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-5.04</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
 447     <para>The source files for the program don't all need to be in the
 448     same directory; the <option>-i</option> option can be used to add
 449     directories to the search path (see <xref
 450     linkend="options-finding-imports">).</para>
 451
 452   </sect1>
 453
 454   <Sect1 id="options-order">
 455     <title>GHC without <option>&ndash;&ndash;make</option></title>
 456
 457     <para>Without <option>&ndash;&ndash;make</option>, GHC will compile one or
 458     more source files given on the command line.</para>
 459
 460     <para>The first phase to run is determined by each input-file
 461     suffix, and the last phase is determined by a flag.  If no
 462     relevant flag is present, then go all the way through linking.
 463     This table summarises:</para>
 464
 465     <informaltable>
 466       <tgroup cols="4">
 467         <colspec align="left">
 468         <colspec align="left">
 469         <colspec align="left">
 470         <colspec align="left">
 471
 472         <thead>
 473           <row>
 474             <entry>Phase of the compilation system</entry>
 475             <entry>Suffix saying &ldquo;start here&rdquo;</entry>
 476             <entry>Flag saying &ldquo;stop after&rdquo;</entry>
 477             <entry>(suffix of) output file</entry>
 478           </row>
 479         </thead>
 480         <tbody>
 481           <row>
 482             <entry>literate pre-processor</entry>
 483             <entry><literal>.lhs</literal></entry>
 484             <entry>-</entry>
 485             <entry><literal>.hs</literal></entry>
 486           </row>
 487
 488           <row>
 489             <entry>C pre-processor (opt.)
 490            </entry>
 491             <entry><literal>.hs</literal> (with
 492             <option>-cpp</option>)</entry>
 493             <entry><option>-E</option></entry>
 494             <entry><literal>.hspp</literal></entry>
 495           </row>
 496
 497           <row>
 498             <entry>Haskell compiler</entry>
 499             <entry><literal>.hs</literal></entry>
 500             <entry><option>-C</option>, <option>-S</option></entry>
 501             <entry><literal>.hc</literal>, <literal>.s</literal></entry>
 502           </row>
 503
 504           <row>
 505             <entry>C compiler (opt.)</entry>
 506             <entry><literal>.hc</literal> or <literal>.c</literal></entry>
 507             <entry><option>-S</option></entry>
 508             <entry><literal>.s</literal></entry>
 509           </row>
 510
 511           <row>
 512             <entry>assembler</entry>
 513             <entry><literal>.s</literal></entry>
 514             <entry><option>-c</option></entry>
 515             <entry><literal>.o</literal></entry>
 516           </row>
 517
 518           <row>
 519             <entry>linker</entry>
 520             <entry><replaceable>other</replaceable></entry>
 521             <entry>-</entry>
 522             <entry><filename>a.out</filename></entry>
 523           </row>
 524         </tbody>
 525       </tgroup>
 526     </informaltable>
 527
 528     <indexterm><primary><option>-C</option></primary></indexterm>
 529     <indexterm><primary><option>-E</option></primary></indexterm>
 530     <indexterm><primary><option>-S</option></primary></indexterm>
 531     <indexterm><primary><option>-c</option></primary></indexterm>
 532
 533     <para>Thus, a common invocation would be: <literal>ghc -c
 534     Foo.hs</literal></para>
 535
 536     <para>Note: What the Haskell compiler proper produces depends on
 537     whether a native-code generator<indexterm><primary>native-code
 538     generator</primary></indexterm> is used (producing assembly
 539     language) or not (producing C).  See <xref
 540     linkend="options-codegen"> for more details.</para>
 541
 542     <para>Note: C pre-processing is optional, the
 543     <option>-ccp</option><indexterm><primary><option>-cpp</option></primary>
 544       </indexterm>flag turns it on.  See <xref
 545     linkend="c-pre-processor"> for more details.</para>
 546
 547     <para>Note: The option <option>-E</option><IndexTerm><Primary>-E
 548     option</Primary></IndexTerm> runs just the pre-processing passes
 549     of the compiler, dumping the result in a file.  Note that this
 550     differs from the previous behaviour of dumping the file to
 551     standard output.</para>
 552   </sect1>
 553
 554   <sect1 id="options-output">
 555     <title>Re-directing the compilation output(s)</title>
 556
 557     <indexterm><primary>output-directing options</primary></indexterm>
 558     <indexterm><primary>redirecting compilation output</primary></indexterm>
 559
 560
 561     <variablelist>
 562       <varlistentry>
 563         <term><option>-o</option></term>
 564         <indexterm><primary><option>-o</option></primary></indexterm>
 565         <listitem>
 566           <para>GHC's compiled output normally goes into a
 567           <filename>.hc</filename>, <filename>.o</filename>, etc.,
 568           file, depending on the last-run compilation phase.  The
 569           option <option>-o foo</option><IndexTerm><Primary>-o
 570           option</Primary></IndexTerm> re-directs the output of that
 571           last-run phase to file <filename>foo</filename>.</para>
 572
 573           <para>Note: this &ldquo;feature&rdquo; can be
 574           counterintuitive: <command>ghc -C -o foo.o foo.hs</command>
 575           will put the intermediate C code in the file
 576           <filename>foo.o</filename>, name notwithstanding!</para>
 577         </listitem>
 578       </varlistentry>
 579
 580       <varlistentry>
 581         <term><option>-odir</option></term>
 582         <indexterm><primary><option>-odir</option></primary></indexterm>
 583         <listitem>
 584           <para>The <option>-o</option> option isn't of much use if
 585           you have <emphasis>several</emphasis> input files&hellip;
 586           Non-interface output files are normally put in the same
 587           directory as their corresponding input file came from.  You
 588           may specify that they be put in another directory using the
 589           <option>-odir &lt;dir&gt;</option><IndexTerm><Primary>-odir
 590           &lt;dir&gt; option</Primary></IndexTerm> (the &ldquo;Oh,
 591           dear&rdquo; option).  For example:</para>
 592
 593 <Screen>
 594 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
 595 </Screen>
 596
 597           <para>The output files, <filename>Foo.o</filename>,
 598           <filename>Bar.o</filename>, and
 599           <filename>Bumble.o</filename> would be put into a
 600           subdirectory named after the architecture of the executing
 601           machine (<filename>sun4</filename>,
 602           <filename>mips</filename>, etc).  The directory must already
 603           exist; it won't be created.</para>
 604
 605           <para>Note that the <option>-odir</option> option does
 606           <emphasis>not</emphasis> affect where the interface files
 607           are put.  In the above example, they would still be put in
 608           <filename>parse/Foo.hi</filename>,
 609           <filename>parse/Bar.hi</filename>, and
 610           <filename>gurgle/Bumble.hi</filename>.</para>
 611         </listitem>
 612       </varlistentry>
 613
 614       <varlistentry>
 615         <term><option>-ohi</option>  <replaceable>file</replaceable></term>
 616         <indexterm><primary><option>-ohi</option></primary>
 617         </indexterm>
 618         <listitem>
 619           <para>The interface output may be directed to another file
 620           <filename>bar2/Wurble.iface</filename> with the option
 621           <option>-ohi bar2/Wurble.iface</option> (not
 622           recommended).</para>
 623
 624           <para>WARNING: if you redirect the interface file somewhere
 625           that GHC can't find it, then the recompilation checker may
 626           get confused (at the least, you won't get any recompilation
 627           avoidance).  We recommend using a combination of
 628           <option>-hidir</option> and <option>-hisuf</option> options
 629           instead, if possible.</para>
 630
 631           <para>To avoid generating an interface at all, you could use
 632           this option to redirect the interface into the bit bucket:
 633           <literal>-ohi /dev/null</literal>, for example.</para>
 634         </listitem>
 635       </varlistentry>
 636
 637       <varlistentry>
 638         <term><option>-hidir</option>  <replaceable>directory</replaceable></term>
 639         <indexterm><primary><option>-hidir</option></primary>
 640         </indexterm>
 641         <listitem>
 642           <para>Redirects all generated interface files into
 643           <replaceable>directory</replaceable>, instead of the default
 644           which is to place the interface file in the same directory
 645           as the source file.</para>
 646         </listitem>
 647       </varlistentry>
 648
 649       <varlistentry>
 650         <term><option>-osuf</option> <replaceable>suffix</replaceable></term>
 651         <term><option>-hisuf</option> <replaceable>suffix</replaceable></term>
 652         <term><option>-hcsuf</option> <replaceable>suffix</replaceable></term>
 653         <indexterm><primary><option>-osuf</option></primary></indexterm>
 654         <indexterm><primary><option>-hisuf</option></primary></indexterm>
 655         <indexterm><primary><option>-hcsuf</option></primary></indexterm>
 656         <listitem>
 657           <para>EXOTICA: The <option>-osuf</option>
 658           <replaceable>suffix</replaceable> will change the
 659           <literal>.o</literal> file suffix for object files to
 660           whatever you specify.  We use this when compiling libraries,
 661           so that objects for the profiling versions of the libraries
 662           don't clobber the normal ones.</para>
 663
 664           <para>Similarly, the <option>-hisuf</option>
 665           <replaceable>suffix</replaceable> will change the
 666           <literal>.hi</literal> file suffix for non-system interface
 667           files (see <XRef LinkEnd="hi-options">).</para>
 668
 669           <para>Finally, the option <option>-hcsuf</option>
 670           <replaceable>suffix</replaceable> will change the
 671           <literal>.hc</literal> file suffix for compiler-generated
 672           intermediate C files.</para>
 673
 674           <para>The <option>-hisuf</option>/<option>-osuf</option>
 675           game is useful if you want to compile a program with both
 676           GHC and HBC (say) in the same directory.  Let HBC use the
 677           standard <filename>.hi</filename>/<filename>.o</filename>
 678           suffixes; add <option>-hisuf g&lowbar;hi -osuf
 679           g&lowbar;o</option> to your <command>make</command> rule for
 680           GHC compiling&hellip;</para>
 681         </listitem>
 682       </varlistentry>
 683     </variablelist>
 684
 685     <sect2 id="keeping-intermediates">
 686       <title>Keeping Intermediate Files</title>
 687       <indexterm><primary>intermediate files, saving</primary>
 688       </indexterm>
 689       <indexterm><primary><literal>.hc</literal> files, saving</primary>
 690       </indexterm>
 691       <indexterm><primary><literal>.s</literal> files, saving</primary>
 692       </indexterm>
 693
 694
 695       <para>The following options are useful for keeping certain
 696       intermediate files around, when normally GHC would throw these
 697       away after compilation:</para>
 698
 699       <variablelist>
 700         <varlistentry>
 701           <term><option>-keep-hc-files</option></term>
 702           <indexterm>
 703             <primary><option>-keep-hc-files</option></primary>
 704           </indexterm>
 705           <listitem>
 706             <para>Keep intermediate <literal>.hc</literal> files when
 707             doing <literal>.hs</literal>-to-<literal>.o</literal>
 708             compilations via C (NOTE: <literal>.hc</literal> files
 709             aren't generated when using the native code generator, you
 710             may need to use <option>-fvia-C</option> to force them
 711             to be produced).</para>
 712           </listitem>
 713         </varlistentry>
 714
 715         <varlistentry>
 716           <term><option>-keep-s-files</option></term>
 717           <indexterm>
 718             <primary><option>-keep-s-files</option></primary>
 719           </indexterm>
 720           <listitem>
 721             <para>Keep intermediate <literal>.s</literal> files.</para>
 722           </listitem>
 723         </varlistentry>
 724
 725         <varlistentry>
 726           <term><option>-keep-raw-s-files</option></term>
 727           <indexterm>
 728             <primary><option>-keep-raw-s-files</option></primary>
 729           </indexterm>
 730           <listitem>
 731             <para>Keep intermediate <literal>.raw-s</literal> files.
 732             These are the direct output from the C compiler, before
 733             GHC does &ldquo;assembly mangling&rdquo; to produce the
 734             <literal>.s</literal> file.  Again, these are not produced
 735             when using the native code generator.</para>
 736           </listitem>
 737         </varlistentry>
 738
 739         <varlistentry>
 740           <term><option>-keep-tmp-files</option></term>
 741           <indexterm>
 742             <primary><option>-keep-tmp-files</option></primary>
 743           </indexterm>
 744           <indexterm>
 745             <primary>temporary files</primary>
 746             <secondary>keeping</secondary>
 747           </indexterm>
 748           <listitem>
 749             <para>Instructs the GHC driver not to delete any of its
 750             temporary files, which it normally keeps in
 751             <literal>/tmp</literal> (or possibly elsewhere; see <xref
 752             linkend="temp-files">).  Running GHC with
 753             <option>-v</option> will show you what temporary files
 754             were generated along the way.</para>
 755           </listitem>
 756         </varlistentry>
 757       </variablelist>
 758     </sect2>
 759
 760     <sect2 id="temp-files">
 761       <title>Redirecting temporary files</title>
 762
 763       <indexterm>
 764         <primary>temporary files</primary>
 765         <secondary>redirecting</secondary>
 766       </indexterm>
 767
 768       <variablelist>
 769         <varlistentry>
 770           <term><option>-tmpdir</option></term>
 771           <indexterm><primary><option>-tmpdir</option></primary></indexterm>
 772           <listitem>
 773             <para>If you have trouble because of running out of space
 774             in <filename>/tmp</filename> (or wherever your
 775             installation thinks temporary files should go), you may
 776             use the <option>-tmpdir
 777             &lt;dir&gt;</option><IndexTerm><Primary>-tmpdir
 778             &lt;dir&gt; option</Primary></IndexTerm> option to specify
 779             an alternate directory.  For example, <option>-tmpdir
 780             .</option> says to put temporary files in the current
 781             working directory.</para>
 782
 783             <para>Alternatively, use your <Constant>TMPDIR</Constant>
 784             environment variable.<IndexTerm><Primary>TMPDIR
 785             environment variable</Primary></IndexTerm> Set it to the
 786             name of the directory where temporary files should be put.
 787             GCC and other programs will honour the
 788             <Constant>TMPDIR</Constant> variable as well.</para>
 789
 790             <para>Even better idea: Set the
 791             <Constant>DEFAULT_TMPDIR</Constant> make variable when
 792             building GHC, and never worry about
 793             <Constant>TMPDIR</Constant> again. (see the build
 794             documentation).</para>
 795           </listitem>
 796         </varlistentry>
 797       </variablelist>
 798     </sect2>
 799
 800   </sect1>
 801
 802   <sect1 id="options-sanity">
 803     <title>Warnings and sanity-checking</title>
 804
 805     <indexterm><primary>sanity-checking options</primary></indexterm>
 806     <indexterm><primary>warnings</primary></indexterm>
 807
 808
 809     <para>GHC has a number of options that select which types of
 810     non-fatal error messages, otherwise known as warnings, can be
 811     generated during compilation.  By default, you get a standard set
 812     of warnings which are generally likely to indicate bugs in your
 813     program.  These are:
 814     <option>-fwarn-overlpapping-patterns</option>,
 815     <option>-fwarn-deprecations</option>,
 816     <option>-fwarn-duplicate-exports</option>,
 817     <option>-fwarn-missing-fields</option>, and
 818     <option>-fwarn-missing-methods</option>.  The following flags are
 819     simple ways to select standard &ldquo;packages&rdquo; of warnings:
 820     </para>
 821
 822     <VariableList>
 823
 824       <varlistentry>
 825         <term><option>-W</option>:</term>
 826         <listitem>
 827           <IndexTerm><Primary>-W option</Primary></IndexTerm>
 828           <para>Provides the standard warnings plus
 829           <option>-fwarn-incomplete-patterns</option>,
 830           <option>-fwarn-unused-matches</option>,
 831           <option>-fwarn-unused-imports</option>,
 832           <option>-fwarn-misc</option>, and
 833           <option>-fwarn-unused-binds</option>.</para>
 834         </listitem>
 835       </varlistentry>
 836
 837       <varlistentry>
 838         <term><option>-w</option>:</term>
 839         <listitem>
 840           <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
 841           <para>Turns off all warnings, including the standard ones.</para>
 842         </listitem>
 843       </varlistentry>
 844
 845       <varlistentry>
 846         <term><option>-Wall</option>:</term>
 847         <listitem>
 848           <indexterm><primary><option>-Wall</option></primary></indexterm>
 849           <para>Turns on all warning options.</para>
 850         </listitem>
 851       </varlistentry>
 852
 853     </variablelist>
 854
 855     <para>The full set of warning options is described below.  To turn
 856     off any warning, simply give the corresponding
 857     <option>-fno-warn-...</option> option on the command line.</para>
 858
 859     <variablelist>
 860
 861       <varlistentry>
 862         <term><option>-fwarn-deprecations</option>:</term>
 863         <listitem>
 864           <indexterm><primary><option>-fwarn-deprecations</option></primary>
 865           </indexterm>
 866           <indexterm><primary>deprecations</primary></indexterm>
 867           <para>Causes a warning to be emitted when a deprecated
 868           function or type is used.  Entities can be marked as
 869           deprecated using a pragma, see <xref
 870           linkend="deprecated-pragma">.</para>
 871         </listitem>
 872       </varlistentry>
 873
 874       <varlistentry>
 875         <term><option>-fwarn-duplicate-exports</option>:</term>
 876         <listitem>
 877           <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
 878           <indexterm><primary>duplicate exports, warning</primary></indexterm>
 879           <indexterm><primary>export lists, duplicates</primary></indexterm>
 880
 881           <para>Have the compiler warn about duplicate entries in
 882           export lists. This is useful information if you maintain
 883           large export lists, and want to avoid the continued export
 884           of a definition after you've deleted (one) mention of it in
 885           the export list.</para>
 886
 887           <para>This option is on by default.</para>
 888         </listitem>
 889       </varlistentry>
 890
 891       <varlistentry>
 892         <term><option>-fwarn-hi-shadowing</option>:</term>
 893         <listitem>
 894           <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
 895           <indexterm><primary>shadowing</primary>
 896             <secondary>interface files</secondary></indexterm>
 897
 898           <para>Causes the compiler to emit a warning when a module or
 899           interface file in the current directory is shadowing one
 900           with the same module name in a library or other
 901           directory.</para>
 902         </listitem>
 903       </varlistentry>
 904
 905       <varlistentry>
 906         <term><option>-fwarn-incomplete-patterns</option>:</term>
 907         <listitem>
 908           <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
 909           <indexterm><primary>incomplete patterns, warning</primary></indexterm>
 910           <indexterm><primary>patterns, incomplete</primary></indexterm>
 911
 912           <para>Similarly for incomplete patterns, the function
 913           <function>g</function> below will fail when applied to
 914           non-empty lists, so the compiler will emit a warning about
 915           this when <option>-fwarn-incomplete-patterns</option> is
 916           enabled.</para>
 917
 918 <programlisting>
 919 g [] = 2
 920 </programlisting>
 921
 922           <para>This option isn't enabled be default because it can be
 923           a bit noisy, and it doesn't always indicate a bug in the
 924           program.  However, it's generally considered good practice
 925           to cover all the cases in your functions.</para>
 926         </listitem>
 927       </varlistentry>
 928
 929       <varlistentry>
 930         <term><option>-fwarn-misc</option>:</term>
 931         <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
 932         <listitem>
 933           <para>Turns on warnings for various harmless but untidy
 934           things.  This currently includes: importing a type with
 935           <literal>(..)</literal> when the export is abstract, and
 936           listing duplicate class assertions in a qualified type.</para>
 937         </listitem>
 938       </varlistentry>
 939
 940       <varlistentry>
 941         <term><option>-fwarn-missing-fields</option>:</term>
 942         <listitem>
 943           <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
 944           <indexterm><primary>missing fields, warning</primary></indexterm>
 945           <indexterm><primary>fields, missing</primary></indexterm>
 946
 947           <para>This option is on by default, and warns you whenever
 948           the construction of a labelled field constructor isn't
 949           complete, missing initializers for one or more fields. While
 950           not an error (the missing fields are initialised with
 951           bottoms), it is often an indication of a programmer error.</para>
 952         </listitem>
 953       </varlistentry>
 954
 955       <varlistentry>
 956         <term><option>-fwarn-missing-methods</option>:</term>
 957         <listitem>
 958           <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
 959           <indexterm><primary>missing methods, warning</primary></indexterm>
 960           <indexterm><primary>methods, missing</primary></indexterm>
 961
 962           <para>This option is on by default, and warns you whenever
 963           an instance declaration is missing one or more methods, and
 964           the corresponding class declaration has no default
 965           declaration for them.</para>
 966         </listitem>
 967       </varlistentry>
 968
 969       <varlistentry>
 970         <term><option>-fwarn-missing-signatures</option>:</term>
 971         <listitem>
 972           <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
 973           <indexterm><primary>type signatures, missing</primary></indexterm>
 974
 975           <para>If you would like GHC to check that every top-level
 976           function/value has a type signature, use the
 977           <option>-fwarn-missing-signatures</option> option.  This
 978           option is off by default.</para>
 979         </listitem>
 980       </varlistentry>
 981
 982       <varlistentry>
 983         <term><option>-fwarn-name-shadowing</option>:</term>
 984         <listitem>
 985           <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
 986           <indexterm><primary>shadowing, warning</primary></indexterm>
 987
 988           <para>This option causes a warning to be emitted whenever an
 989           inner-scope value has the same name as an outer-scope value,
 990           i.e. the inner value shadows the outer one.  This can catch
 991           typographical errors that turn into hard-to-find bugs, e.g.,
 992           in the inadvertent cyclic definition <literal>let x = ... x
 993           ... in</literal>.</para>
 994
 995           <para>Consequently, this option does
 996           <emphasis>will</emphasis> complain about cyclic recursive
 997           definitions.</para>
 998         </listitem>
 999       </varlistentry>
1000
1001       <varlistentry>
1002         <term><option>-fwarn-overlapping-patterns</option>:</term>
1003         <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1004         <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1005         <indexterm><primary>patterns, overlapping</primary></indexterm>
1006         <listitem>
1007           <para>By default, the compiler will warn you if a set of
1008           patterns are overlapping, i.e.,</para>
1009
1010 <programlisting>
1011 f :: String -&#62; Int
1012 f []     = 0
1013 f (_:xs) = 1
1014 f "2"    = 2
1015 </programlisting>
1016
1017           <para>where the last pattern match in <Function>f</Function>
1018           won't ever be reached, as the second pattern overlaps
1019           it. More often than not, redundant patterns is a programmer
1020           mistake/error, so this option is enabled by default.</para>
1021         </listitem>
1022       </varlistentry>
1023
1024       <varlistentry>
1025         <term><option>-fwarn-simple-patterns</option>:</term>
1026         <listitem>
1027           <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1028           </indexterm>
1029           <para>Causes the compiler to warn about lambda-bound
1030           patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1031           Normally, these aren't treated as incomplete patterns by
1032           <option>-fwarn-incomplete-patterns</option>.</para>
1033         </listitem>
1034       </varlistentry>
1035
1036       <varlistentry>
1037         <term><option>-fwarn-type-defaults</option>:</term>
1038         <listitem>
1039           <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1040           <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1041           <para>Have the compiler warn/inform you where in your source
1042           the Haskell defaulting mechanism for numeric types kicks
1043           in. This is useful information when converting code from a
1044           context that assumed one default into one with another,
1045           e.g., the `default default' for Haskell 1.4 caused the
1046           otherwise unconstrained value <Constant>1</Constant> to be
1047           given the type <literal>Int</literal>, whereas Haskell 98
1048           defaults it to <literal>Integer</literal>.  This may lead to
1049           differences in performance and behaviour, hence the
1050           usefulness of being non-silent about this.</para>
1051
1052           <para>This warning is off by default.</para>
1053         </listitem>
1054       </varlistentry>
1055
1056       <varlistentry>
1057         <term><option>-fwarn-unused-binds</option>:</term>
1058         <listitem>
1059           <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1060           <indexterm><primary>unused binds, warning</primary></indexterm>
1061           <indexterm><primary>binds, unused</primary></indexterm>
1062           <para>Report any function definitions (and local bindings)
1063           which are unused.  For top-level functions, the warning is
1064           only given if the binding is not exported.</para>
1065         </listitem>
1066       </varlistentry>
1067
1068       <varlistentry>
1069         <term><option>-fwarn-unused-imports</option>:</term>
1070         <listitem>
1071           <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1072           <indexterm><primary>unused imports, warning</primary></indexterm>
1073           <indexterm><primary>imports, unused</primary></indexterm>
1074
1075           <para>Report any objects that are explicitly imported but
1076           never used.</para>
1077         </listitem>
1078       </varlistentry>
1079
1080       <varlistentry>
1081         <term><option>-fwarn-unused-matches</option>:</term>
1082         <listitem>
1083           <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1084           <indexterm><primary>unused matches, warning</primary></indexterm>
1085           <indexterm><primary>matches, unused</primary></indexterm>
1086
1087           <para>Report all unused variables which arise from pattern
1088           matches, including patterns consisting of a single variable.
1089           For instance <literal>f x y = []</literal> would report
1090           <VarName>x</VarName> and <VarName>y</VarName> as unused.  To
1091           eliminate the warning, all unused variables can be replaced
1092           with wildcards.</para>
1093         </listitem>
1094       </varlistentry>
1095
1096     </VariableList>
1097
1098     <para>If you're feeling really paranoid, the
1099     <option>-dcore-lint</option>
1100     option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1101     is a good choice.  It turns on heavyweight intra-pass
1102     sanity-checking within GHC.  (It checks GHC's sanity, not
1103     yours.)</para>
1104
1105   </sect1>
1106
1107   &separate;
1108   &packages;
1109
1110   <sect1 id="options-optimise">
1111     <title>Optimisation (code improvement)</title>
1112
1113     <indexterm><primary>optimisation</primary></indexterm>
1114     <indexterm><primary>improvement, code</primary></indexterm>
1115
1116     <para>The <option>-O*</option> options specify convenient
1117     &ldquo;packages&rdquo; of optimisation flags; the
1118     <option>-f*</option> options described later on specify
1119     <emphasis>individual</emphasis> optimisations to be turned on/off;
1120     the <option>-m*</option> options specify
1121     <emphasis>machine-specific</emphasis> optimisations to be turned
1122     on/off.</para>
1123
1124     <sect2 id="optimise-pkgs">
1125       <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1126
1127       <para>There are <emphasis>many</emphasis> options that affect
1128       the quality of code produced by GHC.  Most people only have a
1129       general goal, something like &ldquo;Compile quickly&rdquo; or
1130       &ldquo;Make my program run like greased lightning.&rdquo; The
1131       following &ldquo;packages&rdquo; of optimisations (or lack
1132       thereof) should suffice.</para>
1133
1134       <para>Once you choose a <option>-O*</option>
1135       &ldquo;package,&rdquo; stick with it&mdash;don't chop and
1136       change.  Modules' interfaces <emphasis>will</emphasis> change
1137       with a shift to a new <option>-O*</option> option, and you may
1138       have to recompile a large chunk of all importing modules before
1139       your program can again be run safely (see <XRef
1140       LinkEnd="recomp">).</para>
1141
1142       <variablelist>
1143
1144         <varlistentry>
1145           <term>No <option>-O*</option>-type option specified:</term>
1146           <indexterm><primary>-O* not specified</primary></indexterm>
1147           <listitem>
1148             <para>This is taken to mean: &ldquo;Please compile
1149             quickly; I'm not over-bothered about compiled-code
1150             quality.&rdquo; So, for example: <command>ghc -c
1151             Foo.hs</command></para>
1152           </listitem>
1153         </varlistentry>
1154
1155         <varlistentry>
1156           <term><option>-O0</option>:</term>
1157           <indexterm><primary><option>-O0</option></primary></indexterm>
1158           <listitem>
1159             <para>Means &ldquo;turn off all optimisation&rdquo;,
1160             reverting to the same settings as if no
1161             <option>-O</option> options had been specified.  Saying
1162             <option>-O0</option> can be useful if
1163             eg. <command>make</command> has inserted a
1164             <option>-O</option> on the command line already.</para>
1165           </listitem>
1166         </varlistentry>
1167
1168         <varlistentry>
1169           <term><option>-O</option> or <option>-O1</option>:</term>
1170           <indexterm><primary>-O option</primary></indexterm>
1171           <indexterm><primary>-O1 option</primary></indexterm>
1172           <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1173           <listitem>
1174             <para>Means: &ldquo;Generate good-quality code without
1175             taking too long about it.&rdquo; Thus, for example:
1176             <command>ghc -c -O Main.lhs</command></para>
1177           </listitem>
1178         </varlistentry>
1179
1180         <varlistentry>
1181           <term><option>-O2</option>:</term>
1182           <indexterm><primary>-O2 option</primary></indexterm>
1183           <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1184           <listitem>
1185             <para>Means: &ldquo;Apply every non-dangerous
1186             optimisation, even if it means significantly longer
1187             compile times.&rdquo;</para>
1188
1189             <para>The avoided &ldquo;dangerous&rdquo; optimisations
1190             are those that can make runtime or space
1191             <emphasis>worse</emphasis> if you're unlucky.  They are
1192             normally turned on or off individually.</para>
1193
1194             <para>At the moment, <option>-O2</option> is
1195             <emphasis>unlikely</emphasis> to produce better code than
1196             <option>-O</option>.</para>
1197           </listitem>
1198         </varlistentry>
1199
1200         <varlistentry>
1201           <term><option>-Ofile &lt;file&gt;</option>:</term>
1202           <indexterm><primary>-Ofile &lt;file&gt; option</primary></indexterm>
1203           <indexterm><primary>optimising, customised</primary></indexterm>
1204           <listitem>
1205             <para>(NOTE: not supported yet in GHC 5.x.  Please ask if
1206             you're interested in this.)</para>
1207
1208             <para>For those who need <emphasis>absolute</emphasis>
1209             control over <emphasis>exactly</emphasis> what options are
1210             used (e.g., compiler writers, sometimes :-), a list of
1211             options can be put in a file and then slurped in with
1212             <option>-Ofile</option>.</para>
1213
1214             <para>In that file, comments are of the
1215             <literal>&num;</literal>-to-end-of-line variety; blank
1216             lines and most whitespace is ignored.</para>
1217
1218             <para>Please ask if you are baffled and would like an
1219             example of <option>-Ofile</option>!</para>
1220           </listitem>
1221         </varlistentry>
1222       </variablelist>
1223
1224       <para>We don't use a <option>-O*</option> flag for day-to-day
1225       work.  We use <option>-O</option> to get respectable speed;
1226       e.g., when we want to measure something.  When we want to go for
1227       broke, we tend to use <option>-O -fvia-C</option> (and we go for
1228       lots of coffee breaks).</para>
1229
1230       <para>The easiest way to see what <option>-O</option> (etc.)
1231       &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1232       then stand back in amazement.</para>
1233     </sect2>
1234
1235     <sect2 id="options-f">
1236       <title><option>-f*</option>: platform-independent flags</title>
1237
1238       <indexterm><primary>-f* options (GHC)</primary></indexterm>
1239       <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1240
1241       <para>These flags turn on and off individual optimisations.
1242       They are normally set via the <option>-O</option> options
1243       described above, and as such, you shouldn't need to set any of
1244       them explicitly (indeed, doing so could lead to unexpected
1245       results).  However, there are one or two that may be of
1246       interest:</para>
1247
1248       <variablelist>
1249         <varlistentry>
1250           <term><option>-fexcess-precision</option>:</term>
1251           <listitem>
1252             <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1253             <para>When this option is given, intermediate floating
1254             point values can have a <emphasis>greater</emphasis>
1255             precision/range than the final type.  Generally this is a
1256             good thing, but some programs may rely on the exact
1257             precision/range of
1258             <literal>Float</literal>/<literal>Double</literal> values
1259             and should not use this option for their compilation.</para>
1260           </listitem>
1261         </varlistentry>
1262
1263         <varlistentry>
1264           <term><option>-fignore-asserts</option>:</term>
1265           <listitem>
1266             <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1267             <para>Causes GHC to ignore uses of the function
1268             <literal>Exception.assert</literal> in source code (in
1269             other words, rewriting <literal>Exception.assert p
1270             e</literal> to <literal>e</literal> (see <xref
1271             linkend="sec-assertions">).  This flag is turned on by
1272             <option>-O</option>.
1273             </para>
1274           </listitem>
1275         </varlistentry>
1276
1277         <varlistentry>
1278           <term><option>-fno-strictness</option></term>
1279           <indexterm><primary><option>-fno-strictness</option></primary>
1280           </indexterm>
1281           <listitem>
1282             <para>Turns off the strictness analyser; sometimes it eats
1283             too many cycles.</para>
1284           </listitem>
1285         </varlistentry>
1286
1287         <varlistentry>
1288           <term><option>-fno-cpr-analyse</option></term>
1289           <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1290           </indexterm>
1291           <listitem>
1292             <para>Turns off the CPR (constructed product result)
1293             analysis; it is somewhat experimental.</para>
1294           </listitem>
1295         </varlistentry>
1296
1297         <varlistentry>
1298           <term><option>-funbox-strict-fields</option>:</term>
1299           <listitem>
1300             <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1301             <indexterm><primary>strict constructor fields</primary></indexterm>
1302             <indexterm><primary>constructor fields, strict</primary></indexterm>
1303
1304             <para>This option causes all constructor fields which are
1305             marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
1306             unpacked if possible.  For example:</para>
1307
1308 <ProgramListing>
1309 data T = T !Float !Float
1310 </ProgramListing>
1311
1312             <para>will create a constructor <literal>T</literal>
1313             containing two unboxed floats if the
1314             <option>-funbox-strict-fields</option> flag is given.
1315             This may not always be an optimisation: if the
1316             <Function>T</Function> constructor is scrutinised and the
1317             floats passed to a non-strict function for example, they
1318             will have to be reboxed (this is done automatically by the
1319             compiler).</para>
1320
1321             <para>This option should only be used in conjunction with
1322             <option>-O</option>, in order to expose unfoldings to the
1323             compiler so the reboxing can be removed as often as
1324             possible.  For example:</para>
1325
1326 <ProgramListing>
1327 f :: T -&#62; Float
1328 f (T f1 f2) = f1 + f2
1329 </ProgramListing>
1330
1331             <para>The compiler will avoid reboxing
1332             <Function>f1</Function> and <Function>f2</Function> by
1333             inlining <Function>+</Function> on floats, but only when
1334             <option>-O</option> is on.</para>
1335
1336             <para>Any single-constructor data is eligible for
1337             unpacking; for example</para>
1338
1339 <ProgramListing>
1340 data T = T !(Int,Int)
1341 </ProgramListing>
1342
1343             <para>will store the two <literal>Int</literal>s directly
1344             in the <Function>T</Function> constructor, by flattening
1345             the pair.  Multi-level unpacking is also supported:</para>
1346
1347 <ProgramListing>
1348 data T = T !S
1349 data S = S !Int !Int
1350 </ProgramListing>
1351
1352             <para>will store two unboxed <literal>Int&num;</literal>s
1353             directly in the <Function>T</Function> constructor.</para>
1354           </listitem>
1355         </varlistentry>
1356
1357         <varlistentry>
1358           <term><option>-funfolding-update-in-place&lt;n&gt;</option></term>
1359           <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1360           <listitem>
1361             <para>Switches on an experimental "optimisation".
1362             Switching it on makes the compiler a little keener to
1363             inline a function that returns a constructor, if the
1364             context is that of a thunk.
1365 <ProgramListing>
1366    x = plusInt a b
1367 </ProgramListing>
1368             If we inlined plusInt we might get an opportunity to use
1369             update-in-place for the thunk 'x'.</para>
1370           </listitem>
1371         </varlistentry>
1372
1373         <varlistentry>
1374           <term><option>-funfolding-creation-threshold&lt;n&gt;</option>:</term>
1375           <listitem>
1376             <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1377             <indexterm><primary>inlining, controlling</primary></indexterm>
1378             <indexterm><primary>unfolding, controlling</primary></indexterm>
1379
1380             <para>(Default: 45) Governs the maximum size that GHC will
1381             allow a function unfolding to be.   (An unfolding has a
1382             &ldquo;size&rdquo; that reflects the cost in terms of
1383             &ldquo;code bloat&rdquo; of expanding that unfolding at
1384             at a call site. A bigger function would be assigned a
1385             bigger cost.) </para>
1386
1387             <para> Consequences: (a) nothing larger than this will be
1388             inlined (unless it has an INLINE pragma); (b) nothing
1389             larger than this will be spewed into an interface
1390             file. </para>
1391
1392
1393             <para> Increasing this figure is more likely to result in longer
1394             compile times than faster code.  The next option is more
1395             useful:</para>
1396           </listitem>
1397         </varlistentry>
1398
1399         <varlistentry>
1400           <term><option>-funfolding-use-threshold&lt;n&gt;</option>:</term>
1401           <listitem>
1402             <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1403             <indexterm><primary>inlining, controlling</primary></indexterm>
1404             <indexterm><primary>unfolding, controlling</primary></indexterm>
1405
1406             <para>(Default: 8) This is the magic cut-off figure for
1407             unfolding: below this size, a function definition will be
1408             unfolded at the call-site, any bigger and it won't.  The
1409             size computed for a function depends on two things: the
1410             actual size of the expression minus any discounts that
1411             apply (see <option>-funfolding-con-discount</option>).</para>
1412           </listitem>
1413         </varlistentry>
1414       </variablelist>
1415
1416     </sect2>
1417
1418   </sect1>
1419
1420   &phases;
1421
1422   <sect1 id="sec-using-concurrent">
1423 <title>Using Concurrent Haskell</title>
1424
1425              <indexterm><primary>Concurrent Haskell&mdash;use</primary></indexterm>
1426
1427 <para>
1428 GHC supports Concurrent Haskell by default, without requiring a
1429 special option or libraries compiled in a certain way.  To get access
1430 to the support libraries for Concurrent Haskell, just import
1431 <literal>Control.Concurrent</literal> (details are in the accompanying
1432 library documentation).</para>
1433
1434 <para>
1435 RTS options are provided for modifying the behaviour of the threaded
1436 runtime system.  See <XRef LinkEnd="parallel-rts-opts">.
1437 </para>
1438
1439 <para>
1440 Concurrent Haskell is described in more detail in the documentation
1441 for the <literal>Control.Concurrent</literal> module.
1442 </para>
1443
1444 </Sect1>
1445
1446 <Sect1 id="sec-using-parallel">
1447 <title>Using Parallel Haskell</title>
1448
1449 <para>
1450 <indexterm><primary>Parallel Haskell&mdash;use</primary></indexterm>
1451 </para>
1452
1453 <para>
1454 &lsqb;You won't be able to execute parallel Haskell programs unless PVM3
1455 (Parallel Virtual Machine, version 3) is installed at your site.&rsqb;
1456 </Para>
1457
1458 <para>
1459 To compile a Haskell program for parallel execution under PVM, use the
1460 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
1461 option</Primary></IndexTerm> both when compiling <Emphasis>and
1462 linking</Emphasis>.  You will probably want to <Literal>import
1463 Parallel</Literal> into your Haskell modules.
1464 </Para>
1465
1466 <para>
1467 To run your parallel program, once PVM is going, just invoke it
1468 &ldquo;as normal&rdquo;.  The main extra RTS option is
1469 <Option>-qp&lt;n&gt;</Option>, to say how many PVM
1470 &ldquo;processors&rdquo; your program to run on.  (For more details of
1471 all relevant RTS options, please see <XRef
1472 LinkEnd="parallel-rts-opts">.)
1473 </para>
1474
1475 <para>
1476 In truth, running Parallel Haskell programs and getting information
1477 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1478 PVM, detailed in the following sections.
1479 </para>
1480
1481 <Sect2 id="pvm-dummies">
1482 <Title>Dummy's guide to using PVM</Title>
1483
1484 <para>
1485 <indexterm><primary>PVM, how to use</primary></indexterm>
1486 <indexterm><primary>Parallel Haskell&mdash;PVM use</primary></indexterm>
1487 Before you can run a parallel program under PVM, you must set the
1488 required environment variables (PVM's idea, not ours); something like,
1489 probably in your <filename>.cshrc</filename> or equivalent:
1490
1491 <ProgramListing>
1492 setenv PVM_ROOT /wherever/you/put/it
1493 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1494 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1495 </ProgramListing>
1496
1497 </para>
1498
1499 <para>
1500 Creating and/or controlling your &ldquo;parallel machine&rdquo; is a purely-PVM
1501 business; nothing specific to Parallel Haskell. The following paragraphs
1502 describe how to configure your parallel machine interactively.
1503 </Para>
1504
1505 <Para>
1506 If you use parallel Haskell regularly on the same machine configuration it
1507 is a good idea to maintain a file with all machine names and to make the
1508 environment variable PVM_HOST_FILE point to this file. Then you can avoid
1509 the interactive operations described below by just saying
1510 </Para>
1511
1512 <ProgramListing>
1513 pvm $PVM_HOST_FILE
1514 </ProgramListing>
1515
1516 <Para>
1517 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
1518 machine.  You can then do various things to control/monitor your
1519 &ldquo;parallel machine;&rdquo; the most useful being:
1520 </para>
1521
1522 <para>
1523 <InformalTable>
1524 <TGroup Cols=2>
1525 <ColSpec Align="Left">
1526 <TBody>
1527
1528 <row>
1529 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1530 <entry>exit <command>pvm</command>, leaving it running</entry>
1531 </row>
1532
1533 <row>
1534 <entry><command>halt</command></entry>
1535 <entry>kill off this &ldquo;parallel machine&rdquo; &amp; exit</entry>
1536 </row>
1537
1538 <row>
1539 <entry><command>add &lt;host&gt;</command></entry>
1540 <entry>add <command>&lt;host&gt;</command> as a processor</entry>
1541 </row>
1542
1543 <row>
1544 <entry><command>delete &lt;host&gt;</command></entry>
1545 <entry>delete <command>&lt;host&gt;</command></entry>
1546 </row>
1547
1548 <row>
1549 <entry><command>reset</command></entry>
1550 <entry>kill what's going, but leave PVM up</entry>
1551 </row>
1552
1553 <row>
1554 <entry><command>conf</command></entry>
1555 <entry>list the current configuration</entry>
1556 </row>
1557
1558 <row>
1559 <entry><command>ps</command></entry>
1560 <entry>report processes' status</entry>
1561 </row>
1562
1563 <row>
1564 <entry><command>pstat &lt;pid&gt;</command></entry>
1565 <entry>status of a particular process</entry>
1566 </row>
1567
1568 </TBody>
1569 </TGroup>
1570 </InformalTable>
1571 </para>
1572
1573 <para>
1574 The PVM documentation can tell you much, much more about <command>pvm</command>!
1575 </para>
1576
1577 </sect2>
1578
1579 <Sect2 id="par-profiles">
1580 <Title>Parallelism profiles</Title>
1581
1582 <para>
1583 <indexterm><primary>parallelism profiles</primary></indexterm>
1584 <indexterm><primary>profiles, parallelism</primary></indexterm>
1585 <indexterm><primary>visualisation tools</primary></indexterm>
1586 </para>
1587
1588 <para>
1589 With Parallel Haskell programs, we usually don't care about the
1590 results&mdash;only with &ldquo;how parallel&rdquo; it was!  We want pretty pictures.
1591 </para>
1592
1593 <Para>
1594 Parallelism profiles (&agrave; la <Command>hbcpp</Command>) can be generated with the
1595 <Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option.  The
1596 per-processor profiling info is dumped into files named
1597 <Filename>&lt;full-path&gt;&lt;program&gt;.gr</Filename>.  These are then munged into a PostScript picture,
1598 which you can then display.  For example, to run your program
1599 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
1600 </Para>
1601
1602 <Para>
1603
1604 <Screen>
1605 <prompt>&dollar;</prompt> ./a.out +RTS -qP -qp8
1606 <prompt>&dollar;</prompt> grs2gr *.???.gr &#62; temp.gr # combine the 8 .gr files into one
1607 <prompt>&dollar;</prompt> gr2ps -O temp.gr              # cvt to .ps; output in temp.ps
1608 <prompt>&dollar;</prompt> ghostview -seascape temp.ps   # look at it!
1609 </Screen>
1610
1611 </Para>
1612
1613 <para>
1614 The scripts for processing the parallelism profiles are distributed
1615 in <filename>ghc/utils/parallel/</filename>.
1616 </para>
1617
1618 </sect2>
1619
1620 <Sect2>
1621 <Title>Other useful info about running parallel programs</Title>
1622
1623 <Para>
1624 The &ldquo;garbage-collection statistics&rdquo; RTS options can be useful for
1625 seeing what parallel programs are doing.  If you do either
1626 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
1627 you'll get mutator, garbage-collection, etc., times on standard
1628 error. The standard error of all PE's other than the `main thread'
1629 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1630 </para>
1631
1632 <para>
1633 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1634 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1635 </para>
1636
1637 </sect2>
1638
1639 <Sect2 id="parallel-rts-opts">
1640 <title>RTS options for Concurrent/Parallel Haskell
1641 </title>
1642
1643 <para>
1644 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1645 <indexterm><primary>RTS options, parallel</primary></indexterm>
1646 <indexterm><primary>Concurrent Haskell&mdash;RTS options</primary></indexterm>
1647 <indexterm><primary>Parallel Haskell&mdash;RTS options</primary></indexterm>
1648 </para>
1649
1650 <para>
1651 Besides the usual runtime system (RTS) options
1652 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1653 for concurrent/parallel execution.
1654 </para>
1655
1656 <para>
1657 <VariableList>
1658
1659 <VarListEntry>
1660 <Term><Option>-qp&lt;N&gt;</Option>:</Term>
1661 <ListItem>
1662 <Para>
1663 <IndexTerm><Primary>-qp&lt;N&gt; RTS option</Primary></IndexTerm>
1664 (PARALLEL ONLY) Use <Literal>&lt;N&gt;</Literal> PVM processors to run this program;
1665 the default is 2.
1666 </para>
1667 </listitem>
1668 </varlistentry>
1669 <varlistentry>
1670 <term><option>-C[&lt;us&gt;]</option>:</term>
1671 <listitem>
1672 <para>
1673 <indexterm><primary>-C&lt;us&gt; RTS option</primary></indexterm> Sets
1674 the context switch interval to <literal>&lt;s&gt;</literal> seconds.
1675 A context switch will occur at the next heap block allocation after
1676 the timer expires (a heap block allocation occurs every 4k of
1677 allocation).  With <option>-C0</option> or <option>-C</option>,
1678 context switches will occur as often as possible (at every heap block
1679 allocation).  By default, context switches occur every 20ms
1680 milliseconds.  Note that GHC's internal timer ticks every 20ms, and
1681 the context switch timer is always a multiple of this timer, so 20ms
1682 is the maximum granularity available for timed context switches.
1683 </para>
1684 </listitem>
1685 </varlistentry>
1686 <varlistentry>
1687 <term><option>-q[v]</option>:</term>
1688 <listitem>
1689 <para>
1690 <indexterm><primary>-q RTS option</primary></indexterm>
1691 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1692 in the file <FIlename>&lt;program&gt;.qp</FIlename>.  In the style of <command>hbcpp</command>, this profile
1693 records the movement of threads between the green (runnable) and red
1694 (blocked) queues.  If you specify the verbose suboption (<option>-qv</option>), the
1695 green queue is split into green (for the currently running thread
1696 only) and amber (for other runnable threads).  We do not recommend
1697 that you use the verbose suboption if you are planning to use the
1698 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
1699 check (with <Option>-C</Option>).
1700 -->
1701 </Para>
1702 </ListItem>
1703 </VarListEntry>
1704 <VarListEntry>
1705 <Term><Option>-qt&lt;num&gt;</Option>:</Term>
1706 <ListItem>
1707 <Para>
1708 <IndexTerm><Primary>-qt&lt;num&gt; RTS option</Primary></IndexTerm>
1709 (PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
1710 threads per processor to <Literal>&lt;num&gt;</Literal>.  The default is
1711 32.  Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
1712 the heap for thread state and stack objects.  (For 32-bit machines, this
1713 translates to 4K bytes, and for 64-bit machines, 8K bytes.)
1714 </Para>
1715 </ListItem>
1716 </VarListEntry>
1717 <!-- no more -HWL
1718 <VarListEntry>
1719 <Term><Option>-d</Option>:</Term>
1720 <ListItem>
1721 <Para>
1722 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
1723 (PARALLEL ONLY) Turn on debugging.  It pops up one xterm (or GDB, or
1724 something&hellip;) per PVM processor.  We use the standard <Command>debugger</Command>
1725 script that comes with PVM3, but we sometimes meddle with the
1726 <Command>debugger2</Command> script.  We include ours in the GHC distribution,
1727 in <Filename>ghc/utils/pvm/</Filename>.
1728 </Para>
1729 </ListItem>
1730 </VarListEntry>
1731 -->
1732 <VarListEntry>
1733 <Term><Option>-qe&lt;num&gt;</Option>:</Term>
1734 <ListItem>
1735 <Para>
1736 <IndexTerm><Primary>-qe&lt;num&gt; RTS option
1737 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
1738 i.e. the number of pending sparks per processor to
1739 <Literal>&lt;num&gt;</Literal>. The default is 100. A larger number may be
1740 appropriate if your program generates large amounts of parallelism
1741 initially.
1742 </Para>
1743 </ListItem>
1744 </VarListEntry>
1745 <VarListEntry>
1746 <Term><Option>-qQ&lt;num&gt;</Option>:</Term>
1747 <ListItem>
1748 <Para>
1749 <IndexTerm><Primary>-qQ&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1750 (PARALLEL ONLY) Set the size of packets transmitted between processors
1751 to <Literal>&lt;num&gt;</Literal>. The default is 1024 words. A larger number may be
1752 appropriate if your machine has a high communication cost relative to
1753 computation speed.
1754 </Para>
1755 </ListItem>
1756 </VarListEntry>
1757 <VarListEntry>
1758 <Term><Option>-qh&lt;num&gt;</Option>:</Term>
1759 <ListItem>
1760 <Para>
1761 <IndexTerm><Primary>-qh&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1762 (PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
1763 &lt;num&gt;-1 (0 means infinity). By default GUM uses full-subgraph
1764 packing, i.e. the entire subgraph with the requested closure as root is
1765 transmitted (provided it fits into one packet). Choosing a smaller value
1766 reduces the amount of pre-fetching of work done in GUM. This can be
1767 advantageous for improving data locality but it can also worsen the balance
1768 of the load in the system.
1769 </Para>
1770 </ListItem>
1771 </VarListEntry>
1772 <VarListEntry>
1773 <Term><Option>-qg&lt;num&gt;</Option>:</Term>
1774 <ListItem>
1775 <Para>
1776 <IndexTerm><Primary>-qg&lt;num&gt; RTS option
1777 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
1778 scheme. This option affects the
1779 generation of global addresses when transferring data. Global addresses are
1780 globally unique identifiers required to maintain sharing in the distributed
1781 graph structure. Currently this is a binary option. With &lt;num&gt;=0 full globalisation is used
1782 (default). This means a global address is generated for every closure that
1783 is transmitted. With &lt;num&gt;=1 a thunk-only globalisation scheme is
1784 used, which generated global address only for thunks. The latter case may
1785 lose sharing of data but has a reduced overhead in packing graph structures
1786 and maintaining internal tables of global addresses.
1787 </Para>
1788 </ListItem>
1789 </VarListEntry>
1790 </VariableList>
1791 </para>
1792
1793 </sect2>
1794
1795 </Sect1>
1796
1797   <sect1 id="options-platform">
1798     <title>Platform-specific Flags</title>
1799
1800     <indexterm><primary>-m* options</primary></indexterm>
1801     <indexterm><primary>platform-specific options</primary></indexterm>
1802     <indexterm><primary>machine-specific options</primary></indexterm>
1803
1804     <para>Some flags only make sense for particular target
1805     platforms.</para>
1806
1807     <variablelist>
1808
1809       <varlistentry>
1810         <term><option>-mv8</option>:</term>
1811         <listitem>
1812           <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1813           only)</primary></indexterm> Means to pass the like-named
1814           option to GCC; it says to use the Version 8 SPARC
1815           instructions, notably integer multiply and divide.  The
1816           similiar <option>-m*</option> GCC options for SPARC also
1817           work, actually.</para>
1818         </listitem>
1819       </varlistentry>
1820
1821       <varlistentry>
1822         <term><option>-monly-[32]-regs</option>:</term>
1823         <listitem>
1824           <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1825           option (iX86 only)</primary></indexterm> GHC tries to
1826           &ldquo;steal&rdquo; four registers from GCC, for performance
1827           reasons; it almost always works.  However, when GCC is
1828           compiling some modules with four stolen registers, it will
1829           crash, probably saying:
1830
1831 <Screen>
1832 Foo.hc:533: fixed or forbidden register was spilled.
1833 This may be due to a compiler bug or to impossible asm
1834 statements or clauses.
1835 </Screen>
1836
1837           Just give some registers back with
1838           <option>-monly-N-regs</option>.  Try `3' first, then `2'.
1839           If `2' doesn't work, please report the bug to us.</para>
1840         </listitem>
1841       </varlistentry>
1842     </variablelist>
1843
1844   </sect1>
1845
1846 &runtime;
1847
1848 <sect1 id="ext-core">
1849   <title>Generating External Core Files</title>
1850
1851   <indexterm><primary>intermediate code generation</primary></indexterm>
1852
1853   <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
1854   to a file as a side-effect of compilation. Core files, which are given the suffix
1855   <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1856   tools.  The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz"
1857   <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1858   and sample tools (in Haskell)
1859   for manipulating Core files are available in the GHC source distribution
1860   directory <literal>/fptools/ghc/utils/ext-core</literal>.
1861   Note that the format of <literal>.hcr</literal>
1862   files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1863   for debugging purposes (<xref linkend="options-debugging">).</para>
1864
1865     <variablelist>
1866
1867         <varlistentry>
1868           <term><option>-fext-core</option></term>
1869           <indexterm>
1870             <primary><option>-fext-core</option></primary>
1871           </indexterm>
1872           <listitem>
1873             <para>Generate <literal>.hcr</literal> files.</para>
1874           </listitem>
1875         </varlistentry>
1876
1877     </variablelist>
1878
1879 </sect1>
1880
1881 &debug;
1882 &flags;
1883
1884 </Chapter>
1885
1886 <!-- Emacs stuff:
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