path: root/transform.e

~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ ~~ Code transformation facility ~~
~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~
~   The process of producing an executable binary out of Evocation involves
~ various bootstrapping phases during which code operates under different
~ constraints, and must be written with different styles. In some cases,
~ substantially the same code must be output multiple times in slightly
~ different ways, and it would be both arduous and verbose to write each of
~ these directly.
~
~   To solve this problem, this file implements a concept of code
~ transformation. There are two transforms, the label transform and the
~ log-load transform, each of which takes a string containing Evocation source
~ code and produces compiled code that has been modified to operate in a
~ specific way. The transforms rely on the label facility provided by
~ labels.e, and expect to run from within label-loop.
~
~   The label transform produces code that uses one label per word it defines,
~ to statically reference everything. Thus, when output to an executable
~ binary, this code will function without external dependencies. The tradeoff
~ is that it has no way to reference data that exists only at runtime.
~
~   The log-load transform relies on labels, but doesn't add any of its own.
~ It produces a compiled routine which, when run, dynamically looks up all the
~ references in the log, and appends the original code to the log. This adds
~ work that must be done when the runtime starts up, but the benefit is that
~ it can reference data that doesn't exist at compile-time. Most crucially,
~ it can reference the "here" and "latest" pointers in the log, which are
~ required for all the usual word-definition stuff to work, and whose
~ addresses are not known until runtime.
~
~   The log-load transform may also be useful for experimental tasks such as
~ creating additional, independent logs, or injecting Evocation into another
~ process's address space.
~
~   Please notice that both these transforms, in different ways, navigate the
~ same underlying design tension: The Forth compilation model hardcodes
~ references at the time compilation happens, and Evocation makes the choice
~ to not decide the address of the log until runtime. Thus the label transform
~ can't be sufficient on its own. Other Forths avoid this problem by
~ hardcoding an address for the log, or by using OS-provided load-time
~ symbol relocation. Evocation, however, does it on hard mode, mostly for fun.
~
~   Because it was clear from early on that the label transform couldn't stand
~ alone, and that another one would be necessary, we've refrained from adding
~ too many features to it. Since we have multiple transforms, they should each
~ be kept simple and well-defined, so that they can be composed in creative
~ new ways down the line. When adding additional behavior, always give thought
~ to whether it belongs in an existing transform or a new one.
~
~
~ About the label transform
~ ~~~~~~~~~~~~~~~~~~~~~~~~~
~
~   The label transform operates on code that compiles itself, and ensures
~ that the result of the compilation is suitable to be included in an
~ executable binary as words that are statically referenced by their
~ addresses. To achieve this, it causes each newly-defined word to have a
~ corresponding label whose value is the offset of its codeword, and it causes
~ all compiled invocations of other words to be resolved by using these labels.
~ The label transform is suitable for code that must be directly invoked by
~ the warm-start routine provided by execution.e.
~
~   The most fundamental technique the label transform performs is to separate
~ words that run in compile mode from words that run immediately.  There is no
~ distinction made between words running in immediate mode, and words declared
~ as immediate. Immediate words are looked up and executed based on their
~ "real", currently-executing definitions. Compiled words, including
~ literals, are looked up via the label facility.
~
~   Since the label facility is able to resolve forward references, there is
~ no hard requirement that everything in the file be topologically sorted.
~ However, the transform will refuse to create forward references to compiled
~ words. If you want them, you can create them by hand by calling use-label
~ yourself. This restriction is in place because allowing forward references
~ would be a significant difference from un-transformed code that could easily
~ become confusing, and because it simplifies the implementation a bit.
~
~   Compilation words do make extensive reference to the global variables
~ "here" and "latest". In particular, flow-control words such as if-else
~ expect the log to have recent compilation outputs on it, and to be able to
~ mutate them in-place. In order to make this work, we provide temporary
~ values of these two variables which point to the location of the output
~ buffer. This allows pointer resolution to work correctly without additional
~ effort, but notice that the buffer's address will differ from the address
~ the resulting program loads itself at. There's no simple way to avoid this
~ concern, since the variables must point to one of those addresses or the
~ other, not both.
~
~   We resolve the issue by running our own, alternate versions of the words
~ "create", ":", ";", and ";asm" which use the label facility to compute the
~ addresses that will be needed at runtime. These alternates run instead of
~ the normal versions of these words. The code being compiled is responsible
~ for not doing anything else that would rely on "here" and "latest" matching
~ their runtime addresses, though it is otherwise allowed to modify and rely
~ on them in all the usual ways. The alternate versions are defined in this
~ file as their own words, "Lcreate", "L:", "L;", and "L;asm".
~ TODO note L@' and L!'
~
~   Note that these alternates are applied via a purely lexical
~ transformation: when a word would be looked up in the dictionary to
~ interpret, first check if it's one of these. That means the transformation
~ won't apply to indirect callers of these words, nor to tick-quotes of them.
~ The code being compiled is responsible for not doing either of those things.
~
~   Notably, the transformation uses the same "interpreter-flags" variable as
~ the rest of Evocation. There's no need to keep it separate like there is
~ with the other variables. This makes it easy to change modes.
~
~   The label transformation and its alternates rely on various labels, all of
~ which must be defined elsewhere, lest the label loop fail to converge:
~ "lit", "origin", "docol", "exit", ":", ";", and ";asm".
~
~   All of these limitations result in the compiled code being, in effect,
~ written in a dialect which is like Evocation, but more restricted. This is
~ acceptable, because the label transform is intended for compiling code that
~ is an early part of Evocation itself, and the necessary code has all been
~ written to follow these restrictions.
~
~
~ About the log-load transform
~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~
~   The log-load transform also operates on code that compiles itself; it
~ produces a compiled routine which, when run, appends the original code to
~ the log. As the routine is run, each reference to another word is resolved
~ by looking up the name of the target word in the log. Furthermore, these
~ lookups are done using log-load-find, defined in log-load.e, which accepts
~ a pointer to the log's base address as a parameter. See that file for more
~ explanation of what the log is and why it's important. Thus, unlike normal
~ accesses to the log, this routine doesn't rely on already having the log's
~ base address hardcoded into it at the time of its own compilation. The
~ log-load transform is suitable for implementing the core responsibilities of
~ the warm-start routine provided by execution.e, relying on only a few
~ specific words that it statically references via labels.
~
~   Much like the label transform, the log-load transform provides alternate
~ versions of certain immediate words used in word definition. Also like the
~ label transform, it provides its own copies of "here" and "latest".
~
~   The log-load transform provides alternates for a significantly broader set
~ of words than the label transform, including all the flow-control words such
~ as if-else. It runs its own alternates immediately, but unlike the label
~ transform, immediate execution for the log-load transform is not actually
~ immediate; it is compiled into words which will have those immediate effects
~ at the time the generated routine is run. The generated routine can itself
~ be thought of as a compilation process, producing its output on the log, so
~ doing things later for us still means doing them immediately during the
~ routine.
~
~   The log-load transform does impose a no-forward-references requirement,
~ though it is applied at the time the routine is run, rather than at the time
~ of the transformation.
~
~   The log-load transformation and its alternates rely on the following
~ labels, all of which must be defined elsewhere: TODO


~ Buffer- and address-management helpers
~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~
~   The facilities in this section are used as helper code in the
~ implementations of both transforms.

~ TODO all this buffer stuff should be in its own file
~ (buffer size -- buffer address)
: read-to-buffer
  dup allocate dup dup
  ~ (buffer size, buffer address, word start, output point)
  { key
    ~ Exit if it's a zero byte.
    dup not {
      ~ Make sure to pack the zero to serve as a null terminator.
      pack8
      drop drop swap drop exit } if
    dup is-space
      { ~ (buffer size, buffer address, word start, output point, key)
        ~ Tuck the key out of the way until we've done some stuff.
        3unroll
        ~ If it's a space character, first check if we just consumed the magic
        ~ word...
        2dup swap - 8 = dup {
          drop
          ~ Add a null terminator so we can use stringcmp
          dup 0 swap !
          ~ Check for the magic word
          over s" pyrzqxgl" stringcmp 0 =
          } if
        { ~ It's magic, so exit.
          ~ Make sure to pack a zero to serve as a null terminator.
          0 pack8
          drop drop drop swap drop exit }
        { ~ It's not magic, so reset the word start. Of course whitespace is
          ~ not a word but this will help us keep track of things.
          3roll pack8
          swap drop dup } if-else }
      { ~ (buffer size, buffer address, word start, output point, key)
        ~ Tuck the key out of the way again.
        3unroll
        ~ Check if the word just started and the previous character is space.
        2dup = dup { drop dup @ is-space } if
          { ~ If so, this is the actual first character of the word.
            drop swap pack8 dup }
          { ~ If not, leave the word start alone.
            3roll pack8 } if-else } if-else } forever ;


~   In logical terms, this modifies an input buffer metadata structure
~ in-place to push a new, zeroed one into the start of the linked list formed
~ through the next-source field.
~
~   In physical terms, it works by allocating a new structure, copying the
~ fields of the existing one into it, and zeroing the existing one. That's
~ necessary because otherwise we'd need a mutable handle (a pointer to a
~ pointer) to update the start of the list, and there's no way to do that with
~ the main-input-buffer variable working the way it presently does.
~
~ (input buffer metadata pointer --)
: push-input-buffer
  allocate-input-buffer-metadata
  ~ (original metadata pointer, new metadata pointer)
  2dup swap 6 8 * memcopy
  ~ (original metadata pointer, new metadata pointer)
  swap dup zero-input-buffer-metadata
  input-buffer-next-source ! ;


~   This does the inverse of push-input-buffer. In the event that the
~ next-source field is null, it zeroes the buffer.
~
~   Note, however, that it doesn't deallocate the memory, because that's not
~ how memory allocation on the log works. If necessary, it can be deallocated
~ with "forget", though as usual that requires careful planning.
~
~ (input buffer metadata pointer --)
: pop-input-buffer
  dup input-buffer-next-source @
  ~ (original metadata pointer, next source metadata pointer)
  dup { 6 8 * memcopy }
      { drop zero-input-buffer-metadata } if-else ;


~ TODO rename this to transformation-state
: transform-state-saved-here ;
: transform-state-saved-latest 8 + ;
: transform-state-output-buffer-start 2 8 * + ;
: allocate-transform-state
  3 8 * allocate
  dup transform-state-saved-here 0 swap !
  dup transform-state-saved-latest 0 swap !
  dup transform-state-output-buffer-start 0 swap ! ;
allocate-transform-state s" transform-state" variable


~   When calling the label facility during a transformation, it's necessary
~ to use the real, non-wrapped "here" and "latest".
: swap-transform-variables
  here @ transform-state transform-state-saved-here @
  here ! transform-state transform-state-saved-here !
  latest @ transform-state transform-state-saved-latest @
  latest ! transform-state transform-state-saved-latest ! ;

~   We deal with a few address spaces. There's the "host" address space, the
~ space this process performing the compilation is using for itself. There's
~ the "target" address space, the address space that will exist later, when
~ the program we've compiled is running.
~
~   Then there's "offsets", which are relative to the start of the output
~ buffer. For clarity's sake, we always refer to these as offsets, rather than
~ as addresses.
~
~   When we define labels for compiled words, we set their values to be
~ offsets pointing to the generated codeword. This is done by "Lcreate". We
~ then need to convert them either to the host or the target address space,
~ depending on how we're using them.
~
~   There's no approach here that isn't confusing, but the hope is that by
~ using offsets, so that we always have to convert them regardless of what
~ we're doing with them, we won't miss a spot where conversion needs to
~ happen.
~
~ (output offset -- target address)
: offset-to-target-address-space
  ~ Don't transform null pointers.
  dup { swap-transform-variables L@' origin swap-transform-variables + } if ;

~ (target address -- output offset)
: target-address-space-to-offset
  ~ Don't transform null pointers.
  dup { swap-transform-variables L@' origin swap-transform-variables - } if ;

~ (output offset -- host address)
: offset-to-host-address-space
  ~ Don't transform null pointers
  dup { transform-state transform-state-output-buffer-start @ + } if ;

~ (host address --output offset)
: host-address-space-to-offset
  ~ Don't transform null pointers
  dup { transform-state transform-state-output-buffer-start @ - } if ;

~ (host address inside the output buffer -- target address)
: host-address-space-to-target
  host-address-space-to-offset offset-to-target-address-space ;

~ (target address -- host address)
: target-address-space-to-host
  target-address-space-to-offset offset-to-host-address-space ;


: describe-transformation
  ."    active here " here @ .hex64 space ." latest " latest @ .hex64 newline
  ."     saved here " transform-state transform-state-saved-here
  @ .hex64 space
  ." latest " transform-state transform-state-saved-latest @ .hex64 newline
  ."   output start " transform-state transform-state-output-buffer-start
  @ .hex64 newline ;


~ Label transform implementation
~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~
~   The following code is all part of implementing the label transform. For
~ conceptual overview, see the top of this file.


~   This is the alternate version of "create" for use with the label
~ transform. Its code is the same as the regular "create" except as noted
~ below. It is likely to be extremely useful to read and understand "create"
~ in interpret.e before attempting to understand "Lcreate".
: Lcreate
  dup stringlen 1 + dup 3unroll
  here @ 10 + 3unroll memmove
  here @

  ~   This value of "latest" is going into the generated output, so we need
  ~ to map it to the target address space. It's stored in the host address
  ~ space to make immediate words work as expected, so the appropriate
  ~ conversion is host-address-space-to-target.
  latest @ host-address-space-to-target pack64
  0 pack8
  0 pack8
  +
  8 packalign
  here @ latest !

  ~   Now we're immediately after the word header, which is where the codeword
  ~ will be. This is the value the label should taken on, so we set it.
  dup host-address-space-to-offset
  here @ 10 +
  swap-transform-variables
  intern-label set-label
  swap-transform-variables

  here ! ;


~   This is the alternate version of ":" for use with the label transform. Its
~ code is the same as the regular "create" except as noted below. It is likely
~ to be extremely useful to read and understand ":" in interpret.e before
~ attempting to understand "L:".
: L:
  ~ This calls "Lcreate" instead of "create".
  word value@ Lcreate dropstring

  ~ This looks up "docol" by label.
  swap-transform-variables
  L@' docol-codeword-value
  L@' origin
  swap-transform-variables
  + ,

  latest @ hide-entry ] ;


~   This is the alternate version of ";" for use with the label transform. Its
~ code is the same as the regular "create" except as noted below. It is likely
~ to be extremely useful to read and understand ";" in interpret.e before
~ attempting to understand "L;".
: L;
  ~ This looks up "exit" by label.
  swap-transform-variables
  L@' exit
  swap-transform-variables
  offset-to-target-address-space ,

  latest @ unhide-entry

  ~   Since [ is an immediate word, we have to go to extra trouble to compile
  ~ it as part of ;.
  [ ' [ entry-to-execution-token , ]
  ; make-immediate


~   This is the alternate version of ";asm" for use with the label transform.
~ Its code is the same as the regular "create" except as noted below. It is
~ likely to be extremely useful to read and understand ";asm" in interpret.e
~ before attempting to understand "L;asm".
: L;asm
  here @ pack-next 8 packalign here !
  latest @ dup unhide-entry entry-to-execution-token
  ~ The codeword needs to be transformed to the target address space.
  dup 8 + host-address-space-to-target
  swap !

  ~   Since [ is an immediate word, we have to go to extra trouble to compile
  ~ it as part of ;asm.
  [ ' [ entry-to-execution-token , ]
  ; make-immediate


~   Because docol requires it, we provide a special mini-version of the label
~ system. We only do L@' and L!', because that's all we need. These are real
~ labels; there can be arbitrarily many of them, and they can have forward
~ references.
~
~   The value that's accepted is in the host address space; the label is set
~ to an offset; and the value that's returned is in the target address space.
~
~ (-- value)
: label-L@'-alternate
  word value@

  swap-transform-variables
  intern-label
  use-label
  swap-transform-variables

  dropstring-with-result

  offset-to-target-address-space
  ; make-immediate


~ (value --)
: label-L!'-alternate
  host-address-space-to-offset

  word value@

  swap-transform-variables
  intern-label
  swap-transform-variables

  dropstring-with-result

  swap-transform-variables
  set-label
  swap-transform-variables
  ; make-immediate


~   This implements the label transform for a single word. It is directly
~ analogous to "interpret", and reading interpret.e may help in understanding
~ it, though it's meant to still make sense on its own.
~
~ It expects to be called from "label-transform", below, which loops.
~
~ (-- done)
: label-transform-one
  word

  ~ If no word was returned, exit.
  dup 0 = { drop 0 exit } if

  ~ The string is on the top of the stack, so to get a pointer to it we get
  ~ the stack address.
  ~ (string)
  value@

  ~ If it's the magic word, end the transformation.
  dup s" pyrzqxgl" stringcmp 0 = { drop dropstring 1 exit } if

  ~   Check whether it's one of the words we have alternates for, and look up
  ~ the alternate if so.
  dup 0 swap
  ~ (name as stack string, name pointer, placeholder, name pointer)
  dup s" create" stringcmp 0 = { swap drop ' Lcreate swap } if
  dup s" :" stringcmp 0 = { swap drop ' L: swap } if
  dup s" ;" stringcmp 0 = { swap drop ' L; swap } if
  dup s" ;asm" stringcmp 0 = { swap drop ' L;asm swap } if
  dup s" L@'" stringcmp 0 = { swap drop ' label-L@'-alternate swap } if
  dup s" L!'" stringcmp 0 = { swap drop ' label-L!'-alternate swap } if
  drop swap
  ~ (name as stack string, 0 or alternate entry pointer, name pointer)

  ~   If an alternate was found, the alternate will be used in immediate mode.
  ~ If not, we look up the word in the regular, non-transformed dictionary
  ~ and use that for immediate mode.
  over { dup
         transform-state transform-state-saved-latest @ swap find-in
         3roll drop swap } unless
  ~ (name as stack string, immediate entry pointer, name pointer)

  ~   In regular "interpret", we would check whether we found the word before
  ~ checking the mode. However, we have three different places words could
  ~ come from, so that's not a simple notion. So, we check the mode first.
  interpreter-flags @ 0x01 & {
    ~   If we're in compile mode, there's still a chance it's an immediate
    ~ word. First check whether we have an immediate entry, then if so, check
    ~ that entry's flags. Notice that this means the generated code can't
    ~ override an immediate word with a non-immediate word of the same name.
    over dup { entry-flags@ 0x01 & not } { not } if-else

    {
      ~   Either there was no immediate entry, or the immediate entry wasn't
      ~ flagged as an immediate word. So we check whether this could be a
      ~ compilation.
      ~
      ~   To do this, we need to look the word up in the output buffer. We
      ~ can't easily traverse the next-entry pointers in the output buffer's
      ~ dictionary, so we check the label. Since we don't know the word's name
      ~ statically, this is a rare scenario where we can't use the abbreviated
      ~ label syntax, but that's easy enough.
      ~
      ~   Even though we've ruled out the possibility that the word is only
      ~ ever used immediately, it is still possible that there's some reason
      ~ the word doesn't exist. In particular, it could be an integer literal.
      ~ If we were to call use-label first, that would count as a requirement
      ~ that the label must eventually be set. We don't want to require that
      ~ quite yet, so we call find-label.
      ~
      ~   This check is the means by which forward references are disallowed:
      ~ On the very first pass, a forward-referenced label won't exist yet, so
      ~ transform will give a "no such word" error, which in an ideal world
      ~ would prevent there from being a subsequent pass, but at the very
      ~ least it will ensure the output isn't a valid ELF.
      dup
      swap-transform-variables
      find-label
      swap-transform-variables
      {
        ~   It exists, so we declare our use of it (that's also the only way to
        ~ get a value for it).
        swap-transform-variables
        intern-label use-label
        swap-transform-variables

        ~   Labels point to codewords (because that's what "Lcreate" does),
        ~ which is already what we want to output.
        ~
        ~   An important caveat: Though it would require something weird to be
        ~ happening, such as a forced forward reference, the label may be
        ~ zero! We need to allow for that possibility by not examining the
        ~ contents of a nonexistent entry.
        ~
        ~   Fortunately we don't have to look at it, just append it to the log
        ~ and clean up.
        offset-to-target-address-space , drop dropstring 0 exit
      } if

      ~   If we got here, we're in compile mode, no label was found, and even
      ~ if there was a candidate for an immediate word it wasn't flagged as
      ~ immediate. There are two possibilities: It's genuinely missing, or it's
      ~ an integer literal. We decline to run the candidate immediate entry,
      ~ even if it exists, because that's not the correct semantics.
      ~
      ~   If the word is genuinely missing, we want to make sure we make it
      ~ all the way to the not-found error-handling code at the end, because
      ~ that will be way easier to debug than doing the wrong thing will. Way,
      ~ way easier. Far less staring at numbers.
      ~
      ~   Anyway, we no longer need the immediate entry pointer, so we drop
      ~ it.
      drop drop
    } {
      ~   If we get here, we're in compile mode, but there was a candidate
      ~ entry for an immediate word, and it was indeed flagged as immediate.
      ~ So, we run it and exit.
      drop dropstring-with-result entry-to-execution-token execute
      0 exit
    } if-else

    ~   This is the end of the compile-mode branch. As you can see by tracing
    ~ through all the above cases, if we got here, the two possibilities are
    ~ that the word is genuinely missing, or it's an integer literal.
    ~
    ~   Please notice that these are the same two possibilities remaining at
    ~ the end of the immediate-mode branch, below.
  } {
    ~   If we got here, we're in interpret mode. There are three
    ~ possibilities: there's an immediate word which we should run; it's an
    ~ integer literal; or the word is genuinely missing.
    ~
    ~   If the immediate entry pointer is non-zero, run it and exit.
    over {
      drop dropstring-with-result entry-to-execution-token execute
      0 exit
    } if

    ~   There was no immediate word, so either it's an integer literal or
    ~ the word is genuinely missing. Please notice that these are the same two
    ~ possibilities remaining at the end of the compile-mode branch, above.
    ~
    ~   We no longer need the immediate-mode pointer, so drop it.
    drop drop
  } if-else
  ~ (name as stack string)

  ~   If we got here, one of two things is true: the word is an integer
  ~ literal, or it's genuinely missing. We know this because both the mode
  ~ cases above end with these as the only two remaining possibilities. So
  ~
  ~   Check whether it's an integer literal. As before, we get the stack
  ~ address and use it as a string pointer.
  value@ read-integer 0 = {
    ~ It's a number.
    interpreter-flags @ 0x01 & {
      ~ We're in compile mode; append first "lit", then the number, to the
      ~ log. The version of "lit" we use is found by label, so it'll be the
      ~ one that exists when this code is ultimately run.
      dropstring-with-result

      ~ We look up "lit" as a label.
      swap-transform-variables L@' lit swap-transform-variables
      offset-to-target-address-space
      , ,
      0 exit
    } if

    ~ We're in interpret mode; push the number to the stack. Or at least, that's
    ~ what the code we're interpreting will see. Really it's already on the
    ~ stack, just clean everything else up and leave it there.
    dropstring-with-result
    0 exit
  } if

  ~   If it's neither in the dictionary nor a number, just print an error.
  ~
  ~   It's really important, when maintaining this code, to make sure that all
  ~ the possible ways the word can fail to exist, end up here. Doing anything
  ~ else is going to result in many hours of trying to untangle the
  ~ consequences of incorrect behavior, after-the-fact.
  s" No such word: " emitstring value@ emitstring newline dropstring 0 ;


~   This implements the label transform for all words in a region given as an
~ input string. It is directly analogous to "quit", in interpret.e, but is far
~ more complex.
~
~ (output buffer start, output point, input string pointer
~  -- output buffer start, output point)
: label-transform
  main-input-buffer dup push-input-buffer
  ~ TODO the arguments for this seem to be backwards from the documentation
  swap attach-string-to-input-buffer

  ~   Save the old values of "here" and "latest", and set the initial values
  ~ of the internal ones. These values need to persist across iterations,
  ~ since client code will make its own updates to them and then rely on those
  ~ updates having taken effect. So we do the swap just once, here outside the
  ~ loop, and set it back when the loop ends.
  here @ transform-state transform-state-saved-here !
  latest @ transform-state transform-state-saved-latest !
  over transform-state transform-state-output-buffer-start !
  here !
  0 latest !
  ~ Now the stack has nothing of ours on it, so client code can do its thing.

  ~   It's important that the stack has nothing of ours on it that persists
  ~ across iterations, so that client code can add and remove stuff there as
  ~ it sees fit.
  { label-transform-one
    ~ (done)

    ~  When the loop is done, get the real values of "here" and "latest"
    ~ back. The internal "here" is also the output point, and will become our
    ~ return value. The internal "latest" is discarded.
    { here @
      transform-state transform-state-saved-here @ here !
      transform-state transform-state-saved-latest @ latest !
      ~ (output point)

      ~   Though we don't actually use transform-state outside of this
      ~ invocation, for tidiness we zero it out.
      0 transform-state transform-state-saved-here !
      0 transform-state transform-state-saved-latest !
      0 transform-state transform-state-output-buffer-start !

      ~  Also put the input source back how it was.
      main-input-buffer pop-input-buffer

      exit } if } forever ;


~ Log-load transform implementation
~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~
~   The following code is all part of implementing the log-load transform.
~ For conceptual overview, see the top of this file.


~   This is the alternate version of "create" for use with the log-load
~ transform. This one is quite unlike the regular "create"; rather than
~ creating an entry on the log directly, its job is to output words which,
~ when they're later executed, will do create's job.
~
~   The implementations of log-load-find-execution-token and log-load-create
~ are in log-load.e.
~
~ (string pointer --)
: log-load-create-alternate
  swap-transform-variables
  ~   Looking these up in reverse order saves us some stack juggling. Does it
  ~ help readability, or hurt it? Who can say...
  L@' log-load-create
  L@' litstring
  swap-transform-variables
  offset-to-target-address-space ,     ~ litstring
  swap here @ swap packstring 8 packalign here !
  offset-to-target-address-space ,     ~ log-load-create
  ;


~   This is the alternate version of ":" for use with the log-load transform.
~ Its code is the same as the regular ":" except as noted below. It is likely
~ to be extremely useful to read and understand ":" in interpret.e before
~ attempting to understand "log-load-colon-alternate".
: log-load-colon-alternate
  ~ This calls "log-load-create" instead of "create".
  word value@ log-load-create-alternate dropstring

  ~   We generate code that looks up "docol" by name, runs it to get the
  ~ codeword pointer, then finally appends it to the entry.
  swap-transform-variables
  ~ As usual, we do these in reverse.
  L@' log-load-comma
  L@' execute
  L@' log-load-find-execution-token
  L@' litstring
  swap-transform-variables

  offset-to-target-address-space ,     ~ litstring
  here @ s" docol" packstring 8 packalign here !
  offset-to-target-address-space ,     ~ log-load-find-execution-token
  offset-to-target-address-space ,     ~ execute
  offset-to-target-address-space ,     ~ log-load-comma

  ~   This is where we would mark the entry hidden, but we don't do that. It
  ~ won't shadow anything and it won't be called until the entire log-load
  ~ routine has finished.

  ~   Switching between immediate and compile mode is one of the very few
  ~ things that happens NOW, while the log-load transform is actually running.
  ]
  ;


~   This is the alternate version of ";" for use with the log-load transform.
~ Its code is the same as the regular ";" except as noted below. It is
~ likely to be extremely useful to read and understand ";" in interpret.e
~ before attempting to understand "log-load-semicolon-alternate".
: log-load-semicolon-alternate
  ~   We generate code that looks up "exit" by name and appends it to the
  ~ entry.
  swap-transform-variables
  ~ As usual, we do these in reverse.
  L@' log-load-comma
  L@' log-load-find-execution-token
  L@' litstring
  swap-transform-variables

  offset-to-target-address-space ,     ~ litstring
  here @ s" exit" packstring 8 packalign here !
  offset-to-target-address-space ,     ~ log-load-find-execution-token
  offset-to-target-address-space ,     ~ log-load-comma

  ~ This is where we would unhide the entry, but again, we don't do that.

  ~   Since [ is an immediate word, we have to go to extra trouble to compile
  ~ it as part of ;.
  [ ' [ entry-to-execution-token , ]
  ; make-immediate


~   This is the alternate version of ";asm" for use with the log-load
~ transform. Its code is the same as the regular "create" except as noted
~ below. It is likely to be extremely useful to read and understand ";asm" in
~ interpret.e before attempting to understand "log-load;asm".
: log-load-semicolon-assembly-alternate
  ~ here @ pack-next 8 packalign here !
  ~ latest @ dup unhide-entry entry-to-execution-token
  ~ ~ The codeword needs to be transformed to the target address space.
  ~ dup 8 + host-address-space-to-target
  ~ swap !

  ~ ~   Since [ is an immediate word, we have to go to extra trouble to compile
  ~ ~ it as part of ;asm.
  ~ [ ' [ entry-to-execution-token , ]
  ; make-immediate


~   Because docol requires it, we provide a special mini-version of the label
~ system. We only do L@' and L!', because that's all we need. Unlike the
~ version of this feature for the label transform, for the log-load transform,
~ we heavily restrict the use-case.
~
~   The implementation strategy is that we ignore the label name, and store
~ the value on the stack when the generated log-load routine runs. So, each
~ instance of L@' must be closely followed by a matching instance of L!'. Each
~ label can only ever be used exactly once, and it must be a backward
~ reference. Furthermore, there is a very tight restriction on what can be
~ on the stack. The easiest way to explain it is by showing the interface of
~ these words from the transformed code's perspective:
~
~   L!' is (preserved value, value of label
~           -- value of label, preserved value)
~   L@' is (value of label, preserved value
~           -- preserved value, value of label)
~
~   The preserved value is simply another item on the stack, which the label
~ takes pains not to interfere with.
~
~   There is no adjustment done on the saved value, since it's created in the
~ target address space and then also used in the target address space. It
~ wouldn't actually be necessary to use this at all, since checking "here"
~ would be sufficient, but then the code would have to do something different
~ depending on which transform it's running under, and there'd have to be a
~ mechanism for that.
~
~   If that sounds super complex: All we actually do is read a label name,
~ ignore it, and output a call to swap.
~
~   This is sufficient to implement docol, and that's probably the only thing
~ it should be used for.
: log-load-L@'-alternate
  word dropstring

  swap-transform-variables
  L@' swap
  swap-transform-variables

  offset-to-target-address-space ,     ~ swap
  ; make-immediate

: log-load-L!'-alternate
  word dropstring

  swap-transform-variables
  L@' swap
  swap-transform-variables

  offset-to-target-address-space ,     ~ swap
  ; make-immediate

~   This implements the log-load transform for a single word. It is directly
~ analogous to "interpret", and reading interpret.e may help in understanding
~ it, though it's meant to still make sense on its own.
~
~ It expects to be called from "log-load-transform", below, which loops.
~
~ (-- done)
: log-load-transform-one
  word

  ~ If no word was returned, exit.
  dup 0 = { drop 0 exit } if

  ~ The string is on the top of the stack, so to get a pointer to it we get
  ~ the stack address.
  ~ (string)
  value@

  ~ If it's the magic word, end the transformation.
  dup s" pyrzqxgl" stringcmp 0 = { drop dropstring 1 exit } if

  ~   Check whether it's one of the words we have alternates for, and look up
  ~ the alternate if so.
  0 swap
  ~ (name as stack string, placeholder, name pointer)
  dup s" create" stringcmp 0 = {
    swap drop ' log-load-create-alternate swap } if
  dup s" :" stringcmp 0 = {
    swap drop ' log-load-colon-alternate swap } if
  dup s" ;" stringcmp 0 = {
    swap drop ' log-load-semicolon-alternate swap } if
  dup s" ;asm" stringcmp 0 = {
    swap drop ' log-load-semicolon-assembly-alternate swap } if
  dup s" L@'" stringcmp 0 = { swap drop ' log-load-L@'-alternate swap } if
  dup s" L!'" stringcmp 0 = { swap drop ' log-load-L!'-alternate swap } if
  drop
  ~ (name as stack string, 0 or alternate entry pointer)

  ~   If we have an alternate, we want to run that now, regardless of what
  ~ mode we're in. They're all flagged as immediate, but we don't even bother
  ~ checking, because it doesn't fully describe their behavior anyway. With
  ~ this transform there's three potential times at which we might execute
  ~ things, not two. The alternates are more immediate than immediate; they
  ~ run NOW, during the transformation.
  dup {
    dropstring-with-result entry-to-execution-token execute
    0 exit
  } if
  drop
  ~ (name as stack string)

  ~   Now we might have a compiled word, an immediate word, or an integer
  ~ literal. Recall that the word won't actually be looked up until the
  ~ routine we're producing is run - that's the whole point - so there's no
  ~ check we can perform now that will tell us whether the word we have exists
  ~ in the eventual log. Instead, we invert the usual fallback order and
  ~ check whether the word could be an integer literal. If it is, we'll
  ~ handle that; if not, we'll assume it'll eventually exist.
  ~
  ~   This means that code that's run with the log-load transform can't
  ~ shadow an integer literal with a word definition. Oh, so limiting.
  value@ read-integer 0 = {
    ~ It's a number.
    dropstring-with-result

    interpreter-flags @ 0x01 & {
      ~ We're in compile mode, so we want to generate code which will compile
      ~ the number.

      swap-transform-variables
      ~ Just like in log-load-create-alternate, we do these in reverse.
      L@' log-load-comma
      L@' log-load-comma
      L@' litstring
      swap-transform-variables

      offset-to-target-address-space ,     ~ litstring
      here @ s" lit" packstring 8 packalign here !
      offset-to-target-address-space ,     ~ log-load-comma
      swap ,                               ~ the value
      offset-to-target-address-space ,     ~ log-load-comma

      0 exit
    } if

    ~ We're in interpret mode, so we want to generate code which will push the
    ~ number to the stack.
    swap-transform-variables L@' lit swap-transform-variables
    offset-to-target-address-space , ,
    0 exit
  } if
  ~ (name as stack string)

  ~   We know it's a regular word, and we're assuming it will exist at
  ~ runtime. We of course have no way to check what flags it will have, which
  ~ means immediate words don't work with this transform. We still treat it
  ~ differently based on whether we're in compile mode.
  interpreter-flags @ 0x01 & {
    ~ We're in compile mode. We compile code that compiles the word.
    value@
    swap-transform-variables
    ~ Just like in log-load-create-alternate, we do these in reverse.
    L@' log-load-comma
    L@' log-load-find-execution-token
    L@' litstring
    swap-transform-variables

    offset-to-target-address-space ,     ~ litstring
    3roll here @ swap packstring 8 packalign here !
    offset-to-target-address-space ,     ~ log-load-find-execution-token
    offset-to-target-address-space ,     ~ log-load-comma
    dropstring 0 exit
  } if
  ~ (name as stack string)

  ~   We're in immediate mode. We compile code that runs the word immediately.
  ~ We check whether there's a label for the word; if there is, we'll output
  ~ that. Otherwise we'll output code that looks it up in the log and runs it.
  ~
  ~   Just like in label-transform, we use find-label to check whether a label
  ~ exists without declaring a dependency on it, then if it does, we do
  ~ use-label to ask for its value.
  ~
  ~   There's one additional wrinkle to remember here: We're running inside
  ~ the label loop, and warm-start appears before all the normal words in the
  ~ executable. So all the labels we'll be checking are forwared references,
  ~ and on the very first pass they definitely won't be defined. That's fine
  ~ though, they will exist on all subsequent passes, so things will
  ~ definitely still converge.
  ~
  ~   The first pass will never accidentally think it succeeded, because even
  ~ the reference to L' cold-start from the ELF header is a forward reference
  ~ and won't exist on the first pass.
  value@
  swap-transform-variables
  find-label
  swap-transform-variables
  {
    ~   Again just like in label-transform, we declare our use of the label
    ~ and get a value for it.
    value@
    swap-transform-variables
    intern-label use-label
    swap-transform-variables

    ~   Like in label-transform, this is a codeword pointer, so we just output
    ~ it directly. Also as before, because we don't have to examine it, we
    ~ don't have to do anything special in the case where it's zero due to the
    ~ way the label loop works.
    offset-to-target-address-space , dropstring 0 exit
    dropstring 0 exit
  } if

  ~   There's no label for the word; that means it wasn't statically
  ~ compiled-in to the target executable. So we output code that looks up the
  ~ word by name on the log, then calls it.
  value@
  swap-transform-variables
  ~ This is reverse order again.
  L@' execute
  L@' log-load-find-execution-token
  L@' litstring
  swap-transform-variables

  offset-to-target-address-space ,     ~ litstring
  3roll here @ swap packstring 8 packalign here !
  offset-to-target-address-space ,     ~ log-load-find-execution-token
  offset-to-target-address-space ,     ~ execute

  ~ There's no such thing as not finding the word, with this transform. So
  ~ we just exit.
  dropstring 0 ;


~   This implements the log-load transform for all words in a region given as
~ an input string. It is directly analogous to "quit", in interpret.e, but is
~ far more complex.
~
~ (output buffer start, output point, input string pointer
~  -- output buffer start, output point)
: log-load-transform
  main-input-buffer dup push-input-buffer
  ~ TODO the arguments for this seem to be backwards from the documentation
  swap attach-string-to-input-buffer

  ~   Save the old values of "here" and "latest", and set the initial values
  ~ of the internal ones. These values need to persist across iterations,
  ~ since client code will make its own updates to them and then rely on those
  ~ updates having taken effect. So we do the swap just once, here outside the
  ~ loop, and set it back when the loop ends.
  here @ transform-state transform-state-saved-here !
  latest @ transform-state transform-state-saved-latest !
  over transform-state transform-state-output-buffer-start !
  here !
  0 latest !
  ~ Now the stack has nothing of ours on it, so client code can do its thing.

  ~   It's important that the stack has nothing of ours on it that persists
  ~ across iterations, so that client code can add and remove stuff there as
  ~ it sees fit.
  { log-load-transform-one
    ~ (done)

    ~  When the loop is done, get the real values of "here" and "latest"
    ~ back. The internal "here" is also the output point, and will become our
    ~ return value. The internal "latest" is discarded.
    { here @
      transform-state transform-state-saved-here @ here !
      transform-state transform-state-saved-latest @ latest !
      ~ (output point)

      ~   Though we don't actually use transform-state outside of this
      ~ invocation, for tidiness we zero it out.
      0 transform-state transform-state-saved-here !
      0 transform-state transform-state-saved-latest !
      0 transform-state transform-state-output-buffer-start !

      ~  Also put the input source back how it was.
      main-input-buffer pop-input-buffer

      exit } if } forever ;