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>-ddump-core-stats</option>
441 <indexterm><primary><option>-ddump-core-stats</option></primary></indexterm>
444 <para>Print a one-line summary of the size of the Core program
445 at the end of the optimisation pipeline.</para>
451 <option>-dfaststring-stats</option>
452 <indexterm><primary><option>-dfaststring-stats</option></primary></indexterm>
455 <para>Show statistics for the usage of fast strings by the
462 <option>-dppr-debug</option>
463 <indexterm><primary><option>-dppr-debug</option></primary></indexterm>
466 <para>Debugging output is in one of several
467 “styles.” Take the printing of types, for
468 example. In the “user” style (the default), the
469 compiler's internal ideas about types are presented in
470 Haskell source-level syntax, insofar as possible. In the
471 “debug” style (which is the default for
472 debugging output), the types are printed in with explicit
473 foralls, and variables have their unique-id attached (so you
474 can check for things that look the same but aren't). This
475 flag makes debugging output appear in the more verbose debug
482 <sect2 id="formatting dumps">
483 <title>Formatting dumps</title>
485 <indexterm><primary>formatting dumps</primary></indexterm>
490 <option>-dppr-user-length</option>
491 <indexterm><primary><option>-dppr-user-length</option></primary></indexterm>
494 <para>In error messages, expressions are printed to a
495 certain “depth”, with subexpressions beyond the
496 depth replaced by ellipses. This flag sets the
497 depth. Its default value is 5.</para>
503 <option>-dppr-colsNNN</option>
504 <indexterm><primary><option>-dppr-colsNNN</option></primary></indexterm>
507 <para>Set the width of debugging output. Use this if your code is wrapping too much.
508 For example: <option>-dppr-cols200</option>.</para>
514 <option>-dppr-case-as-let</option>
515 <indexterm><primary><option>-dppr-case-as-let</option></primary></indexterm>
518 <para>Print single alternative case expressions as though they were strict
519 let expressions. This is helpful when your code does a lot of unboxing.</para>
525 <option>-dno-debug-output</option>
526 <indexterm><primary><option>-dno-debug-output</option></primary></indexterm>
529 <para>Suppress any unsolicited debugging output. When GHC
530 has been built with the <literal>DEBUG</literal> option it
531 occasionally emits debug output of interest to developers.
532 The extra output can confuse the testing framework and
533 cause bogus test failures, so this flag is provided to
541 <sect2 id="supression">
542 <title>Suppressing unwanted information</title>
544 <indexterm><primary>suppression</primary></indexterm>
546 Core dumps contain a large amount of information. Depending on what you are doing, not all of it will be useful.
547 Use these flags to suppress the parts that you are not interested in.
552 <option>-dsuppress-all</option>
553 <indexterm><primary><option>-dsuppress-all</option></primary></indexterm>
556 <para>Suppress everything that can be suppressed, except for unique ids as this often
557 makes the printout ambiguous. If you just want to see the overall structure of
558 the code, then start here.</para>
564 <option>-dsuppress-uniques</option>
565 <indexterm><primary><option>-dsuppress-uniques</option></primary></indexterm>
568 <para>Suppress the printing of uniques. This may make
569 the printout ambiguous (e.g. unclear where an occurrence of 'x' is bound), but
570 it makes the output of two compiler runs have many fewer gratuitous differences,
571 so you can realistically apply <command>diff</command>. Once <command>diff</command>
572 has shown you where to look, you can try again without <option>-dsuppress-uniques</option></para>
578 <option>-dsuppress-idinfo</option>
579 <indexterm><primary><option>-dsuppress-idinfo</option></primary></indexterm>
582 <para>Suppress extended information about identifiers where they are bound. This includes
583 strictness information and inliner templates. Using this flag can cut the size
584 of the core dump in half, due to the lack of inliner templates</para>
590 <option>-dsuppress-module-prefixes</option>
591 <indexterm><primary><option>-dsuppress-module-prefixes</option></primary></indexterm>
594 <para>Suppress the printing of module qualification prefixes.
595 This is the <constant>Data.List</constant> in <constant>Data.List.length</constant>.</para>
601 <option>-dsuppress-type-signatures</option>
602 <indexterm><primary><option>-dsuppress-type-signatures</option></primary></indexterm>
605 <para>Suppress the printing of type signatures.</para>
611 <option>-dsuppress-type-applications</option>
612 <indexterm><primary><option>-dsuppress-type-applications</option></primary></indexterm>
615 <para>Suppress the printing of type applications.</para>
621 <option>-dsuppress-coercions</option>
622 <indexterm><primary><option>-dsuppress-coercions</option></primary></indexterm>
625 <para>Suppress the printing of type coercions.</para>
631 <sect2 id="checking-consistency">
632 <title>Checking for consistency</title>
634 <indexterm><primary>consistency checks</primary></indexterm>
635 <indexterm><primary>lint</primary></indexterm>
641 <option>-dcore-lint</option>
642 <indexterm><primary><option>-dcore-lint</option></primary></indexterm>
645 <para>Turn on heavyweight intra-pass sanity-checking within
646 GHC, at Core level. (It checks GHC's sanity, not yours.)</para>
652 <option>-dstg-lint</option>:
653 <indexterm><primary><option>-dstg-lint</option></primary></indexterm>
656 <para>Ditto for STG level. (NOTE: currently doesn't work).</para>
662 <option>-dcmm-lint</option>:
663 <indexterm><primary><option>-dcmm-lint</option></primary></indexterm>
666 <para>Ditto for C-- level.</para>
674 <title>How to read Core syntax (from some <option>-ddump</option>
677 <indexterm><primary>reading Core syntax</primary></indexterm>
678 <indexterm><primary>Core syntax, how to read</primary></indexterm>
680 <para>Let's do this by commenting an example. It's from doing
681 <option>-ddump-ds</option> on this code:
684 skip2 m = m : skip2 (m+2)
687 Before we jump in, a word about names of things. Within GHC,
688 variables, type constructors, etc., are identified by their
689 “Uniques.” These are of the form `letter' plus
690 `number' (both loosely interpreted). The `letter' gives some idea
691 of where the Unique came from; e.g., <literal>_</literal>
692 means “built-in type variable”; <literal>t</literal>
693 means “from the typechecker”; <literal>s</literal>
694 means “from the simplifier”; and so on. The `number'
695 is printed fairly compactly in a `base-62' format, which everyone
696 hates except me (WDP).</para>
698 <para>Remember, everything has a “Unique” and it is
699 usually printed out when debugging, in some form or another. So
700 here we go…</para>
704 Main.skip2{-r1L6-} :: _forall_ a$_4 =>{{Num a$_4}} -> a$_4 -> [a$_4]
706 --# `r1L6' is the Unique for Main.skip2;
707 --# `_4' is the Unique for the type-variable (template) `a'
708 --# `{{Num a$_4}}' is a dictionary argument
712 --# `_NI_' means "no (pragmatic) information" yet; it will later
713 --# evolve into the GHC_PRAGMA info that goes into interface files.
716 /\ _4 -> \ d.Num.t4Gt ->
719 +.t4Hg :: _4 -> _4 -> _4
721 +.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt
723 fromInt.t4GS :: Int{-2i-} -> _4
725 fromInt.t4GS = (fromInt{-r3JX-} _4) d.Num.t4Gt
727 --# The `+' class method (Unique: r3JH) selects the addition code
728 --# from a `Num' dictionary (now an explicit lambda'd argument).
729 --# Because Core is 2nd-order lambda-calculus, type applications
730 --# and lambdas (/\) are explicit. So `+' is first applied to a
731 --# type (`_4'), then to a dictionary, yielding the actual addition
732 --# function that we will use subsequently...
734 --# We play the exact same game with the (non-standard) class method
735 --# `fromInt'. Unsurprisingly, the type `Int' is wired into the
745 } in fromInt.t4GS ds.d4Qz
747 --# `I# 2#' is just the literal Int `2'; it reflects the fact that
748 --# GHC defines `data Int = I# Int#', where Int# is the primitive
749 --# unboxed type. (see relevant info about unboxed types elsewhere...)
751 --# The `!' after `I#' indicates that this is a *saturated*
752 --# application of the `I#' data constructor (i.e., not partially
755 skip2.t3Ja :: _4 -> [_4]
759 let { ds.d4QQ :: [_4]
765 ds.d4QY = +.t4Hg m.r1H4 lit.t4Hb
766 } in skip2.t3Ja ds.d4QY
774 <para>(“It's just a simple functional language” is an
775 unregisterised trademark of Peyton Jones Enterprises, plc.)</para>
780 <title>Unregisterised compilation</title>
781 <indexterm><primary>unregisterised compilation</primary></indexterm>
783 <para>The term "unregisterised" really means "compile via vanilla
784 C", disabling some of the platform-specific tricks that GHC
785 normally uses to make programs go faster. When compiling
786 unregisterised, GHC simply generates a C file which is compiled
789 <para>Unregisterised compilation can be useful when porting GHC to
790 a new machine, since it reduces the prerequisite tools to
791 <command>gcc</command>, <command>as</command>, and
792 <command>ld</command> and nothing more, and furthermore the amount
793 of platform-specific code that needs to be written in order to get
794 unregisterised compilation going is usually fairly small.</para>
796 <para>Unregisterised compilation cannot be selected at
797 compile-time; you have to build GHC with the appropriate options
798 set. Consult the GHC Building Guide for details.</para>
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