`d into an ".sls" library.

 ../build/so-rktio/rktio.rktl (generated) and
 ../../lib/librktio.{so,dylib,dll} (generated) - Created when building
         the "io" layer, the "rktio.rktl" file contains FFI descriptions
         that are `included` by "io.sls" and "librktio.{so,dylib,dll}"
         is the shared library that implements rktio.

         CAUTION: The makefile here doesn't track dependencies for
         rktio, so use `make rktio` if you change its implementation.

 primitive/*.ss - for "expander.sls", tables of bindings for
         primitive linklet instances; see "From primitives to modules"
         below for more information.

 convert.rkt - A "schemify"-based linklet-to-library-body compiler,
         which is used to convert a ".rktl" file to a ".scm" file to
         inclusion in an ".sls" library.

 demo/*.ss - Chez Scheme scripts to check that a library basically
         works. For example "demo/regexp.ss" runs the regexp matcher
         on a few examples. To run "demo/*.ss", use `make *-demo`.

 other *.rkt - Racket scripts like "convert.rkt" or comparisons like
         "demo/regexp.rkt". For example, you can run "demo/regexp.rkt"
         and compare the reported timing to "demo/regexp.ss".

From Primitives to Modules
--------------------------

The "expander" layer, as turned into a Chez Scheme library by
"expander.sls", synthesizes primitive Racket modules such as
`'#%kernel` and `'#%network`. The content of those primitive _modules_
at the expander layer is based on primitive _instances_ (which are just
hash tables) as populated by tables in the "primitive" directory. For
example, "primitive/network.scm" defines the content of the
`'#network` primitive instance, which is turned into the primitive
`'#%network` module by the expander layer, which is reexported by the
`racket/network` module that is implemented as plain Racket code. The
Racket implementation in "../racket" provides those same primitive
instances to the macro expander.

Running "demo/expander.ss"
--------------------------

A `make expander-demo` builds and tries the expander on simple
examples, including loading `racket/base` from source.

Dumping Linklets and Schemified Linklets
----------------------------------------

See "Inspecting Compiler Passes" in the Racket reference manual.

Safety and Debugging Mode
-------------------------

If you make changes to files in "rumble", you should turn off
`UNSAFE_COMP` in the makefile.

You may want to turn on `DEBUG_COMP` in the makefile, so that
backtraces provide expression-specific source locations instead of
just procedure-specific source locations. Enabling `DEBUG_COMP` makes
the Racket CS implementation take up twice as much memory and take
twice as long to load.

Turning on `DEBUG_COMP` affects only the Racket CS implementation. To
preserve per-expression locations on compiled Racket code, set
`PLT_CS_DEBUG`. See also "JIT versus Machine Code" for a suggestion on
setting `PLT_ZO_PATH`.

When you change "rumble" or other layers, you can continue to use
Racket modules that were previously compiled to ".zo" form... usually,
but inlining optimizations and similar compiler choices can break
compatibility.

FFI Differences
---------------

Compared to the Racket BC implementation, Racket CS's FFI behaves in
several different ways:

 * The `make-sized-byte-string` function always raises an exception,
   because a foreign address cannot be turned into a byte string whose
   content is stored in the foreign address. The options are to copy
   the foreign content to/from a byte string or use `ptr-ref` and
   `ptr-set!` to read and write at the address.

 * When `_bytes` is used as an argument type, beware that a byte
   string is not implicitly terminated with a NUL byte. When `_bytes`
   is used as a result type, the C result is copied into a fresh byte
   string.

 * A `_gcpointer` can only refer to the start of an allocated object,
   and never the interior of an 'atomic-interior allocation. Like
   Racket BC, `_gcpointer` is equivalent to `_pointer` for
   sending values to a foreign procedure, return values from a
   callback that is called from foreign code, or for `ptr-set!`. For
   the other direction (receiving a foreign result, `ptr-ref`, and
   receiving values in a callback), the received pointer must
   correspond to the content of a byte string or vector.

 * An immobile cell must be modified only through its original pointer
   or a reconstructed `_gcpointer`. If it is cast or reconstructed as
   a `_pointer`, setting the cell will not cooperate correctly with
   the garbage collector.

 * Memory allocated with 'nonatomic works only in limited ways. It
   cannot be usefully passed to foreign functions, since the layout is
   not actually an array of pointers.

Threads, Threads, Atomicity, Atomicity, and Atomicity
-----------------------------------------------------

Racket's thread layer does not use Chez Scheme threads. Chez Scheme
threads correspond to OS threads. Racket threads are implemented in
terms of engines at the Rumble layer. At the same time, futures and
places use Chez Scheme threads, and so parts of Rumble are meant to be
thread-safe with respect to Chez Scheme and OS threads. The FFI also
exposes elements of Chez Scheme / OS threads.

As a result of these layers, there are multiple ways to implement
atomic regions:

 * For critical sections with respect to Chez Scheme / OS threads, use
   a mutex or a spinlock.

   For example, `eq?` and `eqv?`-based hash tables can be accessed
   concurrently from Chez Scheme threads as long as they are guarded
   by a mutex or spinlock. In contrast, `equal?`-based hash table
   operations are not atomic from the Racket perspective, so they
   can't be locked by a mutex or spinlock; they use Racket-thread
   locks, instead.

   Chez Scheme deactivates a thread that is blocked on a mutex, as
   long as interrupts are not disabled, so you don't have to worry
   about waiting on a mutex blocking GCs. However, if a lock guards a
   value that is also used by a GC callback, then interrupts should be
   disabled before taking the lock to avoid deadlock.

 * For critical sections at the Racket level, there are multiple
   possibilities:

     - The Racket "thread" layer provides `start-atomic` and
       `end-atomic` to prevent Racket-thread swaps.

       These are the same operations as provided by
       `ffi/unsafe/atomic`.

     - Disabling Chez Scheme interrupts will also disable Racket
       thread swaps, since a thread swap via engines depends on a
       timer interrupt --- unless something explicitly blocks via the
       Racket thread scheduler, such as with `(sleep)`.

       Disable interrupts for atomicity only at the Rumble level where
       no Racket-level callbacks are not involved. Also, beware that
       disabling interrupts will prevent GC interrupts.

       The Racket "thread" layer provides `start-atomic/no-interrupts`
       and `end-atomic/no-interrupts` for both declaring atomicity at
       the Racket level and turning off Chez Scheme interrupts. The
       combination is useful for implementing functionality that might
       be called in response to a GC and might also be called by
       normal (non-atomic) code; the implementation of logging at the
       "io" layer might be the only use case.

     - The implementation of engines and continuations uses its own
       flag to protect regions where an engine timeout should not
       happen, such as when the metacontinuation is being manipulated.
       That flag is managed by `start-uninterrupted` and
       `end-uninterrupted` in "rumble/interrupt.ss".

       It may be tempting to use that flag for other purposes, as a
       cheap way to disable thread swaps. For now, don't do that.

Performance Notes
-----------------

The best-case scenario for performance is currently the default
configuration:

 * `UNSAFE_COMP` is enabled in "Makefile" --- currently on by default.

   Effectiveness: Can mean a 10-20% improvement in loading
   `racket/base` from source. Since the implementation is in pretty
   good shape, `UNSAFE_COMP` is enabled by default.

 * `DEBUG_COMP` not enabled --- or, if you enable it, run `make
   strip`.

   Effectiveness: Avoids increasing the load time for the Rumble and
   other layers by 30-50%.

 * `PLT_CS_DEBUG` not set --- an environment variable similar to
   `DEBUG_COMP`, but applies to code compiled by Racket CS.

   Effectiveness: Avoids improvement to stack traces, but also avoids
   increases load time and memory use of Racket programs by as much as
   50%.

Inlining Expectations
---------------------

Chez Scheme will inline small Rumble functions as long as the inlined
function body refers only to primitives and other identifiers that are
explicitly defined in the Rumble library body. The "explicily defined"
part can be tricky, particularly since inling will consider the
function body after other inlining is already applied. For example,
given

  (define (list? v)
    (or (null? v)
        (and (pair? v)
             (slow-list? v))))

  (define (slow-list? v)
    (let loop ([v v] [depth 0])
      ....))

then `list?` probably will not be inlined, because the call to
`slow-list?` within `list?` is likely to be inlined, so that `list?`
ends up calling the `loop` function nested within `slow-list?` (and
`loop` is not defined at the level of the library body).

The `$app/no-inline` primitive is useful to prevent unproductive
inlining, particularly to enable other, productive inlining. For
example,

  (define (list? v)
    (or (null? v)
        (and (pair? v)
             (#%$app/no-inline slow-list? v))))

is likely to make `list?` inlinable, since the reference to
`slow-list?` is preserved.

Chez Scheme will inline small Rumble functions independent of the
amount of inlining that has already happened at the call site of the
Rumble function. To prevent code explosion and endless inlining
cycles, however, it will not perform further inlining of a Rumble
function that is referenced by code introduced by inlining another
Rumble function.

Using a macro to force inlining can be ok, but that technique should
be used sparingly. Note that a reference to a primitive in a macro
will be compiled as a safe reference to the primitive, since the
conversion of a primitive reference to unsafe or not based on
`(optimize-level)` is an expansion-time operation. So, macros that are
meant to expand to uses of unsafe operations should refer to the
operations using `#3%`; beware that such a reference will stay unsafe,
even if `UNSAFE_COMP` is disabled in the makefile.
