1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="using-ghc">
3 <title>Using GHC</title>
5 <indexterm><primary>GHC, using</primary></indexterm>
6 <indexterm><primary>using GHC</primary></indexterm>
9 <title>Options overview</title>
11 <para>GHC's behaviour is controlled by
12 <firstterm>options</firstterm>, which for historical reasons are
13 also sometimes referred to as command-line flags or arguments.
14 Options can be specified in three ways:</para>
17 <title>Command-line arguments</title>
19 <indexterm><primary>structure, command-line</primary></indexterm>
20 <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
21 <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
23 <para>An invocation of GHC takes the following form:</para>
29 <para>Command-line arguments are either options or file names.</para>
31 <para>Command-line options begin with <literal>-</literal>.
32 They may <emphasis>not</emphasis> be grouped:
33 <option>-vO</option> is different from <option>-v -O</option>.
34 Options need not precede filenames: e.g., <literal>ghc *.o -o
35 foo</literal>. All options are processed and then applied to
36 all files; you cannot, for example, invoke <literal>ghc -c -O1
37 Foo.hs -O2 Bar.hs</literal> to apply different optimisation
38 levels to the files <filename>Foo.hs</filename> and
39 <filename>Bar.hs</filename>.</para>
42 <sect2 id="source-file-options">
43 <title>Command line options in source files</title>
45 <indexterm><primary>source-file options</primary></indexterm>
47 <para>Sometimes it is useful to make the connection between a
48 source file and the command-line options it requires quite
49 tight. For instance, if a Haskell source file uses GHC
50 extensions, it will always need to be compiled with the
51 <option>-fglasgow-exts</option> option. Rather than maintaining
52 the list of per-file options in a <filename>Makefile</filename>,
53 it is possible to do this directly in the source file using the
54 <literal>OPTIONS_GHC</literal> pragma <indexterm><primary>OPTIONS_GHC
55 pragma</primary></indexterm>:</para>
58 {-# OPTIONS_GHC -fglasgow-exts #-}
63 <para><literal>OPTIONS_GHC</literal> pragmas are only looked for at
64 the top of your source files, upto the first
65 (non-literate,non-empty) line not containing
66 <literal>OPTIONS_GHC</literal>. Multiple <literal>OPTIONS_GHC</literal>
67 pragmas are recognised. Do not put comments before, or on the same line
68 as, the <literal>OPTIONS_GHC</literal> pragma.</para>
70 <para>Note that your command shell does not
71 get to the source file options, they are just included literally
72 in the array of command-line arguments the compiler
73 maintains internally, so you'll be desperately disappointed if
74 you try to glob etc. inside <literal>OPTIONS_GHC</literal>.</para>
76 <para>NOTE: the contents of OPTIONS_GHC are prepended to the
77 command-line options, so you <emphasis>do</emphasis> have the
78 ability to override OPTIONS_GHC settings via the command
81 <para>It is not recommended to move all the contents of your
82 Makefiles into your source files, but in some circumstances, the
83 <literal>OPTIONS_GHC</literal> pragma is the Right Thing. (If you
84 use <option>-keep-hc-file</option> and have OPTION flags in
85 your module, the OPTIONS_GHC will get put into the generated .hc
90 <title>Setting options in GHCi</title>
92 <para>Options may also be modified from within GHCi, using the
93 <literal>:set</literal> command. See <xref linkend="ghci-set"/>
94 for more details.</para>
98 <sect1 id="static-dynamic-flags">
99 <title>Static, Dynamic, and Mode options</title>
100 <indexterm><primary>static</primary><secondary>options</secondary>
102 <indexterm><primary>dynamic</primary><secondary>options</secondary>
104 <indexterm><primary>mode</primary><secondary>options</secondary>
107 <para>Each of GHC's command line options is classified as
108 <firstterm>static</firstterm>, <firstterm>dynamic</firstterm> or
109 <firstterm>mode</firstterm>:</para>
113 <term>Mode flags</term>
115 <para>For example, <option>--make</option> or <option>-E</option>.
116 There may only be a single mode flag on the command line. The
117 available modes are listed in <xref linkend="modes"/>.</para>
121 <term>Dynamic Flags</term>
123 <para>Most non-mode flags fall into this category. A dynamic flag
124 may be used on the command line, in a
125 <literal>GHC_OPTIONS</literal> pragma in a source file, or set
126 using <literal>:set</literal> in GHCi.</para>
130 <term>Static Flags</term>
132 <para>A few flags are "static", which means they can only be used on
133 the command-line, and remain in force over the entire GHC/GHCi
139 <para>The flag reference tables (<xref
140 linkend="flag-reference"/>) lists the status of each flag.</para>
142 <para>There are a few flags that are static except that they can
143 also be used with GHCi's <literal>:set</literal> command; these
144 are listed as “static/<literal>:set</literal>” in the
148 <sect1 id="file-suffixes">
149 <title>Meaningful file suffixes</title>
151 <indexterm><primary>suffixes, file</primary></indexterm>
152 <indexterm><primary>file suffixes for GHC</primary></indexterm>
154 <para>File names with “meaningful” suffixes (e.g.,
155 <filename>.lhs</filename> or <filename>.o</filename>) cause the
156 “right thing” to happen to those files.</para>
161 <term><filename>.hs</filename></term>
163 <para>A Haskell module.</para>
169 <filename>.lhs</filename>
170 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
173 <para>A “literate Haskell” module.</para>
178 <term><filename>.hi</filename></term>
180 <para>A Haskell interface file, probably
181 compiler-generated.</para>
186 <term><filename>.hc</filename></term>
188 <para>Intermediate C file produced by the Haskell
194 <term><filename>.c</filename></term>
196 <para>A C file not produced by the Haskell
202 <term><filename>.s</filename></term>
204 <para>An assembly-language source file, usually produced by
210 <term><filename>.o</filename></term>
212 <para>An object file, produced by an assembler.</para>
217 <para>Files with other suffixes (or without suffixes) are passed
218 straight to the linker.</para>
223 <title>Modes of operation</title>
225 <para>GHC's behaviour is firstly controlled by a mode flag. Only
226 one of these flags may be given, but it does not necessarily need
227 to be the first option on the command-line. The available modes
233 <cmdsynopsis><command>ghc --interactive</command>
235 <indexterm><primary>interactive mode</primary></indexterm>
236 <indexterm><primary>ghci</primary></indexterm>
239 <para>Interactive mode, which is also available as
240 <command>ghci</command>. Interactive mode is described in
241 more detail in <xref linkend="ghci"/>.</para>
247 <cmdsynopsis><command>ghc --make</command>
249 <indexterm><primary>make mode</primary></indexterm>
250 <indexterm><primary><option>––make</option></primary></indexterm>
253 <para>In this mode, GHC will build a multi-module Haskell
254 program automatically, figuring out dependencies for itself.
255 If you have a straightforward Haskell program, this is
256 likely to be much easier, and faster, than using
257 <command>make</command>. Make mode is described in <xref
258 linkend="make-mode"/>.</para>
264 <cmdsynopsis><command>ghc -e</command>
265 <arg choice='plain'><replaceable>expr</replaceable></arg>
267 <indexterm><primary>eval mode</primary></indexterm>
270 <para>Expression-evaluation mode. This is very similar to
271 interactive mode, except that there is a single expression
272 to evaluate (<replaceable>expr</replaceable>) which is given
273 on the command line. See <xref linkend="eval-mode"/> for
281 <command>ghc -E</command>
282 <command>ghc -c</command>
283 <command>ghc -S</command>
284 <command>ghc -c</command>
286 <indexterm><primary><option>-E</option></primary></indexterm>
287 <indexterm><primary><option>-C</option></primary></indexterm>
288 <indexterm><primary><option>-S</option></primary></indexterm>
289 <indexterm><primary><option>-c</option></primary></indexterm>
292 <para>This is the traditional batch-compiler mode, in which
293 GHC can compile source files one at a time, or link objects
294 together into an executable. This mode also applies if
295 there is no other mode flag specified on the command line,
296 in which case it means that the specified files should be
297 compiled and then linked to form a program. See <xref
298 linkend="options-order"/>.</para>
305 <command>ghc -M</command>
307 <indexterm><primary>dependency-generation mode</primary></indexterm>
310 <para>Dependency-generation mode. In this mode, GHC can be
311 used to generate dependency information suitable for use in
312 a <literal>Makefile</literal>. See <xref
313 linkend="makefile-dependencies"/>.</para>
320 <command>ghc --mk-dll</command>
322 <indexterm><primary>DLL-creation mode</primary></indexterm>
325 <para>DLL-creation mode (Windows only). See <xref
326 linkend="win32-dlls-create"/>.</para>
333 <command>ghc --help</command> <command>ghc -?</command>
335 <indexterm><primary><option>––help</option></primary></indexterm>
338 <para>Cause GHC to spew a long usage message to standard
339 output and then exit.</para>
346 <command>ghc --supported-languages</command>
348 <indexterm><primary><option>––supported-languages</option></primary></indexterm>
351 <para>Print the supported language extensions.</para>
358 <command>ghc --info</command>
360 <indexterm><primary><option>––info</option></primary></indexterm>
363 <para>Print information about the compiler.</para>
370 <command>ghc --version</command>
371 <command>ghc -V</command>
373 <indexterm><primary><option>-V</option></primary></indexterm>
374 <indexterm><primary><option>––version</option></primary></indexterm>
377 <para>Print a one-line string including GHC's version number.</para>
384 <command>ghc --numeric-version</command>
386 <indexterm><primary><option>––numeric-version</option></primary></indexterm>
389 <para>Print GHC's numeric version number only.</para>
396 <command>ghc --print-libdir</command>
398 <indexterm><primary><option>––print-libdir</option></primary></indexterm>
401 <para>Print the path to GHC's library directory. This is
402 the top of the directory tree containing GHC's libraries,
403 interfaces, and include files (usually something like
404 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
406 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
407 in the package configuration file
408 (see <xref linkend="packages"/>).</para>
414 <sect2 id="make-mode">
415 <title>Using <command>ghc</command> <option>––make</option></title>
416 <indexterm><primary><option>––make</option></primary></indexterm>
417 <indexterm><primary>separate compilation</primary></indexterm>
419 <para>When given the <option>––make</option> option,
420 GHC will build a multi-module Haskell program by following
421 dependencies from one or more root modules (usually just
422 <literal>Main</literal>). For example, if your
423 <literal>Main</literal> module is in a file called
424 <filename>Main.hs</filename>, you could compile and link the
425 program like this:</para>
428 ghc ––make Main.hs
431 <para>The command line may contain any number of source file
432 names or module names; GHC will figure out all the modules in
433 the program by following the imports from these initial modules.
434 It will then attempt to compile each module which is out of
435 date, and finally, if there is a <literal>Main</literal> module,
436 the program will also be linked into an executable.</para>
438 <para>The main advantages to using <literal>ghc
439 ––make</literal> over traditional
440 <literal>Makefile</literal>s are:</para>
444 <para>GHC doesn't have to be restarted for each compilation,
445 which means it can cache information between compilations.
446 Compiling a multi-module program with <literal>ghc
447 ––make</literal> can be up to twice as fast as
448 running <literal>ghc</literal> individually on each source
452 <para>You don't have to write a <literal>Makefile</literal>.</para>
453 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
456 <para>GHC re-calculates the dependencies each time it is
457 invoked, so the dependencies never get out of sync with the
462 <para>Any of the command-line options described in the rest of
463 this chapter can be used with
464 <option>––make</option>, but note that any options
465 you give on the command line will apply to all the source files
466 compiled, so if you want any options to apply to a single source
467 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
468 pragma (see <xref linkend="source-file-options"/>).</para>
470 <para>If the program needs to be linked with additional objects
471 (say, some auxiliary C code), then the object files can be
472 given on the command line and GHC will include them when linking
473 the executable.</para>
475 <para>Note that GHC can only follow dependencies if it has the
476 source file available, so if your program includes a module for
477 which there is no source file, even if you have an object and an
478 interface file for the module, then GHC will complain. The
479 exception to this rule is for package modules, which may or may
480 not have source files.</para>
482 <para>The source files for the program don't all need to be in
483 the same directory; the <option>-i</option> option can be used
484 to add directories to the search path (see <xref
485 linkend="search-path"/>).</para>
488 <sect2 id="eval-mode">
489 <title>Expression evaluation mode</title>
491 <para>This mode is very similar to interactive mode, except that
492 there is a single expression to evaluate which is specified on
493 the command line as an argument to the <option>-e</option>
497 ghc -e <replaceable>expr</replaceable>
500 <para>Haskell source files may be named on the command line, and
501 they will be loaded exactly as in interactive mode. The
502 expression is evaluated in the context of the loaded
505 <para>For example, to load and run a Haskell program containing
506 a module <literal>Main</literal>, we might say</para>
509 ghc -e Main.main Main.hs
512 <para>or we can just use this mode to evaluate expressions in
513 the context of the <literal>Prelude</literal>:</para>
516 $ ghc -e "interact (unlines.map reverse.lines)"
522 <sect2 id="options-order">
523 <title>Batch compiler mode</title>
525 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
526 given on the command line.</para>
528 <para>The first phase to run is determined by each input-file
529 suffix, and the last phase is determined by a flag. If no
530 relevant flag is present, then go all the way through to linking.
531 This table summarises:</para>
535 <colspec align="left"/>
536 <colspec align="left"/>
537 <colspec align="left"/>
538 <colspec align="left"/>
542 <entry>Phase of the compilation system</entry>
543 <entry>Suffix saying “start here”</entry>
544 <entry>Flag saying “stop after”</entry>
545 <entry>(suffix of) output file</entry>
550 <entry>literate pre-processor</entry>
551 <entry><literal>.lhs</literal></entry>
553 <entry><literal>.hs</literal></entry>
557 <entry>C pre-processor (opt.) </entry>
558 <entry><literal>.hs</literal> (with
559 <option>-cpp</option>)</entry>
560 <entry><option>-E</option></entry>
561 <entry><literal>.hspp</literal></entry>
565 <entry>Haskell compiler</entry>
566 <entry><literal>.hs</literal></entry>
567 <entry><option>-C</option>, <option>-S</option></entry>
568 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
572 <entry>C compiler (opt.)</entry>
573 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
574 <entry><option>-S</option></entry>
575 <entry><literal>.s</literal></entry>
579 <entry>assembler</entry>
580 <entry><literal>.s</literal></entry>
581 <entry><option>-c</option></entry>
582 <entry><literal>.o</literal></entry>
586 <entry>linker</entry>
587 <entry><replaceable>other</replaceable></entry>
589 <entry><filename>a.out</filename></entry>
595 <indexterm><primary><option>-C</option></primary></indexterm>
596 <indexterm><primary><option>-E</option></primary></indexterm>
597 <indexterm><primary><option>-S</option></primary></indexterm>
598 <indexterm><primary><option>-c</option></primary></indexterm>
600 <para>Thus, a common invocation would be: </para>
603 ghc -c Foo.hs</screen>
605 <para>to compile the Haskell source file
606 <filename>Foo.hs</filename> to an object file
607 <filename>Foo.o</filename>.</para>
609 <para>Note: What the Haskell compiler proper produces depends on
610 whether a native-code generator<indexterm><primary>native-code
611 generator</primary></indexterm> is used (producing assembly
612 language) or not (producing C). See <xref
613 linkend="options-codegen"/> for more details.</para>
615 <para>Note: C pre-processing is optional, the
616 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
617 flag turns it on. See <xref linkend="c-pre-processor"/> for more
620 <para>Note: The option <option>-E</option><indexterm><primary>-E
621 option</primary></indexterm> runs just the pre-processing passes
622 of the compiler, dumping the result in a file.</para>
624 <sect3 id="overriding-suffixes">
625 <title>Overriding the default behaviour for a file</title>
627 <para>As described above, the way in which a file is processed by GHC
628 depends on its suffix. This behaviour can be overriden using the
629 <option>-x</option> option:</para>
633 <term><option>-x</option> <replaceable>suffix</replaceable>
634 <indexterm><primary><option>-x</option></primary>
637 <para>Causes all files following this option on the command
638 line to be processed as if they had the suffix
639 <replaceable>suffix</replaceable>. For example, to compile a
640 Haskell module in the file <literal>M.my-hs</literal>,
641 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
650 <sect1 id="options-help">
651 <title>Help and verbosity options</title>
653 <indexterm><primary>help options</primary></indexterm>
654 <indexterm><primary>verbosity options</primary></indexterm>
656 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
657 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
662 <indexterm><primary><option>-n</option></primary></indexterm>
665 <para>Does a dry-run, i.e. GHC goes through all the motions
666 of compiling as normal, but does not actually run any
667 external commands.</para>
674 <indexterm><primary><option>-v</option></primary></indexterm>
677 <para>The <option>-v</option> option makes GHC
678 <emphasis>verbose</emphasis>: it reports its version number
679 and shows (on stderr) exactly how it invokes each phase of
680 the compilation system. Moreover, it passes the
681 <option>-v</option> flag to most phases; each reports its
682 version number (and possibly some other information).</para>
684 <para>Please, oh please, use the <option>-v</option> option
685 when reporting bugs! Knowing that you ran the right bits in
686 the right order is always the first thing we want to
693 <option>-v</option><replaceable>n</replaceable>
694 <indexterm><primary><option>-v</option></primary></indexterm>
697 <para>To provide more control over the compiler's verbosity,
698 the <option>-v</option> flag takes an optional numeric
699 argument. Specifying <option>-v</option> on its own is
700 equivalent to <option>-v3</option>, and the other levels
701 have the following meanings:</para>
705 <term><option>-v0</option></term>
707 <para>Disable all non-essential messages (this is the
713 <term><option>-v1</option></term>
715 <para>Minimal verbosity: print one line per
716 compilation (this is the default when
717 <option>––make</option> or
718 <option>––interactive</option> is on).</para>
723 <term><option>-v2</option></term>
725 <para>Print the name of each compilation phase as it
726 is executed. (equivalent to
727 <option>-dshow-passes</option>).</para>
732 <term><option>-v3</option></term>
734 <para>The same as <option>-v2</option>, except that in
735 addition the full command line (if appropriate) for
736 each compilation phase is also printed.</para>
741 <term><option>-v4</option></term>
743 <para>The same as <option>-v3</option> except that the
744 intermediate program representation after each
745 compilation phase is also printed (excluding
746 preprocessed and C/assembly files).</para>
754 <term><option>-ferror-spans</option>
755 <indexterm><primary><option>-ferror-spans</option></primary>
759 <para>Causes GHC to emit the full source span of the
760 syntactic entity relating to an error message. Normally, GHC
761 emits the source location of the start of the syntactic
764 <para>For example:</para>
766 <screen>test.hs:3:6: parse error on input `where'</screen>
768 <para>becomes:</para>
770 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
772 <para>And multi-line spans are possible too:</para>
774 <screen>test.hs:(5,4)-(6,7):
775 Conflicting definitions for `a'
776 Bound at: test.hs:5:4
778 In the binding group for: a, b, a</screen>
780 <para>Note that line numbers start counting at one, but
781 column numbers start at zero. This choice was made to
782 follow existing convention (i.e. this is how Emacs does
788 <term><option>-H</option><replaceable>size</replaceable>
789 <indexterm><primary><option>-H</option></primary></indexterm>
792 <para>Set the minimum size of the heap to
793 <replaceable>size</replaceable>.
794 This option is equivalent to
795 <literal>+RTS -H<replaceable>size</replaceable></literal>,
796 see <xref linkend="rts-options-gc" />.
802 <term><option>-Rghc-timing</option>
803 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
806 <para>Prints a one-line summary of timing statistics for the
807 GHC run. This option is equivalent to
808 <literal>+RTS -tstderr</literal>, see <xref
809 linkend="rts-options-gc" />.
818 <sect1 id="options-sanity">
819 <title>Warnings and sanity-checking</title>
821 <indexterm><primary>sanity-checking options</primary></indexterm>
822 <indexterm><primary>warnings</primary></indexterm>
825 <para>GHC has a number of options that select which types of
826 non-fatal error messages, otherwise known as warnings, can be
827 generated during compilation. By default, you get a standard set
828 of warnings which are generally likely to indicate bugs in your
830 <option>-fwarn-overlapping-patterns</option>,
831 <option>-fwarn-deprecations</option>,
832 <option>-fwarn-duplicate-exports</option>,
833 <option>-fwarn-missing-fields</option>, and
834 <option>-fwarn-missing-methods</option>. The following flags are
835 simple ways to select standard “packages” of warnings:
841 <term><option>-W</option>:</term>
843 <indexterm><primary>-W option</primary></indexterm>
844 <para>Provides the standard warnings plus
845 <option>-fwarn-incomplete-patterns</option>,
846 <option>-fwarn-dodgy-imports</option>,
847 <option>-fwarn-unused-matches</option>,
848 <option>-fwarn-unused-imports</option>, and
849 <option>-fwarn-unused-binds</option>.</para>
854 <term><option>-Wall</option>:</term>
856 <indexterm><primary><option>-Wall</option></primary></indexterm>
857 <para>Turns on all warning options that indicate potentially
858 suspicious code. The warnings that are
859 <emphasis>not</emphasis> enabled by <option>-Wall</option>
861 <option>-fwarn-simple-patterns</option>,
862 <option>-fwarn-tabs</option>,
863 <option>-fwarn-incomplete-record-updates</option>,
864 <option>-fwarn-monomorphism-restriction</option>, and
865 <option>-fwarn-implicit-prelude</option>.</para>
870 <term><option>-w</option>:</term>
872 <indexterm><primary><option>-w</option></primary></indexterm>
873 <para>Turns off all warnings, including the standard ones and
874 those that <literal>-Wall</literal> doesn't enable.</para>
879 <term><option>-Werror</option>:</term>
881 <indexterm><primary><option>-Werror</option></primary></indexterm>
882 <para>Makes any warning into a fatal error. Useful so that you don't
883 miss warnings when doing batch compilation. </para>
889 <para>The full set of warning options is described below. To turn
890 off any warning, simply give the corresponding
891 <option>-fno-warn-...</option> option on the command line.</para>
896 <term><option>-fwarn-deprecations</option>:</term>
898 <indexterm><primary><option>-fwarn-deprecations</option></primary>
900 <indexterm><primary>deprecations</primary></indexterm>
901 <para>Causes a warning to be emitted when a deprecated
902 function or type is used. Entities can be marked as
903 deprecated using a pragma, see <xref
904 linkend="deprecated-pragma"/>.</para>
906 <para>This option is on by default.</para>
911 <term><option>-fwarn-dodgy-imports</option>:</term>
913 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
915 <para>Causes a warning to be emitted when a a datatype
916 <literal>T</literal> is imported
917 with all constructors, i.e. <literal>T(..)</literal>, but has been
918 exported abstractly, i.e. <literal>T</literal>.</para>
923 <term><option>-fwarn-duplicate-exports</option>:</term>
925 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
926 <indexterm><primary>duplicate exports, warning</primary></indexterm>
927 <indexterm><primary>export lists, duplicates</primary></indexterm>
929 <para>Have the compiler warn about duplicate entries in
930 export lists. This is useful information if you maintain
931 large export lists, and want to avoid the continued export
932 of a definition after you've deleted (one) mention of it in
933 the export list.</para>
935 <para>This option is on by default.</para>
940 <term><option>-fwarn-hi-shadowing</option>:</term>
942 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
943 <indexterm><primary>shadowing</primary>
944 <secondary>interface files</secondary></indexterm>
946 <para>Causes the compiler to emit a warning when a module or
947 interface file in the current directory is shadowing one
948 with the same module name in a library or other
954 <term><option>-fwarn-implicit-prelude</option>:</term>
956 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
957 <indexterm><primary>implicit prelude, warning</primary></indexterm>
958 <para>Have the compiler warn if the Prelude is implicitly
959 imported. This happens unless either the Prelude module is
960 explicitly imported with an <literal>import ... Prelude ...</literal>
961 line, or this implicit import is disabled (either by
962 <option>-fno-implicit-prelude</option> or a
963 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
965 <para>Note that no warning is given for syntax that implicitly
966 refers to the Prelude, even if <option>-fno-implicit-prelude</option>
967 would change whether it refers to the Prelude.
968 For example, no warning is given when
969 <literal>368</literal> means
970 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
971 (where <literal>Prelude</literal> refers to the actual Prelude module,
972 regardless of the imports of the module being compiled).</para>
974 <para>This warning is off by default.</para>
979 <term><option>-fwarn-incomplete-patterns</option>:</term>
981 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
982 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
983 <indexterm><primary>patterns, incomplete</primary></indexterm>
985 <para>Similarly for incomplete patterns, the function
986 <function>g</function> below will fail when applied to
987 non-empty lists, so the compiler will emit a warning about
988 this when <option>-fwarn-incomplete-patterns</option> is
995 <para>This option isn't enabled by default because it can be
996 a bit noisy, and it doesn't always indicate a bug in the
997 program. However, it's generally considered good practice
998 to cover all the cases in your functions.</para>
1003 <term><option>-fwarn-incomplete-record-updates</option>:</term>
1005 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
1006 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
1007 <indexterm><primary>record updates, incomplete</primary></indexterm>
1010 <function>f</function> below will fail when applied to
1011 <literal>Bar</literal>, so the compiler will emit a warning about
1012 this when <option>-fwarn-incomplete-record-updates</option> is
1016 data Foo = Foo { x :: Int }
1020 f foo = foo { x = 6 }
1023 <para>This option isn't enabled by default because it can be
1024 very noisy, and it often doesn't indicate a bug in the
1031 <option>-fwarn-missing-fields</option>:
1032 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1033 <indexterm><primary>missing fields, warning</primary></indexterm>
1034 <indexterm><primary>fields, missing</primary></indexterm>
1038 <para>This option is on by default, and warns you whenever
1039 the construction of a labelled field constructor isn't
1040 complete, missing initializers for one or more fields. While
1041 not an error (the missing fields are initialised with
1042 bottoms), it is often an indication of a programmer error.</para>
1047 <term><option>-fwarn-missing-methods</option>:</term>
1049 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1050 <indexterm><primary>missing methods, warning</primary></indexterm>
1051 <indexterm><primary>methods, missing</primary></indexterm>
1053 <para>This option is on by default, and warns you whenever
1054 an instance declaration is missing one or more methods, and
1055 the corresponding class declaration has no default
1056 declaration for them.</para>
1057 <para>The warning is suppressed if the method name
1058 begins with an underscore. Here's an example where this is useful:
1061 _simpleFn :: a -> String
1062 complexFn :: a -> a -> String
1063 complexFn x y = ... _simpleFn ...
1065 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1066 never <literal>_simpleFn</literal>; and (b)
1067 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1073 <term><option>-fwarn-missing-signatures</option>:</term>
1075 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1076 <indexterm><primary>type signatures, missing</primary></indexterm>
1078 <para>If you would like GHC to check that every top-level
1079 function/value has a type signature, use the
1080 <option>-fwarn-missing-signatures</option> option. As part of
1081 the warning GHC also reports the inferred type. The
1082 option is off by default.</para>
1087 <term><option>-fwarn-name-shadowing</option>:</term>
1089 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1090 <indexterm><primary>shadowing, warning</primary></indexterm>
1092 <para>This option causes a warning to be emitted whenever an
1093 inner-scope value has the same name as an outer-scope value,
1094 i.e. the inner value shadows the outer one. This can catch
1095 typographical errors that turn into hard-to-find bugs, e.g.,
1096 in the inadvertent capture of what would be a recursive call in
1097 <literal>f = ... let f = id in ... f ...</literal>.</para>
1102 <term><option>-fwarn-orphans</option>:</term>
1104 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1105 <indexterm><primary>orphan instances, warning</primary></indexterm>
1106 <indexterm><primary>orphan rules, warning</primary></indexterm>
1108 <para>This option causes a warning to be emitted whenever the
1109 module contains an "orphan" instance declaration or rewrite rule.
1110 An instance declartion is an orphan if it appears in a module in
1111 which neither the class nor the type being instanced are declared
1112 in the same module. A rule is an orphan if it is a rule for a
1113 function declared in another module. A module containing any
1114 orphans is called an orphan module.</para>
1115 <para>The trouble with orphans is that GHC must pro-actively read the interface
1116 files for all orphan modules, just in case their instances or rules
1117 play a role, whether or not the module's interface would otherwise
1118 be of any use. Other things being equal, avoid orphan modules.</para>
1124 <option>-fwarn-overlapping-patterns</option>:
1125 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1126 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1127 <indexterm><primary>patterns, overlapping</primary></indexterm>
1130 <para>By default, the compiler will warn you if a set of
1131 patterns are overlapping, e.g.,</para>
1134 f :: String -> Int
1140 <para>where the last pattern match in <function>f</function>
1141 won't ever be reached, as the second pattern overlaps
1142 it. More often than not, redundant patterns is a programmer
1143 mistake/error, so this option is enabled by default.</para>
1148 <term><option>-fwarn-simple-patterns</option>:</term>
1150 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1152 <para>Causes the compiler to warn about lambda-bound
1153 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1154 Normally, these aren't treated as incomplete patterns by
1155 <option>-fwarn-incomplete-patterns</option>.</para>
1156 <para>“Lambda-bound patterns” includes all places where there is a single pattern,
1157 including list comprehensions and do-notation. In these cases, a pattern-match
1158 failure is quite legitimate, and triggers filtering (list comprehensions) or
1159 the monad <literal>fail</literal> operation (monads). For example:
1161 f :: [Maybe a] -> [a]
1162 f xs = [y | Just y <- xs]
1164 Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1165 these probably-innocent cases, which is why the flag is off by default. </para>
1170 <term><option>-fwarn-tabs</option>:</term>
1172 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1173 <indexterm><primary>tabs, warning</primary></indexterm>
1174 <para>Have the compiler warn if there are tabs in your source
1177 <para>This warning is off by default.</para>
1182 <term><option>-fwarn-type-defaults</option>:</term>
1184 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1185 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1186 <para>Have the compiler warn/inform you where in your source
1187 the Haskell defaulting mechanism for numeric types kicks
1188 in. This is useful information when converting code from a
1189 context that assumed one default into one with another,
1190 e.g., the ‘default default’ for Haskell 1.4 caused the
1191 otherwise unconstrained value <constant>1</constant> to be
1192 given the type <literal>Int</literal>, whereas Haskell 98
1193 defaults it to <literal>Integer</literal>. This may lead to
1194 differences in performance and behaviour, hence the
1195 usefulness of being non-silent about this.</para>
1197 <para>This warning is off by default.</para>
1202 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1204 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1205 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1206 <para>Have the compiler warn/inform you where in your source
1207 the Haskell Monomorphism Restriction is applied. If applied silently
1208 the MR can give rise to unexpected behaviour, so it can be helpful
1209 to have an explicit warning that it is being applied.</para>
1211 <para>This warning is off by default.</para>
1216 <term><option>-fwarn-unused-binds</option>:</term>
1218 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1219 <indexterm><primary>unused binds, warning</primary></indexterm>
1220 <indexterm><primary>binds, unused</primary></indexterm>
1221 <para>Report any function definitions (and local bindings)
1222 which are unused. For top-level functions, the warning is
1223 only given if the binding is not exported.</para>
1224 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1225 mentioned in the right hand side of another definition that is used, or (c) the
1226 function it defines begins with an underscore. The last case provides a
1227 way to suppress unused-binding warnings selectively. </para>
1228 <para> Notice that a variable
1229 is reported as unused even if it appears in the right-hand side of another
1230 unused binding. </para>
1235 <term><option>-fwarn-unused-imports</option>:</term>
1237 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1238 <indexterm><primary>unused imports, warning</primary></indexterm>
1239 <indexterm><primary>imports, unused</primary></indexterm>
1241 <para>Report any modules that are explicitly imported but
1242 never used. However, the form <literal>import M()</literal> is
1243 never reported as an unused import, because it is a useful idiom
1244 for importing instance declarations, which are anonymous in Haskell.</para>
1249 <term><option>-fwarn-unused-matches</option>:</term>
1251 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1252 <indexterm><primary>unused matches, warning</primary></indexterm>
1253 <indexterm><primary>matches, unused</primary></indexterm>
1255 <para>Report all unused variables which arise from pattern
1256 matches, including patterns consisting of a single variable.
1257 For instance <literal>f x y = []</literal> would report
1258 <varname>x</varname> and <varname>y</varname> as unused. The
1259 warning is suppressed if the variable name begins with an underscore, thus:
1269 <para>If you're feeling really paranoid, the
1270 <option>-dcore-lint</option>
1271 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1272 is a good choice. It turns on heavyweight intra-pass
1273 sanity-checking within GHC. (It checks GHC's sanity, not
1280 <sect1 id="options-optimise">
1281 <title>Optimisation (code improvement)</title>
1283 <indexterm><primary>optimisation</primary></indexterm>
1284 <indexterm><primary>improvement, code</primary></indexterm>
1286 <para>The <option>-O*</option> options specify convenient
1287 “packages” of optimisation flags; the
1288 <option>-f*</option> options described later on specify
1289 <emphasis>individual</emphasis> optimisations to be turned on/off;
1290 the <option>-m*</option> options specify
1291 <emphasis>machine-specific</emphasis> optimisations to be turned
1294 <sect2 id="optimise-pkgs">
1295 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1297 <para>There are <emphasis>many</emphasis> options that affect
1298 the quality of code produced by GHC. Most people only have a
1299 general goal, something like “Compile quickly” or
1300 “Make my program run like greased lightning.” The
1301 following “packages” of optimisations (or lack
1302 thereof) should suffice.</para>
1304 <para>Note that higher optimisation levels cause more
1305 cross-module optimisation to be performed, which can have an
1306 impact on how much of your program needs to be recompiled when
1307 you change something. This is one reason to stick to
1308 no-optimisation when developing code.</para>
1314 No <option>-O*</option>-type option specified:
1315 <indexterm><primary>-O* not specified</primary></indexterm>
1318 <para>This is taken to mean: “Please compile
1319 quickly; I'm not over-bothered about compiled-code
1320 quality.” So, for example: <command>ghc -c
1321 Foo.hs</command></para>
1327 <option>-O0</option>:
1328 <indexterm><primary><option>-O0</option></primary></indexterm>
1331 <para>Means “turn off all optimisation”,
1332 reverting to the same settings as if no
1333 <option>-O</option> options had been specified. Saying
1334 <option>-O0</option> can be useful if
1335 eg. <command>make</command> has inserted a
1336 <option>-O</option> on the command line already.</para>
1342 <option>-O</option> or <option>-O1</option>:
1343 <indexterm><primary>-O option</primary></indexterm>
1344 <indexterm><primary>-O1 option</primary></indexterm>
1345 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1348 <para>Means: “Generate good-quality code without
1349 taking too long about it.” Thus, for example:
1350 <command>ghc -c -O Main.lhs</command></para>
1356 <option>-O2</option>:
1357 <indexterm><primary>-O2 option</primary></indexterm>
1358 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1361 <para>Means: “Apply every non-dangerous
1362 optimisation, even if it means significantly longer
1363 compile times.”</para>
1365 <para>The avoided “dangerous” optimisations
1366 are those that can make runtime or space
1367 <emphasis>worse</emphasis> if you're unlucky. They are
1368 normally turned on or off individually.</para>
1370 <para>At the moment, <option>-O2</option> is
1371 <emphasis>unlikely</emphasis> to produce better code than
1372 <option>-O</option>.</para>
1378 <option>-Ofile <file></option>:
1379 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1380 <indexterm><primary>optimising, customised</primary></indexterm>
1383 <para>(NOTE: not supported since GHC 4.x. Please ask if
1384 you're interested in this.)</para>
1386 <para>For those who need <emphasis>absolute</emphasis>
1387 control over <emphasis>exactly</emphasis> what options are
1388 used (e.g., compiler writers, sometimes :-), a list of
1389 options can be put in a file and then slurped in with
1390 <option>-Ofile</option>.</para>
1392 <para>In that file, comments are of the
1393 <literal>#</literal>-to-end-of-line variety; blank
1394 lines and most whitespace is ignored.</para>
1396 <para>Please ask if you are baffled and would like an
1397 example of <option>-Ofile</option>!</para>
1402 <para>We don't use a <option>-O*</option> flag for day-to-day
1403 work. We use <option>-O</option> to get respectable speed;
1404 e.g., when we want to measure something. When we want to go for
1405 broke, we tend to use <option>-O2 -fvia-C</option> (and we go for
1406 lots of coffee breaks).</para>
1408 <para>The easiest way to see what <option>-O</option> (etc.)
1409 “really mean” is to run with <option>-v</option>,
1410 then stand back in amazement.</para>
1413 <sect2 id="options-f">
1414 <title><option>-f*</option>: platform-independent flags</title>
1416 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1417 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1419 <para>These flags turn on and off individual optimisations.
1420 They are normally set via the <option>-O</option> options
1421 described above, and as such, you shouldn't need to set any of
1422 them explicitly (indeed, doing so could lead to unexpected
1423 results). However, there are one or two that may be of
1428 <term><option>-fexcess-precision</option>:</term>
1430 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1431 <para>When this option is given, intermediate floating
1432 point values can have a <emphasis>greater</emphasis>
1433 precision/range than the final type. Generally this is a
1434 good thing, but some programs may rely on the exact
1436 <literal>Float</literal>/<literal>Double</literal> values
1437 and should not use this option for their compilation.</para>
1442 <term><option>-fignore-asserts</option>:</term>
1444 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1445 <para>Causes GHC to ignore uses of the function
1446 <literal>Exception.assert</literal> in source code (in
1447 other words, rewriting <literal>Exception.assert p
1448 e</literal> to <literal>e</literal> (see <xref
1449 linkend="assertions"/>). This flag is turned on by
1450 <option>-O</option>.
1457 <option>-fno-cse</option>
1458 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1461 <para>Turns off the common-sub-expression elimination optimisation.
1462 Can be useful if you have some <literal>unsafePerformIO</literal>
1463 expressions that you don't want commoned-up.</para>
1469 <option>-fno-strictness</option>
1470 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1473 <para>Turns off the strictness analyser; sometimes it eats
1474 too many cycles.</para>
1480 <option>-fno-full-laziness</option>
1481 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1484 <para>Turns off the full laziness optimisation (also known as
1485 let-floating). Full laziness increases sharing, which can lead
1486 to increased memory residency.</para>
1488 <para>NOTE: GHC doesn't implement complete full-laziness.
1489 When optimisation in on, and
1490 <option>-fno-full-laziness</option> is not given, some
1491 transformations that increase sharing are performed, such
1492 as extracting repeated computations from a loop. These
1493 are the same transformations that a fully lazy
1494 implementation would do, the difference is that GHC
1495 doesn't consistently apply full-laziness, so don't rely on
1502 <option>-fno-state-hack</option>
1503 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1506 <para>Turn off the "state hack" whereby any lambda with a
1507 <literal>State#</literal> token as argument is considered to be
1508 single-entry, hence it is considered OK to inline things inside
1509 it. This can improve performance of IO and ST monad code, but it
1510 runs the risk of reducing sharing.</para>
1516 <option>-fno-state-hack</option>
1517 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1520 <para>Turn off the "state hack" whereby any lambda with a
1521 <literal>State#</literal> token as argument is considered to be
1522 single-entry, hence it is considered OK to inline things inside
1523 it. This can improve performance of IO and ST monad code, but it
1524 runs the risk of reducing sharing.</para>
1530 <option>-fomit-interface-pragmas</option>
1531 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
1534 <para>Tells GHC to omit all inessential information from the interface file
1535 generated for the module being compiled (say M). This means that a module
1536 importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
1537 their unfoldings, strictness info, etc. Hence, for example,
1538 no function exported by M will be inlined
1539 into an importing module. The benefit is that modules that import M will
1540 need to be recompiled less often (only when M's exports change their type,
1541 not when they change their implementation).
1548 <option>-fignore-interface-pragmas</option>
1549 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
1552 <para>Tells GHC to ignore all inessential information when reading interface files.
1553 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
1554 for a function, GHC will ignore that information.</para>
1560 <option>-funbox-strict-fields</option>:
1561 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1562 <indexterm><primary>strict constructor fields</primary></indexterm>
1563 <indexterm><primary>constructor fields, strict</primary></indexterm>
1566 <para>This option causes all constructor fields which are
1567 marked strict (i.e. “!”) to be unboxed or
1568 unpacked if possible. It is equivalent to adding an
1569 <literal>UNPACK</literal> pragma to every strict
1570 constructor field (see <xref
1571 linkend="unpack-pragma"/>).</para>
1573 <para>This option is a bit of a sledgehammer: it might
1574 sometimes make things worse. Selectively unboxing fields
1575 by using <literal>UNPACK</literal> pragmas might be
1582 <option>-funfolding-update-in-place=<replaceable>n</replaceable></option>
1583 <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1586 <para>Switches on an experimental "optimisation".
1587 Switching it on makes the compiler a little keener to
1588 inline a function that returns a constructor, if the
1589 context is that of a thunk.
1593 If we inlined plusInt we might get an opportunity to use
1594 update-in-place for the thunk 'x'.</para>
1600 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
1601 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1602 <indexterm><primary>inlining, controlling</primary></indexterm>
1603 <indexterm><primary>unfolding, controlling</primary></indexterm>
1606 <para>(Default: 45) Governs the maximum size that GHC will
1607 allow a function unfolding to be. (An unfolding has a
1608 “size” that reflects the cost in terms of
1609 “code bloat” of expanding that unfolding at
1610 at a call site. A bigger function would be assigned a
1611 bigger cost.) </para>
1613 <para> Consequences: (a) nothing larger than this will be
1614 inlined (unless it has an INLINE pragma); (b) nothing
1615 larger than this will be spewed into an interface
1619 <para> Increasing this figure is more likely to result in longer
1620 compile times than faster code. The next option is more
1626 <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
1628 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1629 <indexterm><primary>inlining, controlling</primary></indexterm>
1630 <indexterm><primary>unfolding, controlling</primary></indexterm>
1632 <para>(Default: 8) This is the magic cut-off figure for
1633 unfolding: below this size, a function definition will be
1634 unfolded at the call-site, any bigger and it won't. The
1635 size computed for a function depends on two things: the
1636 actual size of the expression minus any discounts that
1637 apply (see <option>-funfolding-con-discount</option>).</para>
1648 <sect1 id="using-concurrent">
1649 <title>Using Concurrent Haskell</title>
1650 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1652 <para>GHC supports Concurrent Haskell by default, without requiring a
1653 special option or libraries compiled in a certain way. To get access to
1654 the support libraries for Concurrent Haskell, just import
1656 url="../libraries/base/Control-Concurrent.html"><literal>Control.Concurrent</literal></ulink>. More information on Concurrent Haskell is provided in the documentation for that module.</para>
1658 <para>The following RTS option(s) affect the behaviour of Concurrent
1659 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1663 <term><option>-C<replaceable>s</replaceable></option></term>
1665 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1666 Sets the context switch interval to <replaceable>s</replaceable>
1667 seconds. A context switch will occur at the next heap block
1668 allocation after the timer expires (a heap block allocation occurs
1669 every 4k of allocation). With <option>-C0</option> or
1670 <option>-C</option>, context switches will occur as often as
1671 possible (at every heap block allocation). By default, context
1672 switches occur every 20ms.</para>
1678 <sect1 id="using-smp">
1679 <title>Using SMP parallelism</title>
1680 <indexterm><primary>parallelism</primary>
1682 <indexterm><primary>SMP</primary>
1685 <para>GHC supports running Haskell programs in parallel on an SMP
1686 (symmetric multiprocessor).</para>
1688 <para>There's a fine distinction between
1689 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
1690 parallelism is all about making your program run
1691 <emphasis>faster</emphasis> by making use of multiple processors
1692 simultaneously. Concurrency, on the other hand, is a means of
1693 abstraction: it is a convenient way to structure a program that must
1694 respond to multiple asynchronous events.</para>
1696 <para>However, the two terms are certainly related. By making use of
1697 multiple CPUs it is possible to run concurrent threads in parallel,
1698 and this is exactly what GHC's SMP parallelism support does. But it
1699 is also possible to obtain performance improvements with parallelism
1700 on programs that do not use concurrency. This section describes how to
1701 use GHC to compile and run parallel programs, in <xref
1702 linkend="lang-parallel" /> we desribe the language features that affect
1705 <sect2 id="parallel-options">
1706 <title>Options to enable SMP parallelism</title>
1708 <para>In order to make use of multiple CPUs, your program must be
1709 linked with the <option>-threaded</option> option (see <xref
1710 linkend="options-linker" />). Then, to run a program on multiple
1711 CPUs, use the RTS <option>-N</option> option:</para>
1715 <term><option>-N<replaceable>x</replaceable></option></term>
1717 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1718 Use <replaceable>x</replaceable> simultaneous threads when
1719 running the program. Normally <replaceable>x</replaceable>
1720 should be chosen to match the number of CPU cores on the machine.
1721 There is no means (currently) by which this value may vary after
1722 the program has started.</para>
1724 <para>For example, on a dual-core machine we would probably use
1725 <literal>+RTS -N2 -RTS</literal>.</para>
1727 <para>Whether hyperthreading cores should be counted or not is an
1728 open question; please feel free to experiment and let us know what
1729 results you find.</para>
1736 <title>Hints for using SMP parallelism</title>
1738 <para>Add the <literal>-sstderr</literal> RTS option when
1739 running the program to see timing stats, which will help to tell you
1740 whether your program got faster by using more CPUs or not. If the user
1741 time is greater than
1742 the elapsed time, then the program used more than one CPU. You should
1743 also run the program without <literal>-N</literal> for comparison.</para>
1745 <para>GHC's parallelism support is new and experimental. It may make your
1746 program go faster, or it might slow it down - either way, we'd be
1747 interested to hear from you.</para>
1749 <para>One significant limitation with the current implementation is that
1750 the garbage collector is still single-threaded, and all execution must
1751 stop when GC takes place. This can be a significant bottleneck in a
1752 parallel program, especially if your program does a lot of GC. If this
1753 happens to you, then try reducing the cost of GC by tweaking the GC
1754 settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
1755 allocation area size is a good start.</para>
1759 <sect1 id="options-platform">
1760 <title>Platform-specific Flags</title>
1762 <indexterm><primary>-m* options</primary></indexterm>
1763 <indexterm><primary>platform-specific options</primary></indexterm>
1764 <indexterm><primary>machine-specific options</primary></indexterm>
1766 <para>Some flags only make sense for particular target
1772 <term><option>-monly-[32]-regs</option>:</term>
1774 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1775 option (iX86 only)</primary></indexterm> GHC tries to
1776 “steal” four registers from GCC, for performance
1777 reasons; it almost always works. However, when GCC is
1778 compiling some modules with four stolen registers, it will
1779 crash, probably saying:
1782 Foo.hc:533: fixed or forbidden register was spilled.
1783 This may be due to a compiler bug or to impossible asm
1784 statements or clauses.
1787 Just give some registers back with
1788 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1789 If `2' doesn't work, please report the bug to us.</para>
1798 <sect1 id="ext-core">
1799 <title>Generating and compiling External Core Files</title>
1801 <indexterm><primary>intermediate code generation</primary></indexterm>
1803 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1804 to a file as a side-effect of compilation. Core files, which are given the suffix
1805 <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1806 tools. The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz">
1807 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1808 and sample tools (in Haskell)
1809 for manipulating Core files are available in the GHC source distribution
1810 directory <literal>/fptools/ghc/utils/ext-core</literal>.
1811 Note that the format of <literal>.hcr</literal>
1812 files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1813 for debugging purposes (<xref linkend="options-debugging"/>).</para>
1815 <para>The Core format natively supports notes which you can add to
1816 your source code using the <literal>CORE</literal> pragma (see <xref
1817 linkend="pragmas"/>).</para>
1823 <option>-fext-core</option>
1824 <indexterm><primary><option>-fext-core</option></primary></indexterm>
1827 <para>Generate <literal>.hcr</literal> files.</para>
1833 <para>GHC can also read in External Core files as source; just give the <literal>.hcr</literal> file on
1834 the command line, instead of the <literal>.hs</literal> or <literal>.lhs</literal> Haskell source.
1835 A current infelicity is that you need to give the <literal>-fglasgow-exts</literal> flag too, because
1836 ordinary Haskell 98, when translated to External Core, uses things like rank-2 types.</para>
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