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
 578           <para>Note: on Windows, if the result is an executable file, the
 579           extension "<filename>.exe</filename>" is added if the specified filename
 580             does not already have an extension.  Thus
 581            <programlisting>
 582                 ghc -o foo Main.hs
 583            </programlisting>
 584           will compile and link the module <filename>Main.hs</filename>, and put the
 585           resulting executable in <filename>foo.exe</filename> (not <filename>foo</filename>).
 586           </para>
 587         </listitem>
 588       </varlistentry>
 589
 590       <varlistentry>
 591         <term><option>-odir</option></term>
 592         <indexterm><primary><option>-odir</option></primary></indexterm>
 593         <listitem>
 594           <para>The <option>-o</option> option isn't of much use if
 595           you have <emphasis>several</emphasis> input files&hellip;
 596           Non-interface output files are normally put in the same
 597           directory as their corresponding input file came from.  You
 598           may specify that they be put in another directory using the
 599           <option>-odir &lt;dir&gt;</option><IndexTerm><Primary>-odir
 600           &lt;dir&gt; option</Primary></IndexTerm> (the &ldquo;Oh,
 601           dear&rdquo; option).  For example:</para>
 602
 603 <Screen>
 604 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
 605 </Screen>
 606
 607           <para>The output files, <filename>Foo.o</filename>,
 608           <filename>Bar.o</filename>, and
 609           <filename>Bumble.o</filename> would be put into a
 610           subdirectory named after the architecture of the executing
 611           machine (<filename>sun4</filename>,
 612           <filename>mips</filename>, etc).  The directory must already
 613           exist; it won't be created.</para>
 614
 615           <para>Note that the <option>-odir</option> option does
 616           <emphasis>not</emphasis> affect where the interface files
 617           are put.  In the above example, they would still be put in
 618           <filename>parse/Foo.hi</filename>,
 619           <filename>parse/Bar.hi</filename>, and
 620           <filename>gurgle/Bumble.hi</filename>.</para>
 621         </listitem>
 622       </varlistentry>
 623
 624       <varlistentry>
 625         <term><option>-ohi</option>  <replaceable>file</replaceable></term>
 626         <indexterm><primary><option>-ohi</option></primary>
 627         </indexterm>
 628         <listitem>
 629           <para>The interface output may be directed to another file
 630           <filename>bar2/Wurble.iface</filename> with the option
 631           <option>-ohi bar2/Wurble.iface</option> (not
 632           recommended).</para>
 633
 634           <para>WARNING: if you redirect the interface file somewhere
 635           that GHC can't find it, then the recompilation checker may
 636           get confused (at the least, you won't get any recompilation
 637           avoidance).  We recommend using a combination of
 638           <option>-hidir</option> and <option>-hisuf</option> options
 639           instead, if possible.</para>
 640
 641           <para>To avoid generating an interface at all, you could use
 642           this option to redirect the interface into the bit bucket:
 643           <literal>-ohi /dev/null</literal>, for example.</para>
 644         </listitem>
 645       </varlistentry>
 646
 647       <varlistentry>
 648         <term><option>-hidir</option>  <replaceable>directory</replaceable></term>
 649         <indexterm><primary><option>-hidir</option></primary>
 650         </indexterm>
 651         <listitem>
 652           <para>Redirects all generated interface files into
 653           <replaceable>directory</replaceable>, instead of the default
 654           which is to place the interface file in the same directory
 655           as the source file.</para>
 656         </listitem>
 657       </varlistentry>
 658
 659       <varlistentry>
 660         <term><option>-osuf</option> <replaceable>suffix</replaceable></term>
 661         <term><option>-hisuf</option> <replaceable>suffix</replaceable></term>
 662         <term><option>-hcsuf</option> <replaceable>suffix</replaceable></term>
 663         <indexterm><primary><option>-osuf</option></primary></indexterm>
 664         <indexterm><primary><option>-hisuf</option></primary></indexterm>
 665         <indexterm><primary><option>-hcsuf</option></primary></indexterm>
 666         <listitem>
 667           <para>EXOTICA: The <option>-osuf</option>
 668           <replaceable>suffix</replaceable> will change the
 669           <literal>.o</literal> file suffix for object files to
 670           whatever you specify.  We use this when compiling libraries,
 671           so that objects for the profiling versions of the libraries
 672           don't clobber the normal ones.</para>
 673
 674           <para>Similarly, the <option>-hisuf</option>
 675           <replaceable>suffix</replaceable> will change the
 676           <literal>.hi</literal> file suffix for non-system interface
 677           files (see <XRef LinkEnd="hi-options">).</para>
 678
 679           <para>Finally, the option <option>-hcsuf</option>
 680           <replaceable>suffix</replaceable> will change the
 681           <literal>.hc</literal> file suffix for compiler-generated
 682           intermediate C files.</para>
 683
 684           <para>The <option>-hisuf</option>/<option>-osuf</option>
 685           game is particularly useful if you want to compile a program both with and without
 686             profiling, in the same directory.  You can say:
 687             <Screen>
 688               ghc ...
 689             </Screen>
 690             to get the ordinary version, and
 691             <Screen>
 692               ghc ... -osuf prof.o -hisuf prof.hi -prof -auto-all
 693             </Screen>
 694             to get the profiled version.</para>
 695         </listitem>
 696       </varlistentry>
 697     </variablelist>
 698
 699     <sect2 id="keeping-intermediates">
 700       <title>Keeping Intermediate Files</title>
 701       <indexterm><primary>intermediate files, saving</primary>
 702       </indexterm>
 703       <indexterm><primary><literal>.hc</literal> files, saving</primary>
 704       </indexterm>
 705       <indexterm><primary><literal>.s</literal> files, saving</primary>
 706       </indexterm>
 707
 708
 709       <para>The following options are useful for keeping certain
 710       intermediate files around, when normally GHC would throw these
 711       away after compilation:</para>
 712
 713       <variablelist>
 714         <varlistentry>
 715           <term><option>-keep-hc-files</option></term>
 716           <indexterm>
 717             <primary><option>-keep-hc-files</option></primary>
 718           </indexterm>
 719           <listitem>
 720             <para>Keep intermediate <literal>.hc</literal> files when
 721             doing <literal>.hs</literal>-to-<literal>.o</literal>
 722             compilations via C (NOTE: <literal>.hc</literal> files
 723             aren't generated when using the native code generator, you
 724             may need to use <option>-fvia-C</option> to force them
 725             to be produced).</para>
 726           </listitem>
 727         </varlistentry>
 728
 729         <varlistentry>
 730           <term><option>-keep-s-files</option></term>
 731           <indexterm>
 732             <primary><option>-keep-s-files</option></primary>
 733           </indexterm>
 734           <listitem>
 735             <para>Keep intermediate <literal>.s</literal> files.</para>
 736           </listitem>
 737         </varlistentry>
 738
 739         <varlistentry>
 740           <term><option>-keep-raw-s-files</option></term>
 741           <indexterm>
 742             <primary><option>-keep-raw-s-files</option></primary>
 743           </indexterm>
 744           <listitem>
 745             <para>Keep intermediate <literal>.raw-s</literal> files.
 746             These are the direct output from the C compiler, before
 747             GHC does &ldquo;assembly mangling&rdquo; to produce the
 748             <literal>.s</literal> file.  Again, these are not produced
 749             when using the native code generator.</para>
 750           </listitem>
 751         </varlistentry>
 752
 753         <varlistentry>
 754           <term><option>-keep-tmp-files</option></term>
 755           <indexterm>
 756             <primary><option>-keep-tmp-files</option></primary>
 757           </indexterm>
 758           <indexterm>
 759             <primary>temporary files</primary>
 760             <secondary>keeping</secondary>
 761           </indexterm>
 762           <listitem>
 763             <para>Instructs the GHC driver not to delete any of its
 764             temporary files, which it normally keeps in
 765             <literal>/tmp</literal> (or possibly elsewhere; see <xref
 766             linkend="temp-files">).  Running GHC with
 767             <option>-v</option> will show you what temporary files
 768             were generated along the way.</para>
 769           </listitem>
 770         </varlistentry>
 771       </variablelist>
 772     </sect2>
 773
 774     <sect2 id="temp-files">
 775       <title>Redirecting temporary files</title>
 776
 777       <indexterm>
 778         <primary>temporary files</primary>
 779         <secondary>redirecting</secondary>
 780       </indexterm>
 781
 782       <variablelist>
 783         <varlistentry>
 784           <term><option>-tmpdir</option></term>
 785           <indexterm><primary><option>-tmpdir</option></primary></indexterm>
 786           <listitem>
 787             <para>If you have trouble because of running out of space
 788             in <filename>/tmp</filename> (or wherever your
 789             installation thinks temporary files should go), you may
 790             use the <option>-tmpdir
 791             &lt;dir&gt;</option><IndexTerm><Primary>-tmpdir
 792             &lt;dir&gt; option</Primary></IndexTerm> option to specify
 793             an alternate directory.  For example, <option>-tmpdir
 794             .</option> says to put temporary files in the current
 795             working directory.</para>
 796
 797             <para>Alternatively, use your <Constant>TMPDIR</Constant>
 798             environment variable.<IndexTerm><Primary>TMPDIR
 799             environment variable</Primary></IndexTerm> Set it to the
 800             name of the directory where temporary files should be put.
 801             GCC and other programs will honour the
 802             <Constant>TMPDIR</Constant> variable as well.</para>
 803
 804             <para>Even better idea: Set the
 805             <Constant>DEFAULT_TMPDIR</Constant> make variable when
 806             building GHC, and never worry about
 807             <Constant>TMPDIR</Constant> again. (see the build
 808             documentation).</para>
 809           </listitem>
 810         </varlistentry>
 811       </variablelist>
 812     </sect2>
 813
 814   </sect1>
 815
 816   <sect1 id="options-sanity">
 817     <title>Warnings and sanity-checking</title>
 818
 819     <indexterm><primary>sanity-checking options</primary></indexterm>
 820     <indexterm><primary>warnings</primary></indexterm>
 821
 822
 823     <para>GHC has a number of options that select which types of
 824     non-fatal error messages, otherwise known as warnings, can be
 825     generated during compilation.  By default, you get a standard set
 826     of warnings which are generally likely to indicate bugs in your
 827     program.  These are:
 828     <option>-fwarn-overlpapping-patterns</option>,
 829     <option>-fwarn-deprecations</option>,
 830     <option>-fwarn-duplicate-exports</option>,
 831     <option>-fwarn-missing-fields</option>, and
 832     <option>-fwarn-missing-methods</option>.  The following flags are
 833     simple ways to select standard &ldquo;packages&rdquo; of warnings:
 834     </para>
 835
 836     <VariableList>
 837
 838       <varlistentry>
 839         <term><option>-W</option>:</term>
 840         <listitem>
 841           <IndexTerm><Primary>-W option</Primary></IndexTerm>
 842           <para>Provides the standard warnings plus
 843           <option>-fwarn-incomplete-patterns</option>,
 844           <option>-fwarn-unused-matches</option>,
 845           <option>-fwarn-unused-imports</option>,
 846           <option>-fwarn-misc</option>, and
 847           <option>-fwarn-unused-binds</option>.</para>
 848         </listitem>
 849       </varlistentry>
 850
 851       <varlistentry>
 852         <term><option>-w</option>:</term>
 853         <listitem>
 854           <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
 855           <para>Turns off all warnings, including the standard ones.</para>
 856         </listitem>
 857       </varlistentry>
 858
 859       <varlistentry>
 860         <term><option>-Wall</option>:</term>
 861         <listitem>
 862           <indexterm><primary><option>-Wall</option></primary></indexterm>
 863           <para>Turns on all warning options.</para>
 864         </listitem>
 865       </varlistentry>
 866
 867       <varlistentry>
 868         <term><option>-Werror</option>:</term>
 869         <listitem>
 870           <indexterm><primary><option>-Werror</option></primary></indexterm>
 871           <para>Makes any warning into a fatal error. Useful so that you don't
 872             miss warnings when doing batch compilation. </para>
 873         </listitem>
 874       </varlistentry>
 875
 876     </variablelist>
 877
 878     <para>The full set of warning options is described below.  To turn
 879     off any warning, simply give the corresponding
 880     <option>-fno-warn-...</option> option on the command line.</para>
 881
 882     <variablelist>
 883
 884       <varlistentry>
 885         <term><option>-fwarn-deprecations</option>:</term>
 886         <listitem>
 887           <indexterm><primary><option>-fwarn-deprecations</option></primary>
 888           </indexterm>
 889           <indexterm><primary>deprecations</primary></indexterm>
 890           <para>Causes a warning to be emitted when a deprecated
 891           function or type is used.  Entities can be marked as
 892           deprecated using a pragma, see <xref
 893           linkend="deprecated-pragma">.</para>
 894         </listitem>
 895       </varlistentry>
 896
 897       <varlistentry>
 898         <term><option>-fwarn-duplicate-exports</option>:</term>
 899         <listitem>
 900           <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
 901           <indexterm><primary>duplicate exports, warning</primary></indexterm>
 902           <indexterm><primary>export lists, duplicates</primary></indexterm>
 903
 904           <para>Have the compiler warn about duplicate entries in
 905           export lists. This is useful information if you maintain
 906           large export lists, and want to avoid the continued export
 907           of a definition after you've deleted (one) mention of it in
 908           the export list.</para>
 909
 910           <para>This option is on by default.</para>
 911         </listitem>
 912       </varlistentry>
 913
 914       <varlistentry>
 915         <term><option>-fwarn-hi-shadowing</option>:</term>
 916         <listitem>
 917           <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
 918           <indexterm><primary>shadowing</primary>
 919             <secondary>interface files</secondary></indexterm>
 920
 921           <para>Causes the compiler to emit a warning when a module or
 922           interface file in the current directory is shadowing one
 923           with the same module name in a library or other
 924           directory.</para>
 925         </listitem>
 926       </varlistentry>
 927
 928       <varlistentry>
 929         <term><option>-fwarn-incomplete-patterns</option>:</term>
 930         <listitem>
 931           <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
 932           <indexterm><primary>incomplete patterns, warning</primary></indexterm>
 933           <indexterm><primary>patterns, incomplete</primary></indexterm>
 934
 935           <para>Similarly for incomplete patterns, the function
 936           <function>g</function> below will fail when applied to
 937           non-empty lists, so the compiler will emit a warning about
 938           this when <option>-fwarn-incomplete-patterns</option> is
 939           enabled.</para>
 940
 941 <programlisting>
 942 g [] = 2
 943 </programlisting>
 944
 945           <para>This option isn't enabled be default because it can be
 946           a bit noisy, and it doesn't always indicate a bug in the
 947           program.  However, it's generally considered good practice
 948           to cover all the cases in your functions.</para>
 949         </listitem>
 950       </varlistentry>
 951
 952       <varlistentry>
 953         <term><option>-fwarn-misc</option>:</term>
 954         <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
 955         <listitem>
 956           <para>Turns on warnings for various harmless but untidy
 957           things.  This currently includes: importing a type with
 958           <literal>(..)</literal> when the export is abstract, and
 959           listing duplicate class assertions in a qualified type.</para>
 960         </listitem>
 961       </varlistentry>
 962
 963       <varlistentry>
 964         <term><option>-fwarn-missing-fields</option>:</term>
 965         <listitem>
 966           <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
 967           <indexterm><primary>missing fields, warning</primary></indexterm>
 968           <indexterm><primary>fields, missing</primary></indexterm>
 969
 970           <para>This option is on by default, and warns you whenever
 971           the construction of a labelled field constructor isn't
 972           complete, missing initializers for one or more fields. While
 973           not an error (the missing fields are initialised with
 974           bottoms), it is often an indication of a programmer error.</para>
 975         </listitem>
 976       </varlistentry>
 977
 978       <varlistentry>
 979         <term><option>-fwarn-missing-methods</option>:</term>
 980         <listitem>
 981           <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
 982           <indexterm><primary>missing methods, warning</primary></indexterm>
 983           <indexterm><primary>methods, missing</primary></indexterm>
 984
 985           <para>This option is on by default, and warns you whenever
 986           an instance declaration is missing one or more methods, and
 987           the corresponding class declaration has no default
 988           declaration for them.</para>
 989         </listitem>
 990       </varlistentry>
 991
 992       <varlistentry>
 993         <term><option>-fwarn-missing-signatures</option>:</term>
 994         <listitem>
 995           <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
 996           <indexterm><primary>type signatures, missing</primary></indexterm>
 997
 998           <para>If you would like GHC to check that every top-level
 999           function/value has a type signature, use the
1000           <option>-fwarn-missing-signatures</option> option.  This
1001           option is off by default.</para>
1002         </listitem>
1003       </varlistentry>
1004
1005       <varlistentry>
1006         <term><option>-fwarn-name-shadowing</option>:</term>
1007         <listitem>
1008           <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1009           <indexterm><primary>shadowing, warning</primary></indexterm>
1010
1011           <para>This option causes a warning to be emitted whenever an
1012           inner-scope value has the same name as an outer-scope value,
1013           i.e. the inner value shadows the outer one.  This can catch
1014           typographical errors that turn into hard-to-find bugs, e.g.,
1015           in the inadvertent cyclic definition <literal>let x = ... x
1016           ... in</literal>.</para>
1017
1018           <para>Consequently, this option does
1019           <emphasis>will</emphasis> complain about cyclic recursive
1020           definitions.</para>
1021         </listitem>
1022       </varlistentry>
1023
1024       <varlistentry>
1025         <term><option>-fwarn-overlapping-patterns</option>:</term>
1026         <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1027         <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1028         <indexterm><primary>patterns, overlapping</primary></indexterm>
1029         <listitem>
1030           <para>By default, the compiler will warn you if a set of
1031           patterns are overlapping, i.e.,</para>
1032
1033 <programlisting>
1034 f :: String -&#62; Int
1035 f []     = 0
1036 f (_:xs) = 1
1037 f "2"    = 2
1038 </programlisting>
1039
1040           <para>where the last pattern match in <Function>f</Function>
1041           won't ever be reached, as the second pattern overlaps
1042           it. More often than not, redundant patterns is a programmer
1043           mistake/error, so this option is enabled by default.</para>
1044         </listitem>
1045       </varlistentry>
1046
1047       <varlistentry>
1048         <term><option>-fwarn-simple-patterns</option>:</term>
1049         <listitem>
1050           <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1051           </indexterm>
1052           <para>Causes the compiler to warn about lambda-bound
1053           patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1054           Normally, these aren't treated as incomplete patterns by
1055           <option>-fwarn-incomplete-patterns</option>.</para>
1056         </listitem>
1057       </varlistentry>
1058
1059       <varlistentry>
1060         <term><option>-fwarn-type-defaults</option>:</term>
1061         <listitem>
1062           <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1063           <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1064           <para>Have the compiler warn/inform you where in your source
1065           the Haskell defaulting mechanism for numeric types kicks
1066           in. This is useful information when converting code from a
1067           context that assumed one default into one with another,
1068           e.g., the `default default' for Haskell 1.4 caused the
1069           otherwise unconstrained value <Constant>1</Constant> to be
1070           given the type <literal>Int</literal>, whereas Haskell 98
1071           defaults it to <literal>Integer</literal>.  This may lead to
1072           differences in performance and behaviour, hence the
1073           usefulness of being non-silent about this.</para>
1074
1075           <para>This warning is off by default.</para>
1076         </listitem>
1077       </varlistentry>
1078
1079       <varlistentry>
1080         <term><option>-fwarn-unused-binds</option>:</term>
1081         <listitem>
1082           <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1083           <indexterm><primary>unused binds, warning</primary></indexterm>
1084           <indexterm><primary>binds, unused</primary></indexterm>
1085           <para>Report any function definitions (and local bindings)
1086           which are unused.  For top-level functions, the warning is
1087           only given if the binding is not exported.</para>
1088         </listitem>
1089       </varlistentry>
1090
1091       <varlistentry>
1092         <term><option>-fwarn-unused-imports</option>:</term>
1093         <listitem>
1094           <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1095           <indexterm><primary>unused imports, warning</primary></indexterm>
1096           <indexterm><primary>imports, unused</primary></indexterm>
1097
1098           <para>Report any objects that are explicitly imported but
1099           never used.</para>
1100         </listitem>
1101       </varlistentry>
1102
1103       <varlistentry>
1104         <term><option>-fwarn-unused-matches</option>:</term>
1105         <listitem>
1106           <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1107           <indexterm><primary>unused matches, warning</primary></indexterm>
1108           <indexterm><primary>matches, unused</primary></indexterm>
1109
1110           <para>Report all unused variables which arise from pattern
1111           matches, including patterns consisting of a single variable.
1112           For instance <literal>f x y = []</literal> would report
1113           <VarName>x</VarName> and <VarName>y</VarName> as unused.  To
1114           eliminate the warning, all unused variables can be replaced
1115           with wildcards.</para>
1116         </listitem>
1117       </varlistentry>
1118
1119     </VariableList>
1120
1121     <para>If you're feeling really paranoid, the
1122     <option>-dcore-lint</option>
1123     option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1124     is a good choice.  It turns on heavyweight intra-pass
1125     sanity-checking within GHC.  (It checks GHC's sanity, not
1126     yours.)</para>
1127
1128   </sect1>
1129
1130   &separate;
1131   &packages;
1132
1133   <sect1 id="options-optimise">
1134     <title>Optimisation (code improvement)</title>
1135
1136     <indexterm><primary>optimisation</primary></indexterm>
1137     <indexterm><primary>improvement, code</primary></indexterm>
1138
1139     <para>The <option>-O*</option> options specify convenient
1140     &ldquo;packages&rdquo; of optimisation flags; the
1141     <option>-f*</option> options described later on specify
1142     <emphasis>individual</emphasis> optimisations to be turned on/off;
1143     the <option>-m*</option> options specify
1144     <emphasis>machine-specific</emphasis> optimisations to be turned
1145     on/off.</para>
1146
1147     <sect2 id="optimise-pkgs">
1148       <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1149
1150       <para>There are <emphasis>many</emphasis> options that affect
1151       the quality of code produced by GHC.  Most people only have a
1152       general goal, something like &ldquo;Compile quickly&rdquo; or
1153       &ldquo;Make my program run like greased lightning.&rdquo; The
1154       following &ldquo;packages&rdquo; of optimisations (or lack
1155       thereof) should suffice.</para>
1156
1157       <para>Once you choose a <option>-O*</option>
1158       &ldquo;package,&rdquo; stick with it&mdash;don't chop and
1159       change.  Modules' interfaces <emphasis>will</emphasis> change
1160       with a shift to a new <option>-O*</option> option, and you may
1161       have to recompile a large chunk of all importing modules before
1162       your program can again be run safely (see <XRef
1163       LinkEnd="recomp">).</para>
1164
1165       <variablelist>
1166
1167         <varlistentry>
1168           <term>No <option>-O*</option>-type option specified:</term>
1169           <indexterm><primary>-O* not specified</primary></indexterm>
1170           <listitem>
1171             <para>This is taken to mean: &ldquo;Please compile
1172             quickly; I'm not over-bothered about compiled-code
1173             quality.&rdquo; So, for example: <command>ghc -c
1174             Foo.hs</command></para>
1175           </listitem>
1176         </varlistentry>
1177
1178         <varlistentry>
1179           <term><option>-O0</option>:</term>
1180           <indexterm><primary><option>-O0</option></primary></indexterm>
1181           <listitem>
1182             <para>Means &ldquo;turn off all optimisation&rdquo;,
1183             reverting to the same settings as if no
1184             <option>-O</option> options had been specified.  Saying
1185             <option>-O0</option> can be useful if
1186             eg. <command>make</command> has inserted a
1187             <option>-O</option> on the command line already.</para>
1188           </listitem>
1189         </varlistentry>
1190
1191         <varlistentry>
1192           <term><option>-O</option> or <option>-O1</option>:</term>
1193           <indexterm><primary>-O option</primary></indexterm>
1194           <indexterm><primary>-O1 option</primary></indexterm>
1195           <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1196           <listitem>
1197             <para>Means: &ldquo;Generate good-quality code without
1198             taking too long about it.&rdquo; Thus, for example:
1199             <command>ghc -c -O Main.lhs</command></para>
1200           </listitem>
1201         </varlistentry>
1202
1203         <varlistentry>
1204           <term><option>-O2</option>:</term>
1205           <indexterm><primary>-O2 option</primary></indexterm>
1206           <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1207           <listitem>
1208             <para>Means: &ldquo;Apply every non-dangerous
1209             optimisation, even if it means significantly longer
1210             compile times.&rdquo;</para>
1211
1212             <para>The avoided &ldquo;dangerous&rdquo; optimisations
1213             are those that can make runtime or space
1214             <emphasis>worse</emphasis> if you're unlucky.  They are
1215             normally turned on or off individually.</para>
1216
1217             <para>At the moment, <option>-O2</option> is
1218             <emphasis>unlikely</emphasis> to produce better code than
1219             <option>-O</option>.</para>
1220           </listitem>
1221         </varlistentry>
1222
1223         <varlistentry>
1224           <term><option>-Ofile &lt;file&gt;</option>:</term>
1225           <indexterm><primary>-Ofile &lt;file&gt; option</primary></indexterm>
1226           <indexterm><primary>optimising, customised</primary></indexterm>
1227           <listitem>
1228             <para>(NOTE: not supported yet in GHC 5.x.  Please ask if
1229             you're interested in this.)</para>
1230
1231             <para>For those who need <emphasis>absolute</emphasis>
1232             control over <emphasis>exactly</emphasis> what options are
1233             used (e.g., compiler writers, sometimes :-), a list of
1234             options can be put in a file and then slurped in with
1235             <option>-Ofile</option>.</para>
1236
1237             <para>In that file, comments are of the
1238             <literal>&num;</literal>-to-end-of-line variety; blank
1239             lines and most whitespace is ignored.</para>
1240
1241             <para>Please ask if you are baffled and would like an
1242             example of <option>-Ofile</option>!</para>
1243           </listitem>
1244         </varlistentry>
1245       </variablelist>
1246
1247       <para>We don't use a <option>-O*</option> flag for day-to-day
1248       work.  We use <option>-O</option> to get respectable speed;
1249       e.g., when we want to measure something.  When we want to go for
1250       broke, we tend to use <option>-O -fvia-C</option> (and we go for
1251       lots of coffee breaks).</para>
1252
1253       <para>The easiest way to see what <option>-O</option> (etc.)
1254       &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1255       then stand back in amazement.</para>
1256     </sect2>
1257
1258     <sect2 id="options-f">
1259       <title><option>-f*</option>: platform-independent flags</title>
1260
1261       <indexterm><primary>-f* options (GHC)</primary></indexterm>
1262       <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1263
1264       <para>These flags turn on and off individual optimisations.
1265       They are normally set via the <option>-O</option> options
1266       described above, and as such, you shouldn't need to set any of
1267       them explicitly (indeed, doing so could lead to unexpected
1268       results).  However, there are one or two that may be of
1269       interest:</para>
1270
1271       <variablelist>
1272         <varlistentry>
1273           <term><option>-fexcess-precision</option>:</term>
1274           <listitem>
1275             <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1276             <para>When this option is given, intermediate floating
1277             point values can have a <emphasis>greater</emphasis>
1278             precision/range than the final type.  Generally this is a
1279             good thing, but some programs may rely on the exact
1280             precision/range of
1281             <literal>Float</literal>/<literal>Double</literal> values
1282             and should not use this option for their compilation.</para>
1283           </listitem>
1284         </varlistentry>
1285
1286         <varlistentry>
1287           <term><option>-fignore-asserts</option>:</term>
1288           <listitem>
1289             <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1290             <para>Causes GHC to ignore uses of the function
1291             <literal>Exception.assert</literal> in source code (in
1292             other words, rewriting <literal>Exception.assert p
1293             e</literal> to <literal>e</literal> (see <xref
1294             linkend="sec-assertions">).  This flag is turned on by
1295             <option>-O</option>.
1296             </para>
1297           </listitem>
1298         </varlistentry>
1299
1300         <varlistentry>
1301           <term><option>-fno-strictness</option></term>
1302           <indexterm><primary><option>-fno-strictness</option></primary>
1303           </indexterm>
1304           <listitem>
1305             <para>Turns off the strictness analyser; sometimes it eats
1306             too many cycles.</para>
1307           </listitem>
1308         </varlistentry>
1309
1310         <varlistentry>
1311           <term><option>-fno-cpr-analyse</option></term>
1312           <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1313           </indexterm>
1314           <listitem>
1315             <para>Turns off the CPR (constructed product result)
1316             analysis; it is somewhat experimental.</para>
1317           </listitem>
1318         </varlistentry>
1319
1320         <varlistentry>
1321           <term><option>-funbox-strict-fields</option>:</term>
1322           <listitem>
1323             <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1324             <indexterm><primary>strict constructor fields</primary></indexterm>
1325             <indexterm><primary>constructor fields, strict</primary></indexterm>
1326
1327             <para>This option causes all constructor fields which are
1328             marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
1329             unpacked if possible.  For example:</para>
1330
1331 <ProgramListing>
1332 data T = T !Float !Float
1333 </ProgramListing>
1334
1335             <para>will create a constructor <literal>T</literal>
1336             containing two unboxed floats if the
1337             <option>-funbox-strict-fields</option> flag is given.
1338             This may not always be an optimisation: if the
1339             <Function>T</Function> constructor is scrutinised and the
1340             floats passed to a non-strict function for example, they
1341             will have to be reboxed (this is done automatically by the
1342             compiler).</para>
1343
1344             <para>This option should only be used in conjunction with
1345             <option>-O</option>, in order to expose unfoldings to the
1346             compiler so the reboxing can be removed as often as
1347             possible.  For example:</para>
1348
1349 <ProgramListing>
1350 f :: T -&#62; Float
1351 f (T f1 f2) = f1 + f2
1352 </ProgramListing>
1353
1354             <para>The compiler will avoid reboxing
1355             <Function>f1</Function> and <Function>f2</Function> by
1356             inlining <Function>+</Function> on floats, but only when
1357             <option>-O</option> is on.</para>
1358
1359             <para>Any single-constructor data is eligible for
1360             unpacking; for example</para>
1361
1362 <ProgramListing>
1363 data T = T !(Int,Int)
1364 </ProgramListing>
1365
1366             <para>will store the two <literal>Int</literal>s directly
1367             in the <Function>T</Function> constructor, by flattening
1368             the pair.  Multi-level unpacking is also supported:</para>
1369
1370 <ProgramListing>
1371 data T = T !S
1372 data S = S !Int !Int
1373 </ProgramListing>
1374
1375             <para>will store two unboxed <literal>Int&num;</literal>s
1376             directly in the <Function>T</Function> constructor.</para>
1377           </listitem>
1378         </varlistentry>
1379
1380         <varlistentry>
1381           <term><option>-funfolding-update-in-place&lt;n&gt;</option></term>
1382           <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1383           <listitem>
1384             <para>Switches on an experimental "optimisation".
1385             Switching it on makes the compiler a little keener to
1386             inline a function that returns a constructor, if the
1387             context is that of a thunk.
1388 <ProgramListing>
1389    x = plusInt a b
1390 </ProgramListing>
1391             If we inlined plusInt we might get an opportunity to use
1392             update-in-place for the thunk 'x'.</para>
1393           </listitem>
1394         </varlistentry>
1395
1396         <varlistentry>
1397           <term><option>-funfolding-creation-threshold&lt;n&gt;</option>:</term>
1398           <listitem>
1399             <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1400             <indexterm><primary>inlining, controlling</primary></indexterm>
1401             <indexterm><primary>unfolding, controlling</primary></indexterm>
1402
1403             <para>(Default: 45) Governs the maximum size that GHC will
1404             allow a function unfolding to be.   (An unfolding has a
1405             &ldquo;size&rdquo; that reflects the cost in terms of
1406             &ldquo;code bloat&rdquo; of expanding that unfolding at
1407             at a call site. A bigger function would be assigned a
1408             bigger cost.) </para>
1409
1410             <para> Consequences: (a) nothing larger than this will be
1411             inlined (unless it has an INLINE pragma); (b) nothing
1412             larger than this will be spewed into an interface
1413             file. </para>
1414
1415
1416             <para> Increasing this figure is more likely to result in longer
1417             compile times than faster code.  The next option is more
1418             useful:</para>
1419           </listitem>
1420         </varlistentry>
1421
1422         <varlistentry>
1423           <term><option>-funfolding-use-threshold&lt;n&gt;</option>:</term>
1424           <listitem>
1425             <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1426             <indexterm><primary>inlining, controlling</primary></indexterm>
1427             <indexterm><primary>unfolding, controlling</primary></indexterm>
1428
1429             <para>(Default: 8) This is the magic cut-off figure for
1430             unfolding: below this size, a function definition will be
1431             unfolded at the call-site, any bigger and it won't.  The
1432             size computed for a function depends on two things: the
1433             actual size of the expression minus any discounts that
1434             apply (see <option>-funfolding-con-discount</option>).</para>
1435           </listitem>
1436         </varlistentry>
1437       </variablelist>
1438
1439     </sect2>
1440
1441   </sect1>
1442
1443   &phases;
1444
1445   <sect1 id="sec-using-concurrent">
1446 <title>Using Concurrent Haskell</title>
1447
1448              <indexterm><primary>Concurrent Haskell&mdash;use</primary></indexterm>
1449
1450 <para>
1451 GHC supports Concurrent Haskell by default, without requiring a
1452 special option or libraries compiled in a certain way.  To get access
1453 to the support libraries for Concurrent Haskell, just import
1454 <literal>Control.Concurrent</literal> (details are in the accompanying
1455 library documentation).</para>
1456
1457 <para>
1458 RTS options are provided for modifying the behaviour of the threaded
1459 runtime system.  See <XRef LinkEnd="parallel-rts-opts">.
1460 </para>
1461
1462 <para>
1463 Concurrent Haskell is described in more detail in the documentation
1464 for the <literal>Control.Concurrent</literal> module.
1465 </para>
1466
1467 </Sect1>
1468
1469 <Sect1 id="sec-using-parallel">
1470 <title>Using Parallel Haskell</title>
1471
1472 <para>
1473 <indexterm><primary>Parallel Haskell&mdash;use</primary></indexterm>
1474 </para>
1475
1476 <para>
1477 &lsqb;You won't be able to execute parallel Haskell programs unless PVM3
1478 (Parallel Virtual Machine, version 3) is installed at your site.&rsqb;
1479 </Para>
1480
1481 <para>
1482 To compile a Haskell program for parallel execution under PVM, use the
1483 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
1484 option</Primary></IndexTerm> both when compiling <Emphasis>and
1485 linking</Emphasis>.  You will probably want to <Literal>import
1486 Parallel</Literal> into your Haskell modules.
1487 </Para>
1488
1489 <para>
1490 To run your parallel program, once PVM is going, just invoke it
1491 &ldquo;as normal&rdquo;.  The main extra RTS option is
1492 <Option>-qp&lt;n&gt;</Option>, to say how many PVM
1493 &ldquo;processors&rdquo; your program to run on.  (For more details of
1494 all relevant RTS options, please see <XRef
1495 LinkEnd="parallel-rts-opts">.)
1496 </para>
1497
1498 <para>
1499 In truth, running Parallel Haskell programs and getting information
1500 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1501 PVM, detailed in the following sections.
1502 </para>
1503
1504 <Sect2 id="pvm-dummies">
1505 <Title>Dummy's guide to using PVM</Title>
1506
1507 <para>
1508 <indexterm><primary>PVM, how to use</primary></indexterm>
1509 <indexterm><primary>Parallel Haskell&mdash;PVM use</primary></indexterm>
1510 Before you can run a parallel program under PVM, you must set the
1511 required environment variables (PVM's idea, not ours); something like,
1512 probably in your <filename>.cshrc</filename> or equivalent:
1513
1514 <ProgramListing>
1515 setenv PVM_ROOT /wherever/you/put/it
1516 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1517 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1518 </ProgramListing>
1519
1520 </para>
1521
1522 <para>
1523 Creating and/or controlling your &ldquo;parallel machine&rdquo; is a purely-PVM
1524 business; nothing specific to Parallel Haskell. The following paragraphs
1525 describe how to configure your parallel machine interactively.
1526 </Para>
1527
1528 <Para>
1529 If you use parallel Haskell regularly on the same machine configuration it
1530 is a good idea to maintain a file with all machine names and to make the
1531 environment variable PVM_HOST_FILE point to this file. Then you can avoid
1532 the interactive operations described below by just saying
1533 </Para>
1534
1535 <ProgramListing>
1536 pvm $PVM_HOST_FILE
1537 </ProgramListing>
1538
1539 <Para>
1540 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
1541 machine.  You can then do various things to control/monitor your
1542 &ldquo;parallel machine;&rdquo; the most useful being:
1543 </para>
1544
1545 <para>
1546 <InformalTable>
1547 <TGroup Cols=2>
1548 <ColSpec Align="Left">
1549 <TBody>
1550
1551 <row>
1552 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1553 <entry>exit <command>pvm</command>, leaving it running</entry>
1554 </row>
1555
1556 <row>
1557 <entry><command>halt</command></entry>
1558 <entry>kill off this &ldquo;parallel machine&rdquo; &amp; exit</entry>
1559 </row>
1560
1561 <row>
1562 <entry><command>add &lt;host&gt;</command></entry>
1563 <entry>add <command>&lt;host&gt;</command> as a processor</entry>
1564 </row>
1565
1566 <row>
1567 <entry><command>delete &lt;host&gt;</command></entry>
1568 <entry>delete <command>&lt;host&gt;</command></entry>
1569 </row>
1570
1571 <row>
1572 <entry><command>reset</command></entry>
1573 <entry>kill what's going, but leave PVM up</entry>
1574 </row>
1575
1576 <row>
1577 <entry><command>conf</command></entry>
1578 <entry>list the current configuration</entry>
1579 </row>
1580
1581 <row>
1582 <entry><command>ps</command></entry>
1583 <entry>report processes' status</entry>
1584 </row>
1585
1586 <row>
1587 <entry><command>pstat &lt;pid&gt;</command></entry>
1588 <entry>status of a particular process</entry>
1589 </row>
1590
1591 </TBody>
1592 </TGroup>
1593 </InformalTable>
1594 </para>
1595
1596 <para>
1597 The PVM documentation can tell you much, much more about <command>pvm</command>!
1598 </para>
1599
1600 </sect2>
1601
1602 <Sect2 id="par-profiles">
1603 <Title>Parallelism profiles</Title>
1604
1605 <para>
1606 <indexterm><primary>parallelism profiles</primary></indexterm>
1607 <indexterm><primary>profiles, parallelism</primary></indexterm>
1608 <indexterm><primary>visualisation tools</primary></indexterm>
1609 </para>
1610
1611 <para>
1612 With Parallel Haskell programs, we usually don't care about the
1613 results&mdash;only with &ldquo;how parallel&rdquo; it was!  We want pretty pictures.
1614 </para>
1615
1616 <Para>
1617 Parallelism profiles (&agrave; la <Command>hbcpp</Command>) can be generated with the
1618 <Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option.  The
1619 per-processor profiling info is dumped into files named
1620 <Filename>&lt;full-path&gt;&lt;program&gt;.gr</Filename>.  These are then munged into a PostScript picture,
1621 which you can then display.  For example, to run your program
1622 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
1623 </Para>
1624
1625 <Para>
1626
1627 <Screen>
1628 <prompt>&dollar;</prompt> ./a.out +RTS -qP -qp8
1629 <prompt>&dollar;</prompt> grs2gr *.???.gr &#62; temp.gr # combine the 8 .gr files into one
1630 <prompt>&dollar;</prompt> gr2ps -O temp.gr              # cvt to .ps; output in temp.ps
1631 <prompt>&dollar;</prompt> ghostview -seascape temp.ps   # look at it!
1632 </Screen>
1633
1634 </Para>
1635
1636 <para>
1637 The scripts for processing the parallelism profiles are distributed
1638 in <filename>ghc/utils/parallel/</filename>.
1639 </para>
1640
1641 </sect2>
1642
1643 <Sect2>
1644 <Title>Other useful info about running parallel programs</Title>
1645
1646 <Para>
1647 The &ldquo;garbage-collection statistics&rdquo; RTS options can be useful for
1648 seeing what parallel programs are doing.  If you do either
1649 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
1650 you'll get mutator, garbage-collection, etc., times on standard
1651 error. The standard error of all PE's other than the `main thread'
1652 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1653 </para>
1654
1655 <para>
1656 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1657 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1658 </para>
1659
1660 </sect2>
1661
1662 <Sect2 id="parallel-rts-opts">
1663 <title>RTS options for Concurrent/Parallel Haskell
1664 </title>
1665
1666 <para>
1667 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1668 <indexterm><primary>RTS options, parallel</primary></indexterm>
1669 <indexterm><primary>Concurrent Haskell&mdash;RTS options</primary></indexterm>
1670 <indexterm><primary>Parallel Haskell&mdash;RTS options</primary></indexterm>
1671 </para>
1672
1673 <para>
1674 Besides the usual runtime system (RTS) options
1675 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1676 for concurrent/parallel execution.
1677 </para>
1678
1679 <para>
1680 <VariableList>
1681
1682 <VarListEntry>
1683 <Term><Option>-qp&lt;N&gt;</Option>:</Term>
1684 <ListItem>
1685 <Para>
1686 <IndexTerm><Primary>-qp&lt;N&gt; RTS option</Primary></IndexTerm>
1687 (PARALLEL ONLY) Use <Literal>&lt;N&gt;</Literal> PVM processors to run this program;
1688 the default is 2.
1689 </para>
1690 </listitem>
1691 </varlistentry>
1692 <varlistentry>
1693 <term><option>-C[&lt;us&gt;]</option>:</term>
1694 <listitem>
1695 <para>
1696 <indexterm><primary>-C&lt;us&gt; RTS option</primary></indexterm> Sets
1697 the context switch interval to <literal>&lt;s&gt;</literal> seconds.
1698 A context switch will occur at the next heap block allocation after
1699 the timer expires (a heap block allocation occurs every 4k of
1700 allocation).  With <option>-C0</option> or <option>-C</option>,
1701 context switches will occur as often as possible (at every heap block
1702 allocation).  By default, context switches occur every 20ms
1703 milliseconds.  Note that GHC's internal timer ticks every 20ms, and
1704 the context switch timer is always a multiple of this timer, so 20ms
1705 is the maximum granularity available for timed context switches.
1706 </para>
1707 </listitem>
1708 </varlistentry>
1709 <varlistentry>
1710 <term><option>-q[v]</option>:</term>
1711 <listitem>
1712 <para>
1713 <indexterm><primary>-q RTS option</primary></indexterm>
1714 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1715 in the file <FIlename>&lt;program&gt;.qp</FIlename>.  In the style of <command>hbcpp</command>, this profile
1716 records the movement of threads between the green (runnable) and red
1717 (blocked) queues.  If you specify the verbose suboption (<option>-qv</option>), the
1718 green queue is split into green (for the currently running thread
1719 only) and amber (for other runnable threads).  We do not recommend
1720 that you use the verbose suboption if you are planning to use the
1721 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
1722 check (with <Option>-C</Option>).
1723 -->
1724 </Para>
1725 </ListItem>
1726 </VarListEntry>
1727 <VarListEntry>
1728 <Term><Option>-qt&lt;num&gt;</Option>:</Term>
1729 <ListItem>
1730 <Para>
1731 <IndexTerm><Primary>-qt&lt;num&gt; RTS option</Primary></IndexTerm>
1732 (PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
1733 threads per processor to <Literal>&lt;num&gt;</Literal>.  The default is
1734 32.  Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
1735 the heap for thread state and stack objects.  (For 32-bit machines, this
1736 translates to 4K bytes, and for 64-bit machines, 8K bytes.)
1737 </Para>
1738 </ListItem>
1739 </VarListEntry>
1740 <!-- no more -HWL
1741 <VarListEntry>
1742 <Term><Option>-d</Option>:</Term>
1743 <ListItem>
1744 <Para>
1745 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
1746 (PARALLEL ONLY) Turn on debugging.  It pops up one xterm (or GDB, or
1747 something&hellip;) per PVM processor.  We use the standard <Command>debugger</Command>
1748 script that comes with PVM3, but we sometimes meddle with the
1749 <Command>debugger2</Command> script.  We include ours in the GHC distribution,
1750 in <Filename>ghc/utils/pvm/</Filename>.
1751 </Para>
1752 </ListItem>
1753 </VarListEntry>
1754 -->
1755 <VarListEntry>
1756 <Term><Option>-qe&lt;num&gt;</Option>:</Term>
1757 <ListItem>
1758 <Para>
1759 <IndexTerm><Primary>-qe&lt;num&gt; RTS option
1760 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
1761 i.e. the number of pending sparks per processor to
1762 <Literal>&lt;num&gt;</Literal>. The default is 100. A larger number may be
1763 appropriate if your program generates large amounts of parallelism
1764 initially.
1765 </Para>
1766 </ListItem>
1767 </VarListEntry>
1768 <VarListEntry>
1769 <Term><Option>-qQ&lt;num&gt;</Option>:</Term>
1770 <ListItem>
1771 <Para>
1772 <IndexTerm><Primary>-qQ&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1773 (PARALLEL ONLY) Set the size of packets transmitted between processors
1774 to <Literal>&lt;num&gt;</Literal>. The default is 1024 words. A larger number may be
1775 appropriate if your machine has a high communication cost relative to
1776 computation speed.
1777 </Para>
1778 </ListItem>
1779 </VarListEntry>
1780 <VarListEntry>
1781 <Term><Option>-qh&lt;num&gt;</Option>:</Term>
1782 <ListItem>
1783 <Para>
1784 <IndexTerm><Primary>-qh&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1785 (PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
1786 &lt;num&gt;-1 (0 means infinity). By default GUM uses full-subgraph
1787 packing, i.e. the entire subgraph with the requested closure as root is
1788 transmitted (provided it fits into one packet). Choosing a smaller value
1789 reduces the amount of pre-fetching of work done in GUM. This can be
1790 advantageous for improving data locality but it can also worsen the balance
1791 of the load in the system.
1792 </Para>
1793 </ListItem>
1794 </VarListEntry>
1795 <VarListEntry>
1796 <Term><Option>-qg&lt;num&gt;</Option>:</Term>
1797 <ListItem>
1798 <Para>
1799 <IndexTerm><Primary>-qg&lt;num&gt; RTS option
1800 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
1801 scheme. This option affects the
1802 generation of global addresses when transferring data. Global addresses are
1803 globally unique identifiers required to maintain sharing in the distributed
1804 graph structure. Currently this is a binary option. With &lt;num&gt;=0 full globalisation is used
1805 (default). This means a global address is generated for every closure that
1806 is transmitted. With &lt;num&gt;=1 a thunk-only globalisation scheme is
1807 used, which generated global address only for thunks. The latter case may
1808 lose sharing of data but has a reduced overhead in packing graph structures
1809 and maintaining internal tables of global addresses.
1810 </Para>
1811 </ListItem>
1812 </VarListEntry>
1813 </VariableList>
1814 </para>
1815
1816 </sect2>
1817
1818 </Sect1>
1819
1820   <sect1 id="options-platform">
1821     <title>Platform-specific Flags</title>
1822
1823     <indexterm><primary>-m* options</primary></indexterm>
1824     <indexterm><primary>platform-specific options</primary></indexterm>
1825     <indexterm><primary>machine-specific options</primary></indexterm>
1826
1827     <para>Some flags only make sense for particular target
1828     platforms.</para>
1829
1830     <variablelist>
1831
1832       <varlistentry>
1833         <term><option>-mv8</option>:</term>
1834         <listitem>
1835           <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1836           only)</primary></indexterm> Means to pass the like-named
1837           option to GCC; it says to use the Version 8 SPARC
1838           instructions, notably integer multiply and divide.  The
1839           similiar <option>-m*</option> GCC options for SPARC also
1840           work, actually.</para>
1841         </listitem>
1842       </varlistentry>
1843
1844       <varlistentry>
1845         <term><option>-monly-[32]-regs</option>:</term>
1846         <listitem>
1847           <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1848           option (iX86 only)</primary></indexterm> GHC tries to
1849           &ldquo;steal&rdquo; four registers from GCC, for performance
1850           reasons; it almost always works.  However, when GCC is
1851           compiling some modules with four stolen registers, it will
1852           crash, probably saying:
1853
1854 <Screen>
1855 Foo.hc:533: fixed or forbidden register was spilled.
1856 This may be due to a compiler bug or to impossible asm
1857 statements or clauses.
1858 </Screen>
1859
1860           Just give some registers back with
1861           <option>-monly-N-regs</option>.  Try `3' first, then `2'.
1862           If `2' doesn't work, please report the bug to us.</para>
1863         </listitem>
1864       </varlistentry>
1865     </variablelist>
1866
1867   </sect1>
1868
1869 &runtime;
1870
1871 <sect1 id="ext-core">
1872   <title>Generating External Core Files</title>
1873
1874   <indexterm><primary>intermediate code generation</primary></indexterm>
1875
1876   <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
1877   to a file as a side-effect of compilation. Core files, which are given the suffix
1878   <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1879   tools.  The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz"
1880   <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1881   and sample tools (in Haskell)
1882   for manipulating Core files are available in the GHC source distribution
1883   directory <literal>/fptools/ghc/utils/ext-core</literal>.
1884   Note that the format of <literal>.hcr</literal>
1885   files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1886   for debugging purposes (<xref linkend="options-debugging">).</para>
1887
1888     <variablelist>
1889
1890         <varlistentry>
1891           <term><option>-fext-core</option></term>
1892           <indexterm>
1893             <primary><option>-fext-core</option></primary>
1894           </indexterm>
1895           <listitem>
1896             <para>Generate <literal>.hcr</literal> files.</para>
1897           </listitem>
1898         </varlistentry>
1899
1900     </variablelist>
1901
1902 </sect1>
1903
1904 &debug;
1905 &flags;
1906
1907 </Chapter>
1908
1909 <!-- Emacs stuff:
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