Fold base.git into ghc.git (re #8545)
[ghc.git] / docs / ghci / ghci.tex
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48 \begin{document}
50 \title{The GHCi Draft Design, round 2}
51 \author{MSR Cambridge Haskell Crew \\
52 Microsoft Research Ltd., Cambridge}
54 \maketitle
56 %%%\tableofcontents
57 %%%\newpage
59 %%-----------------------------------------------------------------%%
60 \section{Details}
62 \subsection{Outline of the design}
63 \label{sec:details-intro}
65 The design falls into three major parts:
66 \begin{itemize}
67 \item The compilation manager (CM), which coordinates the
68 system and supplies a HEP-like interface to clients.
69 \item The module compiler (@compile@), which translates individual
70 modules to interpretable or machine code.
71 \item The linker (@link@),
72 which maintains the executable image in interpreted mode.
73 \end{itemize}
75 There are also three auxiliary parts: the finder, which locates
76 source, object and interface files, the summariser, which quickly
77 finds dependency information for modules, and the static info
78 (compiler flags and package details), which is unchanged over the
79 course of a session.
81 This section continues with an overview of the session-lifetime data
82 structures. Then follows the finder (section~\ref{sec:finder}),
83 summariser (section~\ref{sec:summariser}),
84 static info (section~\ref{sec:staticinfo}),
85 and finally the three big sections
86 (\ref{sec:manager},~\ref{sec:compiler},~\ref{sec:linker})
87 on the compilation manager, compiler and linker respectively.
89 \subsubsection*{Some terminology}
91 Lifetimes: the phrase {\bf session lifetime} covers a complete run of
92 GHCI, encompassing multiple recompilation runs. {\bf Module lifetime}
93 is a lot shorter, being that of data needed to translate a single
94 module, but then discarded, for example Core, AbstractC, Stix trees.
96 Data structures with module lifetime are well documented and understood.
97 This document is mostly concerned with session-lifetime data.
98 Most of these structures are ``owned'' by CM, since that's
99 the only major component of GHCI which deals with session-lifetime
100 issues.
102 Modules and packages: {\bf home} refers to modules in this package,
103 precisely the ones tracked and updated by the compilation manager.
104 {\bf Package} refers to all other packages, which are assumed static.
106 \subsubsection*{A summary of all session-lifetime data structures}
108 These structures have session lifetime but not necessarily global
109 visibility. Subsequent sections elaborate who can see what.
110 \begin{itemize}
111 \item {\bf Home Symbol Table (HST)} (owner: CM) holds the post-renaming
112 environments created by compiling each home module.
113 \item {\bf Home Interface Table (HIT)} (owner: CM) holds in-memory
114 representations of the interface file created by compiling
115 each home module.
116 \item {\bf Unlinked Images (UI)} (owner: CM) are executable but as-yet
117 unlinked translations of home modules only.
118 \item {\bf Module Graph (MG)} (owner: CM) is the current module graph.
119 \item {\bf Static Info (SI)} (owner: CM) is the package configuration
120 information (PCI) and compiler flags (FLAGS).
121 \item {\bf Persistent Compiler State (PCS)} (owner: @compile@)
122 is @compile@'s private cache of information about package
123 modules.
124 \item {\bf Persistent Linker State (PLS)} (owner: @link@) is
125 @link@'s private information concerning the the current
126 state of the (in-memory) executable image.
127 \end{itemize}
130 %%-- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --%%
131 \subsection{The finder (\mbox{\tt type Finder})}
132 \label{sec:finder}
134 @Path@ could be an indication of a location in a filesystem, or it
135 could be some more generic kind of resource identifier, a URL for
136 example.
137 \begin{verbatim}
138 data Path = ...
139 \end{verbatim}
141 And some names. @Module@s are now used as primary keys for various
142 maps, so they are given a @Unique@.
143 \begin{verbatim}
144 type ModName = String -- a module name
145 type PkgName = String -- a package name
146 type Module = -- contains ModName and a Unique, at least
147 \end{verbatim}
149 A @ModLocation@ says where a module is, what it's called and in what
150 form it is.
151 \begin{verbatim}
152 data ModLocation = SourceOnly Module Path -- .hs
153 | ObjectCode Module Path Path -- .o, .hi
154 | InPackage Module PkgName
155 -- examine PCI to determine package Path
156 \end{verbatim}
158 The module finder generates @ModLocation@s from @ModName@s. We expect
159 it will assume packages to be static, but we want to be able to track
160 changes in home modules during the session. Specifically, we want to
161 be able to notice that a module's object and interface have been
162 updated, presumably by a compile run outside of the GHCI session.
163 Hence the two-stage type:
164 \begin{verbatim}
165 type Finder = ModName -> IO ModLocation
166 newFinder :: PCI -> IO Finder
167 \end{verbatim}
168 @newFinder@ examines the package information right at the start, but
169 returns an @IO@-typed function which can inspect home module changes
170 later in the session.
173 %%-- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --%%
174 \subsection{The summariser (\mbox{\tt summarise})}
175 \label{sec:summariser}
177 A @ModSummary@ records the minimum information needed to establish the
178 module graph and determine whose source has changed. @ModSummary@s
179 can be created quickly.
180 \begin{verbatim}
181 data ModSummary = ModSummary
182 ModLocation -- location and kind
183 (Maybe (String, Fingerprint))
184 -- source and fingerprint if .hs
185 (Maybe [ModName]) -- imports if .hs or .hi
187 type Fingerprint = ... -- file timestamp, or source checksum?
189 summarise :: ModLocation -> IO ModSummary
190 \end{verbatim}
192 The summary contains the location and source text, and the location
193 contains the name. We would like to remove the assumption that
194 sources live on disk, but I'm not sure this is good enough yet.
196 \ToDo{Should @ModSummary@ contain source text for interface files too?}
197 \ToDo{Also say that @ModIFace@ contains its module's @ModSummary@ (why?).}
200 %%-- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --%%
201 \subsection{Static information (SI)}
202 \label{sec:staticinfo}
204 PCI, the package configuration information, is a list of @PkgInfo@,
205 each containing at least the following:
206 \begin{verbatim}
207 data PkgInfo
208 = PkgInfo PkgName -- my name
209 Path -- path to my base location
210 [PkgName] -- who I depend on
211 [ModName] -- modules I supply
212 [Unlinked] -- paths to my object files
214 type PCI = [PkgInfo]
215 \end{verbatim}
216 The @Path@s in it, including those in the @Unlinked@s, are set up
217 when GHCI starts.
219 FLAGS is a bunch of compiler options. We haven't figured out yet how
220 to partition them into those for the whole session vs those for
221 specific source files, so currently the best we can do is:
222 \begin{verbatim}
223 data FLAGS = ...
224 \end{verbatim}
226 The static information (SI) is the both of these:
227 \begin{verbatim}
228 data SI = SI PCI
230 \end{verbatim}
234 %%-- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --%%
235 \subsection{The Compilation Manager (CM)}
236 \label{sec:manager}
238 \subsubsection{Data structures owned by CM}
240 CM maintains two maps (HST, HIT) and a set (UI). It's important to
241 realise that CM only knows about the map/set-ness, and has no idea
242 what a @ModDetails@, @ModIFace@ or @Linkable@ is. Only @compile@ and
243 @link@ know that, and CM passes these types around without
244 inspecting them.
246 \begin{itemize}
247 \item
248 {\bf Home Symbol Table (HST)} @:: FiniteMap Module ModDetails@
250 The @ModDetails@ (a couple of layers down) contain tycons, classes,
251 instances, etc, collectively known as ``entities''. Referrals from
252 other modules to these entities is direct, with no intervening
253 indirections of any kind; conversely, these entities refer directly
254 to other entities, regardless of module boundaries. HST only holds
255 information for home modules; the corresponding wired-up details
256 for package (non-home) modules are created on demand in the package
257 symbol table (PST) inside the persistent compiler's state (PCS).
259 CM maintains the HST, which is passed to, but not modified by,
260 @compile@. If compilation of a module is successful, @compile@
261 returns the resulting @ModDetails@ (inside the @CompResult@) which
262 CM then adds to HST.
264 CM throws away arbitrarily large parts of HST at the start of a
265 rebuild, and uses @compile@ to incrementally reconstruct it.
267 \item
268 {\bf Home Interface Table (HIT)} @:: FiniteMap Module ModIFace@
270 (Completely private to CM; nobody else sees this).
272 Compilation of a module always creates a @ModIFace@, which contains
273 the unlinked symbol table entries. CM maintains this @FiniteMap@
274 @ModName@ @ModIFace@, with session lifetime. CM never throws away
275 @ModIFace@s, but it does update them, by passing old ones to
276 @compile@ if they exist, and getting new ones back.
278 CM acquires @ModuleIFace@s from @compile@, which it only applies
279 to modules in the home package. As a result, HIT only contains
280 @ModuleIFace@s for modules in the home package. Those from other
281 packages reside in the package interface table (PIT) which is a
282 component of PCS.
284 \item
285 {\bf Unlinked Images (UI)} @:: Set Linkable@
287 The @Linkable@s in UI represent executable but as-yet unlinked
288 module translations. A @Linkable@ can contain the name of an
289 object, archive or DLL file. In interactive mode, it may also be
290 the STG trees derived from translating a module. So @compile@
291 returns a @Linkable@ from each successful run, namely that of
292 translating the module at hand.
294 At link-time, CM supplies @Linkable@s for the upwards closure of
295 all packages which have changed, to @link@. It also examines the
296 @ModSummary@s for all home modules, and by examining their imports
297 and the SI.PCI (package configuration info) it can determine the
298 @Linkable@s from all required imported packages too.
300 @Linkable@s and @ModIFace@s have a close relationship. Each
301 translated module has a corresponding @Linkable@ somewhere.
302 However, there may be @Linkable@s with no corresponding modules
303 (the RTS, for example). Conversely, multiple modules may share a
304 single @Linkable@ -- as is the case for any module from a
305 multi-module package. For these reasons it seems appropriate to
306 keep the two concepts distinct. @Linkable@s also provide
307 information about the sequence in which individual package
308 components should be linked, and that isn't the business of any
309 specific module to know.
311 CM passes @compile@ a module's old @ModIFace@, if it has one, in
312 the hope that the module won't need recompiling. If so, @compile@
313 can just return the new @ModDetails@ created from it, and CM will
314 re-use the old @ModIFace@. If the module {\em is} recompiled (or
315 scheduled to be loaded from disk), @compile@ returns both the
316 new @ModIFace@ and new @Linkable@.
318 \item
319 {\bf Module Graph (MG)} @:: known-only-to-CM@
321 Records, for CM's purposes, the current module graph,
322 up-to-dateness and summaries. More details when I get to them.
323 Only contains home modules.
324 \end{itemize}
325 Probably all this stuff is rolled together into the Persistent CM
326 State (PCMS):
327 \begin{verbatim}
329 emptyPCMS :: IO PCMS
330 \end{verbatim}
332 \subsubsection{What CM implements}
333 It pretty much implements the HEP interface. First, though, define a
334 containing structure for the state of the entire CM system and its
335 subsystems @compile@ and @link@:
336 \begin{verbatim}
337 data CmState
338 = CmState PCMS -- CM's stuff
339 PCS -- compile's stuff
340 PLS -- link's stuff
341 SI -- the static info, never changes
342 Finder -- the finder
343 \end{verbatim}
345 The @CmState@ is threaded through the HEP interface. In reality
346 this might be done using @IORef@s, but for clarity:
347 \begin{verbatim}
348 type ModHandle = ... (opaque to CM/HEP clients) ...
349 type HValue = ... (opaque to CM/HEP clients) ...
351 cmInit :: FLAGS
352 -> [PkgInfo]
353 -> IO CmState
355 cmLoadModule :: CmState
356 -> ModName
357 -> IO (CmState, Either [SDoc] ModHandle)
359 cmGetExpr :: ModHandle
360 -> CmState
361 -> String -> IO (CmState, Either [SDoc] HValue)
363 cmRunExpr :: HValue -> IO () -- don't need CmState here
364 \end{verbatim}
365 Almost all the huff and puff in this document pertains to @cmLoadModule@.
368 \subsubsection{Implementing \mbox{\tt cmInit}}
369 @cmInit@ creates an empty @CmState@ using @emptyPCMS@, @emptyPCS@,
370 @emptyPLS@, making SI from the supplied flags and package info, and
371 by supplying the package info the @newFinder@.
374 \subsubsection{Implementing \mbox{\tt cmLoadModule}}
376 \begin{enumerate}
377 \item {\bf Downsweep:} using @finder@ and @summarise@, chase from
378 the given module to
379 establish the new home module graph (MG). Do not chase into
380 package modules.
381 \item Remove from HIT, HST, UI any modules in the old MG which are
382 not in the new one. The old MG is then replaced by the new one.
383 \item Topologically sort MG to generate a bottom-to-top traversal
384 order, giving a worklist.
385 \item {\bf Upsweep:} call @compile@ on each module in the worklist in
386 turn, passing it
387 the ``correct'' HST, PCS, the old @ModIFace@ if
388 available, and the summary. ``Correct'' HST in the sense that
389 HST contains only the modules in the this module's downward
390 closure, so that @compile@ can construct the correct instance
391 and rule environments simply as the union of those in
392 the module's downward closure.
394 If @compile@ doesn't return a new interface/linkable pair,
395 compilation wasn't necessary. Either way, update HST with
396 the new @ModDetails@, and UI and HIT respectively if a
397 compilation {\em did} occur.
399 Keep going until the root module is successfully done, or
400 compilation fails.
402 \item If the previous step terminated because compilation failed,
403 define the successful set as those modules in successfully
404 completed SCCs, i.e. all @Linkable@s returned by @compile@ excluding
405 those from modules in any cycle which includes the module which failed.
406 Remove from HST, HIT, UI and MG all modules mentioned in MG which
407 are not in the successful set. Call @link@ with the successful
408 set,
409 which should succeed. The net effect is to back off to a point
410 in which those modules which are still aboard are correctly
411 compiled and linked.
413 If the previous step terminated successfully,
414 call @link@ passing it the @Linkable@s in the upward closure of
415 all those modules for which @compile@ produced a new @Linkable@.
416 \end{enumerate}
417 As a small optimisation, do this:
418 \begin{enumerate}
419 \item[3a.] Remove from the worklist any module M where M's source
420 hasn't changed and neither has the source of any module in M's
421 downward closure. This has the effect of not starting the upsweep
422 right at the bottom of the graph when that's not needed.
423 Source-change checking can be done quickly by CM by comparing
424 summaries of modules in MG against corresponding
425 summaries from the old MG.
426 \end{enumerate}
429 %%-- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --%%
430 \subsection{The compiler (\mbox{\tt compile})}
431 \label{sec:compiler}
433 \subsubsection{Data structures owned by \mbox{\tt compile}}
435 {\bf Persistent Compiler State (PCS)} @:: known-only-to-compile@
437 This contains info about foreign packages only, acting as a cache,
438 which is private to @compile@. The cache never becomes out of
439 date. There are three parts to it:
441 \begin{itemize}
442 \item
443 {\bf Package Interface Table (PIT)} @:: FiniteMap Module ModIFace@
445 @compile@ reads interfaces from modules in foreign packages, and
446 caches them in the PIT. Subsequent imports of the same module get
447 them directly out of the PIT, avoiding slow lexing/parsing phases.
448 Because foreign packages are assumed never to become out of date,
449 all contents of PIT remain valid forever. @compile@ of course
450 tries to find package interfaces in PIT in preference to reading
451 them from files.
453 Both successful and failed runs of @compile@ can add arbitrary
454 numbers of new interfaces to the PIT. The failed runs don't matter
455 because we assume that packages are static, so the data cached even
456 by a failed run is valid forever (ie for the rest of the session).
458 \item
459 {\bf Package Symbol Table (PST)} @:: FiniteMap Module ModDetails@
461 Adding an package interface to PIT doesn't make it directly usable
462 to @compile@, because it first needs to be wired (renamed +
463 typechecked) into the sphagetti of the HST. On the other hand,
464 most modules only use a few entities from any imported interface,
465 so wiring-in the interface at PIT-entry time might be a big time
466 waster. Also, wiring in an interface could mean reading other
467 interfaces, and we don't want to do that unnecessarily.
469 The PST avoids these problems by allowing incremental wiring-in to
470 happen. Pieces of foreign interfaces are copied out of the holding
471 pen (HP), renamed, typechecked, and placed in the PST, but only as
472 @compile@ discovers it needs them. In the process of incremental
473 renaming/typechecking, @compile@ may need to read more package
474 interfaces, which are added to the PIT and hence to
475 HP.~\ToDo{How? When?}
477 CM passes the PST to @compile@ and is returned an updated version
478 on both success and failure.
480 \item
481 {\bf Holding Pen (HP)} @:: HoldingPen@
483 HP holds parsed but not-yet renamed-or-typechecked fragments of
484 package interfaces. As typechecking of other modules progresses,
485 fragments are removed (``slurped'') from HP, renamed and
486 typechecked, and placed in PCS.PST (see above). Slurping a
487 fragment may require new interfaces to be read into HP. The hope
488 is, though, that many fragments will never get slurped, reducing
489 the total number of interfaces read (as compared to eager slurping).
491 \end{itemize}
493 PCS is opaque to CM; only @compile@ knows what's in it, and how to
494 update it. Because packages are assumed static, PCS never becomes
495 out of date. So CM only needs to be able to create an empty PCS,
496 with @emptyPCS@, and thence just passes it through @compile@ with
497 no further ado.
499 In return, @compile@ must promise not to store in PCS any
500 information pertaining to the home modules. If it did so, CM would
501 need to have a way to remove this information prior to commencing a
502 rebuild, which conflicts with PCS's opaqueness to CM.
507 \subsubsection{What {\tt compile} does}
508 @compile@ is necessarily somewhat complex. We've decided to do away
509 with private global variables -- they make the design specification
510 less clear, although the implementation might use them. Without
511 further ado:
512 \begin{verbatim}
513 compile :: SI -- obvious
514 -> Finder -- to find modules
515 -> ModSummary -- summary, including source
516 -> Maybe ModIFace
517 -- former summary, if avail
518 -> HST -- for home module ModDetails
519 -> PCS -- IN: the persistent compiler state
521 -> IO CompResult
523 data CompResult
524 = CompOK ModDetails -- new details (== HST additions)
525 (Maybe (ModIFace, Linkable))
526 -- summary and code; Nothing => compilation
527 -- not needed (old summary and code are still valid)
528 PCS -- updated PCS
529 [SDoc] -- warnings
531 | CompErrs PCS -- updated PCS
532 [SDoc] -- warnings and errors
534 data PCS
535 = MkPCS PIT -- package interfaces
536 PST -- post slurping global symtab contribs
537 HoldingPen -- pre slurping interface bits and pieces
539 emptyPCS :: IO PCS -- since CM has no other way to make one
540 \end{verbatim}
541 Although @compile@ is passed three of the global structures (FLAGS,
542 HST and PCS), it only modifies PCS. The rest are modified by CM as it
543 sees fit, from the stuff returned in the @CompResult@.
545 @compile@ is allowed to return an updated PCS even if compilation
546 errors occur, since the information in it pertains only to foreign
547 packages and is assumed to be always-correct.
549 What @compile@ does: \ToDo{A bit vague ... needs refining. How does
550 @finder@ come into the game?}
551 \begin{itemize}
552 \item Figure out if this module needs recompilation.
553 \begin{itemize}
554 \item If there's no old @ModIFace@, it does. Else:
555 \item Compare the @ModSummary@ supplied with that in the
556 old @ModIFace@. If the source has changed, recompilation
557 is needed. Else:
558 \item Compare the usage version numbers in the old @ModIFace@ with
559 those in the imported @ModIFace@s. All needed interfaces
560 for this should be in either HIT or PIT. If any version
561 numbers differ, recompilation is needed.
562 \item Otherwise it isn't needed.
563 \end{itemize}
565 \item
566 If recompilation is not needed, create a new @ModDetails@ from the
567 old @ModIFace@, looking up information in HST and PCS.PST as
568 necessary. Return the new details, a @Nothing@ denoting
569 compilation was not needed, the PCS \ToDo{I don't think the PCS
570 should be updated, but who knows?}, and an empty warning list.
572 \item
573 Otherwise, compilation is needed.
575 If the module is only available in object+interface form, read the
576 interface, make up details, create a linkable pointing at the
577 object code. \ToDo{Does this involve reading any more interfaces? Does
578 it involve updating PST?}
580 Otherwise, translate from source, then create and return: an
581 details, interface, linkable, updated PST, and warnings.
583 When looking for a new interface, search HST, then PCS.PIT, and only
584 then read from disk. In which case add the new interface(s) to
585 PCS.PIT.
587 \ToDo{If compiling a module with a boot-interface file, check the
588 boot interface against the inferred interface.}
589 \end{itemize}
592 \subsubsection{Contents of \mbox{\tt ModDetails},
593 \mbox{\tt ModIFace} and \mbox{\tt HoldingPen}}
594 Only @compile@ can see inside these three types -- they are opaque to
595 everyone else. @ModDetails@ holds the post-renaming,
596 post-typechecking environment created by compiling a module.
598 \begin{verbatim}
599 data ModDetails
600 = ModDetails {
601 moduleExports :: Avails
602 moduleEnv :: GlobalRdrEnv -- == FM RdrName [Name]
603 typeEnv :: FM Name TyThing -- TyThing is in TcEnv.lhs
604 instEnv :: InstEnv
605 fixityEnv :: FM Name Fixity
606 ruleEnv :: FM Id [Rule]
607 }
608 \end{verbatim}
610 @ModIFace@ is nearly the same as @ParsedIFace@ from @RnMonad.lhs@:
611 \begin{verbatim}
612 type ModIFace = ParsedIFace -- not really, but ...
613 data ParsedIface
614 = ParsedIface {
615 pi_mod :: Module, -- Complete with package info
616 pi_vers :: Version, -- Module version number
617 pi_orphan :: WhetherHasOrphans, -- Whether this module has orphans
618 pi_usages :: [ImportVersion OccName], -- Usages
619 pi_exports :: [ExportItem], -- Exports
620 pi_insts :: [RdrNameInstDecl], -- Local instance declarations
621 pi_decls :: [(Version, RdrNameHsDecl)], -- Local definitions
622 pi_fixity :: (Version, [RdrNameFixitySig]), -- Local fixity declarations,
623 -- with their version
624 pi_rules :: (Version, [RdrNameRuleDecl]), -- Rules, with their version
625 pi_deprecs :: [RdrNameDeprecation] -- Deprecations
626 }
627 \end{verbatim}
629 @HoldingPen@ is a cleaned-up version of that found in @RnMonad.lhs@,
630 retaining just the 3 pieces actually comprising the holding pen:
631 \begin{verbatim}
632 data HoldingPen
633 = HoldingPen {
634 iDecls :: DeclsMap, -- A single, global map of Names to decls
636 iInsts :: IfaceInsts,
637 -- The as-yet un-slurped instance decls; this bag is depleted when we
638 -- slurp an instance decl so that we don't slurp the same one twice.
639 -- Each is 'gated' by the names that must be available before
640 -- this instance decl is needed.
642 iRules :: IfaceRules
643 -- Similar to instance decls, only for rules
644 }
645 \end{verbatim}
647 %%-- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --%%
648 \subsection{The linker (\mbox{\tt link})}
649 \label{sec:linker}
651 \subsubsection{Data structures owned by the linker}
653 In the same way that @compile@ has a persistent compiler state (PCS),
654 the linker has a persistent (session-lifetime) state, PLS, the
655 Linker's Persistent State. In batch mode PLS is entirely irrelevant,
656 because there is only a single link step, and can be a unit value
657 ignored by everybody. In interactive mode PLS is composed of the
658 following three parts:
660 \begin{itemize}
661 \item
662 \textbf{The Source Symbol Table (SST)}@ :: FiniteMap RdrName HValue@
663 The source symbol table is used when linking interpreted code.
664 Unlinked interpreted code consists of an STG tree where
665 the leaves are @RdrNames@. The linker's job is to resolve these to
666 actual addresses (the alternative is to resolve these lazily when
667 the code is run, but this requires passing the full symbol table
668 through the interpreter and the repeated lookups will probably be
669 expensive).
671 The source symbol table therefore maps @RdrName@s to @HValue@s, for
672 every @RdrName@ that currently \emph{has} an @HValue@, including all
673 exported functions from object code modules that are currently
674 linked in. Linking therefore turns a @StgTree RdrName@ into an
675 @StgTree HValue@.
677 It is important that we can prune this symbol table by throwing away
678 the mappings for an entire module, whenever we recompile/relink a
679 given module. The representation is therefore probably a two-level
680 mapping, from module names, to function/constructor names, to
681 @HValue@s.
683 \item \textbf{The Object Symbol Table (OST)}@ :: FiniteMap String Addr@
684 This is a lower level symbol table, mapping symbol names in object
685 modules to their addresses in memory. It is used only when
686 resolving the external references in an object module, and contains
687 only entries that are defined in object modules.
689 Why have two symbol tables? Well, there is a clear distinction
690 between the two: the source symbol table maps Haskell symbols to
691 Haskell values, and the object symbol table maps object symbols to
692 addresses. There is some overlap, in that Haskell symbols certainly
693 have addresses, and we could look up a Haskell symbol's address by
694 manufacturing the right object symbol and looking that up in the
695 object symbol table, but this is likely to be slow and would force
696 us to extend the object symbol table with all the symbols
697 ``exported'' by interpreted code. Doing it this way enables us to
698 decouple the object management subsystem from the rest of the linker
699 with a minimal interface; something like
701 \begin{verbatim}
702 loadObject :: Unlinked -> IO Object
703 unloadModule :: Unlinked -> IO ()
704 lookupSymbol :: String -> IO Addr
705 \end{verbatim}
707 Rather unfortunately we need @lookupSymbol@ in order to populate the
708 source symbol table when linking in a new compiled module. Our
709 object management subsystem is currently written in C, so decoupling
710 this interface as much as possible is highly desirable.
712 \item
713 {\bf Linked Image (LI)} @:: no-explicit-representation@
715 LI isn't explicitly represented in the system, but we record it
716 here for completeness anyway. LI is the current set of
717 linked-together module, package and other library fragments
718 constituting the current executable mass. LI comprises:
719 \begin{itemize}
720 \item Machine code (@.o@, @.a@, @.DLL@ file images) in memory.
721 These are loaded from disk when needed, and stored in
722 @malloc@ville. To simplify storage management, they are
723 never freed or reused, since this creates serious
724 complications for storage management. When no longer needed,
725 they are simply abandoned. New linkings of the same object
726 code produces new copies in memory. We hope this not to be
727 too much of a space leak.
728 \item STG trees, which live in the GHCI heap and are managed by the
729 storage manager in the usual way. They are held alive (are
730 reachable) via the @HValue@s in the OST. Such @HValue@s are
731 applications of the interpreter function to the trees
732 themselves. Linking a tree comprises travelling over the
733 tree, replacing all the @Id@s with pointers directly to the
734 relevant @_closure@ labels, as determined by searching the
735 OST. Once the leaves are linked, trees are wrapped with the
736 interpreter function. The resulting @HValue@s then behave
737 indistinguishably from compiled versions of the same code.
738 \end{itemize}
739 Because object code is outside the heap and never deallocated,
740 whilst interpreted code is held alive via the HST, there's no need
741 to have a data structure which ``is'' the linked image.
743 For batch compilation, LI doesn't exist because OST doesn't exist,
744 and because @link@ doesn't load code into memory, instead just
745 invokes the system linker.
747 \ToDo{Do we need to say anything about CAFs and SRTs? Probably ...}
748 \end{itemize}
749 As with PCS, CM has no way to create an initial PLS, so we supply
750 @emptyPLS@ for that purpose.
752 \subsubsection{The linker's interface}
754 In practice, the PLS might be hidden in the I/O monad rather
755 than passed around explicitly. (The same might be true for PCS).
756 Anyway:
758 \begin{verbatim}
759 data PLS -- as described above; opaque to everybody except the linker
761 link :: PCI -> ??? -> [[Linkable]] -> PLS -> IO LinkResult
763 data LinkResult = LinkOK PLS
764 | LinkErrs PLS [SDoc]
766 emptyPLS :: IO PLS -- since CM has no other way to make one
767 \end{verbatim}
769 CM uses @link@ as follows:
771 After repeatedly using @compile@ to compile all modules which are
772 out-of-date, the @link@ is invoked. The @[[Linkable]]@ argument to
773 @link@ represents the list of (recursive groups of) home modules which
774 have been newly compiled, along with @Linkable@s for each of
775 the packages in use (the compilation manager knows which external
776 packages are referenced by the home package). The order of the list
777 is important: it is sorted in such a way that linking any prefix of
778 the list will result in an image with no unresolved references. Note
779 that for batch linking there may be further restrictions; for example
780 it may not be possible to link recursive groups containing libraries.
782 @link@ does the following:
784 \begin{itemize}
785 \item
786 In batch mode, do nothing. In interactive mode,
787 examine the supplied @[[Linkable]]@ to determine which home
788 module @Unlinked@s are new. Remove precisely these @Linkable@s
789 from PLS. (In fact we really need to remove their upwards
790 transitive closure, but I think it is an invariant that CM will
791 supply an upwards transitive closure of new modules).
792 See below for descriptions of @Linkable@ and @Unlinked@.
794 \item
795 Batch system: invoke the external linker to link everything in one go.
796 Interactive: bind the @Unlinked@s for the newly compiled modules,
797 plus those for any newly required packages, into PLS.
799 Note that it is the linker's responsibility to remember which
800 objects and packages have already been linked. By comparing this
801 with the @Linkable@s supplied to @link@, it can determine which
802 of the linkables in LI are out of date
803 \end{itemize}
805 If linking in of a group should fail for some reason, @link@ should
806 not modify its PLS at all. In other words, linking each group
807 is atomic; it either succeeds or fails.
809 \subsubsection*{\mbox{\tt Unlinked} and \mbox{\tt Linkable}}
811 Two important types: @Unlinked@ and @Linkable@. The latter is a
812 higher-level representation involving multiple of the former.
813 An @Unlinked@ is a reference to unlinked executable code, something
814 a linker could take as input:
816 \begin{verbatim}
817 data Unlinked = DotO Path
818 | DotA Path
819 | DotDLL Path
820 | Trees [StgTree RdrName]
821 \end{verbatim}
823 The first three describe the location of a file (presumably)
824 containing the code to link. @Trees@, which only exists in
825 interactive mode, gives a list of @StgTrees@, in which the unresolved
826 references are @RdrNames@ -- hence it's non-linkedness. Once linked,
827 those @RdrNames@ are replaced with pointers to the machine code
828 implementing them.
830 A @Linkable@ gathers together several @Unlinked@s and associates them
831 with either a module or package:
833 \begin{verbatim}
834 data Linkable = LM Module [Unlinked] -- a module
835 | LP PkgName -- a package
836 \end{verbatim}
838 The order of the @Unlinked@s in the list is important, as
839 they are linked in left-to-right order. The @Unlinked@ objects for a
840 particular package can be obtained from the package configuration (see
841 Section \ref{sec:staticinfo}).
843 \ToDo{When adding @Addr@s from an object module to SST, we need to
844 somehow find out the @RdrName@s of the symbols exported by that
845 module.
846 So we'd need to pass in the @ModDetails@ or @ModIFace@ or some such?}
850 %%-----------------------------------------------------------------%%
851 \section{Background ideas}
852 \subsubsection*{Out of date, but correct in spirit}
854 \subsection{Restructuring the system}
856 At the moment @hsc@ compiles one source module into C or assembly.
857 This functionality is pushed inside a function called @compile@,
858 introduced shortly. The main new chunk of code is CM, the compilation manager,
859 which supervises multiple runs of @compile@ so as to create up-to-date
860 translations of a whole bunch of modules, as quickly as possible.
861 CM also employs some minor helper functions, @finder@, @summarise@ and
862 @link@, to do its work.
864 Our intent is to allow CM to be used as the basis either of a
865 multi-module, batch mode compilation system, or to supply an
866 interactive environment similar to that of Hugs.
867 Only minor modifications to the behaviour of @compile@ and @link@
868 are needed to give these different behaviours.
870 CM and @compile@, and, for interactive use, an interpreter, are the
871 main code components. The most important data structure is the global
872 symbol table; much design effort has been expended thereupon.
875 \subsection{How the global symbol table is implemented}
877 The top level symbol table is a @FiniteMap@ @ModuleName@
878 @ModuleDetails@. @ModuleDetails@ contains essentially the environment
879 created by compiling a module. CM manages this finite map, adding and
880 deleting module entries as required.
882 The @ModuleDetails@ for a module @M@ contains descriptions of all
883 tycons, classes, instances, values, unfoldings, etc (henceforth
884 referred to as ``entities''), available from @M@. These are just
885 trees in the GHCI heap. References from other modules to these
886 entities is direct -- when you have a @TyCon@ in your hand, you really
887 have a pointer directly to the @TyCon@ structure in the defining module,
888 rather than some kind of index into a global symbol table. So there
889 is a global symbol table, but it has a distributed (sphagetti-like?)
890 nature.
892 This gives fast and convenient access to tycon, class, instance,
893 etc, information. But because there are no levels of indirection,
894 there's a problem when we replace @M@ with an updated version of @M@.
895 We then need to find all references to entities in the old @M@'s
896 sphagetti, and replace them with pointers to the new @M@'s sphagetti.
897 This problem motivates a large part of the design.
901 \subsection{Implementing incremental recompilation -- simple version}
902 Given the following module graph
903 \begin{verbatim}
904 D
905 / \
906 / \
907 B C
908 \ /
909 \ /
910 A
911 \end{verbatim}
912 (@D@ imports @B@ and @C@, @B@ imports @A@, @C@ imports @A@) the aim is to do the
913 least possible amount of compilation to bring @D@ back up to date. The
914 simplest scheme we can think of is:
915 \begin{itemize}
916 \item {\bf Downsweep}:
917 starting with @D@, re-establish what the current module graph is
918 (it might have changed since last time). This means getting a
919 @ModuleSummary@ of @D@. The summary can be quickly generated,
920 contains @D@'s import lists, and gives some way of knowing whether
921 @D@'s source has changed since the last time it was summarised.
923 Transitively follow summaries from @D@, thereby establishing the
924 module graph.
925 \item
926 Remove from the global symbol table (the @FiniteMap@ @ModuleName@
927 @ModuleDetails@) the upwards closure of all modules in this package
928 which are out-of-date with respect to their previous versions. Also
929 remove all modules no longer reachable from @D@.
930 \item {\bf Upsweep}:
931 Starting at the lowest point in the still-in-date module graph,
932 start compiling upwards, towards @D@. At each module, call
933 @compile@, passing it a @FiniteMap@ @ModuleName@ @ModuleDetails@,
934 and getting a new @ModuleDetails@ for the module, which is added to
935 the map.
937 When compiling a module, the compiler must be able to know which
938 entries in the map are for modules in its strict downwards closure,
939 and which aren't, so that it can manufacture the instance
940 environment correctly (as union of instances in its downwards
941 closure).
942 \item
943 Once @D@ has been compiled, invoke some kind of linking phase
944 if batch compilation. For interactive use, can either do it all
945 at the end, or as you go along.
946 \end{itemize}
947 In this simple world, recompilation visits the upwards closure of
948 all changed modules. That means when a module @M@ is recompiled,
949 we can be sure no-one has any references to entities in the old @M@,
950 because modules importing @M@ will have already been removed from the
951 top-level finite map in the second step above.
953 The upshot is that we don't need to worry about updating links to @M@ in
954 the global symbol table -- there shouldn't be any to update.
955 \ToDo{What about mutually recursive modules?}
957 CM will happily chase through module interfaces in other packages in
958 the downsweep. But it will only process modules in this package
959 during the upsweep. So it assumes that modules in other packages
960 never become out of date. This is a design decision -- we could have
961 decided otherwise.
963 In fact we go further, and require other packages to be compiled,
964 i.e. to consist of a collection of interface files, and one or more
965 source files. CM will never apply @compile@ to a foreign package
966 module, so there's no way a package can be built on the fly from source.
968 We require @compile@ to cache foreign package interfaces it reads, so
969 that subsequent uses don't have to re-read them. The cache never
970 becomes out of date, since we've assumed that the source of foreign
971 packages doesn't change during the course of a session (run of GHCI).
972 As well as caching interfaces, @compile@ must cache, in some sense,
973 the linkable code for modules. In batch compilation this might simply
974 mean remembering the names of object files to link, whereas in
975 interactive mode @compile@ probably needs to load object code into
976 memory in preparation for in-memory linking.
978 Important signatures for this simple scheme are:
979 \begin{verbatim}
980 finder :: ModuleName -> ModLocation
982 summarise :: ModLocation -> IO ModSummary
984 compile :: ModSummary
985 -> FM ModName ModDetails
986 -> IO CompileResult
988 data CompileResult = CompOK ModDetails
989 | CompErr [ErrMsg]
991 link :: [ModLocation] -> [PackageLocation] -> IO Bool -- linked ok?
992 \end{verbatim}
995 \subsection{Implementing incremental recompilation -- clever version}
997 So far, our upsweep, which is the computationally expensive bit,
998 recompiles a module if either its source is out of date, or it
999 imports a module which has been recompiled. Sometimes we know
1000 we can do better than this:
1001 \begin{verbatim}
1002 module B where module A
1003 import A ( f ) {-# NOINLINE f #-}
1004 ... f ... f x = x + 42
1005 \end{verbatim}
1006 If the definition of @f@ is changed to @f x = x + 43@, the simple
1007 upsweep would recompile @B@ unnecessarily. We would like to detect
1008 this situation and avoid propagating recompilation all the way to the
1009 top. There are two parts to this: detecting when a module doesn't
1010 need recompilation, and managing inter-module references in the
1011 global symbol table.
1013 \subsubsection*{Detecting when a module doesn't need recompilation}
1015 To do this, we introduce a new concept: the @ModuleIFace@. This is
1016 effectively an in-memory interface file. References to entities in
1017 other modules are done via strings, rather than being pointers
1018 directly to those entities. Recall that, by comparison,
1019 @ModuleDetails@ do contain pointers directly to the entities they
1020 refer to. So a @ModuleIFace@ is not part of the global symbol table.
1022 As before, compiling a module produces a @ModuleDetails@ (inside the
1023 @CompileResult@), but it also produces a @ModuleIFace@. The latter
1024 records, amongst things, the version numbers of all imported entities
1025 needed for the compilation of that module. @compile@ optionally also
1026 takes the old @ModuleIFace@ as input during compilation:
1027 \begin{verbatim}
1028 data CompileResult = CompOK ModDetails ModIFace
1029 | CompErr [ErrMsg]
1031 compile :: ModSummary
1032 -> FM ModName ModDetails
1033 -> Maybe ModuleIFace
1034 -> IO CompileResult
1035 \end{verbatim}
1036 Now, if the @ModuleSummary@ indicates this module's source hasn't
1037 changed, we only need to recompile it if something it depends on has
1038 changed. @compile@ can detect this by inspecting the imported entity
1039 version numbers in the module's old @ModuleIFace@, and comparing them
1040 with the version numbers from the entities in the modules being
1041 imported. If they are all the same, nothing it depends on has
1042 changed, so there's no point in recompiling.
1044 \subsubsection*{Managing inter-module references in the global symbol table}
1046 In the above example with @A@, @B@ and @f@, the specified change to @f@ would
1047 require @A@ but not @B@ to be recompiled. That generates a new
1048 @ModuleDetails@ for @A@. Problem is, if we leave @B@'s @ModuleDetails@
1049 unchanged, they continue to refer (directly) to the @f@ in @A@'s old
1050 @ModuleDetails@. This is not good, especially if equality between
1051 entities is implemented using pointer equality.
1053 One solution is to throw away @B@'s @ModuleDetails@ and recompile @B@.
1054 But this is precisely what we're trying to avoid, as it's expensive.
1055 Instead, a cheaper mechanism achieves the same thing: recreate @B@'s
1056 details directly from the old @ModuleIFace@. The @ModuleIFace@ will
1057 (textually) mention @f@; @compile@ can then find a pointer to the
1058 up-to-date global symbol table entry for @f@, and place that pointer
1059 in @B@'s @ModuleDetails@. The @ModuleDetails@ are, therefore,
1060 regenerated just by a quick lookup pass over the module's former
1061 @ModuleIFace@. All this applies, of course, only when @compile@ has
1062 concluded it doesn't need to recompile @B@.
1064 Now @compile@'s signature becomes a little clearer. @compile@ has to
1065 recompile the module, generating a fresh @ModuleDetails@ and
1066 @ModuleIFace@, if any of the following hold:
1067 \begin{itemize}
1068 \item
1069 The old @ModuleIFace@ wasn't supplied, for some reason (perhaps
1070 we've never compiled this module before?)
1071 \item
1072 The module's source has changed.
1073 \item
1074 The module's source hasn't changed, but inspection of @ModuleIFaces@
1075 for this and its imports indicates that an imported entity has
1076 changed.
1077 \end{itemize}
1078 If none of those are true, we're in luck: quickly knock up a new
1079 @ModuleDetails@ from the old @ModuleIFace@, and return them both.
1081 As a result, the upsweep still visits all modules in the upwards
1082 closure of those whose sources have changed. However, at some point
1083 we hopefully make a transition from generating new @ModuleDetails@ the
1084 expensive way (recompilation) to a cheap way (recycling old
1085 @ModuleIFaces@). Either way, all modules still get new
1086 @ModuleDetails@, so the global symbol table is correctly
1087 reconstructed.
1090 \subsection{How linking works, roughly}
1092 When @compile@ translates a module, it produces a @ModuleDetails@,
1093 @ModuleIFace@ and a @Linkable@. The @Linkable@ contains the
1094 translated but un-linked code for the module. And when @compile@
1095 ventures into an interface in package it hasn't seen so far, it
1096 copies the package's object code into memory, producing one or more
1097 @Linkable@s. CM keeps track of these linkables.
1099 Once all modules have been @compile@d, CM invokes @link@, supplying
1100 the all the @Linkable@s it knows about. If @compile@ had also been
1101 linking incrementally as it went along, @link@ doesn't have to do
1102 anything. On the other hand, @compile@ could choose not to be
1103 incremental, and leave @link@ to do all the work.
1105 @Linkable@s are opaque to CM. For batch compilation, a @Linkable@
1106 can record just the name of an object file, DLL, archive, or whatever,
1107 in which case the CM's call to @link@ supplies exactly the set of
1108 file names to be linked. @link@ can pass these verbatim to the
1109 standard system linker.
1114 %%-----------------------------------------------------------------%%
1115 \section{Ancient stuff}
1116 \subsubsection*{Should be selectively merged into ``Background ideas''}
1118 \subsection{Overall}
1119 Top level structure is:
1120 \begin{itemize}
1121 \item The Compilation Manager (CM) calculates and maintains module
1122 dependencies, and knows how create up-to-date object or bytecode
1123 for a given module. In doing so it may need to recompile
1124 arbitrary other modules, based on its knowledge of the module
1125 dependencies.
1126 \item On top of the CM are the ``user-level'' services. We envisage
1127 both a HEP-like interface, for interactive use, and an
1128 @hmake@ style batch compiler facility.
1129 \item The CM only deals with inter-module issues. It knows nothing
1130 about how to recompile an individual module, nor where the compiled
1131 result for a module lives, nor how to tell if
1132 a module is up to date, nor how to find the dependencies of a module.
1133 Instead, these services are supplied abstractly to CM via a
1134 @Compiler@ record. To a first approximation, a @Compiler@
1135 contains
1136 the same functionality as @hsc@ has had until now -- the ability to
1137 translate a single Haskell module to C/assembly/object/bytecode.
1139 Different clients of CM (HEP vs @hmake@) may supply different
1140 @Compiler@s, since they need slightly different behaviours.
1141 Specifically, HEP needs a @Compiler@ which creates bytecode
1142 in memory, and knows how to link it, whereas @hmake@ wants
1143 the traditional behaviour of emitting assembly code to disk,
1144 and making no attempt at linkage.
1145 \end{itemize}
1147 \subsection{Open questions}
1148 \begin{itemize}
1149 \item
1150 Error reporting from @open@ and @compile@.
1151 \item
1152 Instance environment management
1153 \item
1154 We probably need to make interface files say what
1155 packages they depend on (so that we can figure out
1156 which packages to load/link).
1157 \item
1158 CM is parameterised both by the client uses and the @Compiler@
1159 supplied. But it doesn't make sense to have a HEP-style client
1160 attached to a @hmake@-style @Compiler@. So, really, the
1161 parameterising entity should contain both aspects, not just the
1162 current @Compiler@ contents.
1163 \end{itemize}
1165 \subsection{Assumptions}
1167 \begin{itemize}
1168 \item Packages other than the "current" one are assumed to be
1169 already compiled.
1170 \item
1171 The "current" package is usually "MAIN",
1172 but we can set it with a command-line flag.
1173 One invocation of ghci has only one "current" package.
1174 \item
1175 Packages are not mutually recursive
1176 \item
1177 All the object code for a package P is in libP.a or libP.dll
1178 \end{itemize}
1180 \subsection{Stuff we need to be able to do}
1181 \begin{itemize}
1182 \item Create the environment in which a module has been translated,
1183 so that interactive queries can be satisfied as if ``in'' that
1184 module.
1185 \end{itemize}
1187 %%-----------------------------------------------------------------%%
1188 \section{The Compilation Manager}
1190 CM (@compilationManager@) is a functor, thus:
1191 \begin{verbatim}
1192 compilationManager :: Compiler -> IO HEP -- IO so that it can create
1193 -- global vars (IORefs)
1195 data HEP = HEP {
1196 load :: ModuleName -> IO (),
1197 compileString :: ModuleName -> String -> IO HValue,
1198 ....
1199 }
1201 newCompiler :: IO Compiler -- ??? this is a peer of compilationManager?
1203 run :: HValue -> IO () -- Run an HValue of type IO ()
1204 -- In HEP?
1205 \end{verbatim}
1207 @load@ is the central action of CM: its job is to bring a module and
1208 all its descendents into an executable state, by doing the following:
1209 \begin{enumerate}
1210 \item
1211 Use @summarise@ to descend the module hierarchy, starting from the
1212 nominated root, creating @ModuleSummary@s, and
1213 building a map @ModuleName@ @->@ @ModuleSummary@. @summarise@
1214 expects to be passed absolute paths to files. Use @finder@ to
1215 convert module names to file paths.
1216 \item
1217 Topologically sort the map,
1218 using dependency info in the @ModuleSummary@s.
1219 \item
1220 Clean up the symbol table by deleting the upward closure of
1221 changed modules.
1222 \item
1223 Working bottom to top, call @compile@ on the upward closure of
1224 all modules whose source has changed. A module's source has
1225 changed when @sourceHasChanged@ indicates there is a difference
1226 between old and new summaries for the module. Update the running
1227 @FiniteMap@ @ModuleName@ @ModuleDetails@ with the new details
1228 for this module. Ditto for the running
1229 @FiniteMap@ @ModuleName@ @ModuleIFace@.
1230 \item
1231 Call @compileDone@ to signify that we've reached the top, so
1232 that the batch system can now link.
1233 \end{enumerate}
1236 %%-----------------------------------------------------------------%%
1237 \section{A compiler}
1239 Most of the system's complexity is hidden inside the functions
1240 supplied in the @Compiler@ record:
1241 \begin{verbatim}
1242 data Compiler = Compiler {
1244 finder :: PackageConf -> [Path] -> IO (ModuleName -> ModuleLocation)
1246 summarise :: ModuleLocation -> IO ModuleSummary
1248 compile :: ModuleSummary
1249 -> Maybe ModuleIFace
1250 -> FiniteMap ModuleName ModuleDetails
1251 -> IO CompileResult
1253 compileDone :: IO ()
1254 compileStarting :: IO () -- still needed? I don't think so.
1255 }
1257 type ModuleName = String (or some such)
1258 type Path = String -- an absolute file name
1259 \end{verbatim}
1261 \subsection{The module \mbox{\tt finder}}
1262 The @finder@, given a package configuration file and a list of
1263 directories to look in, will map module names to @ModuleLocation@s,
1264 in which the @Path@s are filenames, probably with an absolute path
1265 to them.
1266 \begin{verbatim}
1267 data ModuleLocation = SourceOnly Path -- .hs
1268 | ObjectCode Path Path -- .o & .hi
1269 | InPackage Path -- .hi
1270 \end{verbatim}
1271 @SourceOnly@ and @ObjectCode@ are unremarkable. For sanity,
1272 we require that a module's object and interface be in the same
1273 directory. @InPackage@ indicates that the module is in a
1274 different package.
1276 @Module@ values -- perhaps all @Name@ish things -- contain the name of
1277 their package. That's so that
1278 \begin{itemize}
1279 \item Correct code can be generated for in-DLL vs out-of-DLL refs.
1280 \item We don't have version number dependencies for symbols
1281 imported from different packages.
1282 \end{itemize}
1284 Somehow or other, it will be possible to know all the packages
1285 required, so that the for the linker can load them.
1286 We could detect package dependencies by recording them in the
1287 @compile@r's @ModuleIFace@ cache, and with that and the
1288 package config info, figure out the complete set of packages
1289 to link. Or look at the command line args on startup.
1291 \ToDo{Need some way to tell incremental linkers about packages,
1292 since in general we'll need to load and link them before
1293 linking any modules in the current package.}
1296 \subsection{The module \mbox{\tt summarise}r}
1297 Given a filename of a module (\ToDo{presumably source or iface}),
1298 create a summary of it. A @ModuleSummary@ should contain only enough
1299 information for CM to construct an up-to-date picture of the
1300 dependency graph. Rather than expose CM to details of timestamps,
1301 etc, @summarise@ merely provides an up-to-date summary of any module.
1302 CM can extract the list of dependencies from a @ModuleSummary@, but
1303 other than that has no idea what's inside it.
1304 \begin{verbatim}
1305 data ModuleSummary = ... (abstract) ...
1307 depsFromSummary :: ModuleSummary -> [ModuleName] -- module names imported
1308 sourceHasChanged :: ModuleSummary -> ModuleSummary -> Bool
1309 \end{verbatim}
1310 @summarise@ is intended to be fast -- a @stat@ of the source or
1311 interface to see if it has changed, and, if so, a quick semi-parse to
1312 determine the new imports.
1314 \subsection{The module \mbox{\tt compile}r}
1315 @compile@ traffics in @ModuleIFace@s and @ModuleDetails@.
1317 A @ModuleIFace@ is an in-memory representation of the contents of an
1318 interface file, including version numbers, unfoldings and pragmas, and
1319 the linkable code for the module. @ModuleIFace@s are un-renamed,
1320 using @HsSym@/@RdrNames@ rather than (globally distinct) @Names@.
1322 @ModuleDetails@, by contrast, is an in-memory representation of the
1323 static environment created by compiling a module. It is phrased in
1324 terms of post-renaming @Names@, @TyCon@s, etc, so it's basically a
1325 renamed-to-global-uniqueness rendition of a @ModuleIFace@.
1327 In an interactive session, we'll want to be able to evaluate
1328 expressions as if they had been compiled in the scope of some
1329 specified module. This means that the @ModuleDetails@ must contain
1330 the type of everything defined in the module, rather than just the
1331 types of exported stuff. As a consequence, @ModuleIFace@ must also
1332 contain the type of everything, because it should always be possible
1333 to generate a module's @ModuleDetails@ from its @ModuleIFace@.
1335 CM maintains two mappings, one from @ModuleName@s to @ModuleIFace@s,
1336 the other from @ModuleName@s to @ModuleDetail@s. It passes the former
1337 to each call of @compile@. This is used to supply information about
1338 modules compiled prior to this one (lower down in the graph). The
1339 returned @CompileResult@ supplies a new @ModuleDetails@ for the module
1340 if compilation succeeded, and CM adds this to the mapping. The
1341 @CompileResult@ also supplies a new @ModuleIFace@, which is either the
1342 same as that supplied to @compile@, if @compile@ decided not to
1343 retranslate the module, or is the result of a fresh translation (from
1344 source). So these mappings are an explicitly-passed-around part of
1345 the global system state.
1347 @compile@ may also {\em optionally} also accumulate @ModuleIFace@s for
1348 modules in different packages -- that is, interfaces which we read,
1349 but never attempt to recompile source for. Such interfaces, being
1350 from foreign packages, never change, so @compile@ can accumulate them
1351 in perpetuity in a private global variable. Indeed, a major motivator
1352 of this design is to facilitate this caching of interface files,
1353 reading of which is a serious bottleneck for the current compiler.
1355 When CM restarts compilation down at the bottom of the module graph,
1356 it first needs to throw away all \ToDo{all?} @ModuleDetails@ in the
1357 upward closure of the out-of-date modules. So @ModuleDetails@ don't
1358 persist across recompilations. But @ModuleIFace@s do, since they
1359 are conceptually equivalent to interface files.
1362 \subsubsection*{What @compile@ returns}
1363 @compile@ returns a @CompileResult@ to CM.
1364 Note that the @compile@'s foreign-package interface cache can
1365 become augmented even as a result of reading interfaces for a
1366 compilation attempt which ultimately fails, although it will not be
1367 augmented with a new @ModuleIFace@ for the failed module.
1368 \begin{verbatim}
1369 -- CompileResult is not abstract to the Compilation Manager
1370 data CompileResult
1371 = CompOK ModuleIFace
1372 ModuleDetails -- compiled ok, here are new details
1373 -- and new iface
1375 | CompErr [SDoc] -- compilation gave errors
1377 | NoChange -- no change required, meaning:
1378 -- exports, unfoldings, strictness, etc,
1379 -- unchanged, and executable code unchanged
1380 \end{verbatim}
1384 \subsubsection*{Re-establishing local-to-global name mappings}
1385 Consider
1386 \begin{verbatim}
1387 module Upper where module Lower ( f ) where
1388 import Lower ( f ) f = ...
1389 g = ... f ...
1390 \end{verbatim}
1391 When @Lower@ is first compiled, @f@ is allocated a @Unique@
1392 (presumably inside an @Id@ or @Name@?). When @Upper@ is then
1393 compiled, its reference to @f@ is attached directly to the
1394 @Id@ created when compiling @Lower@.
1396 If the definition of @f@ is now changed, but not the type,
1397 unfolding, strictness, or any other thing which affects the way
1398 it should be called, we will have to recompile @Lower@, but not
1399 @Upper@. This creates a problem -- @g@ will then refer to the
1400 the old @Id@ for @f@, not the new one. This may or may not
1401 matter, but it seems safer to ensure that all @Unique@-based
1402 references into child modules are always up to date.
1404 So @compile@ recreates the @ModuleDetails@ for @Upper@ from
1405 the @ModuleIFace@ of @Upper@ and the @ModuleDetails@ of @Lower@.
1407 The rule is: if a module is up to date with respect to its
1408 source, but a child @C@ has changed, then either:
1409 \begin{itemize}
1410 \item On examination of the version numbers in @C@'s
1411 interface/@ModuleIFace@ that we used last time, we discover that
1412 an @Id@/@TyCon@/class/instance we depend on has changed. So
1413 we need to retranslate the module from its source, generating
1414 a new @ModuleIFace@ and @ModuleDetails@.
1415 \item Or: there's nothing in @C@'s interface that we depend on.
1416 So we quickly recreate a new @ModuleDetails@ from the existing
1417 @ModuleIFace@, creating fresh links to the new @Unique@-world
1418 entities in @C@'s new @ModuleDetails@.
1419 \end{itemize}
1421 Upshot: we need to redo @compile@ on all modules all the way up,
1422 rather than just the ones that need retranslation. However, we hope
1423 that most modules won't need retranslation -- just regeneration of the
1424 @ModuleDetails@ from the @ModuleIFace@. In effect, the @ModuleIFace@
1425 is a quickly-compilable representation of the module's contents, just
1426 enough to create the @ModuleDetails@.
1428 \ToDo{Is there anything in @ModuleDetails@ which can't be
1429 recreated from @ModuleIFace@ ?}
1431 So the @ModuleIFace@s persist across calls to @HEP.load@, whereas
1432 @ModuleDetails@ are reconstructed on every compilation pass. This
1433 means that @ModuleIFace@s have the same lifetime as the byte/object
1434 code, and so should somehow contain their code.
1436 The behind-the-scenes @ModuleIFace@ cache has some kind of holding-pen
1437 arrangement, to lazify the copying-out of stuff from it, and thus to
1438 minimise redundant interface reading. \ToDo{Burble burble. More
1439 details.}.
1441 When CM starts working back up the module graph with @compile@, it
1442 needs to remove from the travelling @FiniteMap@ @ModuleName@
1443 @ModuleDetails@ the details for all modules in the upward closure of
1444 the compilation start points. However, since we're going to visit
1445 precisely those modules and no others on the way back up, we might as
1446 well just zap them the old @ModuleDetails@ incrementally. This does
1447 mean that the @FiniteMap@ @ModuleName@ @ModuleDetails@ will be
1448 inconsistent until we reach the top.
1450 In interactive mode, each @compile@ call on a module for which no
1451 object code is available, or for which it is out of date wrt source,
1452 emit bytecode into memory, update the resulting @ModuleIFace@ with the
1453 address of the bytecode image, and link the image.
1455 In batch mode, emit assembly or object code onto disk. Record
1456 somewhere \ToDo{where?} that this object file needs to go into the
1457 final link.
1459 When we reach the top, @compileDone@ is called, to signify that batch
1460 linking can now proceed, if need be.
1462 Modules in other packages never get a @ModuleIFace@ or @ModuleDetails@
1463 entry in CM's maps -- those maps are only for modules in this package.
1464 As previously mentioned, @compile@ may optionally cache @ModuleIFace@s
1465 for foreign package modules. When reading such an interface, we don't
1466 need to read the version info for individual symbols, since foreign
1467 packages are assumed static.
1469 \subsubsection*{What's in a \mbox{\tt ModuleIFace}?}
1471 Current interface file contents?
1474 \subsubsection*{What's in a \mbox{\tt ModuleDetails}?}
1476 There is no global symbol table @:: Name -> ???@. To look up a
1477 @Name@, first extract the @ModuleName@ from it, look that up in
1478 the passed-in @FiniteMap@ @ModuleName@ @ModuleDetails@,
1479 and finally look in the relevant @Env@.
1481 \ToDo{Do we still have the @HoldingPen@, or is it now composed from
1482 per-module bits too?}
1483 \begin{verbatim}
1484 data ModuleDetails = ModuleDetails {
1486 moduleExports :: what it exports (Names)
1487 -- roughly a subset of the .hi file contents
1489 moduleEnv :: RdrName -> Name
1490 -- maps top-level entities in this module to
1491 -- globally distinct (Uniq-ified) Names
1493 moduleDefs :: Bag Name -- All the things in the global symbol table
1494 -- defined by this module
1496 package :: Package -- what package am I in?
1498 lastCompile :: Date -- of last compilation
1500 instEnv :: InstEnv -- local inst env
1501 typeEnv :: Name -> TyThing -- local tycon env?
1502 }
1504 -- A (globally unique) symbol table entry. Note that Ids contain
1505 -- unfoldings.
1506 data TyThing = AClass Class
1507 | ATyCon TyCon
1508 | AnId Id
1509 \end{verbatim}
1510 What's the stuff in @ModuleDetails@ used for?
1511 \begin{itemize}
1512 \item @moduleExports@ so that the stuff which is visible from outside
1513 the module can be calculated.
1514 \item @moduleEnv@: \ToDo{umm err}
1515 \item @moduleDefs@: one reason we want this is so that we can nuke the
1516 global symbol table contribs from this module when it leaves the
1517 system. \ToDo{except ... we don't have a global symbol table any
1518 more.}
1519 \item @package@: we will need to chase arbitrarily deep into the
1520 interfaces of other packages. Of course we don't want to
1521 recompile those, but as we've read their interfaces, we may
1522 as well cache that info. So @package@ indicates whether this
1523 module is in the default package, or, if not, which it is in.
1525 Also, when we come to linking, we'll need to know which
1526 packages are demanded, so we know to load their objects.
1528 \item @lastCompile@: When the module was last compiled. If the
1529 source is older than that, then a recompilation can only be
1530 required if children have changed.
1531 \item @typeEnv@: obvious??
1532 \item @instEnv@: the instances contributed by this module only. The
1533 Report allegedly says that when a module is translated, the
1534 available
1535 instance env is all the instances in the downward closure of
1536 itself in the module graph.
1538 We choose to use this simple representation -- each module
1539 holds just its own instances -- and do the naive thing when
1540 creating an inst env for compilation with. If this turns out
1541 to be a performance problem we'll revisit the design.
1542 \end{itemize}
1546 %%-----------------------------------------------------------------%%
1547 \section{Misc text looking for a home}
1549 \subsection*{Linking}
1551 \ToDo{All this linking stuff is now bogus.}
1553 There's an abstract @LinkState@, which is threaded through the linkery
1554 bits. CM can call @addpkgs@ to notify the linker of packages
1555 required, and it can call @addmods@ to announce modules which need to
1556 be linked. Finally, CM calls @endlink@, after which an executable
1557 image should be ready. The linker may link incrementally, during each
1558 call of @addpkgs@ and @addmods@, or it can just store up names and do
1559 all the linking when @endlink@ is called.
1561 In order that incremental linking is possible, CM should specify
1562 packages and module groups in dependency order, ie, from the bottom up.
1564 \subsection*{In-memory linking of bytecode}
1565 When being HEP-like, @compile@ will translate sources to bytecodes
1566 in memory, with all the bytecode for a module as a contiguous lump
1567 outside the heap. It needs to communicate the addresses of these
1568 lumps to the linker. The linker also needs to know whether a
1569 given module is available as in-memory bytecode, or whether it
1570 needs to load machine code from a file.
1572 I guess @LinkState@ needs to map module names to base addresses
1573 of their loaded images, + the nature of the image, + whether or not
1574 the image has been linked.
1576 \subsection*{On disk linking of object code, to give an executable}
1577 The @LinkState@ in this case is just a list of module and package
1578 names, which @addpkgs@ and @addmods@ add to. The final @endlink@
1579 call can invoke the system linker.
1581 \subsection{Finding out about packages, dependencies, and auxiliary
1582 objects}
1584 Ask the @packages.conf@ file that lives with the driver at the mo.
1586 \ToDo{policy about upward closure?}
1590 \ToDo{record story about how in memory linking is done.}
1592 \ToDo{linker start/stop/initialisation/persistence. Need to
1593 say more about @LinkState@.}
1596 \end{document}