ghc/docs/users_guide/using.sgml

   1 <chapter id="using-ghc">
   2   <title>Using GHC</title>
   3
   4   <indexterm><primary>GHC, using</primary></indexterm>
   5   <indexterm><primary>using GHC</primary></indexterm>
   6
   7   <para>GHC can work in one of three &ldquo;modes&rdquo;:</para>
   8
   9   <variablelist>
  10     <varlistentry>
  11       <term><cmdsynopsis><command>ghc</command>
  12           <arg choice=plain>&ndash;&ndash;interactive</arg>
  13         </cmdsynopsis></term>
  14       <indexterm><primary>interactive mode</primary>
  15       </indexterm>
  16       <indexterm><primary>ghci</primary>
  17       </indexterm>
  18       <listitem>
  19         <para>Interactive mode, which is also available as
  20         <command>ghci</command>.  Interactive mode is described in
  21         more detail in <xref linkend="ghci">.</para>
  22       </listitem>
  23     </varlistentry>
  24
  25     <varlistentry>
  26       <term><cmdsynopsis><command>ghc</command>
  27           <arg choice=plain>&ndash;&ndash;make</arg>
  28         </cmdsynopsis></term>
  29       <indexterm><primary>make mode</primary>
  30       </indexterm>
  31       <indexterm><primary><option>&ndash;&ndash;make</option></primary>
  32       </indexterm>
  33       <listitem>
  34         <para>In this mode, GHC will build a multi-module Haskell
  35         program automatically, figuring out dependencies for itself.
  36         If you have a straightforward Haskell program, this is likely
  37         to be much easier, and faster, than using
  38         <command>make</command>.</para>
  39       </listitem>
  40     </varlistentry>
  41
  42     <varlistentry>
  43       <term><cmdsynopsis>
  44           <command>ghc</command>
  45           <group>
  46             <arg>-E</arg>
  47             <arg>-C</arg>
  48             <arg>-S</arg>
  49             <arg>-c</arg>
  50           </group>
  51         </cmdsynopsis></term>
  52       <indexterm><primary><option>-E</option></primary></indexterm>
  53       <indexterm><primary><option>-C</option></primary></indexterm>
  54       <indexterm><primary><option>-S</option></primary></indexterm>
  55       <indexterm><primary><option>-c</option></primary></indexterm>
  56       <listitem>
  57         <para>This is the traditional batch-compiler mode, in which
  58           GHC can compile source files one at a time, or link objects
  59           together into an executable.</para>
  60       </listitem>
  61     </varlistentry>
  62   </variablelist>
  63
  64   <sect1>
  65     <title>Options overview</title>
  66
  67     <para>GHC's behaviour is controlled by
  68     <firstterm>options</firstterm>, which for historical reasons are
  69     also sometimes referred to as command-line flags or arguments.
  70     Options can be specified in three ways:</para>
  71
  72     <sect2>
  73       <title>Command-line arguments</title>
  74
  75       <indexterm><primary>structure, command-line</primary></indexterm>
  76       <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
  77       <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
  78
  79       <para>An invocation of GHC takes the following form:</para>
  80
  81 <Screen>
  82 ghc [argument...]
  83 </Screen>
  84
  85       <para>Command-line arguments are either options or file names.</para>
  86
  87       <para>Command-line options begin with <literal>-</literal>.
  88       They may <emphasis>not</emphasis> be grouped:
  89       <option>-vO</option> is different from <option>-v -O</option>.
  90       Options need not precede filenames: e.g., <literal>ghc *.o -o
  91       foo</literal>.  All options are processed and then applied to
  92       all files; you cannot, for example, invoke <literal>ghc -c -O1
  93       Foo.hs -O2 Bar.hs</literal> to apply different optimisation
  94       levels to the files <filename>Foo.hs</filename> and
  95       <filename>Bar.hs</filename>.</para>
  96     </sect2>
  97
  98     <Sect2 id="source-file-options">
  99       <title>Command line options in source files</title>
 100
 101       <indexterm><primary>source-file options</primary></indexterm>
 102
 103       <para>Sometimes it is useful to make the connection between a
 104       source file and the command-line options it requires quite
 105       tight. For instance, if a Haskell source file uses GHC
 106       extensions, it will always need to be compiled with the
 107       <option>-fglasgow-exts</option> option.  Rather than maintaining
 108       the list of per-file options in a <filename>Makefile</filename>,
 109       it is possible to do this directly in the source file using the
 110       <literal>OPTIONS</literal> pragma <indexterm><primary>OPTIONS
 111       pragma</primary></indexterm>:</para>
 112
 113 <ProgramListing>
 114 {-# OPTIONS -fglasgow-exts #-}
 115 module X where
 116 ...
 117 </ProgramListing>
 118
 119       <para><literal>OPTIONS</literal> pragmas are only looked for at
 120       the top of your source files, upto the first
 121       (non-literate,non-empty) line not containing
 122       <literal>OPTIONS</literal>. Multiple <literal>OPTIONS</literal>
 123       pragmas are recognised. Note that your command shell does not
 124       get to the source file options, they are just included literally
 125       in the array of command-line arguments the compiler driver
 126       maintains internally, so you'll be desperately disappointed if
 127       you try to glob etc. inside <literal>OPTIONS</literal>.</para>
 128
 129       <para>NOTE: the contents of OPTIONS are prepended to the
 130       command-line options, so you <emphasis>do</emphasis> have the
 131       ability to override OPTIONS settings via the command
 132       line.</para>
 133
 134       <para>It is not recommended to move all the contents of your
 135       Makefiles into your source files, but in some circumstances, the
 136       <literal>OPTIONS</literal> pragma is the Right Thing. (If you
 137       use <option>-keep-hc-file-too</option> and have OPTION flags in
 138       your module, the OPTIONS will get put into the generated .hc
 139       file).</para>
 140     </sect2>
 141
 142     <sect2>
 143       <title>Setting options in GHCi</title>
 144
 145       <para>Options may also be modified from within GHCi, using the
 146       <literal>:set</literal> command.  See <xref linkend="ghci-set">
 147       for more details.</para>
 148     </sect2>
 149   </sect1>
 150
 151   <sect1 id="static-dynamic-flags">
 152     <title>Static vs. Dynamic options</title>
 153     <indexterm><primary>static</primary><secondary>options</secondary>
 154     </indexterm>
 155     <indexterm><primary>dynamic</primary><secondary>options</secondary>
 156     </indexterm>
 157
 158     <para>Each of GHC's command line options is classified as either
 159     <firstterm>static</firstterm> or <firstterm>dynamic</firstterm>.
 160     A static flag may only be specified on the command line, whereas a
 161     dynamic flag may also be given in an <literal>OPTIONS</literal>
 162     pragma in a source file or set from the GHCi command-line with
 163     <literal>:set</literal>.</para>
 164
 165     <para>As a rule of thumb, all the language options are dynamic, as
 166     are the warning options and the debugging options.  The rest are
 167     static, with the notable exceptions of <option>-v</option>,
 168     <option>-cpp</option>, <option>-fasm</option>,
 169     <option>-fvia-C</option>, and <option>-#include</option>.
 170
 171     The flag reference tables (<xref linkend="flag-reference">) lists
 172     the status of each flag.</para>
 173   </sect1>
 174
 175   <sect1 id="file-suffixes">
 176     <title>Meaningful file suffixes</title>
 177
 178     <indexterm><primary>suffixes, file</primary></indexterm>
 179     <indexterm><primary>file suffixes for GHC</primary></indexterm>
 180
 181     <para>File names with &ldquo;meaningful&rdquo; suffixes (e.g.,
 182     <filename>.lhs</filename> or <filename>.o</filename>) cause the
 183     &ldquo;right thing&rdquo; to happen to those files.</para>
 184
 185     <variablelist>
 186
 187       <varlistentry>
 188         <term><filename>.lhs</filename></term>
 189         <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
 190         <listitem>
 191           <para>A &ldquo;literate Haskell&rdquo; module.</para>
 192         </listitem>
 193       </varlistentry>
 194
 195       <varlistentry>
 196         <term><filename>.hs</filename></term>
 197         <listitem>
 198           <para>A not-so-literate Haskell module.</para>
 199         </listitem>
 200       </varlistentry>
 201
 202       <varlistentry>
 203         <term><filename>.hi</filename></term>
 204         <listitem>
 205           <para>A Haskell interface file, probably
 206           compiler-generated.</para>
 207         </listitem>
 208       </varlistentry>
 209
 210       <varlistentry>
 211         <term><filename>.hc</filename></term>
 212         <listitem>
 213           <para>Intermediate C file produced by the Haskell
 214           compiler.</para>
 215         </listitem>
 216       </varlistentry>
 217
 218       <varlistentry>
 219         <term><filename>.c</filename></term>
 220         <listitem>
 221           <para>A C&nbsp;file not produced by the Haskell
 222           compiler.</para>
 223         </listitem>
 224       </varlistentry>
 225
 226       <varlistentry>
 227         <term><filename>.s</filename></term>
 228         <listitem>
 229           <para>An assembly-language source file, usually produced by
 230           the compiler.</para>
 231         </listitem>
 232       </varlistentry>
 233
 234       <varlistentry>
 235         <term><filename>.o</filename></term>
 236         <listitem>
 237           <para>An object file, produced by an assembler.</para>
 238         </listitem>
 239       </varlistentry>
 240     </variablelist>
 241
 242     <para>Files with other suffixes (or without suffixes) are passed
 243     straight to the linker.</para>
 244
 245   </sect1>
 246
 247   <sect1 id="options-help">
 248     <title>Help and verbosity options</title>
 249
 250     <IndexTerm><Primary>help options</Primary></IndexTerm>
 251     <IndexTerm><Primary>verbosity options</Primary></IndexTerm>
 252
 253     <variablelist>
 254       <varlistentry>
 255         <term><option>&ndash;&ndash;help</option></term>
 256         <term><option>-?</option></term>
 257         <indexterm><primary><option>-?</option></primary></indexterm>
 258         <indexterm><primary><option>&ndash;&ndash;help</option></primary></indexterm>
 259         <listitem>
 260           <para>Cause GHC to spew a long usage message to standard
 261           output and then exit.</para>
 262         </listitem>
 263       </varlistentry>
 264
 265       <varlistentry>
 266         <term><option>-v</option></term>
 267         <indexterm><primary><option>-v</option></primary></indexterm>
 268         <listitem>
 269           <para>The <option>-v</option> option makes GHC
 270           <emphasis>verbose</emphasis>: it reports its version number
 271           and shows (on stderr) exactly how it invokes each phase of
 272           the compilation system.  Moreover, it passes the
 273           <option>-v</option> flag to most phases; each reports its
 274           version number (and possibly some other information).</para>
 275
 276           <para>Please, oh please, use the <option>-v</option> option
 277           when reporting bugs!  Knowing that you ran the right bits in
 278           the right order is always the first thing we want to
 279           verify.</para>
 280         </listitem>
 281       </varlistentry>
 282
 283       <varlistentry>
 284         <term><option>-v</option><replaceable>n</replaceable></term>
 285         <indexterm><primary><option>-v</option></primary></indexterm>
 286         <listitem>
 287           <para>To provide more control over the compiler's verbosity,
 288           the <option>-v</option> flag takes an optional numeric
 289           argument.  Specifying <option>-v</option> on its own is
 290           equivalent to <option>-v3</option>, and the other levels
 291           have the following meanings:</para>
 292
 293           <variablelist>
 294             <varlistentry>
 295               <term><option>-v0</option></term>
 296               <listitem>
 297                 <para>Disable all non-essential messages (this is the
 298                 default).</para>
 299               </listitem>
 300             </varlistentry>
 301
 302             <varlistentry>
 303               <term><option>-v1</option></term>
 304               <listitem>
 305                 <para>Minimal verbosity: print one line per
 306                 compilation (this is the default when
 307                 <option>&ndash;&ndash;make</option> or
 308                 <option>&ndash;&ndash;interactive</option> is on).</para>
 309               </listitem>
 310             </varlistentry>
 311
 312             <varlistentry>
 313               <term><option>-v2</option></term>
 314               <listitem>
 315                 <para>Print the name of each compilation phase as it
 316                 is executed. (equivalent to
 317                 <option>-dshow-passes</option>).</para>
 318               </listitem>
 319             </varlistentry>
 320
 321             <varlistentry>
 322               <term><option>-v3</option></term>
 323               <listitem>
 324                 <para>The same as <option>-v2</option>, except that in
 325                 addition the full command line (if appropriate) for
 326                 each compilation phase is also printed.</para>
 327               </listitem>
 328             </varlistentry>
 329
 330             <varlistentry>
 331               <term><option>-v4</option></term>
 332               <listitem>
 333                 <para>The same as <option>-v3</option> except that the
 334                 intermediate program representation after each
 335                 compilation phase is also printed (excluding
 336                 preprocessed and C/assembly files).</para>
 337               </listitem>
 338             </varlistentry>
 339           </variablelist>
 340         </listitem>
 341       </varlistentry>
 342
 343       <varlistentry>
 344         <term><option>&ndash;&ndash;version</option></term>
 345         <indexterm><primary><option>&ndash;&ndash;version</option></primary></indexterm>
 346         <listitem>
 347           <para>Print a one-line string including GHC's version number.</para>
 348         </listitem>
 349       </varlistentry>
 350
 351       <varlistentry>
 352         <term><option>&ndash;&ndash;numeric-version</option></term>
 353         <indexterm><primary><option>&ndash;&ndash;numeric-version</option></primary></indexterm>
 354         <listitem>
 355           <para>Print GHC's numeric version number only.</para>
 356         </listitem>
 357       </varlistentry>
 358
 359       <varlistentry>
 360         <term><option>&ndash;&ndash;print-libdir</option></term>
 361         <indexterm><primary><option>&ndash;&ndash;print-libdir</option></primary></indexterm>
 362         <listitem>
 363           <para>Print the path to GHC's library directory.  This is
 364           the top of the directory tree containing GHC's libraries,
 365           interfaces, and include files (usually something like
 366           <literal>/usr/local/lib/ghc-5.02</literal> on Unix).  This
 367           is the value of
 368           <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary>
 369           </indexterm>in the package configuration file (see <xref
 370           linkend="packages">).</para>
 371         </listitem>
 372       </varlistentry>
 373
 374     </variablelist>
 375   </sect1>
 376
 377   <sect1 id="make-mode">
 378     <title>Using <command>ghc</command> <option>&ndash;&ndash;make</option></title>
 379
 380     <indexterm><primary><option>&ndash;&ndash;make</option></primary>
 381     </indexterm>
 382     <indexterm><primary>separate compilation</primary>
 383     </indexterm>
 384
 385     <para>When given the <option>&ndash;&ndash;make</option> option, GHC will
 386     build a multi-module Haskell program by following dependencies
 387     from a single root module (usually <literal>Main</literal>).  For
 388     example, if your <literal>Main</literal> module is in a file
 389     called <filename>Main.hs</filename>, you could compile and link
 390     the program like this:</para>
 391
 392 <screen>
 393 ghc &ndash;&ndash;make Main.hs
 394 </screen>
 395
 396     <para>The command line must contain one source file or module
 397     name; GHC will figure out all the modules in the program by
 398     following the imports from this initial module.  It will then
 399     attempt to compile each module which is out of date, and finally
 400     if the top module is <literal>Main</literal>, the program
 401     will also be linked into an executable.</para>
 402
 403     <para>The main advantages to using <literal>ghc &ndash;&ndash;make</literal>
 404     over traditional <literal>Makefile</literal>s are:</para>
 405
 406     <itemizedlist>
 407       <listitem>
 408         <para>GHC doesn't have to be restarted for each compilation,
 409         which means it can cache information between compilations.
 410         Compiling a muli-module program with <literal>ghc
 411         &ndash;&ndash;make</literal> can be up to twice as fast as running
 412         <literal>ghc</literal> individually on each source
 413         file.</para>
 414       </listitem>
 415       <listitem>
 416         <para>You don't have to write a
 417         <literal>Makefile</literal>.</para>
 418       </listitem>
 419       <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary>
 420       </indexterm>
 421       <listitem>
 422         <para>GHC re-calculates the dependencies each time it is
 423         invoked, so the dependencies never get out of sync with the
 424         source.</para>
 425       </listitem>
 426     </itemizedlist>
 427
 428     <para>Any of the command-line options described in the rest of
 429     this chapter can be used with <option>&ndash;&ndash;make</option>, but note
 430     that any options you give on the command line will apply to all
 431     the source files compiled, so if you want any options to apply to
 432     a single source file only, you'll need to use an
 433     <literal>OPTIONS</literal> pragma (see <xref
 434     linkend="source-file-options">).</para>
 435
 436     <para>If the program needs to be linked with additional objects
 437     (say, some auxilliary C code), these can be specified on the
 438     command line as usual.</para>
 439
 440     <para>Note that GHC can only follow dependencies if it has the
 441     source file available, so if your program includes a module for
 442     which there is no source file, even if you have an object and an
 443     interface file for the module, then GHC will complain.  The
 444     exception to this rule is for package modules, which may or may
 445     not have source files.</para>
 446   </sect1>
 447
 448   <Sect1 id="options-order">
 449     <title>GHC without <option>&ndash;&ndash;make</option></title>
 450
 451     <para>Without <option>&ndash;&ndash;make</option>, GHC will compile one or
 452     more source files given on the command line.</para>
 453
 454     <para>The first phase to run is determined by each input-file
 455     suffix, and the last phase is determined by a flag.  If no
 456     relevant flag is present, then go all the way through linking.
 457     This table summarises:</para>
 458
 459     <informaltable>
 460       <tgroup cols="4">
 461         <colspec align="left">
 462         <colspec align="left">
 463         <colspec align="left">
 464         <colspec align="left">
 465
 466         <thead>
 467           <row>
 468             <entry>Phase of the compilation system</entry>
 469             <entry>Suffix saying &ldquo;start here&rdquo;</entry>
 470             <entry>Flag saying &ldquo;stop after&rdquo;</entry>
 471             <entry>(suffix of) output file</entry>
 472           </row>
 473         </thead>
 474         <tbody>
 475           <row>
 476             <entry>literate pre-processor</entry>
 477             <entry><literal>.lhs</literal></entry>
 478             <entry>-</entry>
 479             <entry><literal>.hs</literal></entry>
 480           </row>
 481
 482           <row>
 483             <entry>C pre-processor (opt.)
 484            </entry>
 485             <entry><literal>.hs</literal> (with
 486             <option>-cpp</option>)</entry>
 487             <entry><option>-E</option></entry>
 488             <entry><literal>.hspp</literal></entry>
 489           </row>
 490
 491           <row>
 492             <entry>Haskell compiler</entry>
 493             <entry><literal>.hs</literal></entry>
 494             <entry><option>-C</option>, <option>-S</option></entry>
 495             <entry><literal>.hc</literal>, <literal>.s</literal></entry>
 496           </row>
 497
 498           <row>
 499             <entry>C compiler (opt.)</entry>
 500             <entry><literal>.hc</literal> or <literal>.c</literal></entry>
 501             <entry><option>-S</option></entry>
 502             <entry><literal>.s</literal></entry>
 503           </row>
 504
 505           <row>
 506             <entry>assembler</entry>
 507             <entry><literal>.s</literal></entry>
 508             <entry><option>-c</option></entry>
 509             <entry><literal>.o</literal></entry>
 510           </row>
 511
 512           <row>
 513             <entry>linker</entry>
 514             <entry><replaceable>other</replaceable></entry>
 515             <entry>-</entry>
 516             <entry><filename>a.out</filename></entry>
 517           </row>
 518         </tbody>
 519       </tgroup>
 520     </informaltable>
 521
 522     <indexterm><primary><option>-C</option></primary></indexterm>
 523     <indexterm><primary><option>-E</option></primary></indexterm>
 524     <indexterm><primary><option>-S</option></primary></indexterm>
 525     <indexterm><primary><option>-c</option></primary></indexterm>
 526
 527     <para>Thus, a common invocation would be: <literal>ghc -c
 528     Foo.hs</literal></para>
 529
 530     <para>Note: What the Haskell compiler proper produces depends on
 531     whether a native-code generator<indexterm><primary>native-code
 532     generator</primary></indexterm> is used (producing assembly
 533     language) or not (producing C).  See <xref
 534     linkend="options-codegen"> for more details.</para>
 535
 536     <para>Note: C pre-processing is optional, the
 537     <option>-ccp</option><indexterm><primary><option>-cpp</option></primary>
 538       </indexterm>flag turns it on.  See <xref
 539     linkend="c-pre-processor"> for more details.</para>
 540
 541     <para>Note: The option <option>-E</option><IndexTerm><Primary>-E
 542     option</Primary></IndexTerm> runs just the pre-processing passes
 543     of the compiler, dumping the result in a file.  Note that this
 544     differs from the previous behaviour of dumping the file to
 545     standard output.</para>
 546   </sect1>
 547
 548   <sect1 id="options-output">
 549     <title>Re-directing the compilation output(s)</title>
 550
 551     <indexterm><primary>output-directing options</primary></indexterm>
 552     <indexterm><primary>redirecting compilation output</primary></indexterm>
 553
 554
 555     <variablelist>
 556       <varlistentry>
 557         <term><option>-o</option></term>
 558         <indexterm><primary><option>-o</option></primary></indexterm>
 559         <listitem>
 560           <para>GHC's compiled output normally goes into a
 561           <filename>.hc</filename>, <filename>.o</filename>, etc.,
 562           file, depending on the last-run compilation phase.  The
 563           option <option>-o foo</option><IndexTerm><Primary>-o
 564           option</Primary></IndexTerm> re-directs the output of that
 565           last-run phase to file <filename>foo</filename>.</para>
 566
 567           <para>Note: this &ldquo;feature&rdquo; can be
 568           counterintuitive: <command>ghc -C -o foo.o foo.hs</command>
 569           will put the intermediate C code in the file
 570           <filename>foo.o</filename>, name notwithstanding!</para>
 571         </listitem>
 572       </varlistentry>
 573
 574       <varlistentry>
 575         <term><option>-odir</option></term>
 576         <indexterm><primary><option>-odir</option></primary></indexterm>
 577         <listitem>
 578           <para>The <option>-o</option> option isn't of much use if
 579           you have <emphasis>several</emphasis> input files&hellip;
 580           Non-interface output files are normally put in the same
 581           directory as their corresponding input file came from.  You
 582           may specify that they be put in another directory using the
 583           <option>-odir &lt;dir&gt;</option><IndexTerm><Primary>-odir
 584           &lt;dir&gt; option</Primary></IndexTerm> (the &ldquo;Oh,
 585           dear&rdquo; option).  For example:</para>
 586
 587 <Screen>
 588 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
 589 </Screen>
 590
 591           <para>The output files, <filename>Foo.o</filename>,
 592           <filename>Bar.o</filename>, and
 593           <filename>Bumble.o</filename> would be put into a
 594           subdirectory named after the architecture of the executing
 595           machine (<filename>sun4</filename>,
 596           <filename>mips</filename>, etc).  The directory must already
 597           exist; it won't be created.</para>
 598
 599           <para>Note that the <option>-odir</option> option does
 600           <emphasis>not</emphasis> affect where the interface files
 601           are put.  In the above example, they would still be put in
 602           <filename>parse/Foo.hi</filename>,
 603           <filename>parse/Bar.hi</filename>, and
 604           <filename>gurgle/Bumble.hi</filename>.</para>
 605         </listitem>
 606       </varlistentry>
 607
 608       <varlistentry>
 609         <term><option>-ohi</option>  <replaceable>file</replaceable></term>
 610         <indexterm><primary><option>-ohi</option></primary>
 611         </indexterm>
 612         <listitem>
 613           <para>The interface output may be directed to another file
 614           <filename>bar2/Wurble.iface</filename> with the option
 615           <option>-ohi bar2/Wurble.iface</option> (not
 616           recommended).</para>
 617
 618           <para>WARNING: if you redirect the interface file somewhere
 619           that GHC can't find it, then the recompilation checker may
 620           get confused (at the least, you won't get any recompilation
 621           avoidance).  We recommend using a combination of
 622           <option>-hidir</option> and <option>-hisuf</option> options
 623           instead, if possible.</para>
 624
 625           <para>To avoid generating an interface at all, you could use
 626           this option to redirect the interface into the bit bucket:
 627           <literal>-ohi /dev/null</literal>, for example.</para>
 628         </listitem>
 629       </varlistentry>
 630
 631       <varlistentry>
 632         <term><option>-hidir</option>  <replaceable>directory</replaceable></term>
 633         <indexterm><primary><option>-hidir</option></primary>
 634         </indexterm>
 635         <listitem>
 636           <para>Redirects all generated interface files into
 637           <replaceable>directory</replaceable>, instead of the default
 638           which is to place the interface file in the same directory
 639           as the source file.</para>
 640         </listitem>
 641       </varlistentry>
 642
 643       <varlistentry>
 644         <term><option>-osuf</option> <replaceable>suffix</replaceable></term>
 645         <term><option>-hisuf</option> <replaceable>suffix</replaceable></term>
 646         <term><option>-hcsuf</option> <replaceable>suffix</replaceable></term>
 647         <indexterm><primary><option>-osuf</option></primary></indexterm>
 648         <indexterm><primary><option>-hisuf</option></primary></indexterm>
 649         <indexterm><primary><option>-hcsuf</option></primary></indexterm>
 650         <listitem>
 651           <para>EXOTICA: The <option>-osuf</option>
 652           <replaceable>suffix</replaceable> will change the
 653           <literal>.o</literal> file suffix for object files to
 654           whatever you specify.  We use this when compiling libraries,
 655           so that objects for the profiling versions of the libraries
 656           don't clobber the normal ones.</para>
 657
 658           <para>Similarly, the <option>-hisuf</option>
 659           <replaceable>suffix</replaceable> will change the
 660           <literal>.hi</literal> file suffix for non-system interface
 661           files (see <XRef LinkEnd="hi-options">).</para>
 662
 663           <para>Finally, the option <option>-hcsuf</option>
 664           <replaceable>suffix</replaceable> will change the
 665           <literal>.hc</literal> file suffix for compiler-generated
 666           intermediate C files.</para>
 667
 668           <para>The <option>-hisuf</option>/<option>-osuf</option>
 669           game is useful if you want to compile a program with both
 670           GHC and HBC (say) in the same directory.  Let HBC use the
 671           standard <filename>.hi</filename>/<filename>.o</filename>
 672           suffixes; add <option>-hisuf g&lowbar;hi -osuf
 673           g&lowbar;o</option> to your <command>make</command> rule for
 674           GHC compiling&hellip;</para>
 675         </listitem>
 676       </varlistentry>
 677     </variablelist>
 678
 679     <sect2 id="keeping-intermediates">
 680       <title>Keeping Intermediate Files</title>
 681       <indexterm><primary>intermediate files, saving</primary>
 682       </indexterm>
 683       <indexterm><primary><literal>.hc</literal> files, saving</primary>
 684       </indexterm>
 685       <indexterm><primary><literal>.s</literal> files, saving</primary>
 686       </indexterm>
 687
 688
 689       <para>The following options are useful for keeping certain
 690       intermediate files around, when normally GHC would throw these
 691       away after compilation:</para>
 692
 693       <variablelist>
 694         <varlistentry>
 695           <term><option>-keep-hc-files</option></term>
 696           <indexterm>
 697             <primary><option>-keep-hc-files</option></primary>
 698           </indexterm>
 699           <listitem>
 700             <para>Keep intermediate <literal>.hc</literal> files when
 701             doing <literal>.hs</literal>-to-<literal>.o</literal>
 702             compilations via C (NOTE: <literal>.hc</literal> files
 703             aren't generated when using the native code generator, you
 704             may need to use <option>-fvia-C</option> to force them
 705             to be produced).</para>
 706           </listitem>
 707         </varlistentry>
 708
 709         <varlistentry>
 710           <term><option>-keep-s-files</option></term>
 711           <indexterm>
 712             <primary><option>-keep-s-files</option></primary>
 713           </indexterm>
 714           <listitem>
 715             <para>Keep intermediate <literal>.s</literal> files.</para>
 716           </listitem>
 717         </varlistentry>
 718
 719         <varlistentry>
 720           <term><option>-keep-raw-s-files</option></term>
 721           <indexterm>
 722             <primary><option>-keep-raw-s-files</option></primary>
 723           </indexterm>
 724           <listitem>
 725             <para>Keep intermediate <literal>.raw-s</literal> files.
 726             These are the direct output from the C compiler, before
 727             GHC does &ldquo;assembly mangling&rdquo; to produce the
 728             <literal>.s</literal> file.  Again, these are not produced
 729             when using the native code generator.</para>
 730           </listitem>
 731         </varlistentry>
 732
 733         <varlistentry>
 734           <term><option>-keep-tmp-files</option></term>
 735           <indexterm>
 736             <primary><option>-keep-tmp-files</option></primary>
 737           </indexterm>
 738           <indexterm>
 739             <primary>temporary files</primary>
 740             <secondary>keeping</secondary>
 741           </indexterm>
 742           <listitem>
 743             <para>Instructs the GHC driver not to delete any of its
 744             temporary files, which it normally keeps in
 745             <literal>/tmp</literal> (or possibly elsewhere; see <xref
 746             linkend="temp-files">).  Running GHC with
 747             <option>-v</option> will show you what temporary files
 748             were generated along the way.</para>
 749           </listitem>
 750         </varlistentry>
 751       </variablelist>
 752     </sect2>
 753
 754     <sect2 id="temp-files">
 755       <title>Redirecting temporary files</title>
 756
 757       <indexterm>
 758         <primary>temporary files</primary>
 759         <secondary>redirecting</secondary>
 760       </indexterm>
 761
 762       <variablelist>
 763         <varlistentry>
 764           <term><option>-tmpdir</option></term>
 765           <indexterm><primary><option>-tmpdir</option></primary></indexterm>
 766           <listitem>
 767             <para>If you have trouble because of running out of space
 768             in <filename>/tmp</filename> (or wherever your
 769             installation thinks temporary files should go), you may
 770             use the <option>-tmpdir
 771             &lt;dir&gt;</option><IndexTerm><Primary>-tmpdir
 772             &lt;dir&gt; option</Primary></IndexTerm> option to specify
 773             an alternate directory.  For example, <option>-tmpdir
 774             .</option> says to put temporary files in the current
 775             working directory.</para>
 776
 777             <para>Alternatively, use your <Constant>TMPDIR</Constant>
 778             environment variable.<IndexTerm><Primary>TMPDIR
 779             environment variable</Primary></IndexTerm> Set it to the
 780             name of the directory where temporary files should be put.
 781             GCC and other programs will honour the
 782             <Constant>TMPDIR</Constant> variable as well.</para>
 783
 784             <para>Even better idea: Set the
 785             <Constant>DEFAULT_TMPDIR</Constant> make variable when
 786             building GHC, and never worry about
 787             <Constant>TMPDIR</Constant> again. (see the build
 788             documentation).</para>
 789           </listitem>
 790         </varlistentry>
 791       </variablelist>
 792     </sect2>
 793
 794   </sect1>
 795
 796   <sect1 id="options-sanity">
 797     <title>Warnings and sanity-checking</title>
 798
 799     <indexterm><primary>sanity-checking options</primary></indexterm>
 800     <indexterm><primary>warnings</primary></indexterm>
 801
 802
 803     <para>GHC has a number of options that select which types of
 804     non-fatal error messages, otherwise known as warnings, can be
 805     generated during compilation.  By default, you get a standard set
 806     of warnings which are generally likely to indicate bugs in your
 807     program.  These are:
 808     <option>-fwarn-overlpapping-patterns</option>,
 809     <option>-fwarn-deprecations</option>,
 810     <option>-fwarn-duplicate-exports</option>,
 811     <option>-fwarn-missing-fields</option>, and
 812     <option>-fwarn-missing-methods</option>.  The following flags are
 813     simple ways to select standard &ldquo;packages&rdquo; of warnings:
 814     </para>
 815
 816     <VariableList>
 817
 818       <varlistentry>
 819         <term><option>-W</option>:</term>
 820         <listitem>
 821           <IndexTerm><Primary>-W option</Primary></IndexTerm>
 822           <para>Provides the standard warnings plus
 823           <option>-fwarn-incomplete-patterns</option>,
 824           <option>-fwarn-unused-matches</option>,
 825           <option>-fwarn-unused-imports</option>,
 826           <option>-fwarn-misc</option>, and
 827           <option>-fwarn-unused-binds</option>.</para>
 828         </listitem>
 829       </varlistentry>
 830
 831       <varlistentry>
 832         <term><option>-w</option>:</term>
 833         <listitem>
 834           <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
 835           <para>Turns off all warnings, including the standard ones.</para>
 836         </listitem>
 837       </varlistentry>
 838
 839       <varlistentry>
 840         <term><option>-Wall</option>:</term>
 841         <listitem>
 842           <indexterm><primary><option>-Wall</option></primary></indexterm>
 843           <para>Turns on all warning options.</para>
 844         </listitem>
 845       </varlistentry>
 846
 847     </variablelist>
 848
 849     <para>The full set of warning options is described below.  To turn
 850     off any warning, simply give the corresponding
 851     <option>-fno-warn-...</option> option on the command line.</para>
 852
 853     <variablelist>
 854
 855       <varlistentry>
 856         <term><option>-fwarn-deprecations</option>:</term>
 857         <listitem>
 858           <indexterm><primary><option>-fwarn-deprecations</option></primary>
 859           </indexterm>
 860           <indexterm><primary>deprecations</primary></indexterm>
 861           <para>Causes a warning to be emitted when a deprecated
 862           function or type is used.  Entities can be marked as
 863           deprecated using a pragma, see <xref
 864           linkend="deprecated-pragma">.</para>
 865         </listitem>
 866       </varlistentry>
 867
 868       <varlistentry>
 869         <term><option>-fwarn-duplicate-exports</option>:</term>
 870         <listitem>
 871           <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
 872           <indexterm><primary>duplicate exports, warning</primary></indexterm>
 873           <indexterm><primary>export lists, duplicates</primary></indexterm>
 874
 875           <para>Have the compiler warn about duplicate entries in
 876           export lists. This is useful information if you maintain
 877           large export lists, and want to avoid the continued export
 878           of a definition after you've deleted (one) mention of it in
 879           the export list.</para>
 880
 881           <para>This option is on by default.</para>
 882         </listitem>
 883       </varlistentry>
 884
 885       <varlistentry>
 886         <term><option>-fwarn-hi-shadowing</option>:</term>
 887         <listitem>
 888           <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
 889           <indexterm><primary>shadowing</primary>
 890             <secondary>interface files</secondary></indexterm>
 891
 892           <para>Causes the compiler to emit a warning when a module or
 893           interface file in the current directory is shadowing one
 894           with the same module name in a library or other
 895           directory.</para>
 896         </listitem>
 897       </varlistentry>
 898
 899       <varlistentry>
 900         <term><option>-fwarn-incomplete-patterns</option>:</term>
 901         <listitem>
 902           <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
 903           <indexterm><primary>incomplete patterns, warning</primary></indexterm>
 904           <indexterm><primary>patterns, incomplete</primary></indexterm>
 905
 906           <para>Similarly for incomplete patterns, the function
 907           <function>g</function> below will fail when applied to
 908           non-empty lists, so the compiler will emit a warning about
 909           this when <option>-fwarn-incomplete-patterns</option> is
 910           enabled.</para>
 911
 912 <programlisting>
 913 g [] = 2
 914 </programlisting>
 915
 916           <para>This option isn't enabled be default because it can be
 917           a bit noisy, and it doesn't always indicate a bug in the
 918           program.  However, it's generally considered good practice
 919           to cover all the cases in your functions.</para>
 920         </listitem>
 921       </varlistentry>
 922
 923       <varlistentry>
 924         <term><option>-fwarn-misc</option>:</term>
 925         <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
 926         <listitem>
 927           <para>Turns on warnings for various harmless but untidy
 928           things.  This currently includes: importing a type with
 929           <literal>(..)</literal> when the export is abstract, and
 930           listing duplicate class assertions in a qualified type.</para>
 931         </listitem>
 932       </varlistentry>
 933
 934       <varlistentry>
 935         <term><option>-fwarn-missing-fields</option>:</term>
 936         <listitem>
 937           <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
 938           <indexterm><primary>missing fields, warning</primary></indexterm>
 939           <indexterm><primary>fields, missing</primary></indexterm>
 940
 941           <para>This option is on by default, and warns you whenever
 942           the construction of a labelled field constructor isn't
 943           complete, missing initializers for one or more fields. While
 944           not an error (the missing fields are initialised with
 945           bottoms), it is often an indication of a programmer error.</para>
 946         </listitem>
 947       </varlistentry>
 948
 949       <varlistentry>
 950         <term><option>-fwarn-missing-methods</option>:</term>
 951         <listitem>
 952           <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
 953           <indexterm><primary>missing methods, warning</primary></indexterm>
 954           <indexterm><primary>methods, missing</primary></indexterm>
 955
 956           <para>This option is on by default, and warns you whenever
 957           an instance declaration is missing one or more methods, and
 958           the corresponding class declaration has no default
 959           declaration for them.</para>
 960         </listitem>
 961       </varlistentry>
 962
 963       <varlistentry>
 964         <term><option>-fwarn-missing-signatures</option>:</term>
 965         <listitem>
 966           <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
 967           <indexterm><primary>type signatures, missing</primary></indexterm>
 968
 969           <para>If you would like GHC to check that every top-level
 970           function/value has a type signature, use the
 971           <option>-fwarn-missing-signatures</option> option.  This
 972           option is off by default.</para>
 973         </listitem>
 974       </varlistentry>
 975
 976       <varlistentry>
 977         <term><option>-fwarn-name-shadowing</option>:</term>
 978         <listitem>
 979           <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
 980           <indexterm><primary>shadowing, warning</primary></indexterm>
 981
 982           <para>This option causes a warning to be emitted whenever an
 983           inner-scope value has the same name as an outer-scope value,
 984           i.e. the inner value shadows the outer one.  This can catch
 985           typographical errors that turn into hard-to-find bugs, e.g.,
 986           in the inadvertent cyclic definition <literal>let x = ... x
 987           ... in</literal>.</para>
 988
 989           <para>Consequently, this option does
 990           <emphasis>will</emphasis> complain about cyclic recursive
 991           definitions.</para>
 992         </listitem>
 993       </varlistentry>
 994
 995       <varlistentry>
 996         <term><option>-fwarn-overlapping-patterns</option>:</term>
 997         <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
 998         <indexterm><primary>overlapping patterns, warning</primary></indexterm>
 999         <indexterm><primary>patterns, overlapping</primary></indexterm>
1000         <listitem>
1001           <para>By default, the compiler will warn you if a set of
1002           patterns are overlapping, i.e.,</para>
1003
1004 <programlisting>
1005 f :: String -&#62; Int
1006 f []     = 0
1007 f (_:xs) = 1
1008 f "2"    = 2
1009 </programlisting>
1010
1011           <para>where the last pattern match in <Function>f</Function>
1012           won't ever be reached, as the second pattern overlaps
1013           it. More often than not, redundant patterns is a programmer
1014           mistake/error, so this option is enabled by default.</para>
1015         </listitem>
1016       </varlistentry>
1017
1018       <varlistentry>
1019         <term><option>-fwarn-simple-patterns</option>:</term>
1020         <listitem>
1021           <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1022           </indexterm>
1023           <para>Causes the compiler to warn about lambda-bound
1024           patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1025           Normally, these aren't treated as incomplete patterns by
1026           <option>-fwarn-incomplete-patterns</option>.</para>
1027         </listitem>
1028       </varlistentry>
1029
1030       <varlistentry>
1031         <term><option>-fwarn-type-defaults</option>:</term>
1032         <listitem>
1033           <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1034           <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1035           <para>Have the compiler warn/inform you where in your source
1036           the Haskell defaulting mechanism for numeric types kicks
1037           in. This is useful information when converting code from a
1038           context that assumed one default into one with another,
1039           e.g., the `default default' for Haskell 1.4 caused the
1040           otherwise unconstrained value <Constant>1</Constant> to be
1041           given the type <literal>Int</literal>, whereas Haskell 98
1042           defaults it to <literal>Integer</literal>.  This may lead to
1043           differences in performance and behaviour, hence the
1044           usefulness of being non-silent about this.</para>
1045
1046           <para>This warning is off by default.</para>
1047         </listitem>
1048       </varlistentry>
1049
1050       <varlistentry>
1051         <term><option>-fwarn-unused-binds</option>:</term>
1052         <listitem>
1053           <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1054           <indexterm><primary>unused binds, warning</primary></indexterm>
1055           <indexterm><primary>binds, unused</primary></indexterm>
1056           <para>Report any function definitions (and local bindings)
1057           which are unused.  For top-level functions, the warning is
1058           only given if the binding is not exported.</para>
1059         </listitem>
1060       </varlistentry>
1061
1062       <varlistentry>
1063         <term><option>-fwarn-unused-imports</option>:</term>
1064         <listitem>
1065           <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1066           <indexterm><primary>unused imports, warning</primary></indexterm>
1067           <indexterm><primary>imports, unused</primary></indexterm>
1068
1069           <para>Report any objects that are explicitly imported but
1070           never used.</para>
1071         </listitem>
1072       </varlistentry>
1073
1074       <varlistentry>
1075         <term><option>-fwarn-unused-matches</option>:</term>
1076         <listitem>
1077           <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1078           <indexterm><primary>unused matches, warning</primary></indexterm>
1079           <indexterm><primary>matches, unused</primary></indexterm>
1080
1081           <para>Report all unused variables which arise from pattern
1082           matches, including patterns consisting of a single variable.
1083           For instance <literal>f x y = []</literal> would report
1084           <VarName>x</VarName> and <VarName>y</VarName> as unused.  To
1085           eliminate the warning, all unused variables can be replaced
1086           with wildcards.</para>
1087         </listitem>
1088       </varlistentry>
1089
1090     </VariableList>
1091
1092     <para>If you're feeling really paranoid, the
1093     <option>-dcore-lint</option>
1094     option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1095     is a good choice.  It turns on heavyweight intra-pass
1096     sanity-checking within GHC.  (It checks GHC's sanity, not
1097     yours.)</para>
1098
1099   </sect1>
1100
1101   &separate;
1102   &packages;
1103
1104   <sect1 id="options-optimise">
1105     <title>Optimisation (code improvement)</title>
1106
1107     <indexterm><primary>optimisation</primary></indexterm>
1108     <indexterm><primary>improvement, code</primary></indexterm>
1109
1110     <para>The <option>-O*</option> options specify convenient
1111     &ldquo;packages&rdquo; of optimisation flags; the
1112     <option>-f*</option> options described later on specify
1113     <emphasis>individual</emphasis> optimisations to be turned on/off;
1114     the <option>-m*</option> options specify
1115     <emphasis>machine-specific</emphasis> optimisations to be turned
1116     on/off.</para>
1117
1118     <sect2 id="optimise-pkgs">
1119       <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1120
1121       <para>There are <emphasis>many</emphasis> options that affect
1122       the quality of code produced by GHC.  Most people only have a
1123       general goal, something like &ldquo;Compile quickly&rdquo; or
1124       &ldquo;Make my program run like greased lightning.&rdquo; The
1125       following &ldquo;packages&rdquo; of optimisations (or lack
1126       thereof) should suffice.</para>
1127
1128       <para>Once you choose a <option>-O*</option>
1129       &ldquo;package,&rdquo; stick with it&mdash;don't chop and
1130       change.  Modules' interfaces <emphasis>will</emphasis> change
1131       with a shift to a new <option>-O*</option> option, and you may
1132       have to recompile a large chunk of all importing modules before
1133       your program can again be run safely (see <XRef
1134       LinkEnd="recomp">).</para>
1135
1136       <variablelist>
1137
1138         <varlistentry>
1139           <term>No <option>-O*</option>-type option specified:</term>
1140           <indexterm><primary>-O* not specified</primary></indexterm>
1141           <listitem>
1142             <para>This is taken to mean: &ldquo;Please compile
1143             quickly; I'm not over-bothered about compiled-code
1144             quality.&rdquo; So, for example: <command>ghc -c
1145             Foo.hs</command></para>
1146           </listitem>
1147         </varlistentry>
1148
1149         <varlistentry>
1150           <term><option>-O0</option>:</term>
1151           <indexterm><primary><option>-O0</option></primary></indexterm>
1152           <listitem>
1153             <para>Means &ldquo;turn off all optimisation&rdquo;,
1154             reverting to the same settings as if no
1155             <option>-O</option> options had been specified.  Saying
1156             <option>-O0</option> can be useful if
1157             eg. <command>make</command> has inserted a
1158             <option>-O</option> on the command line already.</para>
1159           </listitem>
1160         </varlistentry>
1161
1162         <varlistentry>
1163           <term><option>-O</option> or <option>-O1</option>:</term>
1164           <indexterm><primary>-O option</primary></indexterm>
1165           <indexterm><primary>-O1 option</primary></indexterm>
1166           <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1167           <listitem>
1168             <para>Means: &ldquo;Generate good-quality code without
1169             taking too long about it.&rdquo; Thus, for example:
1170             <command>ghc -c -O Main.lhs</command></para>
1171           </listitem>
1172         </varlistentry>
1173
1174         <varlistentry>
1175           <term><option>-O2</option>:</term>
1176           <indexterm><primary>-O2 option</primary></indexterm>
1177           <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1178           <listitem>
1179             <para>Means: &ldquo;Apply every non-dangerous
1180             optimisation, even if it means significantly longer
1181             compile times.&rdquo;</para>
1182
1183             <para>The avoided &ldquo;dangerous&rdquo; optimisations
1184             are those that can make runtime or space
1185             <emphasis>worse</emphasis> if you're unlucky.  They are
1186             normally turned on or off individually.</para>
1187
1188             <para>At the moment, <option>-O2</option> is
1189             <emphasis>unlikely</emphasis> to produce better code than
1190             <option>-O</option>.</para>
1191           </listitem>
1192         </varlistentry>
1193
1194         <varlistentry>
1195           <term><option>-Ofile &lt;file&gt;</option>:</term>
1196           <indexterm><primary>-Ofile &lt;file&gt; option</primary></indexterm>
1197           <indexterm><primary>optimising, customised</primary></indexterm>
1198           <listitem>
1199             <para>(NOTE: not supported yet in GHC 5.x.  Please ask if
1200             you're interested in this.)</para>
1201
1202             <para>For those who need <emphasis>absolute</emphasis>
1203             control over <emphasis>exactly</emphasis> what options are
1204             used (e.g., compiler writers, sometimes :-), a list of
1205             options can be put in a file and then slurped in with
1206             <option>-Ofile</option>.</para>
1207
1208             <para>In that file, comments are of the
1209             <literal>&num;</literal>-to-end-of-line variety; blank
1210             lines and most whitespace is ignored.</para>
1211
1212             <para>Please ask if you are baffled and would like an
1213             example of <option>-Ofile</option>!</para>
1214           </listitem>
1215         </varlistentry>
1216       </variablelist>
1217
1218       <para>We don't use a <option>-O*</option> flag for day-to-day
1219       work.  We use <option>-O</option> to get respectable speed;
1220       e.g., when we want to measure something.  When we want to go for
1221       broke, we tend to use <option>-O -fvia-C</option> (and we go for
1222       lots of coffee breaks).</para>
1223
1224       <para>The easiest way to see what <option>-O</option> (etc.)
1225       &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1226       then stand back in amazement.</para>
1227     </sect2>
1228
1229     <sect2 id="options-f">
1230       <title><option>-f*</option>: platform-independent flags</title>
1231
1232       <indexterm><primary>-f* options (GHC)</primary></indexterm>
1233       <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1234
1235       <para>These flags turn on and off individual optimisations.
1236       They are normally set via the <option>-O</option> options
1237       described above, and as such, you shouldn't need to set any of
1238       them explicitly (indeed, doing so could lead to unexpected
1239       results).  However, there are one or two that may be of
1240       interest:</para>
1241
1242       <variablelist>
1243         <varlistentry>
1244           <term><option>-fexcess-precision</option>:</term>
1245           <listitem>
1246             <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1247             <para>When this option is given, intermediate floating
1248             point values can have a <emphasis>greater</emphasis>
1249             precision/range than the final type.  Generally this is a
1250             good thing, but some programs may rely on the exact
1251             precision/range of
1252             <literal>Float</literal>/<literal>Double</literal> values
1253             and should not use this option for their compilation.</para>
1254           </listitem>
1255         </varlistentry>
1256
1257         <varlistentry>
1258           <term><option>-fignore-asserts</option>:</term>
1259           <listitem>
1260             <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1261             <para>Causes GHC to ignore uses of the function
1262             <literal>Exception.assert</literal> in source code (in
1263             other words, rewriting <literal>Exception.assert p
1264             e</literal> to <literal>e</literal> (see <xref
1265             linkend="sec-assertions">).  This flag is turned on by
1266             <option>-O</option>.
1267             </para>
1268           </listitem>
1269         </varlistentry>
1270
1271         <varlistentry>
1272           <term><option>-fno-strictness</option></term>
1273           <indexterm><primary><option>-fno-strictness</option></primary>
1274           </indexterm>
1275           <listitem>
1276             <para>Turns off the strictness analyser; sometimes it eats
1277             too many cycles.</para>
1278           </listitem>
1279         </varlistentry>
1280
1281         <varlistentry>
1282           <term><option>-fno-cpr-analyse</option></term>
1283           <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1284           </indexterm>
1285           <listitem>
1286             <para>Turns off the CPR (constructed product result)
1287             analysis; it is somewhat experimental.</para>
1288           </listitem>
1289         </varlistentry>
1290
1291         <varlistentry>
1292           <term><option>-funbox-strict-fields</option>:</term>
1293           <listitem>
1294             <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1295             <indexterm><primary>strict constructor fields</primary></indexterm>
1296             <indexterm><primary>constructor fields, strict</primary></indexterm>
1297
1298             <para>This option causes all constructor fields which are
1299             marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
1300             unpacked if possible.  For example:</para>
1301
1302 <ProgramListing>
1303 data T = T !Float !Float
1304 </ProgramListing>
1305
1306             <para>will create a constructor <literal>T</literal>
1307             containing two unboxed floats if the
1308             <option>-funbox-strict-fields</option> flag is given.
1309             This may not always be an optimisation: if the
1310             <Function>T</Function> constructor is scrutinised and the
1311             floats passed to a non-strict function for example, they
1312             will have to be reboxed (this is done automatically by the
1313             compiler).</para>
1314
1315             <para>This option should only be used in conjunction with
1316             <option>-O</option>, in order to expose unfoldings to the
1317             compiler so the reboxing can be removed as often as
1318             possible.  For example:</para>
1319
1320 <ProgramListing>
1321 f :: T -&#62; Float
1322 f (T f1 f2) = f1 + f2
1323 </ProgramListing>
1324
1325             <para>The compiler will avoid reboxing
1326             <Function>f1</Function> and <Function>f2</Function> by
1327             inlining <Function>+</Function> on floats, but only when
1328             <option>-O</option> is on.</para>
1329
1330             <para>Any single-constructor data is eligible for
1331             unpacking; for example</para>
1332
1333 <ProgramListing>
1334 data T = T !(Int,Int)
1335 </ProgramListing>
1336
1337             <para>will store the two <literal>Int</literal>s directly
1338             in the <Function>T</Function> constructor, by flattening
1339             the pair.  Multi-level unpacking is also supported:</para>
1340
1341 <ProgramListing>
1342 data T = T !S
1343 data S = S !Int !Int
1344 </ProgramListing>
1345
1346             <para>will store two unboxed <literal>Int&num;</literal>s
1347             directly in the <Function>T</Function> constructor.</para>
1348           </listitem>
1349         </varlistentry>
1350
1351         <varlistentry>
1352           <term><option>-funfolding-update-in-place&lt;n&gt;</option></term>
1353           <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1354           <listitem>
1355             <para>Switches on an experimental "optimisation".
1356             Switching it on makes the compiler a little keener to
1357             inline a function that returns a constructor, if the
1358             context is that of a thunk.
1359 <ProgramListing>
1360    x = plusInt a b
1361 </ProgramListing>
1362             If we inlined plusInt we might get an opportunity to use
1363             update-in-place for the thunk 'x'.</para>
1364           </listitem>
1365         </varlistentry>
1366
1367         <varlistentry>
1368           <term><option>-funfolding-creation-threshold&lt;n&gt;</option>:</term>
1369           <listitem>
1370             <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1371             <indexterm><primary>inlining, controlling</primary></indexterm>
1372             <indexterm><primary>unfolding, controlling</primary></indexterm>
1373
1374             <para>(Default: 45) Governs the maximum size that GHC will
1375             allow a function unfolding to be.   (An unfolding has a
1376             &ldquo;size&rdquo; that reflects the cost in terms of
1377             &ldquo;code bloat&rdquo; of expanding that unfolding at
1378             at a call site. A bigger function would be assigned a
1379             bigger cost.) </para>
1380
1381             <para> Consequences: (a) nothing larger than this will be
1382             inlined (unless it has an INLINE pragma); (b) nothing
1383             larger than this will be spewed into an interface
1384             file. </para>
1385
1386
1387             <para> Increasing this figure is more likely to result in longer
1388             compile times than faster code.  The next option is more
1389             useful:</para>
1390           </listitem>
1391         </varlistentry>
1392
1393         <varlistentry>
1394           <term><option>-funfolding-use-threshold&lt;n&gt;</option>:</term>
1395           <listitem>
1396             <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1397             <indexterm><primary>inlining, controlling</primary></indexterm>
1398             <indexterm><primary>unfolding, controlling</primary></indexterm>
1399
1400             <para>(Default: 8) This is the magic cut-off figure for
1401             unfolding: below this size, a function definition will be
1402             unfolded at the call-site, any bigger and it won't.  The
1403             size computed for a function depends on two things: the
1404             actual size of the expression minus any discounts that
1405             apply (see <option>-funfolding-con-discount</option>).</para>
1406           </listitem>
1407         </varlistentry>
1408       </variablelist>
1409
1410     </sect2>
1411
1412   </sect1>
1413
1414 &phases;
1415
1416 <Sect1 id="sec-using-concurrent">
1417 <title>Using Concurrent Haskell</title>
1418
1419 <para>
1420 <indexterm><primary>Concurrent Haskell&mdash;use</primary></indexterm>
1421 </para>
1422
1423 <para>
1424 GHC (as of version 4.00) supports Concurrent Haskell by default,
1425 without requiring a special option or libraries compiled in a certain
1426 way.  To get access to the support libraries for Concurrent Haskell
1427 (i.e. <literal>Concurrent</literal> and friends), use the
1428 <option>-package concurrent</option> option.
1429 </para>
1430
1431 <para>
1432 Three RTS options are provided for modifying the behaviour of the
1433 threaded runtime system.  See the descriptions of
1434 <option>-C[&lt;us&gt;]</option>, <option>-q</option>, and
1435 <option>-t&lt;num&gt;</option> in <XRef LinkEnd="parallel-rts-opts">.
1436 </para>
1437
1438 <para>
1439 Concurrent Haskell is described in more detail in <XRef
1440 LinkEnd="sec-Concurrent">.
1441 </para>
1442
1443 </Sect1>
1444
1445 <Sect1 id="sec-using-parallel">
1446 <title>Using Parallel Haskell</title>
1447
1448 <para>
1449 <indexterm><primary>Parallel Haskell&mdash;use</primary></indexterm>
1450 </para>
1451
1452 <para>
1453 &lsqb;You won't be able to execute parallel Haskell programs unless PVM3
1454 (Parallel Virtual Machine, version 3) is installed at your site.&rsqb;
1455 </Para>
1456
1457 <para>
1458 To compile a Haskell program for parallel execution under PVM, use the
1459 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
1460 option</Primary></IndexTerm> both when compiling <Emphasis>and
1461 linking</Emphasis>.  You will probably want to <Literal>import
1462 Parallel</Literal> into your Haskell modules.
1463 </Para>
1464
1465 <para>
1466 To run your parallel program, once PVM is going, just invoke it
1467 &ldquo;as normal&rdquo;.  The main extra RTS option is
1468 <Option>-qp&lt;n&gt;</Option>, to say how many PVM
1469 &ldquo;processors&rdquo; your program to run on.  (For more details of
1470 all relevant RTS options, please see <XRef
1471 LinkEnd="parallel-rts-opts">.)
1472 </para>
1473
1474 <para>
1475 In truth, running Parallel Haskell programs and getting information
1476 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1477 PVM, detailed in the following sections.
1478 </para>
1479
1480 <Sect2 id="pvm-dummies">
1481 <Title>Dummy's guide to using PVM</Title>
1482
1483 <para>
1484 <indexterm><primary>PVM, how to use</primary></indexterm>
1485 <indexterm><primary>Parallel Haskell&mdash;PVM use</primary></indexterm>
1486 Before you can run a parallel program under PVM, you must set the
1487 required environment variables (PVM's idea, not ours); something like,
1488 probably in your <filename>.cshrc</filename> or equivalent:
1489
1490 <ProgramListing>
1491 setenv PVM_ROOT /wherever/you/put/it
1492 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1493 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1494 </ProgramListing>
1495
1496 </para>
1497
1498 <para>
1499 Creating and/or controlling your &ldquo;parallel machine&rdquo; is a purely-PVM
1500 business; nothing specific to Parallel Haskell. The following paragraphs
1501 describe how to configure your parallel machine interactively.
1502 </Para>
1503
1504 <Para>
1505 If you use parallel Haskell regularly on the same machine configuration it
1506 is a good idea to maintain a file with all machine names and to make the
1507 environment variable PVM_HOST_FILE point to this file. Then you can avoid
1508 the interactive operations described below by just saying
1509 </Para>
1510
1511 <ProgramListing>
1512 pvm $PVM_HOST_FILE
1513 </ProgramListing>
1514
1515 <Para>
1516 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
1517 machine.  You can then do various things to control/monitor your
1518 &ldquo;parallel machine;&rdquo; the most useful being:
1519 </para>
1520
1521 <para>
1522 <InformalTable>
1523 <TGroup Cols=2>
1524 <ColSpec Align="Left">
1525 <TBody>
1526
1527 <row>
1528 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1529 <entry>exit <command>pvm</command>, leaving it running</entry>
1530 </row>
1531
1532 <row>
1533 <entry><command>halt</command></entry>
1534 <entry>kill off this &ldquo;parallel machine&rdquo; &amp; exit</entry>
1535 </row>
1536
1537 <row>
1538 <entry><command>add &lt;host&gt;</command></entry>
1539 <entry>add <command>&lt;host&gt;</command> as a processor</entry>
1540 </row>
1541
1542 <row>
1543 <entry><command>delete &lt;host&gt;</command></entry>
1544 <entry>delete <command>&lt;host&gt;</command></entry>
1545 </row>
1546
1547 <row>
1548 <entry><command>reset</command></entry>
1549 <entry>kill what's going, but leave PVM up</entry>
1550 </row>
1551
1552 <row>
1553 <entry><command>conf</command></entry>
1554 <entry>list the current configuration</entry>
1555 </row>
1556
1557 <row>
1558 <entry><command>ps</command></entry>
1559 <entry>report processes' status</entry>
1560 </row>
1561
1562 <row>
1563 <entry><command>pstat &lt;pid&gt;</command></entry>
1564 <entry>status of a particular process</entry>
1565 </row>
1566
1567 </TBody>
1568 </TGroup>
1569 </InformalTable>
1570 </para>
1571
1572 <para>
1573 The PVM documentation can tell you much, much more about <command>pvm</command>!
1574 </para>
1575
1576 </sect2>
1577
1578 <Sect2 id="par-profiles">
1579 <Title>Parallelism profiles</Title>
1580
1581 <para>
1582 <indexterm><primary>parallelism profiles</primary></indexterm>
1583 <indexterm><primary>profiles, parallelism</primary></indexterm>
1584 <indexterm><primary>visualisation tools</primary></indexterm>
1585 </para>
1586
1587 <para>
1588 With Parallel Haskell programs, we usually don't care about the
1589 results&mdash;only with &ldquo;how parallel&rdquo; it was!  We want pretty pictures.
1590 </para>
1591
1592 <Para>
1593 Parallelism profiles (&agrave; la <Command>hbcpp</Command>) can be generated with the
1594 <Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option.  The
1595 per-processor profiling info is dumped into files named
1596 <Filename>&lt;full-path&gt;&lt;program&gt;.gr</Filename>.  These are then munged into a PostScript picture,
1597 which you can then display.  For example, to run your program
1598 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
1599 </Para>
1600
1601 <Para>
1602
1603 <Screen>
1604 <prompt>&dollar;</prompt> ./a.out +RTS -qP -qp8
1605 <prompt>&dollar;</prompt> grs2gr *.???.gr &#62; temp.gr # combine the 8 .gr files into one
1606 <prompt>&dollar;</prompt> gr2ps -O temp.gr              # cvt to .ps; output in temp.ps
1607 <prompt>&dollar;</prompt> ghostview -seascape temp.ps   # look at it!
1608 </Screen>
1609
1610 </Para>
1611
1612 <para>
1613 The scripts for processing the parallelism profiles are distributed
1614 in <filename>ghc/utils/parallel/</filename>.
1615 </para>
1616
1617 </sect2>
1618
1619 <Sect2>
1620 <Title>Other useful info about running parallel programs</Title>
1621
1622 <Para>
1623 The &ldquo;garbage-collection statistics&rdquo; RTS options can be useful for
1624 seeing what parallel programs are doing.  If you do either
1625 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
1626 you'll get mutator, garbage-collection, etc., times on standard
1627 error. The standard error of all PE's other than the `main thread'
1628 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1629 </para>
1630
1631 <para>
1632 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1633 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1634 </para>
1635
1636 </sect2>
1637
1638 <Sect2 id="parallel-rts-opts">
1639 <title>RTS options for Concurrent/Parallel Haskell
1640 </title>
1641
1642 <para>
1643 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1644 <indexterm><primary>RTS options, parallel</primary></indexterm>
1645 <indexterm><primary>Concurrent Haskell&mdash;RTS options</primary></indexterm>
1646 <indexterm><primary>Parallel Haskell&mdash;RTS options</primary></indexterm>
1647 </para>
1648
1649 <para>
1650 Besides the usual runtime system (RTS) options
1651 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1652 for concurrent/parallel execution.
1653 </para>
1654
1655 <para>
1656 <VariableList>
1657
1658 <VarListEntry>
1659 <Term><Option>-qp&lt;N&gt;</Option>:</Term>
1660 <ListItem>
1661 <Para>
1662 <IndexTerm><Primary>-qp&lt;N&gt; RTS option</Primary></IndexTerm>
1663 (PARALLEL ONLY) Use <Literal>&lt;N&gt;</Literal> PVM processors to run this program;
1664 the default is 2.
1665 </para>
1666 </listitem>
1667 </varlistentry>
1668 <varlistentry>
1669 <term><option>-C[&lt;us&gt;]</option>:</term>
1670 <listitem>
1671 <para>
1672 <indexterm><primary>-C&lt;us&gt; RTS option</primary></indexterm> Sets
1673 the context switch interval to <literal>&lt;s&gt;</literal> seconds.
1674 A context switch will occur at the next heap block allocation after
1675 the timer expires (a heap block allocation occurs every 4k of
1676 allocation).  With <option>-C0</option> or <option>-C</option>,
1677 context switches will occur as often as possible (at every heap block
1678 allocation).  By default, context switches occur every 20ms
1679 milliseconds.  Note that GHC's internal timer ticks every 20ms, and
1680 the context switch timer is always a multiple of this timer, so 20ms
1681 is the maximum granularity available for timed context switches.
1682 </para>
1683 </listitem>
1684 </varlistentry>
1685 <varlistentry>
1686 <term><option>-q[v]</option>:</term>
1687 <listitem>
1688 <para>
1689 <indexterm><primary>-q RTS option</primary></indexterm>
1690 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1691 in the file <FIlename>&lt;program&gt;.qp</FIlename>.  In the style of <command>hbcpp</command>, this profile
1692 records the movement of threads between the green (runnable) and red
1693 (blocked) queues.  If you specify the verbose suboption (<option>-qv</option>), the
1694 green queue is split into green (for the currently running thread
1695 only) and amber (for other runnable threads).  We do not recommend
1696 that you use the verbose suboption if you are planning to use the
1697 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
1698 check (with <Option>-C</Option>).
1699 -->
1700 </Para>
1701 </ListItem>
1702 </VarListEntry>
1703 <VarListEntry>
1704 <Term><Option>-qt&lt;num&gt;</Option>:</Term>
1705 <ListItem>
1706 <Para>
1707 <IndexTerm><Primary>-qt&lt;num&gt; RTS option</Primary></IndexTerm>
1708 (PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
1709 threads per processor to <Literal>&lt;num&gt;</Literal>.  The default is
1710 32.  Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
1711 the heap for thread state and stack objects.  (For 32-bit machines, this
1712 translates to 4K bytes, and for 64-bit machines, 8K bytes.)
1713 </Para>
1714 </ListItem>
1715 </VarListEntry>
1716 <!-- no more -HWL
1717 <VarListEntry>
1718 <Term><Option>-d</Option>:</Term>
1719 <ListItem>
1720 <Para>
1721 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
1722 (PARALLEL ONLY) Turn on debugging.  It pops up one xterm (or GDB, or
1723 something&hellip;) per PVM processor.  We use the standard <Command>debugger</Command>
1724 script that comes with PVM3, but we sometimes meddle with the
1725 <Command>debugger2</Command> script.  We include ours in the GHC distribution,
1726 in <Filename>ghc/utils/pvm/</Filename>.
1727 </Para>
1728 </ListItem>
1729 </VarListEntry>
1730 -->
1731 <VarListEntry>
1732 <Term><Option>-qe&lt;num&gt;</Option>:</Term>
1733 <ListItem>
1734 <Para>
1735 <IndexTerm><Primary>-qe&lt;num&gt; RTS option
1736 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
1737 i.e. the number of pending sparks per processor to
1738 <Literal>&lt;num&gt;</Literal>. The default is 100. A larger number may be
1739 appropriate if your program generates large amounts of parallelism
1740 initially.
1741 </Para>
1742 </ListItem>
1743 </VarListEntry>
1744 <VarListEntry>
1745 <Term><Option>-qQ&lt;num&gt;</Option>:</Term>
1746 <ListItem>
1747 <Para>
1748 <IndexTerm><Primary>-qQ&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1749 (PARALLEL ONLY) Set the size of packets transmitted between processors
1750 to <Literal>&lt;num&gt;</Literal>. The default is 1024 words. A larger number may be
1751 appropriate if your machine has a high communication cost relative to
1752 computation speed.
1753 </Para>
1754 </ListItem>
1755 </VarListEntry>
1756 <VarListEntry>
1757 <Term><Option>-qh&lt;num&gt;</Option>:</Term>
1758 <ListItem>
1759 <Para>
1760 <IndexTerm><Primary>-qh&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1761 (PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
1762 &lt;num&gt;-1 (0 means infinity). By default GUM uses full-subgraph
1763 packing, i.e. the entire subgraph with the requested closure as root is
1764 transmitted (provided it fits into one packet). Choosing a smaller value
1765 reduces the amount of pre-fetching of work done in GUM. This can be
1766 advantageous for improving data locality but it can also worsen the balance
1767 of the load in the system.
1768 </Para>
1769 </ListItem>
1770 </VarListEntry>
1771 <VarListEntry>
1772 <Term><Option>-qg&lt;num&gt;</Option>:</Term>
1773 <ListItem>
1774 <Para>
1775 <IndexTerm><Primary>-qg&lt;num&gt; RTS option
1776 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
1777 scheme. This option affects the
1778 generation of global addresses when transferring data. Global addresses are
1779 globally unique identifiers required to maintain sharing in the distributed
1780 graph structure. Currently this is a binary option. With &lt;num&gt;=0 full globalisation is used
1781 (default). This means a global address is generated for every closure that
1782 is transmitted. With &lt;num&gt;=1 a thunk-only globalisation scheme is
1783 used, which generated global address only for thunks. The latter case may
1784 lose sharing of data but has a reduced overhead in packing graph structures
1785 and maintaining internal tables of global addresses.
1786 </Para>
1787 </ListItem>
1788 </VarListEntry>
1789 </VariableList>
1790 </para>
1791
1792 </sect2>
1793
1794 </Sect1>
1795
1796   <sect1 id="options-platform">
1797     <title>Platform-specific Flags</title>
1798
1799     <indexterm><primary>-m* options</primary></indexterm>
1800     <indexterm><primary>platform-specific options</primary></indexterm>
1801     <indexterm><primary>machine-specific options</primary></indexterm>
1802
1803     <para>Some flags only make sense for particular target
1804     platforms.</para>
1805
1806     <variablelist>
1807
1808       <varlistentry>
1809         <term><option>-mv8</option>:</term>
1810         <listitem>
1811           <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1812           only)</primary></indexterm> Means to pass the like-named
1813           option to GCC; it says to use the Version 8 SPARC
1814           instructions, notably integer multiply and divide.  The
1815           similiar <option>-m*</option> GCC options for SPARC also
1816           work, actually.</para>
1817         </listitem>
1818       </varlistentry>
1819
1820       <varlistentry>
1821         <term><option>-monly-[32]-regs</option>:</term>
1822         <listitem>
1823           <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1824           option (iX86 only)</primary></indexterm> GHC tries to
1825           &ldquo;steal&rdquo; four registers from GCC, for performance
1826           reasons; it almost always works.  However, when GCC is
1827           compiling some modules with four stolen registers, it will
1828           crash, probably saying:
1829
1830 <Screen>
1831 Foo.hc:533: fixed or forbidden register was spilled.
1832 This may be due to a compiler bug or to impossible asm
1833 statements or clauses.
1834 </Screen>
1835
1836           Just give some registers back with
1837           <option>-monly-N-regs</option>.  Try `3' first, then `2'.
1838           If `2' doesn't work, please report the bug to us.</para>
1839         </listitem>
1840       </varlistentry>
1841     </variablelist>
1842
1843   </sect1>
1844
1845 &runtime;
1846
1847 <sect1 id="ext-core">
1848   <title>Generating External Core Files</title>
1849
1850   <indexterm><primary>intermediate code generation</primary></indexterm>
1851
1852   <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
1853   to a file as a side-effect of compilation. Core files, which are given the suffix
1854   <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1855   tools.  The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz"
1856   <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1857   and sample tools (in Haskell)
1858   for manipulating Core files are available in the GHC source distribution
1859   directory <literal>/fptools/ghc/utils/ext-core</literal>.
1860   Note that the format of <literal>.hcr</literal>
1861   files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1862   for debugging purposes (<xref linkend="options-debugging">).</para>
1863
1864     <variablelist>
1865
1866         <varlistentry>
1867           <term><option>-fext-core</option></term>
1868           <indexterm>
1869             <primary><option>-fext-core</option></primary>
1870           </indexterm>
1871           <listitem>
1872             <para>Generate <literal>.hcr</literal> files.</para>
1873           </listitem>
1874         </varlistentry>
1875
1876     </variablelist>
1877
1878 </sect1>
1879
1880 &debug;
1881 &flags;
1882
1883 </Chapter>
1884
1885 <!-- Emacs stuff:
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