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 may contain any number of source file names
 397     or module names; GHC will figure out all the modules in the
 398     program by following the imports from these initial modules.  It
 399     will then attempt to compile each module which is out of date, and
 400     finally if there is a <literal>Main</literal> module, 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), then the object files can be
 438     given on the command line and GHC will include them when linking
 439     the executable.</para>
 440
 441     <para>Note that GHC can only follow dependencies if it has the
 442     source file available, so if your program includes a module for
 443     which there is no source file, even if you have an object and an
 444     interface file for the module, then GHC will complain.  The
 445     exception to this rule is for package modules, which may or may
 446     not have source files.</para>
 447
 448     <para>The source files for the program don't all need to be in the
 449     same directory; the <option>-i</option> option can be used to add
 450     directories to the search path (see <xref
 451     linkend="options-finding-imports">).</para>
 452
 453   </sect1>
 454
 455   <Sect1 id="options-order">
 456     <title>GHC without <option>&ndash;&ndash;make</option></title>
 457
 458     <para>Without <option>&ndash;&ndash;make</option>, GHC will compile one or
 459     more source files given on the command line.</para>
 460
 461     <para>The first phase to run is determined by each input-file
 462     suffix, and the last phase is determined by a flag.  If no
 463     relevant flag is present, then go all the way through linking.
 464     This table summarises:</para>
 465
 466     <informaltable>
 467       <tgroup cols="4">
 468         <colspec align="left">
 469         <colspec align="left">
 470         <colspec align="left">
 471         <colspec align="left">
 472
 473         <thead>
 474           <row>
 475             <entry>Phase of the compilation system</entry>
 476             <entry>Suffix saying &ldquo;start here&rdquo;</entry>
 477             <entry>Flag saying &ldquo;stop after&rdquo;</entry>
 478             <entry>(suffix of) output file</entry>
 479           </row>
 480         </thead>
 481         <tbody>
 482           <row>
 483             <entry>literate pre-processor</entry>
 484             <entry><literal>.lhs</literal></entry>
 485             <entry>-</entry>
 486             <entry><literal>.hs</literal></entry>
 487           </row>
 488
 489           <row>
 490             <entry>C pre-processor (opt.)
 491            </entry>
 492             <entry><literal>.hs</literal> (with
 493             <option>-cpp</option>)</entry>
 494             <entry><option>-E</option></entry>
 495             <entry><literal>.hspp</literal></entry>
 496           </row>
 497
 498           <row>
 499             <entry>Haskell compiler</entry>
 500             <entry><literal>.hs</literal></entry>
 501             <entry><option>-C</option>, <option>-S</option></entry>
 502             <entry><literal>.hc</literal>, <literal>.s</literal></entry>
 503           </row>
 504
 505           <row>
 506             <entry>C compiler (opt.)</entry>
 507             <entry><literal>.hc</literal> or <literal>.c</literal></entry>
 508             <entry><option>-S</option></entry>
 509             <entry><literal>.s</literal></entry>
 510           </row>
 511
 512           <row>
 513             <entry>assembler</entry>
 514             <entry><literal>.s</literal></entry>
 515             <entry><option>-c</option></entry>
 516             <entry><literal>.o</literal></entry>
 517           </row>
 518
 519           <row>
 520             <entry>linker</entry>
 521             <entry><replaceable>other</replaceable></entry>
 522             <entry>-</entry>
 523             <entry><filename>a.out</filename></entry>
 524           </row>
 525         </tbody>
 526       </tgroup>
 527     </informaltable>
 528
 529     <indexterm><primary><option>-C</option></primary></indexterm>
 530     <indexterm><primary><option>-E</option></primary></indexterm>
 531     <indexterm><primary><option>-S</option></primary></indexterm>
 532     <indexterm><primary><option>-c</option></primary></indexterm>
 533
 534     <para>Thus, a common invocation would be: <literal>ghc -c
 535     Foo.hs</literal></para>
 536
 537     <para>Note: What the Haskell compiler proper produces depends on
 538     whether a native-code generator<indexterm><primary>native-code
 539     generator</primary></indexterm> is used (producing assembly
 540     language) or not (producing C).  See <xref
 541     linkend="options-codegen"> for more details.</para>
 542
 543     <para>Note: C pre-processing is optional, the
 544     <option>-ccp</option><indexterm><primary><option>-cpp</option></primary>
 545       </indexterm>flag turns it on.  See <xref
 546     linkend="c-pre-processor"> for more details.</para>
 547
 548     <para>Note: The option <option>-E</option><IndexTerm><Primary>-E
 549     option</Primary></IndexTerm> runs just the pre-processing passes
 550     of the compiler, dumping the result in a file.  Note that this
 551     differs from the previous behaviour of dumping the file to
 552     standard output.</para>
 553   </sect1>
 554
 555   <sect1 id="options-output">
 556     <title>Re-directing the compilation output(s)</title>
 557
 558     <indexterm><primary>output-directing options</primary></indexterm>
 559     <indexterm><primary>redirecting compilation output</primary></indexterm>
 560
 561
 562     <variablelist>
 563       <varlistentry>
 564         <term><option>-o</option></term>
 565         <indexterm><primary><option>-o</option></primary></indexterm>
 566         <listitem>
 567           <para>GHC's compiled output normally goes into a
 568           <filename>.hc</filename>, <filename>.o</filename>, etc.,
 569           file, depending on the last-run compilation phase.  The
 570           option <option>-o foo</option><IndexTerm><Primary>-o
 571           option</Primary></IndexTerm> re-directs the output of that
 572           last-run phase to file <filename>foo</filename>.</para>
 573
 574           <para>Note: this &ldquo;feature&rdquo; can be
 575           counterintuitive: <command>ghc -C -o foo.o foo.hs</command>
 576           will put the intermediate C code in the file
 577           <filename>foo.o</filename>, name notwithstanding!</para>
 578
 579           <para>Note: on Windows, if the result is an executable file, the
 580           extension "<filename>.exe</filename>" is added if the specified filename
 581             does not already have an extension.  Thus
 582            <programlisting>
 583                 ghc -o foo Main.hs
 584            </programlisting>
 585           will compile and link the module <filename>Main.hs</filename>, and put the
 586           resulting executable in <filename>foo.exe</filename> (not <filename>foo</filename>).
 587           </para>
 588         </listitem>
 589       </varlistentry>
 590
 591       <varlistentry>
 592         <term><option>-odir</option></term>
 593         <indexterm><primary><option>-odir</option></primary></indexterm>
 594         <listitem>
 595           <para>The <option>-o</option> option isn't of much use if
 596           you have <emphasis>several</emphasis> input files&hellip;
 597           Non-interface output files are normally put in the same
 598           directory as their corresponding input file came from.  You
 599           may specify that they be put in another directory using the
 600           <option>-odir &lt;dir&gt;</option><IndexTerm><Primary>-odir
 601           &lt;dir&gt; option</Primary></IndexTerm> (the &ldquo;Oh,
 602           dear&rdquo; option).  For example:</para>
 603
 604 <Screen>
 605 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
 606 </Screen>
 607
 608           <para>The output files, <filename>Foo.o</filename>,
 609           <filename>Bar.o</filename>, and
 610           <filename>Bumble.o</filename> would be put into a
 611           subdirectory named after the architecture of the executing
 612           machine (<filename>sun4</filename>,
 613           <filename>mips</filename>, etc).  The directory must already
 614           exist; it won't be created.</para>
 615
 616           <para>Note that the <option>-odir</option> option does
 617           <emphasis>not</emphasis> affect where the interface files
 618           are put.  In the above example, they would still be put in
 619           <filename>parse/Foo.hi</filename>,
 620           <filename>parse/Bar.hi</filename>, and
 621           <filename>gurgle/Bumble.hi</filename>.</para>
 622         </listitem>
 623       </varlistentry>
 624
 625       <varlistentry>
 626         <term><option>-ohi</option>  <replaceable>file</replaceable></term>
 627         <indexterm><primary><option>-ohi</option></primary>
 628         </indexterm>
 629         <listitem>
 630           <para>The interface output may be directed to another file
 631           <filename>bar2/Wurble.iface</filename> with the option
 632           <option>-ohi bar2/Wurble.iface</option> (not
 633           recommended).</para>
 634
 635           <para>WARNING: if you redirect the interface file somewhere
 636           that GHC can't find it, then the recompilation checker may
 637           get confused (at the least, you won't get any recompilation
 638           avoidance).  We recommend using a combination of
 639           <option>-hidir</option> and <option>-hisuf</option> options
 640           instead, if possible.</para>
 641
 642           <para>To avoid generating an interface at all, you could use
 643           this option to redirect the interface into the bit bucket:
 644           <literal>-ohi /dev/null</literal>, for example.</para>
 645         </listitem>
 646       </varlistentry>
 647
 648       <varlistentry>
 649         <term><option>-hidir</option>  <replaceable>directory</replaceable></term>
 650         <indexterm><primary><option>-hidir</option></primary>
 651         </indexterm>
 652         <listitem>
 653           <para>Redirects all generated interface files into
 654           <replaceable>directory</replaceable>, instead of the default
 655           which is to place the interface file in the same directory
 656           as the source file.</para>
 657         </listitem>
 658       </varlistentry>
 659
 660       <varlistentry>
 661         <term><option>-osuf</option> <replaceable>suffix</replaceable></term>
 662         <term><option>-hisuf</option> <replaceable>suffix</replaceable></term>
 663         <term><option>-hcsuf</option> <replaceable>suffix</replaceable></term>
 664         <indexterm><primary><option>-osuf</option></primary></indexterm>
 665         <indexterm><primary><option>-hisuf</option></primary></indexterm>
 666         <indexterm><primary><option>-hcsuf</option></primary></indexterm>
 667         <listitem>
 668           <para>EXOTICA: The <option>-osuf</option>
 669           <replaceable>suffix</replaceable> will change the
 670           <literal>.o</literal> file suffix for object files to
 671           whatever you specify.  We use this when compiling libraries,
 672           so that objects for the profiling versions of the libraries
 673           don't clobber the normal ones.</para>
 674
 675           <para>Similarly, the <option>-hisuf</option>
 676           <replaceable>suffix</replaceable> will change the
 677           <literal>.hi</literal> file suffix for non-system interface
 678           files (see <XRef LinkEnd="hi-options">).</para>
 679
 680           <para>Finally, the option <option>-hcsuf</option>
 681           <replaceable>suffix</replaceable> will change the
 682           <literal>.hc</literal> file suffix for compiler-generated
 683           intermediate C files.</para>
 684
 685           <para>The <option>-hisuf</option>/<option>-osuf</option>
 686           game is particularly useful if you want to compile a program both with and without
 687             profiling, in the same directory.  You can say:
 688             <Screen>
 689               ghc ...
 690             </Screen>
 691             to get the ordinary version, and
 692             <Screen>
 693               ghc ... -osuf prof.o -hisuf prof.hi -prof -auto-all
 694             </Screen>
 695             to get the profiled version.</para>
 696         </listitem>
 697       </varlistentry>
 698     </variablelist>
 699
 700     <sect2 id="keeping-intermediates">
 701       <title>Keeping Intermediate Files</title>
 702       <indexterm><primary>intermediate files, saving</primary>
 703       </indexterm>
 704       <indexterm><primary><literal>.hc</literal> files, saving</primary>
 705       </indexterm>
 706       <indexterm><primary><literal>.s</literal> files, saving</primary>
 707       </indexterm>
 708
 709
 710       <para>The following options are useful for keeping certain
 711       intermediate files around, when normally GHC would throw these
 712       away after compilation:</para>
 713
 714       <variablelist>
 715         <varlistentry>
 716           <term><option>-keep-hc-files</option></term>
 717           <indexterm>
 718             <primary><option>-keep-hc-files</option></primary>
 719           </indexterm>
 720           <listitem>
 721             <para>Keep intermediate <literal>.hc</literal> files when
 722             doing <literal>.hs</literal>-to-<literal>.o</literal>
 723             compilations via C (NOTE: <literal>.hc</literal> files
 724             aren't generated when using the native code generator, you
 725             may need to use <option>-fvia-C</option> to force them
 726             to be produced).</para>
 727           </listitem>
 728         </varlistentry>
 729
 730         <varlistentry>
 731           <term><option>-keep-s-files</option></term>
 732           <indexterm>
 733             <primary><option>-keep-s-files</option></primary>
 734           </indexterm>
 735           <listitem>
 736             <para>Keep intermediate <literal>.s</literal> files.</para>
 737           </listitem>
 738         </varlistentry>
 739
 740         <varlistentry>
 741           <term><option>-keep-raw-s-files</option></term>
 742           <indexterm>
 743             <primary><option>-keep-raw-s-files</option></primary>
 744           </indexterm>
 745           <listitem>
 746             <para>Keep intermediate <literal>.raw-s</literal> files.
 747             These are the direct output from the C compiler, before
 748             GHC does &ldquo;assembly mangling&rdquo; to produce the
 749             <literal>.s</literal> file.  Again, these are not produced
 750             when using the native code generator.</para>
 751           </listitem>
 752         </varlistentry>
 753
 754         <varlistentry>
 755           <term><option>-keep-tmp-files</option></term>
 756           <indexterm>
 757             <primary><option>-keep-tmp-files</option></primary>
 758           </indexterm>
 759           <indexterm>
 760             <primary>temporary files</primary>
 761             <secondary>keeping</secondary>
 762           </indexterm>
 763           <listitem>
 764             <para>Instructs the GHC driver not to delete any of its
 765             temporary files, which it normally keeps in
 766             <literal>/tmp</literal> (or possibly elsewhere; see <xref
 767             linkend="temp-files">).  Running GHC with
 768             <option>-v</option> will show you what temporary files
 769             were generated along the way.</para>
 770           </listitem>
 771         </varlistentry>
 772       </variablelist>
 773     </sect2>
 774
 775     <sect2 id="temp-files">
 776       <title>Redirecting temporary files</title>
 777
 778       <indexterm>
 779         <primary>temporary files</primary>
 780         <secondary>redirecting</secondary>
 781       </indexterm>
 782
 783       <variablelist>
 784         <varlistentry>
 785           <term><option>-tmpdir</option></term>
 786           <indexterm><primary><option>-tmpdir</option></primary></indexterm>
 787           <listitem>
 788             <para>If you have trouble because of running out of space
 789             in <filename>/tmp</filename> (or wherever your
 790             installation thinks temporary files should go), you may
 791             use the <option>-tmpdir
 792             &lt;dir&gt;</option><IndexTerm><Primary>-tmpdir
 793             &lt;dir&gt; option</Primary></IndexTerm> option to specify
 794             an alternate directory.  For example, <option>-tmpdir
 795             .</option> says to put temporary files in the current
 796             working directory.</para>
 797
 798             <para>Alternatively, use your <Constant>TMPDIR</Constant>
 799             environment variable.<IndexTerm><Primary>TMPDIR
 800             environment variable</Primary></IndexTerm> Set it to the
 801             name of the directory where temporary files should be put.
 802             GCC and other programs will honour the
 803             <Constant>TMPDIR</Constant> variable as well.</para>
 804
 805             <para>Even better idea: Set the
 806             <Constant>DEFAULT_TMPDIR</Constant> make variable when
 807             building GHC, and never worry about
 808             <Constant>TMPDIR</Constant> again. (see the build
 809             documentation).</para>
 810           </listitem>
 811         </varlistentry>
 812       </variablelist>
 813     </sect2>
 814
 815   </sect1>
 816
 817   <sect1 id="options-sanity">
 818     <title>Warnings and sanity-checking</title>
 819
 820     <indexterm><primary>sanity-checking options</primary></indexterm>
 821     <indexterm><primary>warnings</primary></indexterm>
 822
 823
 824     <para>GHC has a number of options that select which types of
 825     non-fatal error messages, otherwise known as warnings, can be
 826     generated during compilation.  By default, you get a standard set
 827     of warnings which are generally likely to indicate bugs in your
 828     program.  These are:
 829     <option>-fwarn-overlapping-patterns</option>,
 830     <option>-fwarn-deprecations</option>,
 831     <option>-fwarn-duplicate-exports</option>,
 832     <option>-fwarn-missing-fields</option>, and
 833     <option>-fwarn-missing-methods</option>.  The following flags are
 834     simple ways to select standard &ldquo;packages&rdquo; of warnings:
 835     </para>
 836
 837     <VariableList>
 838
 839       <varlistentry>
 840         <term><option>-W</option>:</term>
 841         <listitem>
 842           <IndexTerm><Primary>-W option</Primary></IndexTerm>
 843           <para>Provides the standard warnings plus
 844           <option>-fwarn-incomplete-patterns</option>,
 845           <option>-fwarn-unused-matches</option>,
 846           <option>-fwarn-unused-imports</option>,
 847           <option>-fwarn-misc</option>, and
 848           <option>-fwarn-unused-binds</option>.</para>
 849         </listitem>
 850       </varlistentry>
 851
 852       <varlistentry>
 853         <term><option>-w</option>:</term>
 854         <listitem>
 855           <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
 856           <para>Turns off all warnings, including the standard ones.</para>
 857         </listitem>
 858       </varlistentry>
 859
 860       <varlistentry>
 861         <term><option>-Wall</option>:</term>
 862         <listitem>
 863           <indexterm><primary><option>-Wall</option></primary></indexterm>
 864           <para>Turns on all warning options.</para>
 865         </listitem>
 866       </varlistentry>
 867
 868       <varlistentry>
 869         <term><option>-Werror</option>:</term>
 870         <listitem>
 871           <indexterm><primary><option>-Werror</option></primary></indexterm>
 872           <para>Makes any warning into a fatal error. Useful so that you don't
 873             miss warnings when doing batch compilation. </para>
 874         </listitem>
 875       </varlistentry>
 876
 877     </variablelist>
 878
 879     <para>The full set of warning options is described below.  To turn
 880     off any warning, simply give the corresponding
 881     <option>-fno-warn-...</option> option on the command line.</para>
 882
 883     <variablelist>
 884
 885       <varlistentry>
 886         <term><option>-fwarn-deprecations</option>:</term>
 887         <listitem>
 888           <indexterm><primary><option>-fwarn-deprecations</option></primary>
 889           </indexterm>
 890           <indexterm><primary>deprecations</primary></indexterm>
 891           <para>Causes a warning to be emitted when a deprecated
 892           function or type is used.  Entities can be marked as
 893           deprecated using a pragma, see <xref
 894           linkend="deprecated-pragma">.</para>
 895         </listitem>
 896       </varlistentry>
 897
 898       <varlistentry>
 899         <term><option>-fwarn-duplicate-exports</option>:</term>
 900         <listitem>
 901           <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
 902           <indexterm><primary>duplicate exports, warning</primary></indexterm>
 903           <indexterm><primary>export lists, duplicates</primary></indexterm>
 904
 905           <para>Have the compiler warn about duplicate entries in
 906           export lists. This is useful information if you maintain
 907           large export lists, and want to avoid the continued export
 908           of a definition after you've deleted (one) mention of it in
 909           the export list.</para>
 910
 911           <para>This option is on by default.</para>
 912         </listitem>
 913       </varlistentry>
 914
 915       <varlistentry>
 916         <term><option>-fwarn-hi-shadowing</option>:</term>
 917         <listitem>
 918           <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
 919           <indexterm><primary>shadowing</primary>
 920             <secondary>interface files</secondary></indexterm>
 921
 922           <para>Causes the compiler to emit a warning when a module or
 923           interface file in the current directory is shadowing one
 924           with the same module name in a library or other
 925           directory.</para>
 926         </listitem>
 927       </varlistentry>
 928
 929       <varlistentry>
 930         <term><option>-fwarn-incomplete-patterns</option>:</term>
 931         <listitem>
 932           <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
 933           <indexterm><primary>incomplete patterns, warning</primary></indexterm>
 934           <indexterm><primary>patterns, incomplete</primary></indexterm>
 935
 936           <para>Similarly for incomplete patterns, the function
 937           <function>g</function> below will fail when applied to
 938           non-empty lists, so the compiler will emit a warning about
 939           this when <option>-fwarn-incomplete-patterns</option> is
 940           enabled.</para>
 941
 942 <programlisting>
 943 g [] = 2
 944 </programlisting>
 945
 946           <para>This option isn't enabled be default because it can be
 947           a bit noisy, and it doesn't always indicate a bug in the
 948           program.  However, it's generally considered good practice
 949           to cover all the cases in your functions.</para>
 950         </listitem>
 951       </varlistentry>
 952
 953       <varlistentry>
 954         <term><option>-fwarn-misc</option>:</term>
 955         <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
 956         <listitem>
 957           <para>Turns on warnings for various harmless but untidy
 958           things.  This currently includes: importing a type with
 959           <literal>(..)</literal> when the export is abstract, and
 960           listing duplicate class assertions in a qualified type.</para>
 961         </listitem>
 962       </varlistentry>
 963
 964       <varlistentry>
 965         <term><option>-fwarn-missing-fields</option>:</term>
 966         <listitem>
 967           <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
 968           <indexterm><primary>missing fields, warning</primary></indexterm>
 969           <indexterm><primary>fields, missing</primary></indexterm>
 970
 971           <para>This option is on by default, and warns you whenever
 972           the construction of a labelled field constructor isn't
 973           complete, missing initializers for one or more fields. While
 974           not an error (the missing fields are initialised with
 975           bottoms), it is often an indication of a programmer error.</para>
 976         </listitem>
 977       </varlistentry>
 978
 979       <varlistentry>
 980         <term><option>-fwarn-missing-methods</option>:</term>
 981         <listitem>
 982           <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
 983           <indexterm><primary>missing methods, warning</primary></indexterm>
 984           <indexterm><primary>methods, missing</primary></indexterm>
 985
 986           <para>This option is on by default, and warns you whenever
 987           an instance declaration is missing one or more methods, and
 988           the corresponding class declaration has no default
 989           declaration for them.</para>
 990           <para>The warning is suppressed if the method name
 991           begins with an underscore.  Here's an example where this is useful:
 992             <programlisting>
 993               class C a where
 994                 _simpleFn :: a -> String
 995                 complexFn :: a -> a -> String
 996                 complexFn x y = ... _simpleFn ...
 997               </programlisting>
 998             The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
 999             never <literal>_simpleFn</literal>; and (b)
1000             instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1001             </para>
1002         </listitem>
1003       </varlistentry>
1004
1005       <varlistentry>
1006         <term><option>-fwarn-missing-signatures</option>:</term>
1007         <listitem>
1008           <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1009           <indexterm><primary>type signatures, missing</primary></indexterm>
1010
1011           <para>If you would like GHC to check that every top-level
1012           function/value has a type signature, use the
1013           <option>-fwarn-missing-signatures</option> option.  This
1014           option is off by default.</para>
1015         </listitem>
1016       </varlistentry>
1017
1018       <varlistentry>
1019         <term><option>-fwarn-name-shadowing</option>:</term>
1020         <listitem>
1021           <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1022           <indexterm><primary>shadowing, warning</primary></indexterm>
1023
1024           <para>This option causes a warning to be emitted whenever an
1025           inner-scope value has the same name as an outer-scope value,
1026           i.e. the inner value shadows the outer one.  This can catch
1027           typographical errors that turn into hard-to-find bugs, e.g.,
1028           in the inadvertent cyclic definition <literal>let x = ... x
1029           ... in</literal>.</para>
1030
1031           <para>Consequently, this option does
1032           <emphasis>will</emphasis> complain about cyclic recursive
1033           definitions.</para>
1034         </listitem>
1035       </varlistentry>
1036
1037       <varlistentry>
1038         <term><option>-fwarn-overlapping-patterns</option>:</term>
1039         <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1040         <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1041         <indexterm><primary>patterns, overlapping</primary></indexterm>
1042         <listitem>
1043           <para>By default, the compiler will warn you if a set of
1044           patterns are overlapping, i.e.,</para>
1045
1046 <programlisting>
1047 f :: String -&#62; Int
1048 f []     = 0
1049 f (_:xs) = 1
1050 f "2"    = 2
1051 </programlisting>
1052
1053           <para>where the last pattern match in <Function>f</Function>
1054           won't ever be reached, as the second pattern overlaps
1055           it. More often than not, redundant patterns is a programmer
1056           mistake/error, so this option is enabled by default.</para>
1057         </listitem>
1058       </varlistentry>
1059
1060       <varlistentry>
1061         <term><option>-fwarn-simple-patterns</option>:</term>
1062         <listitem>
1063           <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1064           </indexterm>
1065           <para>Causes the compiler to warn about lambda-bound
1066           patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1067           Normally, these aren't treated as incomplete patterns by
1068           <option>-fwarn-incomplete-patterns</option>.</para>
1069           <para>``Lambda-bound patterns'' includes all places where there is a single pattern,
1070             including list comprehensions and do-notation.  In these cases, a pattern-match
1071             failure is quite legitimate, and triggers filtering (list comprehensions) or
1072             the monad <literal>fail</literal> operation (monads). For example:
1073             <programlisting>
1074               f :: [Maybe a] -> [a]
1075               f xs = [y | Just y <- xs]
1076               </programlisting>
1077             Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1078             these probably-innocent cases, which is why the flag is off by default. </para>
1079           <para> The <literal>deriving( Read )</literal> mechanism produces monadic code with
1080             pattern matches, so you will also get misleading warnings about the compiler-generated
1081             code.  (This is arguably a Bad Thing, but it's awkward to fix.)</para>
1082
1083         </listitem>
1084       </varlistentry>
1085
1086       <varlistentry>
1087         <term><option>-fwarn-type-defaults</option>:</term>
1088         <listitem>
1089           <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1090           <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1091           <para>Have the compiler warn/inform you where in your source
1092           the Haskell defaulting mechanism for numeric types kicks
1093           in. This is useful information when converting code from a
1094           context that assumed one default into one with another,
1095           e.g., the `default default' for Haskell 1.4 caused the
1096           otherwise unconstrained value <Constant>1</Constant> to be
1097           given the type <literal>Int</literal>, whereas Haskell 98
1098           defaults it to <literal>Integer</literal>.  This may lead to
1099           differences in performance and behaviour, hence the
1100           usefulness of being non-silent about this.</para>
1101
1102           <para>This warning is off by default.</para>
1103         </listitem>
1104       </varlistentry>
1105
1106       <varlistentry>
1107         <term><option>-fwarn-unused-binds</option>:</term>
1108         <listitem>
1109           <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1110           <indexterm><primary>unused binds, warning</primary></indexterm>
1111           <indexterm><primary>binds, unused</primary></indexterm>
1112           <para>Report any function definitions (and local bindings)
1113           which are unused.  For top-level functions, the warning is
1114           only given if the binding is not exported.</para>
1115         </listitem>
1116       </varlistentry>
1117
1118       <varlistentry>
1119         <term><option>-fwarn-unused-imports</option>:</term>
1120         <listitem>
1121           <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1122           <indexterm><primary>unused imports, warning</primary></indexterm>
1123           <indexterm><primary>imports, unused</primary></indexterm>
1124
1125           <para>Report any objects that are explicitly imported but
1126           never used.</para>
1127         </listitem>
1128       </varlistentry>
1129
1130       <varlistentry>
1131         <term><option>-fwarn-unused-matches</option>:</term>
1132         <listitem>
1133           <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1134           <indexterm><primary>unused matches, warning</primary></indexterm>
1135           <indexterm><primary>matches, unused</primary></indexterm>
1136
1137           <para>Report all unused variables which arise from pattern
1138           matches, including patterns consisting of a single variable.
1139           For instance <literal>f x y = []</literal> would report
1140           <VarName>x</VarName> and <VarName>y</VarName> as unused.  The
1141           warning is suppressed if the variable name begins with an underscore, thus:
1142             <programlisting>
1143                f _x = True
1144             </programlisting>
1145           </para>
1146         </listitem>
1147       </varlistentry>
1148
1149     </VariableList>
1150
1151     <para>If you're feeling really paranoid, the
1152     <option>-dcore-lint</option>
1153     option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1154     is a good choice.  It turns on heavyweight intra-pass
1155     sanity-checking within GHC.  (It checks GHC's sanity, not
1156     yours.)</para>
1157
1158   </sect1>
1159
1160   &separate;
1161   &packages;
1162
1163   <sect1 id="options-optimise">
1164     <title>Optimisation (code improvement)</title>
1165
1166     <indexterm><primary>optimisation</primary></indexterm>
1167     <indexterm><primary>improvement, code</primary></indexterm>
1168
1169     <para>The <option>-O*</option> options specify convenient
1170     &ldquo;packages&rdquo; of optimisation flags; the
1171     <option>-f*</option> options described later on specify
1172     <emphasis>individual</emphasis> optimisations to be turned on/off;
1173     the <option>-m*</option> options specify
1174     <emphasis>machine-specific</emphasis> optimisations to be turned
1175     on/off.</para>
1176
1177     <sect2 id="optimise-pkgs">
1178       <title><option>-O*</option>: convenient &ldquo;packages&rdquo; of optimisation flags.</title>
1179
1180       <para>There are <emphasis>many</emphasis> options that affect
1181       the quality of code produced by GHC.  Most people only have a
1182       general goal, something like &ldquo;Compile quickly&rdquo; or
1183       &ldquo;Make my program run like greased lightning.&rdquo; The
1184       following &ldquo;packages&rdquo; of optimisations (or lack
1185       thereof) should suffice.</para>
1186
1187       <para>Once you choose a <option>-O*</option>
1188       &ldquo;package,&rdquo; stick with it&mdash;don't chop and
1189       change.  Modules' interfaces <emphasis>will</emphasis> change
1190       with a shift to a new <option>-O*</option> option, and you may
1191       have to recompile a large chunk of all importing modules before
1192       your program can again be run safely (see <XRef
1193       LinkEnd="recomp">).</para>
1194
1195       <variablelist>
1196
1197         <varlistentry>
1198           <term>No <option>-O*</option>-type option specified:</term>
1199           <indexterm><primary>-O* not specified</primary></indexterm>
1200           <listitem>
1201             <para>This is taken to mean: &ldquo;Please compile
1202             quickly; I'm not over-bothered about compiled-code
1203             quality.&rdquo; So, for example: <command>ghc -c
1204             Foo.hs</command></para>
1205           </listitem>
1206         </varlistentry>
1207
1208         <varlistentry>
1209           <term><option>-O0</option>:</term>
1210           <indexterm><primary><option>-O0</option></primary></indexterm>
1211           <listitem>
1212             <para>Means &ldquo;turn off all optimisation&rdquo;,
1213             reverting to the same settings as if no
1214             <option>-O</option> options had been specified.  Saying
1215             <option>-O0</option> can be useful if
1216             eg. <command>make</command> has inserted a
1217             <option>-O</option> on the command line already.</para>
1218           </listitem>
1219         </varlistentry>
1220
1221         <varlistentry>
1222           <term><option>-O</option> or <option>-O1</option>:</term>
1223           <indexterm><primary>-O option</primary></indexterm>
1224           <indexterm><primary>-O1 option</primary></indexterm>
1225           <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1226           <listitem>
1227             <para>Means: &ldquo;Generate good-quality code without
1228             taking too long about it.&rdquo; Thus, for example:
1229             <command>ghc -c -O Main.lhs</command></para>
1230           </listitem>
1231         </varlistentry>
1232
1233         <varlistentry>
1234           <term><option>-O2</option>:</term>
1235           <indexterm><primary>-O2 option</primary></indexterm>
1236           <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1237           <listitem>
1238             <para>Means: &ldquo;Apply every non-dangerous
1239             optimisation, even if it means significantly longer
1240             compile times.&rdquo;</para>
1241
1242             <para>The avoided &ldquo;dangerous&rdquo; optimisations
1243             are those that can make runtime or space
1244             <emphasis>worse</emphasis> if you're unlucky.  They are
1245             normally turned on or off individually.</para>
1246
1247             <para>At the moment, <option>-O2</option> is
1248             <emphasis>unlikely</emphasis> to produce better code than
1249             <option>-O</option>.</para>
1250           </listitem>
1251         </varlistentry>
1252
1253         <varlistentry>
1254           <term><option>-Ofile &lt;file&gt;</option>:</term>
1255           <indexterm><primary>-Ofile &lt;file&gt; option</primary></indexterm>
1256           <indexterm><primary>optimising, customised</primary></indexterm>
1257           <listitem>
1258             <para>(NOTE: not supported yet in GHC 5.x.  Please ask if
1259             you're interested in this.)</para>
1260
1261             <para>For those who need <emphasis>absolute</emphasis>
1262             control over <emphasis>exactly</emphasis> what options are
1263             used (e.g., compiler writers, sometimes :-), a list of
1264             options can be put in a file and then slurped in with
1265             <option>-Ofile</option>.</para>
1266
1267             <para>In that file, comments are of the
1268             <literal>&num;</literal>-to-end-of-line variety; blank
1269             lines and most whitespace is ignored.</para>
1270
1271             <para>Please ask if you are baffled and would like an
1272             example of <option>-Ofile</option>!</para>
1273           </listitem>
1274         </varlistentry>
1275       </variablelist>
1276
1277       <para>We don't use a <option>-O*</option> flag for day-to-day
1278       work.  We use <option>-O</option> to get respectable speed;
1279       e.g., when we want to measure something.  When we want to go for
1280       broke, we tend to use <option>-O -fvia-C</option> (and we go for
1281       lots of coffee breaks).</para>
1282
1283       <para>The easiest way to see what <option>-O</option> (etc.)
1284       &ldquo;really mean&rdquo; is to run with <option>-v</option>,
1285       then stand back in amazement.</para>
1286     </sect2>
1287
1288     <sect2 id="options-f">
1289       <title><option>-f*</option>: platform-independent flags</title>
1290
1291       <indexterm><primary>-f* options (GHC)</primary></indexterm>
1292       <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1293
1294       <para>These flags turn on and off individual optimisations.
1295       They are normally set via the <option>-O</option> options
1296       described above, and as such, you shouldn't need to set any of
1297       them explicitly (indeed, doing so could lead to unexpected
1298       results).  However, there are one or two that may be of
1299       interest:</para>
1300
1301       <variablelist>
1302         <varlistentry>
1303           <term><option>-fexcess-precision</option>:</term>
1304           <listitem>
1305             <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1306             <para>When this option is given, intermediate floating
1307             point values can have a <emphasis>greater</emphasis>
1308             precision/range than the final type.  Generally this is a
1309             good thing, but some programs may rely on the exact
1310             precision/range of
1311             <literal>Float</literal>/<literal>Double</literal> values
1312             and should not use this option for their compilation.</para>
1313           </listitem>
1314         </varlistentry>
1315
1316         <varlistentry>
1317           <term><option>-fignore-asserts</option>:</term>
1318           <listitem>
1319             <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1320             <para>Causes GHC to ignore uses of the function
1321             <literal>Exception.assert</literal> in source code (in
1322             other words, rewriting <literal>Exception.assert p
1323             e</literal> to <literal>e</literal> (see <xref
1324             linkend="sec-assertions">).  This flag is turned on by
1325             <option>-O</option>.
1326             </para>
1327           </listitem>
1328         </varlistentry>
1329
1330         <varlistentry>
1331           <term><option>-fno-strictness</option></term>
1332           <indexterm><primary><option>-fno-strictness</option></primary>
1333           </indexterm>
1334           <listitem>
1335             <para>Turns off the strictness analyser; sometimes it eats
1336             too many cycles.</para>
1337           </listitem>
1338         </varlistentry>
1339
1340         <varlistentry>
1341           <term><option>-fno-cpr-analyse</option></term>
1342           <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1343           </indexterm>
1344           <listitem>
1345             <para>Turns off the CPR (constructed product result)
1346             analysis; it is somewhat experimental.</para>
1347           </listitem>
1348         </varlistentry>
1349
1350         <varlistentry>
1351           <term><option>-funbox-strict-fields</option>:</term>
1352           <listitem>
1353             <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1354             <indexterm><primary>strict constructor fields</primary></indexterm>
1355             <indexterm><primary>constructor fields, strict</primary></indexterm>
1356
1357             <para>This option causes all constructor fields which are
1358             marked strict (i.e. &ldquo;!&rdquo;) to be unboxed or
1359             unpacked if possible.  For example:</para>
1360
1361 <ProgramListing>
1362 data T = T !Float !Float
1363 </ProgramListing>
1364
1365             <para>will create a constructor <literal>T</literal>
1366             containing two unboxed floats if the
1367             <option>-funbox-strict-fields</option> flag is given.
1368             This may not always be an optimisation: if the
1369             <Function>T</Function> constructor is scrutinised and the
1370             floats passed to a non-strict function for example, they
1371             will have to be reboxed (this is done automatically by the
1372             compiler).</para>
1373
1374             <para>This option should only be used in conjunction with
1375             <option>-O</option>, in order to expose unfoldings to the
1376             compiler so the reboxing can be removed as often as
1377             possible.  For example:</para>
1378
1379 <ProgramListing>
1380 f :: T -&#62; Float
1381 f (T f1 f2) = f1 + f2
1382 </ProgramListing>
1383
1384             <para>The compiler will avoid reboxing
1385             <Function>f1</Function> and <Function>f2</Function> by
1386             inlining <Function>+</Function> on floats, but only when
1387             <option>-O</option> is on.</para>
1388
1389             <para>Any single-constructor data is eligible for
1390             unpacking; for example</para>
1391
1392 <ProgramListing>
1393 data T = T !(Int,Int)
1394 </ProgramListing>
1395
1396             <para>will store the two <literal>Int</literal>s directly
1397             in the <Function>T</Function> constructor, by flattening
1398             the pair.  Multi-level unpacking is also supported:</para>
1399
1400 <ProgramListing>
1401 data T = T !S
1402 data S = S !Int !Int
1403 </ProgramListing>
1404
1405             <para>will store two unboxed <literal>Int&num;</literal>s
1406             directly in the <Function>T</Function> constructor.</para>
1407           </listitem>
1408         </varlistentry>
1409
1410         <varlistentry>
1411           <term><option>-funfolding-update-in-place&lt;n&gt;</option></term>
1412           <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1413           <listitem>
1414             <para>Switches on an experimental "optimisation".
1415             Switching it on makes the compiler a little keener to
1416             inline a function that returns a constructor, if the
1417             context is that of a thunk.
1418 <ProgramListing>
1419    x = plusInt a b
1420 </ProgramListing>
1421             If we inlined plusInt we might get an opportunity to use
1422             update-in-place for the thunk 'x'.</para>
1423           </listitem>
1424         </varlistentry>
1425
1426         <varlistentry>
1427           <term><option>-funfolding-creation-threshold&lt;n&gt;</option>:</term>
1428           <listitem>
1429             <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1430             <indexterm><primary>inlining, controlling</primary></indexterm>
1431             <indexterm><primary>unfolding, controlling</primary></indexterm>
1432
1433             <para>(Default: 45) Governs the maximum size that GHC will
1434             allow a function unfolding to be.   (An unfolding has a
1435             &ldquo;size&rdquo; that reflects the cost in terms of
1436             &ldquo;code bloat&rdquo; of expanding that unfolding at
1437             at a call site. A bigger function would be assigned a
1438             bigger cost.) </para>
1439
1440             <para> Consequences: (a) nothing larger than this will be
1441             inlined (unless it has an INLINE pragma); (b) nothing
1442             larger than this will be spewed into an interface
1443             file. </para>
1444
1445
1446             <para> Increasing this figure is more likely to result in longer
1447             compile times than faster code.  The next option is more
1448             useful:</para>
1449           </listitem>
1450         </varlistentry>
1451
1452         <varlistentry>
1453           <term><option>-funfolding-use-threshold&lt;n&gt;</option>:</term>
1454           <listitem>
1455             <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1456             <indexterm><primary>inlining, controlling</primary></indexterm>
1457             <indexterm><primary>unfolding, controlling</primary></indexterm>
1458
1459             <para>(Default: 8) This is the magic cut-off figure for
1460             unfolding: below this size, a function definition will be
1461             unfolded at the call-site, any bigger and it won't.  The
1462             size computed for a function depends on two things: the
1463             actual size of the expression minus any discounts that
1464             apply (see <option>-funfolding-con-discount</option>).</para>
1465           </listitem>
1466         </varlistentry>
1467       </variablelist>
1468
1469     </sect2>
1470
1471   </sect1>
1472
1473   &phases;
1474
1475   <sect1 id="sec-using-concurrent">
1476 <title>Using Concurrent Haskell</title>
1477
1478              <indexterm><primary>Concurrent Haskell&mdash;use</primary></indexterm>
1479
1480 <para>
1481 GHC supports Concurrent Haskell by default, without requiring a
1482 special option or libraries compiled in a certain way.  To get access
1483 to the support libraries for Concurrent Haskell, just import
1484 <literal>Control.Concurrent</literal> (details are in the accompanying
1485 library documentation).</para>
1486
1487 <para>
1488 RTS options are provided for modifying the behaviour of the threaded
1489 runtime system.  See <XRef LinkEnd="parallel-rts-opts">.
1490 </para>
1491
1492 <para>
1493 Concurrent Haskell is described in more detail in the documentation
1494 for the <literal>Control.Concurrent</literal> module.
1495 </para>
1496
1497 </Sect1>
1498
1499 <Sect1 id="sec-using-parallel">
1500 <title>Using Parallel Haskell</title>
1501
1502 <para>
1503 <indexterm><primary>Parallel Haskell&mdash;use</primary></indexterm>
1504 </para>
1505
1506 <para>
1507 &lsqb;You won't be able to execute parallel Haskell programs unless PVM3
1508 (Parallel Virtual Machine, version 3) is installed at your site.&rsqb;
1509 </Para>
1510
1511 <para>
1512 To compile a Haskell program for parallel execution under PVM, use the
1513 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
1514 option</Primary></IndexTerm> both when compiling <Emphasis>and
1515 linking</Emphasis>.  You will probably want to <Literal>import
1516 Parallel</Literal> into your Haskell modules.
1517 </Para>
1518
1519 <para>
1520 To run your parallel program, once PVM is going, just invoke it
1521 &ldquo;as normal&rdquo;.  The main extra RTS option is
1522 <Option>-qp&lt;n&gt;</Option>, to say how many PVM
1523 &ldquo;processors&rdquo; your program to run on.  (For more details of
1524 all relevant RTS options, please see <XRef
1525 LinkEnd="parallel-rts-opts">.)
1526 </para>
1527
1528 <para>
1529 In truth, running Parallel Haskell programs and getting information
1530 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1531 PVM, detailed in the following sections.
1532 </para>
1533
1534 <Sect2 id="pvm-dummies">
1535 <Title>Dummy's guide to using PVM</Title>
1536
1537 <para>
1538 <indexterm><primary>PVM, how to use</primary></indexterm>
1539 <indexterm><primary>Parallel Haskell&mdash;PVM use</primary></indexterm>
1540 Before you can run a parallel program under PVM, you must set the
1541 required environment variables (PVM's idea, not ours); something like,
1542 probably in your <filename>.cshrc</filename> or equivalent:
1543
1544 <ProgramListing>
1545 setenv PVM_ROOT /wherever/you/put/it
1546 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1547 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1548 </ProgramListing>
1549
1550 </para>
1551
1552 <para>
1553 Creating and/or controlling your &ldquo;parallel machine&rdquo; is a purely-PVM
1554 business; nothing specific to Parallel Haskell. The following paragraphs
1555 describe how to configure your parallel machine interactively.
1556 </Para>
1557
1558 <Para>
1559 If you use parallel Haskell regularly on the same machine configuration it
1560 is a good idea to maintain a file with all machine names and to make the
1561 environment variable PVM_HOST_FILE point to this file. Then you can avoid
1562 the interactive operations described below by just saying
1563 </Para>
1564
1565 <ProgramListing>
1566 pvm $PVM_HOST_FILE
1567 </ProgramListing>
1568
1569 <Para>
1570 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
1571 machine.  You can then do various things to control/monitor your
1572 &ldquo;parallel machine;&rdquo; the most useful being:
1573 </para>
1574
1575 <para>
1576 <InformalTable>
1577 <TGroup Cols=2>
1578 <ColSpec Align="Left">
1579 <TBody>
1580
1581 <row>
1582 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1583 <entry>exit <command>pvm</command>, leaving it running</entry>
1584 </row>
1585
1586 <row>
1587 <entry><command>halt</command></entry>
1588 <entry>kill off this &ldquo;parallel machine&rdquo; &amp; exit</entry>
1589 </row>
1590
1591 <row>
1592 <entry><command>add &lt;host&gt;</command></entry>
1593 <entry>add <command>&lt;host&gt;</command> as a processor</entry>
1594 </row>
1595
1596 <row>
1597 <entry><command>delete &lt;host&gt;</command></entry>
1598 <entry>delete <command>&lt;host&gt;</command></entry>
1599 </row>
1600
1601 <row>
1602 <entry><command>reset</command></entry>
1603 <entry>kill what's going, but leave PVM up</entry>
1604 </row>
1605
1606 <row>
1607 <entry><command>conf</command></entry>
1608 <entry>list the current configuration</entry>
1609 </row>
1610
1611 <row>
1612 <entry><command>ps</command></entry>
1613 <entry>report processes' status</entry>
1614 </row>
1615
1616 <row>
1617 <entry><command>pstat &lt;pid&gt;</command></entry>
1618 <entry>status of a particular process</entry>
1619 </row>
1620
1621 </TBody>
1622 </TGroup>
1623 </InformalTable>
1624 </para>
1625
1626 <para>
1627 The PVM documentation can tell you much, much more about <command>pvm</command>!
1628 </para>
1629
1630 </sect2>
1631
1632 <Sect2 id="par-profiles">
1633 <Title>Parallelism profiles</Title>
1634
1635 <para>
1636 <indexterm><primary>parallelism profiles</primary></indexterm>
1637 <indexterm><primary>profiles, parallelism</primary></indexterm>
1638 <indexterm><primary>visualisation tools</primary></indexterm>
1639 </para>
1640
1641 <para>
1642 With Parallel Haskell programs, we usually don't care about the
1643 results&mdash;only with &ldquo;how parallel&rdquo; it was!  We want pretty pictures.
1644 </para>
1645
1646 <Para>
1647 Parallelism profiles (&agrave; la <Command>hbcpp</Command>) can be generated with the
1648 <Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option.  The
1649 per-processor profiling info is dumped into files named
1650 <Filename>&lt;full-path&gt;&lt;program&gt;.gr</Filename>.  These are then munged into a PostScript picture,
1651 which you can then display.  For example, to run your program
1652 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
1653 </Para>
1654
1655 <Para>
1656
1657 <Screen>
1658 <prompt>&dollar;</prompt> ./a.out +RTS -qP -qp8
1659 <prompt>&dollar;</prompt> grs2gr *.???.gr &#62; temp.gr # combine the 8 .gr files into one
1660 <prompt>&dollar;</prompt> gr2ps -O temp.gr              # cvt to .ps; output in temp.ps
1661 <prompt>&dollar;</prompt> ghostview -seascape temp.ps   # look at it!
1662 </Screen>
1663
1664 </Para>
1665
1666 <para>
1667 The scripts for processing the parallelism profiles are distributed
1668 in <filename>ghc/utils/parallel/</filename>.
1669 </para>
1670
1671 </sect2>
1672
1673 <Sect2>
1674 <Title>Other useful info about running parallel programs</Title>
1675
1676 <Para>
1677 The &ldquo;garbage-collection statistics&rdquo; RTS options can be useful for
1678 seeing what parallel programs are doing.  If you do either
1679 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
1680 you'll get mutator, garbage-collection, etc., times on standard
1681 error. The standard error of all PE's other than the `main thread'
1682 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1683 </para>
1684
1685 <para>
1686 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1687 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1688 </para>
1689
1690 </sect2>
1691
1692 <Sect2 id="parallel-rts-opts">
1693 <title>RTS options for Concurrent/Parallel Haskell
1694 </title>
1695
1696 <para>
1697 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1698 <indexterm><primary>RTS options, parallel</primary></indexterm>
1699 <indexterm><primary>Concurrent Haskell&mdash;RTS options</primary></indexterm>
1700 <indexterm><primary>Parallel Haskell&mdash;RTS options</primary></indexterm>
1701 </para>
1702
1703 <para>
1704 Besides the usual runtime system (RTS) options
1705 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1706 for concurrent/parallel execution.
1707 </para>
1708
1709 <para>
1710 <VariableList>
1711
1712 <VarListEntry>
1713 <Term><Option>-qp&lt;N&gt;</Option>:</Term>
1714 <ListItem>
1715 <Para>
1716 <IndexTerm><Primary>-qp&lt;N&gt; RTS option</Primary></IndexTerm>
1717 (PARALLEL ONLY) Use <Literal>&lt;N&gt;</Literal> PVM processors to run this program;
1718 the default is 2.
1719 </para>
1720 </listitem>
1721 </varlistentry>
1722 <varlistentry>
1723 <term><option>-C[&lt;us&gt;]</option>:</term>
1724 <listitem>
1725 <para>
1726 <indexterm><primary>-C&lt;us&gt; RTS option</primary></indexterm> Sets
1727 the context switch interval to <literal>&lt;s&gt;</literal> seconds.
1728 A context switch will occur at the next heap block allocation after
1729 the timer expires (a heap block allocation occurs every 4k of
1730 allocation).  With <option>-C0</option> or <option>-C</option>,
1731 context switches will occur as often as possible (at every heap block
1732 allocation).  By default, context switches occur every 20ms
1733 milliseconds.  Note that GHC's internal timer ticks every 20ms, and
1734 the context switch timer is always a multiple of this timer, so 20ms
1735 is the maximum granularity available for timed context switches.
1736 </para>
1737 </listitem>
1738 </varlistentry>
1739 <varlistentry>
1740 <term><option>-q[v]</option>:</term>
1741 <listitem>
1742 <para>
1743 <indexterm><primary>-q RTS option</primary></indexterm>
1744 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1745 in the file <FIlename>&lt;program&gt;.qp</FIlename>.  In the style of <command>hbcpp</command>, this profile
1746 records the movement of threads between the green (runnable) and red
1747 (blocked) queues.  If you specify the verbose suboption (<option>-qv</option>), the
1748 green queue is split into green (for the currently running thread
1749 only) and amber (for other runnable threads).  We do not recommend
1750 that you use the verbose suboption if you are planning to use the
1751 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
1752 check (with <Option>-C</Option>).
1753 -->
1754 </Para>
1755 </ListItem>
1756 </VarListEntry>
1757 <VarListEntry>
1758 <Term><Option>-qt&lt;num&gt;</Option>:</Term>
1759 <ListItem>
1760 <Para>
1761 <IndexTerm><Primary>-qt&lt;num&gt; RTS option</Primary></IndexTerm>
1762 (PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
1763 threads per processor to <Literal>&lt;num&gt;</Literal>.  The default is
1764 32.  Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
1765 the heap for thread state and stack objects.  (For 32-bit machines, this
1766 translates to 4K bytes, and for 64-bit machines, 8K bytes.)
1767 </Para>
1768 </ListItem>
1769 </VarListEntry>
1770 <!-- no more -HWL
1771 <VarListEntry>
1772 <Term><Option>-d</Option>:</Term>
1773 <ListItem>
1774 <Para>
1775 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
1776 (PARALLEL ONLY) Turn on debugging.  It pops up one xterm (or GDB, or
1777 something&hellip;) per PVM processor.  We use the standard <Command>debugger</Command>
1778 script that comes with PVM3, but we sometimes meddle with the
1779 <Command>debugger2</Command> script.  We include ours in the GHC distribution,
1780 in <Filename>ghc/utils/pvm/</Filename>.
1781 </Para>
1782 </ListItem>
1783 </VarListEntry>
1784 -->
1785 <VarListEntry>
1786 <Term><Option>-qe&lt;num&gt;</Option>:</Term>
1787 <ListItem>
1788 <Para>
1789 <IndexTerm><Primary>-qe&lt;num&gt; RTS option
1790 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
1791 i.e. the number of pending sparks per processor to
1792 <Literal>&lt;num&gt;</Literal>. The default is 100. A larger number may be
1793 appropriate if your program generates large amounts of parallelism
1794 initially.
1795 </Para>
1796 </ListItem>
1797 </VarListEntry>
1798 <VarListEntry>
1799 <Term><Option>-qQ&lt;num&gt;</Option>:</Term>
1800 <ListItem>
1801 <Para>
1802 <IndexTerm><Primary>-qQ&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1803 (PARALLEL ONLY) Set the size of packets transmitted between processors
1804 to <Literal>&lt;num&gt;</Literal>. The default is 1024 words. A larger number may be
1805 appropriate if your machine has a high communication cost relative to
1806 computation speed.
1807 </Para>
1808 </ListItem>
1809 </VarListEntry>
1810 <VarListEntry>
1811 <Term><Option>-qh&lt;num&gt;</Option>:</Term>
1812 <ListItem>
1813 <Para>
1814 <IndexTerm><Primary>-qh&lt;num&gt; RTS option (parallel)</Primary></IndexTerm>
1815 (PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
1816 &lt;num&gt;-1 (0 means infinity). By default GUM uses full-subgraph
1817 packing, i.e. the entire subgraph with the requested closure as root is
1818 transmitted (provided it fits into one packet). Choosing a smaller value
1819 reduces the amount of pre-fetching of work done in GUM. This can be
1820 advantageous for improving data locality but it can also worsen the balance
1821 of the load in the system.
1822 </Para>
1823 </ListItem>
1824 </VarListEntry>
1825 <VarListEntry>
1826 <Term><Option>-qg&lt;num&gt;</Option>:</Term>
1827 <ListItem>
1828 <Para>
1829 <IndexTerm><Primary>-qg&lt;num&gt; RTS option
1830 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
1831 scheme. This option affects the
1832 generation of global addresses when transferring data. Global addresses are
1833 globally unique identifiers required to maintain sharing in the distributed
1834 graph structure. Currently this is a binary option. With &lt;num&gt;=0 full globalisation is used
1835 (default). This means a global address is generated for every closure that
1836 is transmitted. With &lt;num&gt;=1 a thunk-only globalisation scheme is
1837 used, which generated global address only for thunks. The latter case may
1838 lose sharing of data but has a reduced overhead in packing graph structures
1839 and maintaining internal tables of global addresses.
1840 </Para>
1841 </ListItem>
1842 </VarListEntry>
1843 </VariableList>
1844 </para>
1845
1846 </sect2>
1847
1848 </Sect1>
1849
1850   <sect1 id="options-platform">
1851     <title>Platform-specific Flags</title>
1852
1853     <indexterm><primary>-m* options</primary></indexterm>
1854     <indexterm><primary>platform-specific options</primary></indexterm>
1855     <indexterm><primary>machine-specific options</primary></indexterm>
1856
1857     <para>Some flags only make sense for particular target
1858     platforms.</para>
1859
1860     <variablelist>
1861
1862       <varlistentry>
1863         <term><option>-mv8</option>:</term>
1864         <listitem>
1865           <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1866           only)</primary></indexterm> Means to pass the like-named
1867           option to GCC; it says to use the Version 8 SPARC
1868           instructions, notably integer multiply and divide.  The
1869           similiar <option>-m*</option> GCC options for SPARC also
1870           work, actually.</para>
1871         </listitem>
1872       </varlistentry>
1873
1874       <varlistentry>
1875         <term><option>-monly-[32]-regs</option>:</term>
1876         <listitem>
1877           <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1878           option (iX86 only)</primary></indexterm> GHC tries to
1879           &ldquo;steal&rdquo; four registers from GCC, for performance
1880           reasons; it almost always works.  However, when GCC is
1881           compiling some modules with four stolen registers, it will
1882           crash, probably saying:
1883
1884 <Screen>
1885 Foo.hc:533: fixed or forbidden register was spilled.
1886 This may be due to a compiler bug or to impossible asm
1887 statements or clauses.
1888 </Screen>
1889
1890           Just give some registers back with
1891           <option>-monly-N-regs</option>.  Try `3' first, then `2'.
1892           If `2' doesn't work, please report the bug to us.</para>
1893         </listitem>
1894       </varlistentry>
1895     </variablelist>
1896
1897   </sect1>
1898
1899 &runtime;
1900
1901 <sect1 id="ext-core">
1902   <title>Generating and compiling External Core Files</title>
1903
1904   <indexterm><primary>intermediate code generation</primary></indexterm>
1905
1906   <para>GHC can dump its optimized intermediate code (said to be in &ldquo;Core&rdquo; format)
1907   to a file as a side-effect of compilation. Core files, which are given the suffix
1908   <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1909   tools.  The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz"
1910   <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1911   and sample tools (in Haskell)
1912   for manipulating Core files are available in the GHC source distribution
1913   directory <literal>/fptools/ghc/utils/ext-core</literal>.
1914   Note that the format of <literal>.hcr</literal>
1915   files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1916   for debugging purposes (<xref linkend="options-debugging">).</para>
1917
1918   <para>The Core format natively supports notes which you can add to
1919   your source code using the <literal>CORE</literal> pragma (see <xref
1920   linkend="pragmas">).</para>
1921
1922     <variablelist>
1923
1924         <varlistentry>
1925           <term><option>-fext-core</option></term>
1926           <indexterm>
1927             <primary><option>-fext-core</option></primary>
1928           </indexterm>
1929           <listitem>
1930             <para>Generate <literal>.hcr</literal> files.</para>
1931           </listitem>
1932         </varlistentry>
1933
1934     </variablelist>
1935
1936 <para>GHC can also read in External Core files as source; just give the <literal>.hcr</literal> file on
1937 the command line, instead of the <literal>.hs</literal> or <literal>.lhs</literal> Haskell source.
1938 A current infelicity is that you need to give teh <literal>-fglasgow-exts</literal> flag too, because
1939 ordinary Haskell 98, when translated to External Core, uses things like rank-2 types.</para>
1940 </sect1>
1941
1942 &debug;
1943 &flags;
1944
1945 </Chapter>
1946
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