docs/users_guide/debugging.xml

   1 <?xml version="1.0" encoding="iso-8859-1"?>
   2 <sect1 id="options-debugging">
   3   <title>Debugging the compiler</title>
   4
   5   <indexterm><primary>debugging options (for GHC)</primary></indexterm>
   6
   7   <para>HACKER TERRITORY. HACKER TERRITORY.  (You were warned.)</para>
   8
   9   <sect2 id="dumping-output">
  10     <title>Dumping out compiler intermediate structures</title>
  11
  12     <indexterm><primary>dumping GHC intermediates</primary></indexterm>
  13     <indexterm><primary>intermediate passes, output</primary></indexterm>
  14
  15     <variablelist>
  16       <varlistentry>
  17         <term>
  18           <option>-ddump-</option><replaceable>pass</replaceable>
  19           <indexterm><primary><option>-ddump</option> options</primary></indexterm>
  20         </term>
  21         <listitem>
  22           <para>Make a debugging dump after pass
  23         <literal>&lt;pass&gt;</literal> (may be common enough to need
  24         a short form&hellip;).  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
  28         are:</para>
  29
  30           <variablelist>
  31             <varlistentry>
  32               <term>
  33                 <option>-ddump-parsed</option>:
  34                 <indexterm><primary><option>-ddump-parsed</option></primary></indexterm>
  35               </term>
  36               <listitem>
  37                 <para>parser output</para>
  38               </listitem>
  39             </varlistentry>
  40
  41             <varlistentry>
  42               <term>
  43                 <option>-ddump-rn</option>:
  44                 <indexterm><primary><option>-ddump-rn</option></primary></indexterm>
  45               </term>
  46               <listitem>
  47                 <para>renamer output</para>
  48               </listitem>
  49             </varlistentry>
  50
  51             <varlistentry>
  52               <term>
  53                 <option>-ddump-tc</option>:
  54                 <indexterm><primary><option>-ddump-tc</option></primary></indexterm>
  55               </term>
  56               <listitem>
  57                 <para>typechecker output</para>
  58               </listitem>
  59             </varlistentry>
  60
  61             <varlistentry>
  62               <term>
  63                 <option>-ddump-splices</option>:
  64                 <indexterm><primary><option>-ddump-splices</option></primary></indexterm>
  65               </term>
  66               <listitem>
  67                 <para>Dump Template Haskell expressions that we splice in,
  68                 and what Haskell code the expression evaluates to.</para>
  69               </listitem>
  70             </varlistentry>
  71
  72             <varlistentry>
  73               <term>
  74                 <option>-ddump-types</option>:
  75                 <indexterm><primary><option>-ddump-types</option></primary></indexterm>
  76               </term>
  77               <listitem>
  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
  83               compiler.</para>
  84               </listitem>
  85             </varlistentry>
  86
  87             <varlistentry>
  88               <term>
  89                 <option>-ddump-deriv</option>:
  90                 <indexterm><primary><option>-ddump-deriv</option></primary></indexterm>
  91               </term>
  92               <listitem>
  93                 <para>derived instances</para>
  94               </listitem>
  95             </varlistentry>
  96
  97             <varlistentry>
  98               <term>
  99                 <option>-ddump-ds</option>:
 100                 <indexterm><primary><option>-ddump-ds</option></primary></indexterm>
 101               </term>
 102               <listitem>
 103                 <para>desugarer output</para>
 104               </listitem>
 105             </varlistentry>
 106
 107             <varlistentry>
 108               <term>
 109                 <option>-ddump-spec</option>:
 110                 <indexterm><primary><option>-ddump-spec</option></primary></indexterm>
 111               </term>
 112               <listitem>
 113                 <para>output of specialisation pass</para>
 114               </listitem>
 115             </varlistentry>
 116
 117             <varlistentry>
 118               <term>
 119                 <option>-ddump-rules</option>:
 120                 <indexterm><primary><option>-ddump-rules</option></primary></indexterm>
 121               </term>
 122               <listitem>
 123                 <para>dumps all rewrite rules (including those generated
 124               by the specialisation pass)</para>
 125               </listitem>
 126             </varlistentry>
 127
 128             <varlistentry>
 129               <term>
 130                 <option>-ddump-simpl</option>:
 131                 <indexterm><primary><option>-ddump-simpl</option></primary></indexterm>
 132               </term>
 133               <listitem>
 134                 <para>simplifier output (Core-to-Core passes)</para>
 135               </listitem>
 136             </varlistentry>
 137
 138             <varlistentry>
 139               <term>
 140                 <option>-ddump-inlinings</option>:
 141                 <indexterm><primary><option>-ddump-inlinings</option></primary></indexterm>
 142               </term>
 143               <listitem>
 144                 <para>inlining info from the simplifier</para>
 145               </listitem>
 146             </varlistentry>
 147
 148             <varlistentry>
 149               <term>
 150                 <option>-ddump-cpranal</option>:
 151                 <indexterm><primary><option>-ddump-cpranal</option></primary></indexterm>
 152               </term>
 153               <listitem>
 154                 <para>CPR analyser output</para>
 155               </listitem>
 156             </varlistentry>
 157
 158             <varlistentry>
 159               <term>
 160                 <option>-ddump-stranal</option>:
 161                 <indexterm><primary><option>-ddump-stranal</option></primary></indexterm>
 162               </term>
 163               <listitem>
 164                 <para>strictness analyser output</para>
 165               </listitem>
 166             </varlistentry>
 167
 168             <varlistentry>
 169               <term>
 170                 <option>-ddump-cse</option>:
 171                 <indexterm><primary><option>-ddump-cse</option></primary></indexterm>
 172               </term>
 173               <listitem>
 174                 <para>CSE pass output</para>
 175               </listitem>
 176             </varlistentry>
 177
 178             <varlistentry>
 179               <term>
 180                 <option>-ddump-workwrap</option>:
 181                 <indexterm><primary><option>-ddump-workwrap</option></primary></indexterm>
 182               </term>
 183               <listitem>
 184                 <para>worker/wrapper split output</para>
 185               </listitem>
 186             </varlistentry>
 187
 188             <varlistentry>
 189               <term>
 190                 <option>-ddump-occur-anal</option>:
 191                 <indexterm><primary><option>-ddump-occur-anal</option></primary></indexterm>
 192               </term>
 193               <listitem>
 194                 <para>`occurrence analysis' output</para>
 195               </listitem>
 196             </varlistentry>
 197
 198             <varlistentry>
 199               <term>
 200                 <option>-ddump-prep</option>:
 201                 <indexterm><primary><option>-ddump-prep</option></primary></indexterm>
 202               </term>
 203               <listitem>
 204                 <para>output of core preparation pass</para>
 205               </listitem>
 206             </varlistentry>
 207
 208             <varlistentry>
 209               <term>
 210                 <option>-ddump-stg</option>:
 211                 <indexterm><primary><option>-ddump-stg</option></primary></indexterm>
 212               </term>
 213               <listitem>
 214                 <para>output of STG-to-STG passes</para>
 215               </listitem>
 216             </varlistentry>
 217
 218             <varlistentry>
 219               <term>
 220                 <option>-ddump-flatC</option>:
 221                 <indexterm><primary><option>-ddump-flatC</option></primary></indexterm>
 222               </term>
 223               <listitem>
 224                 <para><emphasis>flattened</emphasis> Abstract&nbsp;C</para>
 225               </listitem>
 226             </varlistentry>
 227
 228             <varlistentry>
 229               <term>
 230                 <option>-ddump-cmm</option>:
 231                 <indexterm><primary><option>-ddump-cmm</option></primary></indexterm>
 232               </term>
 233               <listitem>
 234                 <para>Print the C-- code out.</para>
 235               </listitem>
 236             </varlistentry>
 237
 238             <varlistentry>
 239               <term>
 240                 <option>-ddump-opt-cmm</option>:
 241                 <indexterm><primary><option>-ddump-opt-cmm</option></primary></indexterm>
 242               </term>
 243               <listitem>
 244                 <para>Dump the results of C-- to C-- optimising passes.</para>
 245               </listitem>
 246             </varlistentry>
 247
 248             <varlistentry>
 249               <term>
 250                 <option>-ddump-asm</option>:
 251                 <indexterm><primary><option>-ddump-asm</option></primary></indexterm>
 252               </term>
 253               <listitem>
 254                 <para>assembly language from the native-code generator</para>
 255               </listitem>
 256             </varlistentry>
 257
 258             <varlistentry>
 259               <term>
 260                 <option>-ddump-bcos</option>:
 261                 <indexterm><primary><option>-ddump-bcos</option></primary></indexterm>
 262               </term>
 263               <listitem>
 264                 <para>byte code compiler output</para>
 265               </listitem>
 266             </varlistentry>
 267
 268             <varlistentry>
 269               <term>
 270                 <option>-ddump-foreign</option>:
 271                 <indexterm><primary><option>-ddump-foreign</option></primary></indexterm>
 272               </term>
 273               <listitem>
 274                 <para>dump foreign export stubs</para>
 275               </listitem>
 276             </varlistentry>
 277           </variablelist>
 278         </listitem>
 279       </varlistentry>
 280
 281       <varlistentry>
 282         <term>
 283           <option>-ddump-simpl-phases</option>:
 284           <indexterm><primary><option>-ddump-simpl-phases</option></primary></indexterm>
 285         </term>
 286         <listitem>
 287           <para>Show the output of each run of the simplifier.  Used when even
 288         <option>-dverbose-simpl</option> doesn't cut it.</para>
 289         </listitem>
 290       </varlistentry>
 291
 292       <varlistentry>
 293         <term>
 294           <option>-ddump-simpl-iterations</option>:
 295           <indexterm><primary><option>-ddump-simpl-iterations</option></primary></indexterm>
 296         </term>
 297         <listitem>
 298           <para>Show the output of each <emphasis>iteration</emphasis>
 299         of the simplifier (each run of the simplifier has a maximum
 300         number of iterations, normally 4).  This outputs even more information
 301         than <option>-ddump-simpl-phases</option>.</para>
 302         </listitem>
 303       </varlistentry>
 304
 305       <varlistentry>
 306         <term>
 307           <option>-ddump-simpl-stats</option>
 308           <indexterm><primary><option>-ddump-simpl-stats option</option></primary></indexterm>
 309         </term>
 310         <listitem>
 311           <para>Dump statistics about how many of each kind of
 312         transformation too place.  If you add
 313         <option>-dppr-debug</option> you get more detailed
 314         information.</para>
 315         </listitem>
 316       </varlistentry>
 317
 318       <varlistentry>
 319         <term>
 320           <option>-ddump-if-trace</option>
 321           <indexterm><primary><option>-ddump-if-trace</option></primary></indexterm>
 322         </term>
 323         <listitem>
 324           <para>Make the interface loader be *real* chatty about what it is
 325         upto.</para>
 326         </listitem>
 327       </varlistentry>
 328
 329       <varlistentry>
 330         <term>
 331           <option>-ddump-tc-trace</option>
 332           <indexterm><primary><option>-ddump-tc-trace</option></primary></indexterm>
 333         </term>
 334         <listitem>
 335           <para>Make the type checker be *real* chatty about what it is
 336         upto.</para>
 337         </listitem>
 338       </varlistentry>
 339
 340       <varlistentry>
 341         <term>
 342           <option>-ddump-rn-trace</option>
 343           <indexterm><primary><option>-ddump-rn-trace</option></primary></indexterm>
 344         </term>
 345         <listitem>
 346           <para>Make the renamer be *real* chatty about what it is
 347         upto.</para>
 348         </listitem>
 349       </varlistentry>
 350
 351       <varlistentry>
 352         <term>
 353           <option>-ddump-rn-stats</option>
 354           <indexterm><primary><option>-dshow-rn-stats</option></primary></indexterm>
 355         </term>
 356         <listitem>
 357           <para>Print out summary of what kind of information the renamer
 358         had to bring in.</para>
 359         </listitem>
 360       </varlistentry>
 361
 362       <varlistentry>
 363         <term>
 364           <option>-dverbose-core2core</option>
 365           <indexterm><primary><option>-dverbose-core2core</option></primary></indexterm>
 366         </term>
 367         <term>
 368           <option>-dverbose-stg2stg</option>
 369           <indexterm><primary><option>-dverbose-stg2stg</option></primary></indexterm>
 370         </term>
 371         <listitem>
 372           <para>Show the output of the intermediate Core-to-Core and
 373         STG-to-STG passes, respectively.  (<emphasis>Lots</emphasis>
 374         of output!) So: when we're really desperate:</para>
 375
 376           <screen>
 377 % ghc -noC -O -ddump-simpl -dverbose-simpl -dcore-lint Foo.hs
 378 </screen>
 379
 380         </listitem>
 381       </varlistentry>
 382
 383       <varlistentry>
 384         <term>
 385           <option>-dshow-passes</option>
 386           <indexterm><primary><option>-dshow-passes</option></primary></indexterm>
 387         </term>
 388         <listitem>
 389           <para>Print out each pass name as it happens.</para>
 390         </listitem>
 391       </varlistentry>
 392
 393       <varlistentry>
 394         <term>
 395           <option>-dfaststring-stats</option>
 396           <indexterm><primary><option>-dfaststring-stats</option></primary></indexterm>
 397         </term>
 398         <listitem>
 399           <para>Show statistics for the usage of fast strings by the
 400           compiler.</para>
 401         </listitem>
 402       </varlistentry>
 403
 404       <varlistentry>
 405         <term>
 406           <option>-dppr-debug</option>
 407           <indexterm><primary><option>-dppr-debug</option></primary></indexterm>
 408         </term>
 409         <listitem>
 410           <para>Debugging output is in one of several
 411           &ldquo;styles.&rdquo; Take the printing of types, for
 412           example.  In the &ldquo;user&rdquo; style (the default), the
 413           compiler's internal ideas about types are presented in
 414           Haskell source-level syntax, insofar as possible.  In the
 415           &ldquo;debug&rdquo; style (which is the default for
 416           debugging output), the types are printed in with explicit
 417           foralls, and variables have their unique-id attached (so you
 418           can check for things that look the same but aren't).  This
 419           flag makes debugging output appear in the more verbose debug
 420           style.</para>
 421         </listitem>
 422       </varlistentry>
 423
 424       <varlistentry>
 425         <term>
 426           <option>-dppr-user-length</option>
 427           <indexterm><primary><option>-dppr-user-length</option></primary></indexterm>
 428         </term>
 429         <listitem>
 430           <para>In error messages, expressions are printed to a
 431           certain &ldquo;depth&rdquo;, with subexpressions beyond the
 432           depth replaced by ellipses.  This flag sets the
 433           depth.  Its default value is 5.</para>
 434         </listitem>
 435       </varlistentry>
 436
 437       <varlistentry>
 438         <term>
 439           <option>-dshow-unused-imports</option>
 440           <indexterm><primary><option>-dshow-unused-imports</option></primary></indexterm>
 441         </term>
 442         <listitem>
 443           <para>Have the renamer report what imports does not
 444         contribute.</para>
 445         </listitem>
 446       </varlistentry>
 447     </variablelist>
 448   </sect2>
 449
 450   <sect2 id="checking-consistency">
 451     <title>Checking for consistency</title>
 452
 453     <indexterm><primary>consistency checks</primary></indexterm>
 454     <indexterm><primary>lint</primary></indexterm>
 455
 456     <variablelist>
 457
 458       <varlistentry>
 459         <term>
 460           <option>-dcore-lint</option>
 461           <indexterm><primary><option>-dcore-lint</option></primary></indexterm>
 462         </term>
 463         <listitem>
 464           <para>Turn on heavyweight intra-pass sanity-checking within
 465           GHC, at Core level.  (It checks GHC's sanity, not yours.)</para>
 466         </listitem>
 467       </varlistentry>
 468
 469       <varlistentry>
 470         <term>
 471           <option>-dstg-lint</option>:
 472           <indexterm><primary><option>-dstg-lint</option></primary></indexterm>
 473         </term>
 474         <listitem>
 475           <para>Ditto for STG level. (NOTE: currently doesn't work).</para>
 476         </listitem>
 477       </varlistentry>
 478
 479       <varlistentry>
 480         <term>
 481           <option>-dcmm-lint</option>:
 482           <indexterm><primary><option>-dcmm-lint</option></primary></indexterm>
 483         </term>
 484         <listitem>
 485           <para>Ditto for C-- level.</para>
 486         </listitem>
 487       </varlistentry>
 488
 489     </variablelist>
 490   </sect2>
 491
 492   <sect2>
 493     <title>How to read Core syntax (from some <option>-ddump</option>
 494     flags)</title>
 495
 496     <indexterm><primary>reading Core syntax</primary></indexterm>
 497     <indexterm><primary>Core syntax, how to read</primary></indexterm>
 498
 499     <para>Let's do this by commenting an example.  It's from doing
 500     <option>-ddump-ds</option> on this code:
 501
 502 <programlisting>
 503 skip2 m = m : skip2 (m+2)
 504 </programlisting>
 505
 506     Before we jump in, a word about names of things.  Within GHC,
 507     variables, type constructors, etc., are identified by their
 508     &ldquo;Uniques.&rdquo; These are of the form `letter' plus
 509     `number' (both loosely interpreted).  The `letter' gives some idea
 510     of where the Unique came from; e.g., <literal>&lowbar;</literal>
 511     means &ldquo;built-in type variable&rdquo;; <literal>t</literal>
 512     means &ldquo;from the typechecker&rdquo;; <literal>s</literal>
 513     means &ldquo;from the simplifier&rdquo;; and so on.  The `number'
 514     is printed fairly compactly in a `base-62' format, which everyone
 515     hates except me (WDP).</para>
 516
 517     <para>Remember, everything has a &ldquo;Unique&rdquo; and it is
 518     usually printed out when debugging, in some form or another.  So
 519     here we go&hellip;</para>
 520
 521 <programlisting>
 522 Desugared:
 523 Main.skip2{-r1L6-} :: _forall_ a$_4 =&#62;{{Num a$_4}} -&#62; a$_4 -&#62; [a$_4]
 524
 525 --# `r1L6' is the Unique for Main.skip2;
 526 --# `_4' is the Unique for the type-variable (template) `a'
 527 --# `{{Num a$_4}}' is a dictionary argument
 528
 529 _NI_
 530
 531 --# `_NI_' means "no (pragmatic) information" yet; it will later
 532 --# evolve into the GHC_PRAGMA info that goes into interface files.
 533
 534 Main.skip2{-r1L6-} =
 535     /\ _4 -&#62; \ d.Num.t4Gt -&#62;
 536         let {
 537           {- CoRec -}
 538           +.t4Hg :: _4 -&#62; _4 -&#62; _4
 539           _NI_
 540           +.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt
 541
 542           fromInt.t4GS :: Int{-2i-} -&#62; _4
 543           _NI_
 544           fromInt.t4GS = (fromInt{-r3JX-} _4) d.Num.t4Gt
 545
 546 --# The `+' class method (Unique: r3JH) selects the addition code
 547 --# from a `Num' dictionary (now an explicit lambda'd argument).
 548 --# Because Core is 2nd-order lambda-calculus, type applications
 549 --# and lambdas (/\) are explicit.  So `+' is first applied to a
 550 --# type (`_4'), then to a dictionary, yielding the actual addition
 551 --# function that we will use subsequently...
 552
 553 --# We play the exact same game with the (non-standard) class method
 554 --# `fromInt'.  Unsurprisingly, the type `Int' is wired into the
 555 --# compiler.
 556
 557           lit.t4Hb :: _4
 558           _NI_
 559           lit.t4Hb =
 560               let {
 561                 ds.d4Qz :: Int{-2i-}
 562                 _NI_
 563                 ds.d4Qz = I#! 2#
 564               } in  fromInt.t4GS ds.d4Qz
 565
 566 --# `I# 2#' is just the literal Int `2'; it reflects the fact that
 567 --# GHC defines `data Int = I# Int#', where Int# is the primitive
 568 --# unboxed type.  (see relevant info about unboxed types elsewhere...)
 569
 570 --# The `!' after `I#' indicates that this is a *saturated*
 571 --# application of the `I#' data constructor (i.e., not partially
 572 --# applied).
 573
 574           skip2.t3Ja :: _4 -&#62; [_4]
 575           _NI_
 576           skip2.t3Ja =
 577               \ m.r1H4 -&#62;
 578                   let { ds.d4QQ :: [_4]
 579                         _NI_
 580                         ds.d4QQ =
 581                     let {
 582                       ds.d4QY :: _4
 583                       _NI_
 584                       ds.d4QY = +.t4Hg m.r1H4 lit.t4Hb
 585                     } in  skip2.t3Ja ds.d4QY
 586                   } in
 587                   :! _4 m.r1H4 ds.d4QQ
 588
 589           {- end CoRec -}
 590         } in  skip2.t3Ja
 591 </programlisting>
 592
 593     <para>(&ldquo;It's just a simple functional language&rdquo; is an
 594     unregisterised trademark of Peyton Jones Enterprises, plc.)</para>
 595
 596   </sect2>
 597
 598   <sect2 id="unreg">
 599     <title>Unregisterised compilation</title>
 600     <indexterm><primary>unregisterised compilation</primary></indexterm>
 601
 602     <para>The term "unregisterised" really means "compile via vanilla
 603     C", disabling some of the platform-specific tricks that GHC
 604     normally uses to make programs go faster.  When compiling
 605     unregisterised, GHC simply generates a C file which is compiled
 606     via gcc.</para>
 607
 608     <para>Unregisterised compilation can be useful when porting GHC to
 609     a new machine, since it reduces the prerequisite tools to
 610     <command>gcc</command>, <command>as</command>, and
 611     <command>ld</command> and nothing more, and furthermore the amount
 612     of platform-specific code that needs to be written in order to get
 613     unregisterised compilation going is usually fairly small.</para>
 614
 615    <para>Unregisterised compilation cannot be selected at
 616    compile-time; you have to build GHC with the appropriate options
 617    set.  Consult the GHC Building Guide for details.</para>
 618   </sect2>
 619
 620 </sect1>
 621
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