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-splices</option>:
64 <indexterm><primary><option>-ddump-splices</option></primary></indexterm>
67 <para>Dump Template Haskell expressions that we splice in,
68 and what Haskell code the expression evaluates to.</para>
74 <option>-ddump-types</option>:
75 <indexterm><primary><option>-ddump-types</option></primary></indexterm>
78 <para>Dump a type signature for each value defined at
79 the top level of the module. The list is sorted
80 alphabetically. Using <option>-dppr-debug</option>
81 dumps a type signature for all the imported and
82 system-defined things as well; useful for debugging the
89 <option>-ddump-deriv</option>:
90 <indexterm><primary><option>-ddump-deriv</option></primary></indexterm>
93 <para>derived instances</para>
99 <option>-ddump-ds</option>:
100 <indexterm><primary><option>-ddump-ds</option></primary></indexterm>
103 <para>desugarer output</para>
109 <option>-ddump-spec</option>:
110 <indexterm><primary><option>-ddump-spec</option></primary></indexterm>
113 <para>output of specialisation pass</para>
119 <option>-ddump-rules</option>:
120 <indexterm><primary><option>-ddump-rules</option></primary></indexterm>
123 <para>dumps all rewrite rules specified in this module;
124 see <xref linkend="controlling-rules"/>.
131 <option>-ddump-rule-firings</option>:
132 <indexterm><primary><option>-ddump-rule-firings</option></primary></indexterm>
135 <para>dumps the names of all rules that fired in this module</para>
141 <option>-ddump-rule-rewrites</option>:
142 <indexterm><primary><option>-ddump-rule-rewrites</option></primary></indexterm>
145 <para>dumps detailed information about all rules that fired in
153 <option>-ddump-vect</option>:
154 <indexterm><primary><option>-ddump-vect</option></primary></indexterm>
157 <para>dumps the output of the vectoriser.
164 <option>-ddump-simpl</option>:
165 <indexterm><primary><option>-ddump-simpl</option></primary></indexterm>
168 <para>simplifier output (Core-to-Core passes)</para>
174 <option>-ddump-inlinings</option>:
175 <indexterm><primary><option>-ddump-inlinings</option></primary></indexterm>
178 <para>inlining info from the simplifier</para>
184 <option>-ddump-cpranal</option>:
185 <indexterm><primary><option>-ddump-cpranal</option></primary></indexterm>
188 <para>CPR analyser output</para>
194 <option>-ddump-stranal</option>:
195 <indexterm><primary><option>-ddump-stranal</option></primary></indexterm>
198 <para>strictness analyser output</para>
204 <option>-ddump-cse</option>:
205 <indexterm><primary><option>-ddump-cse</option></primary></indexterm>
208 <para>CSE pass output</para>
214 <option>-ddump-worker-wrapper</option>:
215 <indexterm><primary><option>-ddump-worker-wrapper</option></primary></indexterm>
218 <para>worker/wrapper split output</para>
224 <option>-ddump-occur-anal</option>:
225 <indexterm><primary><option>-ddump-occur-anal</option></primary></indexterm>
228 <para>`occurrence analysis' output</para>
234 <option>-ddump-prep</option>:
235 <indexterm><primary><option>-ddump-prep</option></primary></indexterm>
238 <para>output of core preparation pass</para>
244 <option>-ddump-stg</option>:
245 <indexterm><primary><option>-ddump-stg</option></primary></indexterm>
248 <para>output of STG-to-STG passes</para>
254 <option>-ddump-flatC</option>:
255 <indexterm><primary><option>-ddump-flatC</option></primary></indexterm>
258 <para><emphasis>flattened</emphasis> Abstract C</para>
264 <option>-ddump-cmm</option>:
265 <indexterm><primary><option>-ddump-cmm</option></primary></indexterm>
268 <para>Print the C-- code out.</para>
274 <option>-ddump-opt-cmm</option>:
275 <indexterm><primary><option>-ddump-opt-cmm</option></primary></indexterm>
278 <para>Dump the results of C-- to C-- optimising passes.</para>
284 <option>-ddump-asm</option>:
285 <indexterm><primary><option>-ddump-asm</option></primary></indexterm>
288 <para>assembly language from the native-code generator</para>
294 <option>-ddump-bcos</option>:
295 <indexterm><primary><option>-ddump-bcos</option></primary></indexterm>
298 <para>byte code compiler output</para>
304 <option>-ddump-foreign</option>:
305 <indexterm><primary><option>-ddump-foreign</option></primary></indexterm>
308 <para>dump foreign export stubs</para>
317 <option>-ddump-simpl-phases</option>:
318 <indexterm><primary><option>-ddump-simpl-phases</option></primary></indexterm>
321 <para>Show the output of each run of the simplifier. Used when even
322 <option>-dverbose-core2core</option> doesn't cut it.</para>
328 <option>-ddump-simpl-iterations</option>:
329 <indexterm><primary><option>-ddump-simpl-iterations</option></primary></indexterm>
332 <para>Show the output of each <emphasis>iteration</emphasis>
333 of the simplifier (each run of the simplifier has a maximum
334 number of iterations, normally 4). This outputs even more information
335 than <option>-ddump-simpl-phases</option>.</para>
341 <option>-ddump-simpl-stats</option>
342 <indexterm><primary><option>-ddump-simpl-stats option</option></primary></indexterm>
345 <para>Dump statistics about how many of each kind of
346 transformation too place. If you add
347 <option>-dppr-debug</option> you get more detailed
354 <option>-ddump-if-trace</option>
355 <indexterm><primary><option>-ddump-if-trace</option></primary></indexterm>
358 <para>Make the interface loader be *real* chatty about what it is
365 <option>-ddump-tc-trace</option>
366 <indexterm><primary><option>-ddump-tc-trace</option></primary></indexterm>
369 <para>Make the type checker be *real* chatty about what it is
376 <option>-ddump-vt-trace</option>
377 <indexterm><primary><option>-ddump-tv-trace</option></primary></indexterm>
380 <para>Make the vectoriser be *real* chatty about what it is
387 <option>-ddump-rn-trace</option>
388 <indexterm><primary><option>-ddump-rn-trace</option></primary></indexterm>
391 <para>Make the renamer be *real* chatty about what it is
398 <option>-ddump-rn-stats</option>
399 <indexterm><primary><option>-dshow-rn-stats</option></primary></indexterm>
402 <para>Print out summary of what kind of information the renamer
403 had to bring in.</para>
409 <option>-dverbose-core2core</option>
410 <indexterm><primary><option>-dverbose-core2core</option></primary></indexterm>
413 <option>-dverbose-stg2stg</option>
414 <indexterm><primary><option>-dverbose-stg2stg</option></primary></indexterm>
417 <para>Show the output of the intermediate Core-to-Core and
418 STG-to-STG passes, respectively. (<emphasis>Lots</emphasis>
419 of output!) So: when we're really desperate:</para>
422 % ghc -noC -O -ddump-simpl -dverbose-core2core -dcore-lint Foo.hs
430 <option>-dshow-passes</option>
431 <indexterm><primary><option>-dshow-passes</option></primary></indexterm>
434 <para>Print out each pass name as it happens.</para>
440 <option>-dfaststring-stats</option>
441 <indexterm><primary><option>-dfaststring-stats</option></primary></indexterm>
444 <para>Show statistics for the usage of fast strings by the
451 <option>-dppr-debug</option>
452 <indexterm><primary><option>-dppr-debug</option></primary></indexterm>
455 <para>Debugging output is in one of several
456 “styles.” Take the printing of types, for
457 example. In the “user” style (the default), the
458 compiler's internal ideas about types are presented in
459 Haskell source-level syntax, insofar as possible. In the
460 “debug” style (which is the default for
461 debugging output), the types are printed in with explicit
462 foralls, and variables have their unique-id attached (so you
463 can check for things that look the same but aren't). This
464 flag makes debugging output appear in the more verbose debug
471 <option>-dsuppress-uniques</option>
472 <indexterm><primary><option>-dsuppress-uniques</option></primary></indexterm>
475 <para>Suppress the printing of uniques in debugging output. This may make
476 the printout ambiguous (e.g. unclear where an occurrence of 'x' is bound), but
477 it makes the output of two compiler runs have many fewer gratuitous differences,
478 so you can realistically apply <command>diff</command>. Once <command>diff</command>
479 has shown you where to look, you can try again without <option>-dsuppress-uniques</option></para>
485 <option>-dsuppress-coercions</option>
486 <indexterm><primary><option>-dsuppress-coercions</option></primary></indexterm>
489 <para>Suppress the printing of coercions in Core dumps to make them
496 <option>-dsuppress-module-prefixes</option>
497 <indexterm><primary><option>-dsuppress-module-prefixes</option></primary></indexterm>
500 <para>Suppress the printing of module qualification prefixes in Core dumps to make them easier to read.</para>
506 <option>-dppr-user-length</option>
507 <indexterm><primary><option>-dppr-user-length</option></primary></indexterm>
510 <para>In error messages, expressions are printed to a
511 certain “depth”, with subexpressions beyond the
512 depth replaced by ellipses. This flag sets the
513 depth. Its default value is 5.</para>
519 <option>-dno-debug-output</option>
520 <indexterm><primary><option>-dno-debug-output</option></primary></indexterm>
523 <para>Suppress any unsolicited debugging output. When GHC
524 has been built with the <literal>DEBUG</literal> option it
525 occasionally emits debug output of interest to developers.
526 The extra output can confuse the testing framework and
527 cause bogus test failures, so this flag is provided to
534 <sect2 id="checking-consistency">
535 <title>Checking for consistency</title>
537 <indexterm><primary>consistency checks</primary></indexterm>
538 <indexterm><primary>lint</primary></indexterm>
544 <option>-dcore-lint</option>
545 <indexterm><primary><option>-dcore-lint</option></primary></indexterm>
548 <para>Turn on heavyweight intra-pass sanity-checking within
549 GHC, at Core level. (It checks GHC's sanity, not yours.)</para>
555 <option>-dstg-lint</option>:
556 <indexterm><primary><option>-dstg-lint</option></primary></indexterm>
559 <para>Ditto for STG level. (NOTE: currently doesn't work).</para>
565 <option>-dcmm-lint</option>:
566 <indexterm><primary><option>-dcmm-lint</option></primary></indexterm>
569 <para>Ditto for C-- level.</para>
577 <title>How to read Core syntax (from some <option>-ddump</option>
580 <indexterm><primary>reading Core syntax</primary></indexterm>
581 <indexterm><primary>Core syntax, how to read</primary></indexterm>
583 <para>Let's do this by commenting an example. It's from doing
584 <option>-ddump-ds</option> on this code:
587 skip2 m = m : skip2 (m+2)
590 Before we jump in, a word about names of things. Within GHC,
591 variables, type constructors, etc., are identified by their
592 “Uniques.” These are of the form `letter' plus
593 `number' (both loosely interpreted). The `letter' gives some idea
594 of where the Unique came from; e.g., <literal>_</literal>
595 means “built-in type variable”; <literal>t</literal>
596 means “from the typechecker”; <literal>s</literal>
597 means “from the simplifier”; and so on. The `number'
598 is printed fairly compactly in a `base-62' format, which everyone
599 hates except me (WDP).</para>
601 <para>Remember, everything has a “Unique” and it is
602 usually printed out when debugging, in some form or another. So
603 here we go…</para>
607 Main.skip2{-r1L6-} :: _forall_ a$_4 =>{{Num a$_4}} -> a$_4 -> [a$_4]
609 --# `r1L6' is the Unique for Main.skip2;
610 --# `_4' is the Unique for the type-variable (template) `a'
611 --# `{{Num a$_4}}' is a dictionary argument
615 --# `_NI_' means "no (pragmatic) information" yet; it will later
616 --# evolve into the GHC_PRAGMA info that goes into interface files.
619 /\ _4 -> \ d.Num.t4Gt ->
622 +.t4Hg :: _4 -> _4 -> _4
624 +.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt
626 fromInt.t4GS :: Int{-2i-} -> _4
628 fromInt.t4GS = (fromInt{-r3JX-} _4) d.Num.t4Gt
630 --# The `+' class method (Unique: r3JH) selects the addition code
631 --# from a `Num' dictionary (now an explicit lambda'd argument).
632 --# Because Core is 2nd-order lambda-calculus, type applications
633 --# and lambdas (/\) are explicit. So `+' is first applied to a
634 --# type (`_4'), then to a dictionary, yielding the actual addition
635 --# function that we will use subsequently...
637 --# We play the exact same game with the (non-standard) class method
638 --# `fromInt'. Unsurprisingly, the type `Int' is wired into the
648 } in fromInt.t4GS ds.d4Qz
650 --# `I# 2#' is just the literal Int `2'; it reflects the fact that
651 --# GHC defines `data Int = I# Int#', where Int# is the primitive
652 --# unboxed type. (see relevant info about unboxed types elsewhere...)
654 --# The `!' after `I#' indicates that this is a *saturated*
655 --# application of the `I#' data constructor (i.e., not partially
658 skip2.t3Ja :: _4 -> [_4]
662 let { ds.d4QQ :: [_4]
668 ds.d4QY = +.t4Hg m.r1H4 lit.t4Hb
669 } in skip2.t3Ja ds.d4QY
677 <para>(“It's just a simple functional language” is an
678 unregisterised trademark of Peyton Jones Enterprises, plc.)</para>
683 <title>Unregisterised compilation</title>
684 <indexterm><primary>unregisterised compilation</primary></indexterm>
686 <para>The term "unregisterised" really means "compile via vanilla
687 C", disabling some of the platform-specific tricks that GHC
688 normally uses to make programs go faster. When compiling
689 unregisterised, GHC simply generates a C file which is compiled
692 <para>Unregisterised compilation can be useful when porting GHC to
693 a new machine, since it reduces the prerequisite tools to
694 <command>gcc</command>, <command>as</command>, and
695 <command>ld</command> and nothing more, and furthermore the amount
696 of platform-specific code that needs to be written in order to get
697 unregisterised compilation going is usually fairly small.</para>
699 <para>Unregisterised compilation cannot be selected at
700 compile-time; you have to build GHC with the appropriate options
701 set. Consult the GHC Building Guide for details.</para>
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