1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <sect1 id="options-debugging">
3 <title>Debugging the compiler</title>
5 <indexterm><primary>debugging options (for GHC)</primary></indexterm>
7 <para>HACKER TERRITORY. HACKER TERRITORY. (You were warned.)</para>
9 <sect2 id="dumping-output">
10 <title>Dumping out compiler intermediate structures</title>
12 <indexterm><primary>dumping GHC intermediates</primary></indexterm>
13 <indexterm><primary>intermediate passes, output</primary></indexterm>
18 <option>-ddump-</option><replaceable>pass</replaceable>
19 <indexterm><primary><option>-ddump</option> options</primary></indexterm>
22 <para>Make a debugging dump after pass
23 <literal><pass></literal> (may be common enough to need
24 a short form…). You can get all of these at once
25 (<emphasis>lots</emphasis> of output) by using
26 <option>-v5</option>, or most of them with
27 <option>-v4</option>. Some of the most useful ones
33 <option>-ddump-parsed</option>:
34 <indexterm><primary><option>-ddump-parsed</option></primary></indexterm>
37 <para>parser output</para>
43 <option>-ddump-rn</option>:
44 <indexterm><primary><option>-ddump-rn</option></primary></indexterm>
47 <para>renamer output</para>
53 <option>-ddump-tc</option>:
54 <indexterm><primary><option>-ddump-tc</option></primary></indexterm>
57 <para>typechecker output</para>
63 <option>-ddump-types</option>:
64 <indexterm><primary><option>-ddump-types</option></primary></indexterm>
67 <para>Dump a type signature for each value defined at
68 the top level of the module. The list is sorted
69 alphabetically. Using <option>-dppr-debug</option>
70 dumps a type signature for all the imported and
71 system-defined things as well; useful for debugging the
78 <option>-ddump-deriv</option>:
79 <indexterm><primary><option>-ddump-deriv</option></primary></indexterm>
82 <para>derived instances</para>
88 <option>-ddump-ds</option>:
89 <indexterm><primary><option>-ddump-ds</option></primary></indexterm>
92 <para>desugarer output</para>
98 <option>-ddump-spec</option>:
99 <indexterm><primary><option>-ddump-spec</option></primary></indexterm>
102 <para>output of specialisation pass</para>
108 <option>-ddump-rules</option>:
109 <indexterm><primary><option>-ddump-rules</option></primary></indexterm>
112 <para>dumps all rewrite rules (including those generated
113 by the specialisation pass)</para>
119 <option>-ddump-simpl</option>:
120 <indexterm><primary><option>-ddump-simpl</option></primary></indexterm>
123 <para>simplifier output (Core-to-Core passes)</para>
129 <option>-ddump-inlinings</option>:
130 <indexterm><primary><option>-ddump-inlinings</option></primary></indexterm>
133 <para>inlining info from the simplifier</para>
139 <option>-ddump-cpranal</option>:
140 <indexterm><primary><option>-ddump-cpranal</option></primary></indexterm>
143 <para>CPR analyser output</para>
149 <option>-ddump-stranal</option>:
150 <indexterm><primary><option>-ddump-stranal</option></primary></indexterm>
153 <para>strictness analyser output</para>
159 <option>-ddump-cse</option>:
160 <indexterm><primary><option>-ddump-cse</option></primary></indexterm>
163 <para>CSE pass output</para>
169 <option>-ddump-workwrap</option>:
170 <indexterm><primary><option>-ddump-workwrap</option></primary></indexterm>
173 <para>worker/wrapper split output</para>
179 <option>-ddump-occur-anal</option>:
180 <indexterm><primary><option>-ddump-occur-anal</option></primary></indexterm>
183 <para>`occurrence analysis' output</para>
189 <option>-ddump-stg</option>:
190 <indexterm><primary><option>-ddump-stg</option></primary></indexterm>
193 <para>output of STG-to-STG passes</para>
199 <option>-ddump-flatC</option>:
200 <indexterm><primary><option>-ddump-flatC</option></primary></indexterm>
203 <para><emphasis>flattened</emphasis> Abstract C</para>
209 <option>-ddump-asm</option>:
210 <indexterm><primary><option>-ddump-asm</option></primary></indexterm>
213 <para>assembly language from the native-code generator</para>
219 <option>-ddump-bcos</option>:
220 <indexterm><primary><option>-ddump-bcos</option></primary></indexterm>
223 <para>byte code compiler output</para>
229 <option>-ddump-foreign</option>:
230 <indexterm><primary><option>-ddump-foreign</option></primary></indexterm>
233 <para>dump foreign export stubs</para>
242 <option>-ddump-simpl-iterations</option>:
243 <indexterm><primary><option>-ddump-simpl-iterations</option></primary></indexterm>
246 <para>Show the output of each <emphasis>iteration</emphasis>
247 of the simplifier (each run of the simplifier has a maximum
248 number of iterations, normally 4). Used when even
249 <option>-dverbose-simpl</option> doesn't cut it.</para>
255 <option>-ddump-simpl-stats</option>
256 <indexterm><primary><option>-ddump-simpl-stats option</option></primary></indexterm>
259 <para>Dump statistics about how many of each kind of
260 transformation too place. If you add
261 <option>-dppr-debug</option> you get more detailed
268 <option>-ddump-rn-trace</option>
269 <indexterm><primary><option>-ddump-rn-trace</option></primary></indexterm>
272 <para>Make the renamer be *real* chatty about what it is
279 <option>-ddump-rn-stats</option>
280 <indexterm><primary><option>-dshow-rn-stats</option></primary></indexterm>
283 <para>Print out summary of what kind of information the renamer
284 had to bring in.</para>
290 <option>-dverbose-core2core</option>
291 <indexterm><primary><option>-dverbose-core2core</option></primary></indexterm>
294 <option>-dverbose-stg2stg</option>
295 <indexterm><primary><option>-dverbose-stg2stg</option></primary></indexterm>
298 <para>Show the output of the intermediate Core-to-Core and
299 STG-to-STG passes, respectively. (<emphasis>Lots</emphasis>
300 of output!) So: when we're really desperate:</para>
303 % ghc -noC -O -ddump-simpl -dverbose-simpl -dcore-lint Foo.hs
311 <option>-dfaststring-stats</option>
312 <indexterm><primary><option>-dfaststring-stats</option></primary></indexterm>
315 <para>Show statistics for the usage of fast strings by the
322 <option>-dppr-debug</option>
323 <indexterm><primary><option>-dppr-debug</option></primary></indexterm>
326 <para>Debugging output is in one of several
327 “styles.” Take the printing of types, for
328 example. In the “user” style (the default), the
329 compiler's internal ideas about types are presented in
330 Haskell source-level syntax, insofar as possible. In the
331 “debug” style (which is the default for
332 debugging output), the types are printed in with explicit
333 foralls, and variables have their unique-id attached (so you
334 can check for things that look the same but aren't). This
335 flag makes debugging output appear in the more verbose debug
342 <option>-dppr-user-length</option>
343 <indexterm><primary><option>-dppr-user-length</option></primary></indexterm>
346 <para>In error messages, expressions are printed to a
347 certain “depth”, with subexpressions beyond the
348 depth replaced by ellipses. This flag sets the
355 <option>-dshow-unused-imports</option>
356 <indexterm><primary><option>-dshow-unused-imports</option></primary></indexterm>
359 <para>Have the renamer report what imports does not
366 <sect2 id="checking-consistency">
367 <title>Checking for consistency</title>
369 <indexterm><primary>consistency checks</primary></indexterm>
370 <indexterm><primary>lint</primary></indexterm>
376 <option>-dcore-lint</option>
377 <indexterm><primary><option>-dcore-lint</option></primary></indexterm>
380 <para>Turn on heavyweight intra-pass sanity-checking within
381 GHC, at Core level. (It checks GHC's sanity, not yours.)</para>
387 <option>-dstg-lint</option>:
388 <indexterm><primary><option>-dstg-lint</option></primary></indexterm>
391 <para>Ditto for STG level. (NOTE: currently doesn't work).</para>
399 <title>How to read Core syntax (from some <option>-ddump</option>
402 <indexterm><primary>reading Core syntax</primary></indexterm>
403 <indexterm><primary>Core syntax, how to read</primary></indexterm>
405 <para>Let's do this by commenting an example. It's from doing
406 <option>-ddump-ds</option> on this code:
409 skip2 m = m : skip2 (m+2)
412 Before we jump in, a word about names of things. Within GHC,
413 variables, type constructors, etc., are identified by their
414 “Uniques.” These are of the form `letter' plus
415 `number' (both loosely interpreted). The `letter' gives some idea
416 of where the Unique came from; e.g., <literal>_</literal>
417 means “built-in type variable”; <literal>t</literal>
418 means “from the typechecker”; <literal>s</literal>
419 means “from the simplifier”; and so on. The `number'
420 is printed fairly compactly in a `base-62' format, which everyone
421 hates except me (WDP).</para>
423 <para>Remember, everything has a “Unique” and it is
424 usually printed out when debugging, in some form or another. So
425 here we go…</para>
429 Main.skip2{-r1L6-} :: _forall_ a$_4 =>{{Num a$_4}} -> a$_4 -> [a$_4]
431 --# `r1L6' is the Unique for Main.skip2;
432 --# `_4' is the Unique for the type-variable (template) `a'
433 --# `{{Num a$_4}}' is a dictionary argument
437 --# `_NI_' means "no (pragmatic) information" yet; it will later
438 --# evolve into the GHC_PRAGMA info that goes into interface files.
441 /\ _4 -> \ d.Num.t4Gt ->
444 +.t4Hg :: _4 -> _4 -> _4
446 +.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt
448 fromInt.t4GS :: Int{-2i-} -> _4
450 fromInt.t4GS = (fromInt{-r3JX-} _4) d.Num.t4Gt
452 --# The `+' class method (Unique: r3JH) selects the addition code
453 --# from a `Num' dictionary (now an explicit lambda'd argument).
454 --# Because Core is 2nd-order lambda-calculus, type applications
455 --# and lambdas (/\) are explicit. So `+' is first applied to a
456 --# type (`_4'), then to a dictionary, yielding the actual addition
457 --# function that we will use subsequently...
459 --# We play the exact same game with the (non-standard) class method
460 --# `fromInt'. Unsurprisingly, the type `Int' is wired into the
470 } in fromInt.t4GS ds.d4Qz
472 --# `I# 2#' is just the literal Int `2'; it reflects the fact that
473 --# GHC defines `data Int = I# Int#', where Int# is the primitive
474 --# unboxed type. (see relevant info about unboxed types elsewhere...)
476 --# The `!' after `I#' indicates that this is a *saturated*
477 --# application of the `I#' data constructor (i.e., not partially
480 skip2.t3Ja :: _4 -> [_4]
484 let { ds.d4QQ :: [_4]
490 ds.d4QY = +.t4Hg m.r1H4 lit.t4Hb
491 } in skip2.t3Ja ds.d4QY
499 <para>(“It's just a simple functional language” is an
500 unregisterised trademark of Peyton Jones Enterprises, plc.)</para>
505 <title>Unregisterised compilation</title>
506 <indexterm><primary>unregisterised compilation</primary></indexterm>
508 <para>The term "unregisterised" really means "compile via vanilla
509 C", disabling some of the platform-specific tricks that GHC
510 normally uses to make programs go faster. When compiling
511 unregisterised, GHC simply generates a C file which is compiled
514 <para>Unregisterised compilation can be useful when porting GHC to
515 a new machine, since it reduces the prerequisite tools to
516 <command>gcc</command>, <command>as</command>, and
517 <command>ld</command> and nothing more, and furthermore the amount
518 of platform-specific code that needs to be written in order to get
519 unregisterised compilation going is usually fairly small.</para>
524 <option>-unreg</option>:
525 <indexterm><primary><option>-unreg</option></primary></indexterm>
528 <para>Compile via vanilla ANSI C only, turning off
529 platform-specific optimisations. NOTE: in order to use
530 <option>-unreg</option>, you need to have a set of libraries
531 (including the RTS) built for unregisterised compilation.
532 This amounts to building GHC with way "u" enabled.</para>
541 ;;; Local Variables: ***
543 ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter" "sect1") ***