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         Support for compilation into real machine code in CSL
         =====================================================

First note that this does not really exist yet, and the facilities and
ideas described here are part of an EXPERIMENT rather than stable
and viable reality.

An objective is that a single CSL image file should continue to work
properly with any version of the system. This means that if run on a
computer that uses a machine code where no hard-code compiler is available
the system should run all byte-coded. It also suggests that there should
be support for having multiple sets of machine code in the one image.

The first part of realising this is to arrange that there is a numeric
code value that is available at run-time and that identifies the current
machine architecture. Part of this is set up in the CSL sources in the
file "machine.h" by defining a symbol HARD_CODE_TAG, which is an integer
in the range 0 to 31 (if I ever get close to having more different sorts
of computer than that I will think some more!). The tag found in this way
can be qualified (in csl.c) by setting some of the 0xe0 bits to distinguish
between systems that "machine.h" does not. Eg this MIGHT be used to
distinguish between 486, Pentium and Pentium-Pro optimisations in an
Intel-targetted version, where different optimisations would be useful in
the three cases.  The value of hard_code_tag end up in the range 0 to 255
with 0 reserved to mean "no hard code facility available for this
architecture".  The value of this tag is available at run-time on the
features list (lispsystem!* in Standard Lisp mode) as an entry of the
form (native . nnn).

Hard coded functions live in a set of pages (hard_code_pages). In memory
there will be just one such set of pages, and for a FIRST version at least
their contents will be fixed and not subject to garbage collection. I
hope that eventually it will be possible to have a garbage collector for this
space though, and so the code in CSL makes provision for a second space
of hard code that the main one could be copied across into. In generating
machine code the possibility that even while machine code is being
executed a garbage collection might want to relocate it should be considered,
and for some architectures this thought may be sufficiently dreadful that
it must be avoided even if on other systems it is permitted.

In an image file there can be many different sets of hard code pages.
Because preserving a new image only wants to over-write one of these
(the one for the current machine type) these will not be stored as
part of the main initial image itself, but as items rather like "fasl"
modules. Well actually rather more like the way that help data is stored
in the image file. I will support the idea that for codes 1 to 255
the numeric code -1 to -255 can be passed in to the low-level file
manipulation code in "preserve.c" (just as special values are used for
IMAGE_CODE and HELP_CODE). At startup it can be checked whether a hard code
module of the relevant sort exists, and if so it can be loaded. The format in
the module will be a 2-byte integer showing how many pages are there followed
by dumps of the data to go in each such page. On doing a "preserve" to write
out a new checkpoint image the code will in effect preface the writing of the
root portion of the image with a "(faslout 'HardCode<nn>);
(write-out-the-hard-code)(faslend)" or whatever to put a copy of what is
in memory back into the module. Well, I guess the correct strategy will
be for it to do this if anything has been done that changes the set of
things compiled into machine code.

When hard code is loaded two more things have to be done. Firstly the
code may need to be relocated, both to allow for the address in memory
that it ends up at, and then to fix up places where it refers to entrypoints
into the rest of CSL and to statically allocated data. This will be done by
making the contents of a hard code page contain relevant relocation
information so that it can be scanned when first loaded and the code patched
up. The relocation and patch-up information needs to be designed so that
a single relocation program can deal with all the possible relocation modes
needed by various architectures, even though only a few will be implemented
to start with. Secondly it will be necessary to point actual entrypoints
into code towards the proper bits of compiled code. This last is achieved by
having (in the main heap image) a list (hard_code) that gives such
entrypoints. For every function that has been hard coded for at least one
architecture this has an entry, so overall its structure is:
       ((f-name-1 nargs . details-1) (f-name-2 nargs . details-2) ...)
The value nargs is an integer as used in symbol_set_definition (fns2.c)
and where each "details" is a list of items
       details = ((type-and-page offset . env) ...)
here type-and-page is an integer (a Lisp fixnum). The top 8 bits give the
machine type that this entry refers to. If these bits are zero we are
talking here about a byte-coded definition. Every function that is hard
compiled for some architecture must have a bytecoded definition stored here
(so it can be instated on systems where no hard code is available).
Non-zero values are used when genuine hard code is available. The next
2 bits give four possibilities. The obvious one is when the code-pointer
described here just goes into the natural function cell of the symbol and the
other two function calls get filled with default values. The remaining three
codes are used to make it possible to put a value into just the FN1, FN2 or
FNN call of the function. The bottom 4 bits of the fixnum are TAG_FIXNUM
to indicate that it is a small integer, so that leaves 18 over. These are
used to specify a page-number in the hard code heap. Since pages are
64K or (more usually) 256K bytes large having 18 bits of page selection is
comfortably generous for the moment.  Lisp integer "offset" is a byte
offset within the selected page. "env" is an arbitrary Lisp value (but very
often a vector) that will be placed in the environment cell when the function
is defined. Note that it may often (I hope) be that the same environment
vector will be used for several different machine architectures, and in
such cases the reference will be to a shared object, so space might not
be too badly wasted.

See relocate_hard_code() in restart.c and preserve_hard_code in preserve.c
for some more details.

The following Lisp functions may be used:

   (setq v (make-native n))    create handle on n bytes of native code space
   (putv-native v k w)         put byte w at offset k
   (getv-native v k)           retrieve byte (for checking)
   (putv-native v k w 1/2/4)   as putv-native but the trailing integer arg
   (getv-native v k 1/2/4)     .. says use 1, 2 or 4 byte value.
   (preserve)                  dumps all current native code for re-loading
   (native-address 'lispfn nargs)  get address of entrypoint for function
                               when called with n args
   (native-address n)          integer n selects an address to hand back
                               as an integer. See fns3.c for details

The native code as created must include (put there by the person
generating the code) relocation etc information.

   (symbol-set-native fname args bpsbase offset env)
fname must be a symbol.
(args & 0xff) is the number of args to the function. If other bits of args
being set tell the system NOT to set the other 2 function cells to error
calls, so USUALLY just use args=1,2 or 3.
bpsbase is the value returnned earlier by (make-native nnn). Bytes in it
must have been filled in by (native-putv ..) calls.
offset is the offset within this vector that the entrypoint should be set
to. The offset is needed because the first few bytes of the vector will need
to hold relocation information for when the code is re-loaded. At present
PLEASE start the contents of the vector at byte 8 leaving the first few bytes
untouched. Sometimes (later on) a function taking variable numbers of args
will also have several entrypoints into the same vector - another reason for
having the offset.
env is a thing to put in the environment cell of the function, and this will
be passed as the first argument to any call, so it will probably usefully
be a vector of literal Lisp objects that the function wants to use.

Problem: I maybe want to support cross-compilation of native code, and for
that the function symbol-set-native may need to be told what architecture
the relevant code has been created for?
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