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 --version</command>
359 <command>ghc -V</command>
361 <indexterm><primary><option>-V</option></primary></indexterm>
362 <indexterm><primary><option>––version</option></primary></indexterm>
365 <para>Print a one-line string including GHC's version number.</para>
372 <command>ghc --numeric-version</command>
374 <indexterm><primary><option>––numeric-version</option></primary></indexterm>
377 <para>Print GHC's numeric version number only.</para>
384 <command>ghc --print-libdir</command>
386 <indexterm><primary><option>––print-libdir</option></primary></indexterm>
389 <para>Print the path to GHC's library directory. This is
390 the top of the directory tree containing GHC's libraries,
391 interfaces, and include files (usually something like
392 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
394 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary></indexterm>
395 in the package configuration file
396 (see <xref linkend="packages"/>).</para>
402 <sect2 id="make-mode">
403 <title>Using <command>ghc</command> <option>––make</option></title>
404 <indexterm><primary><option>––make</option></primary></indexterm>
405 <indexterm><primary>separate compilation</primary></indexterm>
407 <para>When given the <option>––make</option> option,
408 GHC will build a multi-module Haskell program by following
409 dependencies from one or more root modules (usually just
410 <literal>Main</literal>). For example, if your
411 <literal>Main</literal> module is in a file called
412 <filename>Main.hs</filename>, you could compile and link the
413 program like this:</para>
416 ghc ––make Main.hs
419 <para>The command line may contain any number of source file
420 names or module names; GHC will figure out all the modules in
421 the program by following the imports from these initial modules.
422 It will then attempt to compile each module which is out of
423 date, and finally, if there is a <literal>Main</literal> module,
424 the program will also be linked into an executable.</para>
426 <para>The main advantages to using <literal>ghc
427 ––make</literal> over traditional
428 <literal>Makefile</literal>s are:</para>
432 <para>GHC doesn't have to be restarted for each compilation,
433 which means it can cache information between compilations.
434 Compiling a multi-module program with <literal>ghc
435 ––make</literal> can be up to twice as fast as
436 running <literal>ghc</literal> individually on each source
440 <para>You don't have to write a <literal>Makefile</literal>.</para>
441 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary></indexterm>
444 <para>GHC re-calculates the dependencies each time it is
445 invoked, so the dependencies never get out of sync with the
450 <para>Any of the command-line options described in the rest of
451 this chapter can be used with
452 <option>––make</option>, but note that any options
453 you give on the command line will apply to all the source files
454 compiled, so if you want any options to apply to a single source
455 file only, you'll need to use an <literal>OPTIONS_GHC</literal>
456 pragma (see <xref linkend="source-file-options"/>).</para>
458 <para>If the program needs to be linked with additional objects
459 (say, some auxiliary C code), then the object files can be
460 given on the command line and GHC will include them when linking
461 the executable.</para>
463 <para>Note that GHC can only follow dependencies if it has the
464 source file available, so if your program includes a module for
465 which there is no source file, even if you have an object and an
466 interface file for the module, then GHC will complain. The
467 exception to this rule is for package modules, which may or may
468 not have source files.</para>
470 <para>The source files for the program don't all need to be in
471 the same directory; the <option>-i</option> option can be used
472 to add directories to the search path (see <xref
473 linkend="search-path"/>).</para>
476 <sect2 id="eval-mode">
477 <title>Expression evaluation mode</title>
479 <para>This mode is very similar to interactive mode, except that
480 there is a single expression to evaluate which is specified on
481 the command line as an argument to the <option>-e</option>
485 ghc -e <replaceable>expr</replaceable>
488 <para>Haskell source files may be named on the command line, and
489 they will be loaded exactly as in interactive mode. The
490 expression is evaluated in the context of the loaded
493 <para>For example, to load and run a Haskell program containing
494 a module <literal>Main</literal>, we might say</para>
497 ghc -e Main.main Main.hs
500 <para>or we can just use this mode to evaluate expressions in
501 the context of the <literal>Prelude</literal>:</para>
504 $ ghc -e "interact (unlines.map reverse.lines)"
510 <sect2 id="options-order">
511 <title>Batch compiler mode</title>
513 <para>In <emphasis>batch mode</emphasis>, GHC will compile one or more source files
514 given on the command line.</para>
516 <para>The first phase to run is determined by each input-file
517 suffix, and the last phase is determined by a flag. If no
518 relevant flag is present, then go all the way through to linking.
519 This table summarises:</para>
523 <colspec align="left"/>
524 <colspec align="left"/>
525 <colspec align="left"/>
526 <colspec align="left"/>
530 <entry>Phase of the compilation system</entry>
531 <entry>Suffix saying “start here”</entry>
532 <entry>Flag saying “stop after”</entry>
533 <entry>(suffix of) output file</entry>
538 <entry>literate pre-processor</entry>
539 <entry><literal>.lhs</literal></entry>
541 <entry><literal>.hs</literal></entry>
545 <entry>C pre-processor (opt.) </entry>
546 <entry><literal>.hs</literal> (with
547 <option>-cpp</option>)</entry>
548 <entry><option>-E</option></entry>
549 <entry><literal>.hspp</literal></entry>
553 <entry>Haskell compiler</entry>
554 <entry><literal>.hs</literal></entry>
555 <entry><option>-C</option>, <option>-S</option></entry>
556 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
560 <entry>C compiler (opt.)</entry>
561 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
562 <entry><option>-S</option></entry>
563 <entry><literal>.s</literal></entry>
567 <entry>assembler</entry>
568 <entry><literal>.s</literal></entry>
569 <entry><option>-c</option></entry>
570 <entry><literal>.o</literal></entry>
574 <entry>linker</entry>
575 <entry><replaceable>other</replaceable></entry>
577 <entry><filename>a.out</filename></entry>
583 <indexterm><primary><option>-C</option></primary></indexterm>
584 <indexterm><primary><option>-E</option></primary></indexterm>
585 <indexterm><primary><option>-S</option></primary></indexterm>
586 <indexterm><primary><option>-c</option></primary></indexterm>
588 <para>Thus, a common invocation would be: </para>
591 ghc -c Foo.hs</screen>
593 <para>to compile the Haskell source file
594 <filename>Foo.hs</filename> to an object file
595 <filename>Foo.o</filename>.</para>
597 <para>Note: What the Haskell compiler proper produces depends on
598 whether a native-code generator<indexterm><primary>native-code
599 generator</primary></indexterm> is used (producing assembly
600 language) or not (producing C). See <xref
601 linkend="options-codegen"/> for more details.</para>
603 <para>Note: C pre-processing is optional, the
604 <option>-cpp</option><indexterm><primary><option>-cpp</option></primary></indexterm>
605 flag turns it on. See <xref linkend="c-pre-processor"/> for more
608 <para>Note: The option <option>-E</option><indexterm><primary>-E
609 option</primary></indexterm> runs just the pre-processing passes
610 of the compiler, dumping the result in a file.</para>
612 <sect3 id="overriding-suffixes">
613 <title>Overriding the default behaviour for a file</title>
615 <para>As described above, the way in which a file is processed by GHC
616 depends on its suffix. This behaviour can be overriden using the
617 <option>-x</option> option:</para>
621 <term><option>-x</option> <replaceable>suffix</replaceable>
622 <indexterm><primary><option>-x</option></primary>
625 <para>Causes all files following this option on the command
626 line to be processed as if they had the suffix
627 <replaceable>suffix</replaceable>. For example, to compile a
628 Haskell module in the file <literal>M.my-hs</literal>,
629 use <literal>ghc -c -x hs M.my-hs</literal>.</para>
638 <sect1 id="options-help">
639 <title>Help and verbosity options</title>
641 <indexterm><primary>help options</primary></indexterm>
642 <indexterm><primary>verbosity options</primary></indexterm>
644 <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
645 and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
650 <indexterm><primary><option>-n</option></primary></indexterm>
653 <para>Does a dry-run, i.e. GHC goes through all the motions
654 of compiling as normal, but does not actually run any
655 external commands.</para>
662 <indexterm><primary><option>-v</option></primary></indexterm>
665 <para>The <option>-v</option> option makes GHC
666 <emphasis>verbose</emphasis>: it reports its version number
667 and shows (on stderr) exactly how it invokes each phase of
668 the compilation system. Moreover, it passes the
669 <option>-v</option> flag to most phases; each reports its
670 version number (and possibly some other information).</para>
672 <para>Please, oh please, use the <option>-v</option> option
673 when reporting bugs! Knowing that you ran the right bits in
674 the right order is always the first thing we want to
681 <option>-v</option><replaceable>n</replaceable>
682 <indexterm><primary><option>-v</option></primary></indexterm>
685 <para>To provide more control over the compiler's verbosity,
686 the <option>-v</option> flag takes an optional numeric
687 argument. Specifying <option>-v</option> on its own is
688 equivalent to <option>-v3</option>, and the other levels
689 have the following meanings:</para>
693 <term><option>-v0</option></term>
695 <para>Disable all non-essential messages (this is the
701 <term><option>-v1</option></term>
703 <para>Minimal verbosity: print one line per
704 compilation (this is the default when
705 <option>––make</option> or
706 <option>––interactive</option> is on).</para>
711 <term><option>-v2</option></term>
713 <para>Print the name of each compilation phase as it
714 is executed. (equivalent to
715 <option>-dshow-passes</option>).</para>
720 <term><option>-v3</option></term>
722 <para>The same as <option>-v2</option>, except that in
723 addition the full command line (if appropriate) for
724 each compilation phase is also printed.</para>
729 <term><option>-v4</option></term>
731 <para>The same as <option>-v3</option> except that the
732 intermediate program representation after each
733 compilation phase is also printed (excluding
734 preprocessed and C/assembly files).</para>
742 <term><option>-ferror-spans</option>
743 <indexterm><primary><option>-ferror-spans</option></primary>
747 <para>Causes GHC to emit the full source span of the
748 syntactic entity relating to an error message. Normally, GHC
749 emits the source location of the start of the syntactic
752 <para>For example:</para>
754 <screen>test.hs:3:6: parse error on input `where'</screen>
756 <para>becomes:</para>
758 <screen>test296.hs:3:6-10: parse error on input `where'</screen>
760 <para>And multi-line spans are possible too:</para>
762 <screen>test.hs:(5,4)-(6,7):
763 Conflicting definitions for `a'
764 Bound at: test.hs:5:4
766 In the binding group for: a, b, a</screen>
768 <para>Note that line numbers start counting at one, but
769 column numbers start at zero. This choice was made to
770 follow existing convention (i.e. this is how Emacs does
776 <term><option>-H</option><replaceable>size</replaceable>
777 <indexterm><primary><option>-H</option></primary></indexterm>
780 <para>Set the minimum size of the heap to
781 <replaceable>size</replaceable>.
782 This option is equivalent to
783 <literal>+RTS -H<replaceable>size</replaceable></literal>,
784 see <xref linkend="rts-options-gc" />.
790 <term><option>-Rghc-timing</option>
791 <indexterm><primary><option>-Rghc-timing</option></primary></indexterm>
794 <para>Prints a one-line summary of timing statistics for the
795 GHC run. This option is equivalent to
796 <literal>+RTS -tstderr</literal>, see <xref
797 linkend="rts-options-gc" />.
806 <sect1 id="options-sanity">
807 <title>Warnings and sanity-checking</title>
809 <indexterm><primary>sanity-checking options</primary></indexterm>
810 <indexterm><primary>warnings</primary></indexterm>
813 <para>GHC has a number of options that select which types of
814 non-fatal error messages, otherwise known as warnings, can be
815 generated during compilation. By default, you get a standard set
816 of warnings which are generally likely to indicate bugs in your
818 <option>-fwarn-overlapping-patterns</option>,
819 <option>-fwarn-deprecations</option>,
820 <option>-fwarn-duplicate-exports</option>,
821 <option>-fwarn-missing-fields</option>, and
822 <option>-fwarn-missing-methods</option>. The following flags are
823 simple ways to select standard “packages” of warnings:
829 <term><option>-W</option>:</term>
831 <indexterm><primary>-W option</primary></indexterm>
832 <para>Provides the standard warnings plus
833 <option>-fwarn-incomplete-patterns</option>,
834 <option>-fwarn-dodgy-imports</option>,
835 <option>-fwarn-unused-matches</option>,
836 <option>-fwarn-unused-imports</option>, and
837 <option>-fwarn-unused-binds</option>.</para>
842 <term><option>-Wall</option>:</term>
844 <indexterm><primary><option>-Wall</option></primary></indexterm>
845 <para>Turns on all warning options that indicate potentially
846 suspicious code. The warnings that are
847 <emphasis>not</emphasis> enabled by <option>-Wall</option>
849 <option>-fwarn-simple-patterns</option>,
850 <option>-fwarn-tabs</option>,
851 <option>-fwarn-incomplete-record-updates</option>,
852 <option>-fwarn-monomorphism-restriction</option>, and
853 <option>-fwarn-implicit-prelude</option>.</para>
858 <term><option>-w</option>:</term>
860 <indexterm><primary><option>-w</option></primary></indexterm>
861 <para>Turns off all warnings, including the standard ones and
862 those that <literal>-Wall</literal> doesn't enable.</para>
867 <term><option>-Werror</option>:</term>
869 <indexterm><primary><option>-Werror</option></primary></indexterm>
870 <para>Makes any warning into a fatal error. Useful so that you don't
871 miss warnings when doing batch compilation. </para>
877 <para>The full set of warning options is described below. To turn
878 off any warning, simply give the corresponding
879 <option>-fno-warn-...</option> option on the command line.</para>
884 <term><option>-fwarn-deprecations</option>:</term>
886 <indexterm><primary><option>-fwarn-deprecations</option></primary>
888 <indexterm><primary>deprecations</primary></indexterm>
889 <para>Causes a warning to be emitted when a deprecated
890 function or type is used. Entities can be marked as
891 deprecated using a pragma, see <xref
892 linkend="deprecated-pragma"/>.</para>
894 <para>This option is on by default.</para>
899 <term><option>-fwarn-dodgy-imports</option>:</term>
901 <indexterm><primary><option>-fwarn-dodgy-imports</option></primary>
903 <para>Causes a warning to be emitted when a a datatype
904 <literal>T</literal> is imported
905 with all constructors, i.e. <literal>T(..)</literal>, but has been
906 exported abstractly, i.e. <literal>T</literal>.</para>
911 <term><option>-fwarn-duplicate-exports</option>:</term>
913 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
914 <indexterm><primary>duplicate exports, warning</primary></indexterm>
915 <indexterm><primary>export lists, duplicates</primary></indexterm>
917 <para>Have the compiler warn about duplicate entries in
918 export lists. This is useful information if you maintain
919 large export lists, and want to avoid the continued export
920 of a definition after you've deleted (one) mention of it in
921 the export list.</para>
923 <para>This option is on by default.</para>
928 <term><option>-fwarn-hi-shadowing</option>:</term>
930 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
931 <indexterm><primary>shadowing</primary>
932 <secondary>interface files</secondary></indexterm>
934 <para>Causes the compiler to emit a warning when a module or
935 interface file in the current directory is shadowing one
936 with the same module name in a library or other
942 <term><option>-fwarn-implicit-prelude</option>:</term>
944 <indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
945 <indexterm><primary>implicit prelude, warning</primary></indexterm>
946 <para>Have the compiler warn if the Prelude is implicitly
947 imported. This happens unless either the Prelude module is
948 explicitly imported with an <literal>import ... Prelude ...</literal>
949 line, or this implicit import is disabled (either by
950 <option>-fno-implicit-prelude</option> or a
951 <literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
953 <para>Note that no warning is given for syntax that implicitly
954 refers to the Prelude, even if <option>-fno-implicit-prelude</option>
955 would change whether it refers to the Prelude.
956 For example, no warning is given when
957 <literal>368</literal> means
958 <literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
959 (where <literal>Prelude</literal> refers to the actual Prelude module,
960 regardless of the imports of the module being compiled).</para>
962 <para>This warning is off by default.</para>
967 <term><option>-fwarn-incomplete-patterns</option>:</term>
969 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
970 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
971 <indexterm><primary>patterns, incomplete</primary></indexterm>
973 <para>Similarly for incomplete patterns, the function
974 <function>g</function> below will fail when applied to
975 non-empty lists, so the compiler will emit a warning about
976 this when <option>-fwarn-incomplete-patterns</option> is
983 <para>This option isn't enabled by default because it can be
984 a bit noisy, and it doesn't always indicate a bug in the
985 program. However, it's generally considered good practice
986 to cover all the cases in your functions.</para>
991 <term><option>-fwarn-incomplete-record-updates</option>:</term>
993 <indexterm><primary><option>-fwarn-incomplete-record-updates</option></primary></indexterm>
994 <indexterm><primary>incomplete record updates, warning</primary></indexterm>
995 <indexterm><primary>record updates, incomplete</primary></indexterm>
998 <function>f</function> below will fail when applied to
999 <literal>Bar</literal>, so the compiler will emit a warning about
1000 this when <option>-fwarn-incomplete-record-updates</option> is
1004 data Foo = Foo { x :: Int }
1008 f foo = foo { x = 6 }
1011 <para>This option isn't enabled by default because it can be
1012 very noisy, and it often doesn't indicate a bug in the
1019 <option>-fwarn-missing-fields</option>:
1020 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
1021 <indexterm><primary>missing fields, warning</primary></indexterm>
1022 <indexterm><primary>fields, missing</primary></indexterm>
1026 <para>This option is on by default, and warns you whenever
1027 the construction of a labelled field constructor isn't
1028 complete, missing initializers for one or more fields. While
1029 not an error (the missing fields are initialised with
1030 bottoms), it is often an indication of a programmer error.</para>
1035 <term><option>-fwarn-missing-methods</option>:</term>
1037 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
1038 <indexterm><primary>missing methods, warning</primary></indexterm>
1039 <indexterm><primary>methods, missing</primary></indexterm>
1041 <para>This option is on by default, and warns you whenever
1042 an instance declaration is missing one or more methods, and
1043 the corresponding class declaration has no default
1044 declaration for them.</para>
1045 <para>The warning is suppressed if the method name
1046 begins with an underscore. Here's an example where this is useful:
1049 _simpleFn :: a -> String
1050 complexFn :: a -> a -> String
1051 complexFn x y = ... _simpleFn ...
1053 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
1054 never <literal>_simpleFn</literal>; and (b)
1055 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1061 <term><option>-fwarn-missing-signatures</option>:</term>
1063 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1064 <indexterm><primary>type signatures, missing</primary></indexterm>
1066 <para>If you would like GHC to check that every top-level
1067 function/value has a type signature, use the
1068 <option>-fwarn-missing-signatures</option> option. As part of
1069 the warning GHC also reports the inferred type. The
1070 option is off by default.</para>
1075 <term><option>-fwarn-name-shadowing</option>:</term>
1077 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1078 <indexterm><primary>shadowing, warning</primary></indexterm>
1080 <para>This option causes a warning to be emitted whenever an
1081 inner-scope value has the same name as an outer-scope value,
1082 i.e. the inner value shadows the outer one. This can catch
1083 typographical errors that turn into hard-to-find bugs, e.g.,
1084 in the inadvertent capture of what would be a recursive call in
1085 <literal>f = ... let f = id in ... f ...</literal>.</para>
1090 <term><option>-fwarn-orphans</option>:</term>
1092 <indexterm><primary><option>-fwarn-orphans</option></primary></indexterm>
1093 <indexterm><primary>orphan instances, warning</primary></indexterm>
1094 <indexterm><primary>orphan rules, warning</primary></indexterm>
1096 <para>This option causes a warning to be emitted whenever the
1097 module contains an "orphan" instance declaration or rewrite rule.
1098 An instance declartion is an orphan if it appears in a module in
1099 which neither the class nor the type being instanced are declared
1100 in the same module. A rule is an orphan if it is a rule for a
1101 function declared in another module. A module containing any
1102 orphans is called an orphan module.</para>
1103 <para>The trouble with orphans is that GHC must pro-actively read the interface
1104 files for all orphan modules, just in case their instances or rules
1105 play a role, whether or not the module's interface would otherwise
1106 be of any use. Other things being equal, avoid orphan modules.</para>
1112 <option>-fwarn-overlapping-patterns</option>:
1113 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1114 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1115 <indexterm><primary>patterns, overlapping</primary></indexterm>
1118 <para>By default, the compiler will warn you if a set of
1119 patterns are overlapping, e.g.,</para>
1122 f :: String -> Int
1128 <para>where the last pattern match in <function>f</function>
1129 won't ever be reached, as the second pattern overlaps
1130 it. More often than not, redundant patterns is a programmer
1131 mistake/error, so this option is enabled by default.</para>
1136 <term><option>-fwarn-simple-patterns</option>:</term>
1138 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1140 <para>Causes the compiler to warn about lambda-bound
1141 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1142 Normally, these aren't treated as incomplete patterns by
1143 <option>-fwarn-incomplete-patterns</option>.</para>
1144 <para>“Lambda-bound patterns” includes all places where there is a single pattern,
1145 including list comprehensions and do-notation. In these cases, a pattern-match
1146 failure is quite legitimate, and triggers filtering (list comprehensions) or
1147 the monad <literal>fail</literal> operation (monads). For example:
1149 f :: [Maybe a] -> [a]
1150 f xs = [y | Just y <- xs]
1152 Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
1153 these probably-innocent cases, which is why the flag is off by default. </para>
1158 <term><option>-fwarn-tabs</option>:</term>
1160 <indexterm><primary><option>-fwarn-tabs</option></primary></indexterm>
1161 <indexterm><primary>tabs, warning</primary></indexterm>
1162 <para>Have the compiler warn if there are tabs in your source
1165 <para>This warning is off by default.</para>
1170 <term><option>-fwarn-type-defaults</option>:</term>
1172 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1173 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1174 <para>Have the compiler warn/inform you where in your source
1175 the Haskell defaulting mechanism for numeric types kicks
1176 in. This is useful information when converting code from a
1177 context that assumed one default into one with another,
1178 e.g., the ‘default default’ for Haskell 1.4 caused the
1179 otherwise unconstrained value <constant>1</constant> to be
1180 given the type <literal>Int</literal>, whereas Haskell 98
1181 defaults it to <literal>Integer</literal>. This may lead to
1182 differences in performance and behaviour, hence the
1183 usefulness of being non-silent about this.</para>
1185 <para>This warning is off by default.</para>
1190 <term><option>-fwarn-monomorphism-restriction</option>:</term>
1192 <indexterm><primary><option>-fwarn-monomorphism-restriction</option></primary></indexterm>
1193 <indexterm><primary>monomorphism restriction, warning</primary></indexterm>
1194 <para>Have the compiler warn/inform you where in your source
1195 the Haskell Monomorphism Restriction is applied. If applied silently
1196 the MR can give rise to unexpected behaviour, so it can be helpful
1197 to have an explicit warning that it is being applied.</para>
1199 <para>This warning is off by default.</para>
1204 <term><option>-fwarn-unused-binds</option>:</term>
1206 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1207 <indexterm><primary>unused binds, warning</primary></indexterm>
1208 <indexterm><primary>binds, unused</primary></indexterm>
1209 <para>Report any function definitions (and local bindings)
1210 which are unused. For top-level functions, the warning is
1211 only given if the binding is not exported.</para>
1212 <para>A definition is regarded as "used" if (a) it is exported, or (b) it is
1213 mentioned in the right hand side of another definition that is used, or (c) the
1214 function it defines begins with an underscore. The last case provides a
1215 way to suppress unused-binding warnings selectively. </para>
1216 <para> Notice that a variable
1217 is reported as unused even if it appears in the right-hand side of another
1218 unused binding. </para>
1223 <term><option>-fwarn-unused-imports</option>:</term>
1225 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1226 <indexterm><primary>unused imports, warning</primary></indexterm>
1227 <indexterm><primary>imports, unused</primary></indexterm>
1229 <para>Report any modules that are explicitly imported but
1230 never used. However, the form <literal>import M()</literal> is
1231 never reported as an unused import, because it is a useful idiom
1232 for importing instance declarations, which are anonymous in Haskell.</para>
1237 <term><option>-fwarn-unused-matches</option>:</term>
1239 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1240 <indexterm><primary>unused matches, warning</primary></indexterm>
1241 <indexterm><primary>matches, unused</primary></indexterm>
1243 <para>Report all unused variables which arise from pattern
1244 matches, including patterns consisting of a single variable.
1245 For instance <literal>f x y = []</literal> would report
1246 <varname>x</varname> and <varname>y</varname> as unused. The
1247 warning is suppressed if the variable name begins with an underscore, thus:
1257 <para>If you're feeling really paranoid, the
1258 <option>-dcore-lint</option>
1259 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1260 is a good choice. It turns on heavyweight intra-pass
1261 sanity-checking within GHC. (It checks GHC's sanity, not
1268 <sect1 id="options-optimise">
1269 <title>Optimisation (code improvement)</title>
1271 <indexterm><primary>optimisation</primary></indexterm>
1272 <indexterm><primary>improvement, code</primary></indexterm>
1274 <para>The <option>-O*</option> options specify convenient
1275 “packages” of optimisation flags; the
1276 <option>-f*</option> options described later on specify
1277 <emphasis>individual</emphasis> optimisations to be turned on/off;
1278 the <option>-m*</option> options specify
1279 <emphasis>machine-specific</emphasis> optimisations to be turned
1282 <sect2 id="optimise-pkgs">
1283 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1285 <para>There are <emphasis>many</emphasis> options that affect
1286 the quality of code produced by GHC. Most people only have a
1287 general goal, something like “Compile quickly” or
1288 “Make my program run like greased lightning.” The
1289 following “packages” of optimisations (or lack
1290 thereof) should suffice.</para>
1292 <para>Note that higher optimisation levels cause more
1293 cross-module optimisation to be performed, which can have an
1294 impact on how much of your program needs to be recompiled when
1295 you change something. This is one reason to stick to
1296 no-optimisation when developing code.</para>
1302 No <option>-O*</option>-type option specified:
1303 <indexterm><primary>-O* not specified</primary></indexterm>
1306 <para>This is taken to mean: “Please compile
1307 quickly; I'm not over-bothered about compiled-code
1308 quality.” So, for example: <command>ghc -c
1309 Foo.hs</command></para>
1315 <option>-O0</option>:
1316 <indexterm><primary><option>-O0</option></primary></indexterm>
1319 <para>Means “turn off all optimisation”,
1320 reverting to the same settings as if no
1321 <option>-O</option> options had been specified. Saying
1322 <option>-O0</option> can be useful if
1323 eg. <command>make</command> has inserted a
1324 <option>-O</option> on the command line already.</para>
1330 <option>-O</option> or <option>-O1</option>:
1331 <indexterm><primary>-O option</primary></indexterm>
1332 <indexterm><primary>-O1 option</primary></indexterm>
1333 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1336 <para>Means: “Generate good-quality code without
1337 taking too long about it.” Thus, for example:
1338 <command>ghc -c -O Main.lhs</command></para>
1344 <option>-O2</option>:
1345 <indexterm><primary>-O2 option</primary></indexterm>
1346 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1349 <para>Means: “Apply every non-dangerous
1350 optimisation, even if it means significantly longer
1351 compile times.”</para>
1353 <para>The avoided “dangerous” optimisations
1354 are those that can make runtime or space
1355 <emphasis>worse</emphasis> if you're unlucky. They are
1356 normally turned on or off individually.</para>
1358 <para>At the moment, <option>-O2</option> is
1359 <emphasis>unlikely</emphasis> to produce better code than
1360 <option>-O</option>.</para>
1366 <option>-Ofile <file></option>:
1367 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1368 <indexterm><primary>optimising, customised</primary></indexterm>
1371 <para>(NOTE: not supported since GHC 4.x. Please ask if
1372 you're interested in this.)</para>
1374 <para>For those who need <emphasis>absolute</emphasis>
1375 control over <emphasis>exactly</emphasis> what options are
1376 used (e.g., compiler writers, sometimes :-), a list of
1377 options can be put in a file and then slurped in with
1378 <option>-Ofile</option>.</para>
1380 <para>In that file, comments are of the
1381 <literal>#</literal>-to-end-of-line variety; blank
1382 lines and most whitespace is ignored.</para>
1384 <para>Please ask if you are baffled and would like an
1385 example of <option>-Ofile</option>!</para>
1390 <para>We don't use a <option>-O*</option> flag for day-to-day
1391 work. We use <option>-O</option> to get respectable speed;
1392 e.g., when we want to measure something. When we want to go for
1393 broke, we tend to use <option>-O2 -fvia-C</option> (and we go for
1394 lots of coffee breaks).</para>
1396 <para>The easiest way to see what <option>-O</option> (etc.)
1397 “really mean” is to run with <option>-v</option>,
1398 then stand back in amazement.</para>
1401 <sect2 id="options-f">
1402 <title><option>-f*</option>: platform-independent flags</title>
1404 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1405 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1407 <para>These flags turn on and off individual optimisations.
1408 They are normally set via the <option>-O</option> options
1409 described above, and as such, you shouldn't need to set any of
1410 them explicitly (indeed, doing so could lead to unexpected
1411 results). However, there are one or two that may be of
1416 <term><option>-fexcess-precision</option>:</term>
1418 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1419 <para>When this option is given, intermediate floating
1420 point values can have a <emphasis>greater</emphasis>
1421 precision/range than the final type. Generally this is a
1422 good thing, but some programs may rely on the exact
1424 <literal>Float</literal>/<literal>Double</literal> values
1425 and should not use this option for their compilation.</para>
1430 <term><option>-fignore-asserts</option>:</term>
1432 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1433 <para>Causes GHC to ignore uses of the function
1434 <literal>Exception.assert</literal> in source code (in
1435 other words, rewriting <literal>Exception.assert p
1436 e</literal> to <literal>e</literal> (see <xref
1437 linkend="assertions"/>). This flag is turned on by
1438 <option>-O</option>.
1445 <option>-fno-cse</option>
1446 <indexterm><primary><option>-fno-cse</option></primary></indexterm>
1449 <para>Turns off the common-sub-expression elimination optimisation.
1450 Can be useful if you have some <literal>unsafePerformIO</literal>
1451 expressions that you don't want commoned-up.</para>
1457 <option>-fno-strictness</option>
1458 <indexterm><primary><option>-fno-strictness</option></primary></indexterm>
1461 <para>Turns off the strictness analyser; sometimes it eats
1462 too many cycles.</para>
1468 <option>-fno-full-laziness</option>
1469 <indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
1472 <para>Turns off the full laziness optimisation (also known as
1473 let-floating). Full laziness increases sharing, which can lead
1474 to increased memory residency.</para>
1476 <para>NOTE: GHC doesn't implement complete full-laziness.
1477 When optimisation in on, and
1478 <option>-fno-full-laziness</option> is not given, some
1479 transformations that increase sharing are performed, such
1480 as extracting repeated computations from a loop. These
1481 are the same transformations that a fully lazy
1482 implementation would do, the difference is that GHC
1483 doesn't consistently apply full-laziness, so don't rely on
1490 <option>-fno-state-hack</option>
1491 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1494 <para>Turn off the "state hack" whereby any lambda with a
1495 <literal>State#</literal> token as argument is considered to be
1496 single-entry, hence it is considered OK to inline things inside
1497 it. This can improve performance of IO and ST monad code, but it
1498 runs the risk of reducing sharing.</para>
1504 <option>-fno-state-hack</option>
1505 <indexterm><primary><option>-fno-state-hack</option></primary></indexterm>
1508 <para>Turn off the "state hack" whereby any lambda with a
1509 <literal>State#</literal> token as argument is considered to be
1510 single-entry, hence it is considered OK to inline things inside
1511 it. This can improve performance of IO and ST monad code, but it
1512 runs the risk of reducing sharing.</para>
1518 <option>-fomit-interface-pragmas</option>
1519 <indexterm><primary><option>-fomit-interface-pragmas</option></primary></indexterm>
1522 <para>Tells GHC to omit all inessential information from the interface file
1523 generated for the module being compiled (say M). This means that a module
1524 importing M will see only the <emphasis>types</emphasis> of the functions that M exports, but not
1525 their unfoldings, strictness info, etc. Hence, for example,
1526 no function exported by M will be inlined
1527 into an importing module. The benefit is that modules that import M will
1528 need to be recompiled less often (only when M's exports change their type,
1529 not when they change their implementation).
1536 <option>-fignore-interface-pragmas</option>
1537 <indexterm><primary><option>-fignore-interface-pragmas</option></primary></indexterm>
1540 <para>Tells GHC to ignore all inessential information when reading interface files.
1541 That is, even if <filename>M.hi</filename> contains unfolding or strictness information
1542 for a function, GHC will ignore that information.</para>
1548 <option>-funbox-strict-fields</option>:
1549 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1550 <indexterm><primary>strict constructor fields</primary></indexterm>
1551 <indexterm><primary>constructor fields, strict</primary></indexterm>
1554 <para>This option causes all constructor fields which are
1555 marked strict (i.e. “!”) to be unboxed or
1556 unpacked if possible. It is equivalent to adding an
1557 <literal>UNPACK</literal> pragma to every strict
1558 constructor field (see <xref
1559 linkend="unpack-pragma"/>).</para>
1561 <para>This option is a bit of a sledgehammer: it might
1562 sometimes make things worse. Selectively unboxing fields
1563 by using <literal>UNPACK</literal> pragmas might be
1570 <option>-funfolding-update-in-place=<replaceable>n</replaceable></option>
1571 <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1574 <para>Switches on an experimental "optimisation".
1575 Switching it on makes the compiler a little keener to
1576 inline a function that returns a constructor, if the
1577 context is that of a thunk.
1581 If we inlined plusInt we might get an opportunity to use
1582 update-in-place for the thunk 'x'.</para>
1588 <option>-funfolding-creation-threshold=<replaceable>n</replaceable></option>:
1589 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1590 <indexterm><primary>inlining, controlling</primary></indexterm>
1591 <indexterm><primary>unfolding, controlling</primary></indexterm>
1594 <para>(Default: 45) Governs the maximum size that GHC will
1595 allow a function unfolding to be. (An unfolding has a
1596 “size” that reflects the cost in terms of
1597 “code bloat” of expanding that unfolding at
1598 at a call site. A bigger function would be assigned a
1599 bigger cost.) </para>
1601 <para> Consequences: (a) nothing larger than this will be
1602 inlined (unless it has an INLINE pragma); (b) nothing
1603 larger than this will be spewed into an interface
1607 <para> Increasing this figure is more likely to result in longer
1608 compile times than faster code. The next option is more
1614 <term><option>-funfolding-use-threshold=<replaceable>n</replaceable></option></term>
1616 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1617 <indexterm><primary>inlining, controlling</primary></indexterm>
1618 <indexterm><primary>unfolding, controlling</primary></indexterm>
1620 <para>(Default: 8) This is the magic cut-off figure for
1621 unfolding: below this size, a function definition will be
1622 unfolded at the call-site, any bigger and it won't. The
1623 size computed for a function depends on two things: the
1624 actual size of the expression minus any discounts that
1625 apply (see <option>-funfolding-con-discount</option>).</para>
1636 <sect1 id="using-concurrent">
1637 <title>Using Concurrent Haskell</title>
1638 <indexterm><primary>Concurrent Haskell</primary><secondary>using</secondary></indexterm>
1640 <para>GHC supports Concurrent Haskell by default, without requiring a
1641 special option or libraries compiled in a certain way. To get access to
1642 the support libraries for Concurrent Haskell, just import
1644 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>
1646 <para>The following RTS option(s) affect the behaviour of Concurrent
1647 Haskell programs:<indexterm><primary>RTS options, concurrent</primary></indexterm></para>
1651 <term><option>-C<replaceable>s</replaceable></option></term>
1653 <para><indexterm><primary><option>-C<replaceable>s</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1654 Sets the context switch interval to <replaceable>s</replaceable>
1655 seconds. A context switch will occur at the next heap block
1656 allocation after the timer expires (a heap block allocation occurs
1657 every 4k of allocation). With <option>-C0</option> or
1658 <option>-C</option>, context switches will occur as often as
1659 possible (at every heap block allocation). By default, context
1660 switches occur every 20ms.</para>
1666 <sect1 id="using-smp">
1667 <title>Using SMP parallelism</title>
1668 <indexterm><primary>parallelism</primary>
1670 <indexterm><primary>SMP</primary>
1673 <para>GHC supports running Haskell programs in parallel on an SMP
1674 (symmetric multiprocessor).</para>
1676 <para>There's a fine distinction between
1677 <emphasis>concurrency</emphasis> and <emphasis>parallelism</emphasis>:
1678 parallelism is all about making your program run
1679 <emphasis>faster</emphasis> by making use of multiple processors
1680 simultaneously. Concurrency, on the other hand, is a means of
1681 abstraction: it is a convenient way to structure a program that must
1682 respond to multiple asynchronous events.</para>
1684 <para>However, the two terms are certainly related. By making use of
1685 multiple CPUs it is possible to run concurrent threads in parallel,
1686 and this is exactly what GHC's SMP parallelism support does. But it
1687 is also possible to obtain performance improvements with parallelism
1688 on programs that do not use concurrency. This section describes how to
1689 use GHC to compile and run parallel programs, in <xref
1690 linkend="lang-parallel" /> we desribe the language features that affect
1693 <sect2 id="parallel-options">
1694 <title>Options to enable SMP parallelism</title>
1696 <para>In order to make use of multiple CPUs, your program must be
1697 linked with the <option>-threaded</option> option (see <xref
1698 linkend="options-linker" />). Then, to run a program on multiple
1699 CPUs, use the RTS <option>-N</option> option:</para>
1703 <term><option>-N<replaceable>x</replaceable></option></term>
1705 <para><indexterm><primary><option>-N<replaceable>x</replaceable></option></primary><secondary>RTS option</secondary></indexterm>
1706 Use <replaceable>x</replaceable> simultaneous threads when
1707 running the program. Normally <replaceable>x</replaceable>
1708 should be chosen to match the number of CPU cores on the machine.
1709 There is no means (currently) by which this value may vary after
1710 the program has started.</para>
1712 <para>For example, on a dual-core machine we would probably use
1713 <literal>+RTS -N2 -RTS</literal>.</para>
1715 <para>Whether hyperthreading cores should be counted or not is an
1716 open question; please feel free to experiment and let us know what
1717 results you find.</para>
1724 <title>Hints for using SMP parallelism</title>
1726 <para>Add the <literal>-sstderr</literal> RTS option when
1727 running the program to see timing stats, which will help to tell you
1728 whether your program got faster by using more CPUs or not. If the user
1729 time is greater than
1730 the elapsed time, then the program used more than one CPU. You should
1731 also run the program without <literal>-N</literal> for comparison.</para>
1733 <para>GHC's parallelism support is new and experimental. It may make your
1734 program go faster, or it might slow it down - either way, we'd be
1735 interested to hear from you.</para>
1737 <para>One significant limitation with the current implementation is that
1738 the garbage collector is still single-threaded, and all execution must
1739 stop when GC takes place. This can be a significant bottleneck in a
1740 parallel program, especially if your program does a lot of GC. If this
1741 happens to you, then try reducing the cost of GC by tweaking the GC
1742 settings (<xref linkend="rts-options-gc" />): enlarging the heap or the
1743 allocation area size is a good start.</para>
1747 <sect1 id="options-platform">
1748 <title>Platform-specific Flags</title>
1750 <indexterm><primary>-m* options</primary></indexterm>
1751 <indexterm><primary>platform-specific options</primary></indexterm>
1752 <indexterm><primary>machine-specific options</primary></indexterm>
1754 <para>Some flags only make sense for particular target
1760 <term><option>-monly-[32]-regs</option>:</term>
1762 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1763 option (iX86 only)</primary></indexterm> GHC tries to
1764 “steal” four registers from GCC, for performance
1765 reasons; it almost always works. However, when GCC is
1766 compiling some modules with four stolen registers, it will
1767 crash, probably saying:
1770 Foo.hc:533: fixed or forbidden register was spilled.
1771 This may be due to a compiler bug or to impossible asm
1772 statements or clauses.
1775 Just give some registers back with
1776 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1777 If `2' doesn't work, please report the bug to us.</para>
1786 <sect1 id="ext-core">
1787 <title>Generating and compiling External Core Files</title>
1789 <indexterm><primary>intermediate code generation</primary></indexterm>
1791 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1792 to a file as a side-effect of compilation. Core files, which are given the suffix
1793 <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1794 tools. The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz">
1795 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1796 and sample tools (in Haskell)
1797 for manipulating Core files are available in the GHC source distribution
1798 directory <literal>/fptools/ghc/utils/ext-core</literal>.
1799 Note that the format of <literal>.hcr</literal>
1800 files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1801 for debugging purposes (<xref linkend="options-debugging"/>).</para>
1803 <para>The Core format natively supports notes which you can add to
1804 your source code using the <literal>CORE</literal> pragma (see <xref
1805 linkend="pragmas"/>).</para>
1811 <option>-fext-core</option>
1812 <indexterm><primary><option>-fext-core</option></primary></indexterm>
1815 <para>Generate <literal>.hcr</literal> files.</para>
1821 <para>GHC can also read in External Core files as source; just give the <literal>.hcr</literal> file on
1822 the command line, instead of the <literal>.hs</literal> or <literal>.lhs</literal> Haskell source.
1823 A current infelicity is that you need to give the <literal>-fglasgow-exts</literal> flag too, because
1824 ordinary Haskell 98, when translated to External Core, uses things like rank-2 types.</para>
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