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-simpl</option>:
132 <indexterm><primary><option>-ddump-simpl</option></primary></indexterm>
135 <para>simplifier output (Core-to-Core passes)</para>
141 <option>-ddump-inlinings</option>:
142 <indexterm><primary><option>-ddump-inlinings</option></primary></indexterm>
145 <para>inlining info from the simplifier</para>
151 <option>-ddump-cpranal</option>:
152 <indexterm><primary><option>-ddump-cpranal</option></primary></indexterm>
155 <para>CPR analyser output</para>
161 <option>-ddump-stranal</option>:
162 <indexterm><primary><option>-ddump-stranal</option></primary></indexterm>
165 <para>strictness analyser output</para>
171 <option>-ddump-cse</option>:
172 <indexterm><primary><option>-ddump-cse</option></primary></indexterm>
175 <para>CSE pass output</para>
181 <option>-ddump-worker-wrapper</option>:
182 <indexterm><primary><option>-ddump-worker-wrapper</option></primary></indexterm>
185 <para>worker/wrapper split output</para>
191 <option>-ddump-occur-anal</option>:
192 <indexterm><primary><option>-ddump-occur-anal</option></primary></indexterm>
195 <para>`occurrence analysis' output</para>
201 <option>-ddump-prep</option>:
202 <indexterm><primary><option>-ddump-prep</option></primary></indexterm>
205 <para>output of core preparation pass</para>
211 <option>-ddump-stg</option>:
212 <indexterm><primary><option>-ddump-stg</option></primary></indexterm>
215 <para>output of STG-to-STG passes</para>
221 <option>-ddump-flatC</option>:
222 <indexterm><primary><option>-ddump-flatC</option></primary></indexterm>
225 <para><emphasis>flattened</emphasis> Abstract C</para>
231 <option>-ddump-cmm</option>:
232 <indexterm><primary><option>-ddump-cmm</option></primary></indexterm>
235 <para>Print the C-- code out.</para>
241 <option>-ddump-opt-cmm</option>:
242 <indexterm><primary><option>-ddump-opt-cmm</option></primary></indexterm>
245 <para>Dump the results of C-- to C-- optimising passes.</para>
251 <option>-ddump-asm</option>:
252 <indexterm><primary><option>-ddump-asm</option></primary></indexterm>
255 <para>assembly language from the native-code generator</para>
261 <option>-ddump-bcos</option>:
262 <indexterm><primary><option>-ddump-bcos</option></primary></indexterm>
265 <para>byte code compiler output</para>
271 <option>-ddump-foreign</option>:
272 <indexterm><primary><option>-ddump-foreign</option></primary></indexterm>
275 <para>dump foreign export stubs</para>
284 <option>-ddump-simpl-phases</option>:
285 <indexterm><primary><option>-ddump-simpl-phases</option></primary></indexterm>
288 <para>Show the output of each run of the simplifier. Used when even
289 <option>-dverbose-core2core</option> doesn't cut it.</para>
295 <option>-ddump-simpl-iterations</option>:
296 <indexterm><primary><option>-ddump-simpl-iterations</option></primary></indexterm>
299 <para>Show the output of each <emphasis>iteration</emphasis>
300 of the simplifier (each run of the simplifier has a maximum
301 number of iterations, normally 4). This outputs even more information
302 than <option>-ddump-simpl-phases</option>.</para>
308 <option>-ddump-simpl-stats</option>
309 <indexterm><primary><option>-ddump-simpl-stats option</option></primary></indexterm>
312 <para>Dump statistics about how many of each kind of
313 transformation too place. If you add
314 <option>-dppr-debug</option> you get more detailed
321 <option>-ddump-if-trace</option>
322 <indexterm><primary><option>-ddump-if-trace</option></primary></indexterm>
325 <para>Make the interface loader be *real* chatty about what it is
332 <option>-ddump-tc-trace</option>
333 <indexterm><primary><option>-ddump-tc-trace</option></primary></indexterm>
336 <para>Make the type checker be *real* chatty about what it is
343 <option>-ddump-rn-trace</option>
344 <indexterm><primary><option>-ddump-rn-trace</option></primary></indexterm>
347 <para>Make the renamer be *real* chatty about what it is
354 <option>-ddump-rn-stats</option>
355 <indexterm><primary><option>-dshow-rn-stats</option></primary></indexterm>
358 <para>Print out summary of what kind of information the renamer
359 had to bring in.</para>
365 <option>-dverbose-core2core</option>
366 <indexterm><primary><option>-dverbose-core2core</option></primary></indexterm>
369 <option>-dverbose-stg2stg</option>
370 <indexterm><primary><option>-dverbose-stg2stg</option></primary></indexterm>
373 <para>Show the output of the intermediate Core-to-Core and
374 STG-to-STG passes, respectively. (<emphasis>Lots</emphasis>
375 of output!) So: when we're really desperate:</para>
378 % ghc -noC -O -ddump-simpl -dverbose-core2core -dcore-lint Foo.hs
386 <option>-dshow-passes</option>
387 <indexterm><primary><option>-dshow-passes</option></primary></indexterm>
390 <para>Print out each pass name as it happens.</para>
396 <option>-dfaststring-stats</option>
397 <indexterm><primary><option>-dfaststring-stats</option></primary></indexterm>
400 <para>Show statistics for the usage of fast strings by the
407 <option>-dppr-debug</option>
408 <indexterm><primary><option>-dppr-debug</option></primary></indexterm>
411 <para>Debugging output is in one of several
412 “styles.” Take the printing of types, for
413 example. In the “user” style (the default), the
414 compiler's internal ideas about types are presented in
415 Haskell source-level syntax, insofar as possible. In the
416 “debug” style (which is the default for
417 debugging output), the types are printed in with explicit
418 foralls, and variables have their unique-id attached (so you
419 can check for things that look the same but aren't). This
420 flag makes debugging output appear in the more verbose debug
427 <option>-dsuppress-uniques</option>
428 <indexterm><primary><option>-dsuppress-uniques</option></primary></indexterm>
431 <para>Suppress the printing of uniques in debugging output. This may make
432 the printout ambiguous (e.g. unclear where an occurrence of 'x' is bound), but
433 it makes the output of two compiler runs have many fewer gratuitous differences,
434 so you can realistically apply <command>diff</command>. Once <command>diff</command>
435 has shown you where to look, you can try again without <option>-dsuppress-uniques</option></para>
441 <option>-dsuppress-coercions</option>
442 <indexterm><primary><option>-dsuppress-coercions</option></primary></indexterm>
445 <para>Suppress the printing of coercions in Core dumps to make them
452 <option>-dsuppress-module-prefixes</option>
453 <indexterm><primary><option>-dsuppress-module-prefixes</option></primary></indexterm>
456 <para>Suppress the printing of module qualification prefixes in Core dumps to make them easier to read.</para>
462 <option>-dppr-user-length</option>
463 <indexterm><primary><option>-dppr-user-length</option></primary></indexterm>
466 <para>In error messages, expressions are printed to a
467 certain “depth”, with subexpressions beyond the
468 depth replaced by ellipses. This flag sets the
469 depth. Its default value is 5.</para>
475 <option>-dno-debug-output</option>
476 <indexterm><primary><option>-dno-debug-output</option></primary></indexterm>
479 <para>Suppress any unsolicited debugging output. When GHC
480 has been built with the <literal>DEBUG</literal> option it
481 occasionally emits debug output of interest to developers.
482 The extra output can confuse the testing framework and
483 cause bogus test failures, so this flag is provided to
490 <sect2 id="checking-consistency">
491 <title>Checking for consistency</title>
493 <indexterm><primary>consistency checks</primary></indexterm>
494 <indexterm><primary>lint</primary></indexterm>
500 <option>-dcore-lint</option>
501 <indexterm><primary><option>-dcore-lint</option></primary></indexterm>
504 <para>Turn on heavyweight intra-pass sanity-checking within
505 GHC, at Core level. (It checks GHC's sanity, not yours.)</para>
511 <option>-dstg-lint</option>:
512 <indexterm><primary><option>-dstg-lint</option></primary></indexterm>
515 <para>Ditto for STG level. (NOTE: currently doesn't work).</para>
521 <option>-dcmm-lint</option>:
522 <indexterm><primary><option>-dcmm-lint</option></primary></indexterm>
525 <para>Ditto for C-- level.</para>
533 <title>How to read Core syntax (from some <option>-ddump</option>
536 <indexterm><primary>reading Core syntax</primary></indexterm>
537 <indexterm><primary>Core syntax, how to read</primary></indexterm>
539 <para>Let's do this by commenting an example. It's from doing
540 <option>-ddump-ds</option> on this code:
543 skip2 m = m : skip2 (m+2)
546 Before we jump in, a word about names of things. Within GHC,
547 variables, type constructors, etc., are identified by their
548 “Uniques.” These are of the form `letter' plus
549 `number' (both loosely interpreted). The `letter' gives some idea
550 of where the Unique came from; e.g., <literal>_</literal>
551 means “built-in type variable”; <literal>t</literal>
552 means “from the typechecker”; <literal>s</literal>
553 means “from the simplifier”; and so on. The `number'
554 is printed fairly compactly in a `base-62' format, which everyone
555 hates except me (WDP).</para>
557 <para>Remember, everything has a “Unique” and it is
558 usually printed out when debugging, in some form or another. So
559 here we go…</para>
563 Main.skip2{-r1L6-} :: _forall_ a$_4 =>{{Num a$_4}} -> a$_4 -> [a$_4]
565 --# `r1L6' is the Unique for Main.skip2;
566 --# `_4' is the Unique for the type-variable (template) `a'
567 --# `{{Num a$_4}}' is a dictionary argument
571 --# `_NI_' means "no (pragmatic) information" yet; it will later
572 --# evolve into the GHC_PRAGMA info that goes into interface files.
575 /\ _4 -> \ d.Num.t4Gt ->
578 +.t4Hg :: _4 -> _4 -> _4
580 +.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt
582 fromInt.t4GS :: Int{-2i-} -> _4
584 fromInt.t4GS = (fromInt{-r3JX-} _4) d.Num.t4Gt
586 --# The `+' class method (Unique: r3JH) selects the addition code
587 --# from a `Num' dictionary (now an explicit lambda'd argument).
588 --# Because Core is 2nd-order lambda-calculus, type applications
589 --# and lambdas (/\) are explicit. So `+' is first applied to a
590 --# type (`_4'), then to a dictionary, yielding the actual addition
591 --# function that we will use subsequently...
593 --# We play the exact same game with the (non-standard) class method
594 --# `fromInt'. Unsurprisingly, the type `Int' is wired into the
604 } in fromInt.t4GS ds.d4Qz
606 --# `I# 2#' is just the literal Int `2'; it reflects the fact that
607 --# GHC defines `data Int = I# Int#', where Int# is the primitive
608 --# unboxed type. (see relevant info about unboxed types elsewhere...)
610 --# The `!' after `I#' indicates that this is a *saturated*
611 --# application of the `I#' data constructor (i.e., not partially
614 skip2.t3Ja :: _4 -> [_4]
618 let { ds.d4QQ :: [_4]
624 ds.d4QY = +.t4Hg m.r1H4 lit.t4Hb
625 } in skip2.t3Ja ds.d4QY
633 <para>(“It's just a simple functional language” is an
634 unregisterised trademark of Peyton Jones Enterprises, plc.)</para>
639 <title>Unregisterised compilation</title>
640 <indexterm><primary>unregisterised compilation</primary></indexterm>
642 <para>The term "unregisterised" really means "compile via vanilla
643 C", disabling some of the platform-specific tricks that GHC
644 normally uses to make programs go faster. When compiling
645 unregisterised, GHC simply generates a C file which is compiled
648 <para>Unregisterised compilation can be useful when porting GHC to
649 a new machine, since it reduces the prerequisite tools to
650 <command>gcc</command>, <command>as</command>, and
651 <command>ld</command> and nothing more, and furthermore the amount
652 of platform-specific code that needs to be written in order to get
653 unregisterised compilation going is usually fairly small.</para>
655 <para>Unregisterised compilation cannot be selected at
656 compile-time; you have to build GHC with the appropriate options
657 set. Consult the GHC Building Guide for details.</para>
663 ;;; Local Variables: ***
664 ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter" "sect1") ***