1 <chapter id="using-ghc">
2 <title>Using GHC</title>
4 <indexterm><primary>GHC, using</primary></indexterm>
5 <indexterm><primary>using GHC</primary></indexterm>
7 <para>GHC can work in one of three “modes”:</para>
11 <term><cmdsynopsis><command>ghc</command>
12 <arg choice=plain>––interactive</arg>
14 <indexterm><primary>interactive mode</primary>
16 <indexterm><primary>ghci</primary>
19 <para>Interactive mode, which is also available as
20 <command>ghci</command>. Interactive mode is described in
21 more detail in <xref linkend="ghci">.</para>
26 <term><cmdsynopsis><command>ghc</command>
27 <arg choice=plain>––make</arg>
29 <indexterm><primary>make mode</primary>
31 <indexterm><primary><option>––make</option></primary>
34 <para>In this mode, GHC will build a multi-module Haskell
35 program automatically, figuring out dependencies for itself.
36 If you have a straightforward Haskell program, this is likely
37 to be much easier, and faster, than using
38 <command>make</command>.</para>
44 <command>ghc</command>
52 <indexterm><primary><option>-E</option></primary></indexterm>
53 <indexterm><primary><option>-C</option></primary></indexterm>
54 <indexterm><primary><option>-S</option></primary></indexterm>
55 <indexterm><primary><option>-c</option></primary></indexterm>
57 <para>This is the traditional batch-compiler mode, in which
58 GHC can compile source files one at a time, or link objects
59 together into an executable.</para>
65 <title>Options overview</title>
67 <para>GHC's behaviour is controlled by
68 <firstterm>options</firstterm>, which for historical reasons are
69 also sometimes referred to as command-line flags or arguments.
70 Options can be specified in three ways:</para>
73 <title>Command-line arguments</title>
75 <indexterm><primary>structure, command-line</primary></indexterm>
76 <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
77 <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
79 <para>An invocation of GHC takes the following form:</para>
85 <para>Command-line arguments are either options or file names.</para>
87 <para>Command-line options begin with <literal>-</literal>.
88 They may <emphasis>not</emphasis> be grouped:
89 <option>-vO</option> is different from <option>-v -O</option>.
90 Options need not precede filenames: e.g., <literal>ghc *.o -o
91 foo</literal>. All options are processed and then applied to
92 all files; you cannot, for example, invoke <literal>ghc -c -O1
93 Foo.hs -O2 Bar.hs</literal> to apply different optimisation
94 levels to the files <filename>Foo.hs</filename> and
95 <filename>Bar.hs</filename>.</para>
98 <Sect2 id="source-file-options">
99 <title>Command line options in source files</title>
101 <indexterm><primary>source-file options</primary></indexterm>
103 <para>Sometimes it is useful to make the connection between a
104 source file and the command-line options it requires quite
105 tight. For instance, if a Haskell source file uses GHC
106 extensions, it will always need to be compiled with the
107 <option>-fglasgow-exts</option> option. Rather than maintaining
108 the list of per-file options in a <filename>Makefile</filename>,
109 it is possible to do this directly in the source file using the
110 <literal>OPTIONS</literal> pragma <indexterm><primary>OPTIONS
111 pragma</primary></indexterm>:</para>
114 {-# OPTIONS -fglasgow-exts #-}
119 <para><literal>OPTIONS</literal> pragmas are only looked for at
120 the top of your source files, upto the first
121 (non-literate,non-empty) line not containing
122 <literal>OPTIONS</literal>. Multiple <literal>OPTIONS</literal>
123 pragmas are recognised. Note that your command shell does not
124 get to the source file options, they are just included literally
125 in the array of command-line arguments the compiler driver
126 maintains internally, so you'll be desperately disappointed if
127 you try to glob etc. inside <literal>OPTIONS</literal>.</para>
129 <para>NOTE: the contents of OPTIONS are prepended to the
130 command-line options, so you <emphasis>do</emphasis> have the
131 ability to override OPTIONS settings via the command
134 <para>It is not recommended to move all the contents of your
135 Makefiles into your source files, but in some circumstances, the
136 <literal>OPTIONS</literal> pragma is the Right Thing. (If you
137 use <option>-keep-hc-file-too</option> and have OPTION flags in
138 your module, the OPTIONS will get put into the generated .hc
143 <title>Setting options in GHCi</title>
145 <para>Options may also be modified from within GHCi, using the
146 <literal>:set</literal> command. See <xref linkend="ghci-set">
147 for more details.</para>
151 <sect1 id="static-dynamic-flags">
152 <title>Static vs. Dynamic options</title>
153 <indexterm><primary>static</primary><secondary>options</secondary>
155 <indexterm><primary>dynamic</primary><secondary>options</secondary>
158 <para>Each of GHC's command line options is classified as either
159 <firstterm>static</firstterm> or <firstterm>dynamic</firstterm>.
160 A static flag may only be specified on the command line, whereas a
161 dynamic flag may also be given in an <literal>OPTIONS</literal>
162 pragma in a source file or set from the GHCi command-line with
163 <literal>:set</literal>.</para>
165 <para>As a rule of thumb, all the language options are dynamic, as
166 are the warning options and the debugging options. The rest are
167 static, with the notable exceptions of <option>-v</option>,
168 <option>-cpp</option>, <option>-fasm</option>,
169 <option>-fvia-C</option>, and <option>-#include</option>.
171 The flag reference tables (<xref linkend="flag-reference">) lists
172 the status of each flag.</para>
175 <sect1 id="file-suffixes">
176 <title>Meaningful file suffixes</title>
178 <indexterm><primary>suffixes, file</primary></indexterm>
179 <indexterm><primary>file suffixes for GHC</primary></indexterm>
181 <para>File names with “meaningful” suffixes (e.g.,
182 <filename>.lhs</filename> or <filename>.o</filename>) cause the
183 “right thing” to happen to those files.</para>
188 <term><filename>.lhs</filename></term>
189 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
191 <para>A “literate Haskell” module.</para>
196 <term><filename>.hs</filename></term>
198 <para>A not-so-literate Haskell module.</para>
203 <term><filename>.hi</filename></term>
205 <para>A Haskell interface file, probably
206 compiler-generated.</para>
211 <term><filename>.hc</filename></term>
213 <para>Intermediate C file produced by the Haskell
219 <term><filename>.c</filename></term>
221 <para>A C file not produced by the Haskell
227 <term><filename>.s</filename></term>
229 <para>An assembly-language source file, usually produced by
235 <term><filename>.o</filename></term>
237 <para>An object file, produced by an assembler.</para>
242 <para>Files with other suffixes (or without suffixes) are passed
243 straight to the linker.</para>
247 <sect1 id="options-help">
248 <title>Help and verbosity options</title>
250 <IndexTerm><Primary>help options</Primary></IndexTerm>
251 <IndexTerm><Primary>verbosity options</Primary></IndexTerm>
255 <term><option>––help</option></term>
256 <term><option>-?</option></term>
257 <indexterm><primary><option>-?</option></primary></indexterm>
258 <indexterm><primary><option>––help</option></primary></indexterm>
260 <para>Cause GHC to spew a long usage message to standard
261 output and then exit.</para>
266 <term><option>-v</option></term>
267 <indexterm><primary><option>-v</option></primary></indexterm>
269 <para>The <option>-v</option> option makes GHC
270 <emphasis>verbose</emphasis>: it reports its version number
271 and shows (on stderr) exactly how it invokes each phase of
272 the compilation system. Moreover, it passes the
273 <option>-v</option> flag to most phases; each reports its
274 version number (and possibly some other information).</para>
276 <para>Please, oh please, use the <option>-v</option> option
277 when reporting bugs! Knowing that you ran the right bits in
278 the right order is always the first thing we want to
284 <term><option>-v</option><replaceable>n</replaceable></term>
285 <indexterm><primary><option>-v</option></primary></indexterm>
287 <para>To provide more control over the compiler's verbosity,
288 the <option>-v</option> flag takes an optional numeric
289 argument. Specifying <option>-v</option> on its own is
290 equivalent to <option>-v3</option>, and the other levels
291 have the following meanings:</para>
295 <term><option>-v0</option></term>
297 <para>Disable all non-essential messages (this is the
303 <term><option>-v1</option></term>
305 <para>Minimal verbosity: print one line per
306 compilation (this is the default when
307 <option>––make</option> or
308 <option>––interactive</option> is on).</para>
313 <term><option>-v2</option></term>
315 <para>Print the name of each compilation phase as it
316 is executed. (equivalent to
317 <option>-dshow-passes</option>).</para>
322 <term><option>-v3</option></term>
324 <para>The same as <option>-v2</option>, except that in
325 addition the full command line (if appropriate) for
326 each compilation phase is also printed.</para>
331 <term><option>-v4</option></term>
333 <para>The same as <option>-v3</option> except that the
334 intermediate program representation after each
335 compilation phase is also printed (excluding
336 preprocessed and C/assembly files).</para>
344 <term><option>––version</option></term>
345 <indexterm><primary><option>––version</option></primary></indexterm>
347 <para>Print a one-line string including GHC's version number.</para>
352 <term><option>––numeric-version</option></term>
353 <indexterm><primary><option>––numeric-version</option></primary></indexterm>
355 <para>Print GHC's numeric version number only.</para>
360 <term><option>––print-libdir</option></term>
361 <indexterm><primary><option>––print-libdir</option></primary></indexterm>
363 <para>Print the path to GHC's library directory. This is
364 the top of the directory tree containing GHC's libraries,
365 interfaces, and include files (usually something like
366 <literal>/usr/local/lib/ghc-5.04</literal> on Unix). This
368 <literal>$libdir</literal><indexterm><primary><literal>libdir</literal></primary>
369 </indexterm>in the package configuration file (see <xref
370 linkend="packages">).</para>
377 <sect1 id="make-mode">
378 <title>Using <command>ghc</command> <option>––make</option></title>
380 <indexterm><primary><option>––make</option></primary>
382 <indexterm><primary>separate compilation</primary>
385 <para>When given the <option>––make</option> option, GHC will
386 build a multi-module Haskell program by following dependencies
387 from a single root module (usually <literal>Main</literal>). For
388 example, if your <literal>Main</literal> module is in a file
389 called <filename>Main.hs</filename>, you could compile and link
390 the program like this:</para>
393 ghc ––make Main.hs
396 <para>The command line may contain any number of source file names
397 or module names; GHC will figure out all the modules in the
398 program by following the imports from these initial modules. It
399 will then attempt to compile each module which is out of date, and
400 finally if there is a <literal>Main</literal> module, the program
401 will also be linked into an executable.</para>
403 <para>The main advantages to using <literal>ghc ––make</literal>
404 over traditional <literal>Makefile</literal>s are:</para>
408 <para>GHC doesn't have to be restarted for each compilation,
409 which means it can cache information between compilations.
410 Compiling a muli-module program with <literal>ghc
411 ––make</literal> can be up to twice as fast as running
412 <literal>ghc</literal> individually on each source
416 <para>You don't have to write a
417 <literal>Makefile</literal>.</para>
419 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary>
422 <para>GHC re-calculates the dependencies each time it is
423 invoked, so the dependencies never get out of sync with the
428 <para>Any of the command-line options described in the rest of
429 this chapter can be used with <option>––make</option>, but note
430 that any options you give on the command line will apply to all
431 the source files compiled, so if you want any options to apply to
432 a single source file only, you'll need to use an
433 <literal>OPTIONS</literal> pragma (see <xref
434 linkend="source-file-options">).</para>
436 <para>If the program needs to be linked with additional objects
437 (say, some auxilliary C code), then the object files can be
438 given on the command line and GHC will include them when linking
439 the executable.</para>
441 <para>Note that GHC can only follow dependencies if it has the
442 source file available, so if your program includes a module for
443 which there is no source file, even if you have an object and an
444 interface file for the module, then GHC will complain. The
445 exception to this rule is for package modules, which may or may
446 not have source files.</para>
448 <para>The source files for the program don't all need to be in the
449 same directory; the <option>-i</option> option can be used to add
450 directories to the search path (see <xref
451 linkend="options-finding-imports">).</para>
455 <Sect1 id="options-order">
456 <title>GHC without <option>––make</option></title>
458 <para>Without <option>––make</option>, GHC will compile one or
459 more source files given on the command line.</para>
461 <para>The first phase to run is determined by each input-file
462 suffix, and the last phase is determined by a flag. If no
463 relevant flag is present, then go all the way through linking.
464 This table summarises:</para>
468 <colspec align="left">
469 <colspec align="left">
470 <colspec align="left">
471 <colspec align="left">
475 <entry>Phase of the compilation system</entry>
476 <entry>Suffix saying “start here”</entry>
477 <entry>Flag saying “stop after”</entry>
478 <entry>(suffix of) output file</entry>
483 <entry>literate pre-processor</entry>
484 <entry><literal>.lhs</literal></entry>
486 <entry><literal>.hs</literal></entry>
490 <entry>C pre-processor (opt.)
492 <entry><literal>.hs</literal> (with
493 <option>-cpp</option>)</entry>
494 <entry><option>-E</option></entry>
495 <entry><literal>.hspp</literal></entry>
499 <entry>Haskell compiler</entry>
500 <entry><literal>.hs</literal></entry>
501 <entry><option>-C</option>, <option>-S</option></entry>
502 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
506 <entry>C compiler (opt.)</entry>
507 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
508 <entry><option>-S</option></entry>
509 <entry><literal>.s</literal></entry>
513 <entry>assembler</entry>
514 <entry><literal>.s</literal></entry>
515 <entry><option>-c</option></entry>
516 <entry><literal>.o</literal></entry>
520 <entry>linker</entry>
521 <entry><replaceable>other</replaceable></entry>
523 <entry><filename>a.out</filename></entry>
529 <indexterm><primary><option>-C</option></primary></indexterm>
530 <indexterm><primary><option>-E</option></primary></indexterm>
531 <indexterm><primary><option>-S</option></primary></indexterm>
532 <indexterm><primary><option>-c</option></primary></indexterm>
534 <para>Thus, a common invocation would be: <literal>ghc -c
535 Foo.hs</literal></para>
537 <para>Note: What the Haskell compiler proper produces depends on
538 whether a native-code generator<indexterm><primary>native-code
539 generator</primary></indexterm> is used (producing assembly
540 language) or not (producing C). See <xref
541 linkend="options-codegen"> for more details.</para>
543 <para>Note: C pre-processing is optional, the
544 <option>-ccp</option><indexterm><primary><option>-cpp</option></primary>
545 </indexterm>flag turns it on. See <xref
546 linkend="c-pre-processor"> for more details.</para>
548 <para>Note: The option <option>-E</option><IndexTerm><Primary>-E
549 option</Primary></IndexTerm> runs just the pre-processing passes
550 of the compiler, dumping the result in a file. Note that this
551 differs from the previous behaviour of dumping the file to
552 standard output.</para>
555 <sect1 id="options-output">
556 <title>Re-directing the compilation output(s)</title>
558 <indexterm><primary>output-directing options</primary></indexterm>
559 <indexterm><primary>redirecting compilation output</primary></indexterm>
564 <term><option>-o</option></term>
565 <indexterm><primary><option>-o</option></primary></indexterm>
567 <para>GHC's compiled output normally goes into a
568 <filename>.hc</filename>, <filename>.o</filename>, etc.,
569 file, depending on the last-run compilation phase. The
570 option <option>-o foo</option><IndexTerm><Primary>-o
571 option</Primary></IndexTerm> re-directs the output of that
572 last-run phase to file <filename>foo</filename>.</para>
574 <para>Note: this “feature” can be
575 counterintuitive: <command>ghc -C -o foo.o foo.hs</command>
576 will put the intermediate C code in the file
577 <filename>foo.o</filename>, name notwithstanding!</para>
579 <para>Note: on Windows, if the result is an executable file, the
580 extension "<filename>.exe</filename>" is added if the specified filename
581 does not already have an extension. Thus
585 will compile and link the module <filename>Main.hs</filename>, and put the
586 resulting executable in <filename>foo.exe</filename> (not <filename>foo</filename>).
592 <term><option>-odir</option></term>
593 <indexterm><primary><option>-odir</option></primary></indexterm>
595 <para>The <option>-o</option> option isn't of much use if
596 you have <emphasis>several</emphasis> input files…
597 Non-interface output files are normally put in the same
598 directory as their corresponding input file came from. You
599 may specify that they be put in another directory using the
600 <option>-odir <dir></option><IndexTerm><Primary>-odir
601 <dir> option</Primary></IndexTerm> (the “Oh,
602 dear” option). For example:</para>
605 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
608 <para>The output files, <filename>Foo.o</filename>,
609 <filename>Bar.o</filename>, and
610 <filename>Bumble.o</filename> would be put into a
611 subdirectory named after the architecture of the executing
612 machine (<filename>sun4</filename>,
613 <filename>mips</filename>, etc). The directory must already
614 exist; it won't be created.</para>
616 <para>Note that the <option>-odir</option> option does
617 <emphasis>not</emphasis> affect where the interface files
618 are put. In the above example, they would still be put in
619 <filename>parse/Foo.hi</filename>,
620 <filename>parse/Bar.hi</filename>, and
621 <filename>gurgle/Bumble.hi</filename>.</para>
626 <term><option>-ohi</option> <replaceable>file</replaceable></term>
627 <indexterm><primary><option>-ohi</option></primary>
630 <para>The interface output may be directed to another file
631 <filename>bar2/Wurble.iface</filename> with the option
632 <option>-ohi bar2/Wurble.iface</option> (not
635 <para>WARNING: if you redirect the interface file somewhere
636 that GHC can't find it, then the recompilation checker may
637 get confused (at the least, you won't get any recompilation
638 avoidance). We recommend using a combination of
639 <option>-hidir</option> and <option>-hisuf</option> options
640 instead, if possible.</para>
642 <para>To avoid generating an interface at all, you could use
643 this option to redirect the interface into the bit bucket:
644 <literal>-ohi /dev/null</literal>, for example.</para>
649 <term><option>-hidir</option> <replaceable>directory</replaceable></term>
650 <indexterm><primary><option>-hidir</option></primary>
653 <para>Redirects all generated interface files into
654 <replaceable>directory</replaceable>, instead of the default
655 which is to place the interface file in the same directory
656 as the source file.</para>
661 <term><option>-osuf</option> <replaceable>suffix</replaceable></term>
662 <term><option>-hisuf</option> <replaceable>suffix</replaceable></term>
663 <term><option>-hcsuf</option> <replaceable>suffix</replaceable></term>
664 <indexterm><primary><option>-osuf</option></primary></indexterm>
665 <indexterm><primary><option>-hisuf</option></primary></indexterm>
666 <indexterm><primary><option>-hcsuf</option></primary></indexterm>
668 <para>EXOTICA: The <option>-osuf</option>
669 <replaceable>suffix</replaceable> will change the
670 <literal>.o</literal> file suffix for object files to
671 whatever you specify. We use this when compiling libraries,
672 so that objects for the profiling versions of the libraries
673 don't clobber the normal ones.</para>
675 <para>Similarly, the <option>-hisuf</option>
676 <replaceable>suffix</replaceable> will change the
677 <literal>.hi</literal> file suffix for non-system interface
678 files (see <XRef LinkEnd="hi-options">).</para>
680 <para>Finally, the option <option>-hcsuf</option>
681 <replaceable>suffix</replaceable> will change the
682 <literal>.hc</literal> file suffix for compiler-generated
683 intermediate C files.</para>
685 <para>The <option>-hisuf</option>/<option>-osuf</option>
686 game is particularly useful if you want to compile a program both with and without
687 profiling, in the same directory. You can say:
691 to get the ordinary version, and
693 ghc ... -osuf prof.o -hisuf prof.hi -prof -auto-all
695 to get the profiled version.</para>
700 <sect2 id="keeping-intermediates">
701 <title>Keeping Intermediate Files</title>
702 <indexterm><primary>intermediate files, saving</primary>
704 <indexterm><primary><literal>.hc</literal> files, saving</primary>
706 <indexterm><primary><literal>.s</literal> files, saving</primary>
710 <para>The following options are useful for keeping certain
711 intermediate files around, when normally GHC would throw these
712 away after compilation:</para>
716 <term><option>-keep-hc-files</option></term>
718 <primary><option>-keep-hc-files</option></primary>
721 <para>Keep intermediate <literal>.hc</literal> files when
722 doing <literal>.hs</literal>-to-<literal>.o</literal>
723 compilations via C (NOTE: <literal>.hc</literal> files
724 aren't generated when using the native code generator, you
725 may need to use <option>-fvia-C</option> to force them
726 to be produced).</para>
731 <term><option>-keep-s-files</option></term>
733 <primary><option>-keep-s-files</option></primary>
736 <para>Keep intermediate <literal>.s</literal> files.</para>
741 <term><option>-keep-raw-s-files</option></term>
743 <primary><option>-keep-raw-s-files</option></primary>
746 <para>Keep intermediate <literal>.raw-s</literal> files.
747 These are the direct output from the C compiler, before
748 GHC does “assembly mangling” to produce the
749 <literal>.s</literal> file. Again, these are not produced
750 when using the native code generator.</para>
755 <term><option>-keep-tmp-files</option></term>
757 <primary><option>-keep-tmp-files</option></primary>
760 <primary>temporary files</primary>
761 <secondary>keeping</secondary>
764 <para>Instructs the GHC driver not to delete any of its
765 temporary files, which it normally keeps in
766 <literal>/tmp</literal> (or possibly elsewhere; see <xref
767 linkend="temp-files">). Running GHC with
768 <option>-v</option> will show you what temporary files
769 were generated along the way.</para>
775 <sect2 id="temp-files">
776 <title>Redirecting temporary files</title>
779 <primary>temporary files</primary>
780 <secondary>redirecting</secondary>
785 <term><option>-tmpdir</option></term>
786 <indexterm><primary><option>-tmpdir</option></primary></indexterm>
788 <para>If you have trouble because of running out of space
789 in <filename>/tmp</filename> (or wherever your
790 installation thinks temporary files should go), you may
791 use the <option>-tmpdir
792 <dir></option><IndexTerm><Primary>-tmpdir
793 <dir> option</Primary></IndexTerm> option to specify
794 an alternate directory. For example, <option>-tmpdir
795 .</option> says to put temporary files in the current
796 working directory.</para>
798 <para>Alternatively, use your <Constant>TMPDIR</Constant>
799 environment variable.<IndexTerm><Primary>TMPDIR
800 environment variable</Primary></IndexTerm> Set it to the
801 name of the directory where temporary files should be put.
802 GCC and other programs will honour the
803 <Constant>TMPDIR</Constant> variable as well.</para>
805 <para>Even better idea: Set the
806 <Constant>DEFAULT_TMPDIR</Constant> make variable when
807 building GHC, and never worry about
808 <Constant>TMPDIR</Constant> again. (see the build
809 documentation).</para>
817 <sect1 id="options-sanity">
818 <title>Warnings and sanity-checking</title>
820 <indexterm><primary>sanity-checking options</primary></indexterm>
821 <indexterm><primary>warnings</primary></indexterm>
824 <para>GHC has a number of options that select which types of
825 non-fatal error messages, otherwise known as warnings, can be
826 generated during compilation. By default, you get a standard set
827 of warnings which are generally likely to indicate bugs in your
829 <option>-fwarn-overlapping-patterns</option>,
830 <option>-fwarn-deprecations</option>,
831 <option>-fwarn-duplicate-exports</option>,
832 <option>-fwarn-missing-fields</option>, and
833 <option>-fwarn-missing-methods</option>. The following flags are
834 simple ways to select standard “packages” of warnings:
840 <term><option>-W</option>:</term>
842 <IndexTerm><Primary>-W option</Primary></IndexTerm>
843 <para>Provides the standard warnings plus
844 <option>-fwarn-incomplete-patterns</option>,
845 <option>-fwarn-unused-matches</option>,
846 <option>-fwarn-unused-imports</option>,
847 <option>-fwarn-misc</option>, and
848 <option>-fwarn-unused-binds</option>.</para>
853 <term><option>-w</option>:</term>
855 <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
856 <para>Turns off all warnings, including the standard ones.</para>
861 <term><option>-Wall</option>:</term>
863 <indexterm><primary><option>-Wall</option></primary></indexterm>
864 <para>Turns on all warning options.</para>
869 <term><option>-Werror</option>:</term>
871 <indexterm><primary><option>-Werror</option></primary></indexterm>
872 <para>Makes any warning into a fatal error. Useful so that you don't
873 miss warnings when doing batch compilation. </para>
879 <para>The full set of warning options is described below. To turn
880 off any warning, simply give the corresponding
881 <option>-fno-warn-...</option> option on the command line.</para>
886 <term><option>-fwarn-deprecations</option>:</term>
888 <indexterm><primary><option>-fwarn-deprecations</option></primary>
890 <indexterm><primary>deprecations</primary></indexterm>
891 <para>Causes a warning to be emitted when a deprecated
892 function or type is used. Entities can be marked as
893 deprecated using a pragma, see <xref
894 linkend="deprecated-pragma">.</para>
899 <term><option>-fwarn-duplicate-exports</option>:</term>
901 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
902 <indexterm><primary>duplicate exports, warning</primary></indexterm>
903 <indexterm><primary>export lists, duplicates</primary></indexterm>
905 <para>Have the compiler warn about duplicate entries in
906 export lists. This is useful information if you maintain
907 large export lists, and want to avoid the continued export
908 of a definition after you've deleted (one) mention of it in
909 the export list.</para>
911 <para>This option is on by default.</para>
916 <term><option>-fwarn-hi-shadowing</option>:</term>
918 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
919 <indexterm><primary>shadowing</primary>
920 <secondary>interface files</secondary></indexterm>
922 <para>Causes the compiler to emit a warning when a module or
923 interface file in the current directory is shadowing one
924 with the same module name in a library or other
930 <term><option>-fwarn-incomplete-patterns</option>:</term>
932 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
933 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
934 <indexterm><primary>patterns, incomplete</primary></indexterm>
936 <para>Similarly for incomplete patterns, the function
937 <function>g</function> below will fail when applied to
938 non-empty lists, so the compiler will emit a warning about
939 this when <option>-fwarn-incomplete-patterns</option> is
946 <para>This option isn't enabled be default because it can be
947 a bit noisy, and it doesn't always indicate a bug in the
948 program. However, it's generally considered good practice
949 to cover all the cases in your functions.</para>
954 <term><option>-fwarn-misc</option>:</term>
955 <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
957 <para>Turns on warnings for various harmless but untidy
958 things. This currently includes: importing a type with
959 <literal>(..)</literal> when the export is abstract, and
960 listing duplicate class assertions in a qualified type.</para>
965 <term><option>-fwarn-missing-fields</option>:</term>
967 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
968 <indexterm><primary>missing fields, warning</primary></indexterm>
969 <indexterm><primary>fields, missing</primary></indexterm>
971 <para>This option is on by default, and warns you whenever
972 the construction of a labelled field constructor isn't
973 complete, missing initializers for one or more fields. While
974 not an error (the missing fields are initialised with
975 bottoms), it is often an indication of a programmer error.</para>
980 <term><option>-fwarn-missing-methods</option>:</term>
982 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
983 <indexterm><primary>missing methods, warning</primary></indexterm>
984 <indexterm><primary>methods, missing</primary></indexterm>
986 <para>This option is on by default, and warns you whenever
987 an instance declaration is missing one or more methods, and
988 the corresponding class declaration has no default
989 declaration for them.</para>
990 <para>The warning is suppressed if the method name
991 begins with an underscore. Here's an example where this is useful:
994 _simpleFn :: a -> String
995 complexFn :: a -> a -> String
996 complexFn x y = ... _simpleFn ...
998 The idea is that: (a) users of the class will only call <literal>complexFn</literal>;
999 never <literal>_simpleFn</literal>; and (b)
1000 instance declarations can define either <literal>complexFn</literal> or <literal>_simpleFn</literal>.
1006 <term><option>-fwarn-missing-signatures</option>:</term>
1008 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
1009 <indexterm><primary>type signatures, missing</primary></indexterm>
1011 <para>If you would like GHC to check that every top-level
1012 function/value has a type signature, use the
1013 <option>-fwarn-missing-signatures</option> option. This
1014 option is off by default.</para>
1019 <term><option>-fwarn-name-shadowing</option>:</term>
1021 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
1022 <indexterm><primary>shadowing, warning</primary></indexterm>
1024 <para>This option causes a warning to be emitted whenever an
1025 inner-scope value has the same name as an outer-scope value,
1026 i.e. the inner value shadows the outer one. This can catch
1027 typographical errors that turn into hard-to-find bugs, e.g.,
1028 in the inadvertent cyclic definition <literal>let x = ... x
1029 ... in</literal>.</para>
1031 <para>Consequently, this option does
1032 <emphasis>will</emphasis> complain about cyclic recursive
1038 <term><option>-fwarn-overlapping-patterns</option>:</term>
1039 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1040 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1041 <indexterm><primary>patterns, overlapping</primary></indexterm>
1043 <para>By default, the compiler will warn you if a set of
1044 patterns are overlapping, i.e.,</para>
1047 f :: String -> Int
1053 <para>where the last pattern match in <Function>f</Function>
1054 won't ever be reached, as the second pattern overlaps
1055 it. More often than not, redundant patterns is a programmer
1056 mistake/error, so this option is enabled by default.</para>
1061 <term><option>-fwarn-simple-patterns</option>:</term>
1063 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1065 <para>Causes the compiler to warn about lambda-bound
1066 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1067 Normally, these aren't treated as incomplete patterns by
1068 <option>-fwarn-incomplete-patterns</option>.</para>
1073 <term><option>-fwarn-type-defaults</option>:</term>
1075 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1076 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1077 <para>Have the compiler warn/inform you where in your source
1078 the Haskell defaulting mechanism for numeric types kicks
1079 in. This is useful information when converting code from a
1080 context that assumed one default into one with another,
1081 e.g., the `default default' for Haskell 1.4 caused the
1082 otherwise unconstrained value <Constant>1</Constant> to be
1083 given the type <literal>Int</literal>, whereas Haskell 98
1084 defaults it to <literal>Integer</literal>. This may lead to
1085 differences in performance and behaviour, hence the
1086 usefulness of being non-silent about this.</para>
1088 <para>This warning is off by default.</para>
1093 <term><option>-fwarn-unused-binds</option>:</term>
1095 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1096 <indexterm><primary>unused binds, warning</primary></indexterm>
1097 <indexterm><primary>binds, unused</primary></indexterm>
1098 <para>Report any function definitions (and local bindings)
1099 which are unused. For top-level functions, the warning is
1100 only given if the binding is not exported.</para>
1105 <term><option>-fwarn-unused-imports</option>:</term>
1107 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1108 <indexterm><primary>unused imports, warning</primary></indexterm>
1109 <indexterm><primary>imports, unused</primary></indexterm>
1111 <para>Report any objects that are explicitly imported but
1117 <term><option>-fwarn-unused-matches</option>:</term>
1119 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1120 <indexterm><primary>unused matches, warning</primary></indexterm>
1121 <indexterm><primary>matches, unused</primary></indexterm>
1123 <para>Report all unused variables which arise from pattern
1124 matches, including patterns consisting of a single variable.
1125 For instance <literal>f x y = []</literal> would report
1126 <VarName>x</VarName> and <VarName>y</VarName> as unused. The
1127 warning is suppressed if the variable name begins with an underscore, thus:
1137 <para>If you're feeling really paranoid, the
1138 <option>-dcore-lint</option>
1139 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1140 is a good choice. It turns on heavyweight intra-pass
1141 sanity-checking within GHC. (It checks GHC's sanity, not
1149 <sect1 id="options-optimise">
1150 <title>Optimisation (code improvement)</title>
1152 <indexterm><primary>optimisation</primary></indexterm>
1153 <indexterm><primary>improvement, code</primary></indexterm>
1155 <para>The <option>-O*</option> options specify convenient
1156 “packages” of optimisation flags; the
1157 <option>-f*</option> options described later on specify
1158 <emphasis>individual</emphasis> optimisations to be turned on/off;
1159 the <option>-m*</option> options specify
1160 <emphasis>machine-specific</emphasis> optimisations to be turned
1163 <sect2 id="optimise-pkgs">
1164 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1166 <para>There are <emphasis>many</emphasis> options that affect
1167 the quality of code produced by GHC. Most people only have a
1168 general goal, something like “Compile quickly” or
1169 “Make my program run like greased lightning.” The
1170 following “packages” of optimisations (or lack
1171 thereof) should suffice.</para>
1173 <para>Once you choose a <option>-O*</option>
1174 “package,” stick with it—don't chop and
1175 change. Modules' interfaces <emphasis>will</emphasis> change
1176 with a shift to a new <option>-O*</option> option, and you may
1177 have to recompile a large chunk of all importing modules before
1178 your program can again be run safely (see <XRef
1179 LinkEnd="recomp">).</para>
1184 <term>No <option>-O*</option>-type option specified:</term>
1185 <indexterm><primary>-O* not specified</primary></indexterm>
1187 <para>This is taken to mean: “Please compile
1188 quickly; I'm not over-bothered about compiled-code
1189 quality.” So, for example: <command>ghc -c
1190 Foo.hs</command></para>
1195 <term><option>-O0</option>:</term>
1196 <indexterm><primary><option>-O0</option></primary></indexterm>
1198 <para>Means “turn off all optimisation”,
1199 reverting to the same settings as if no
1200 <option>-O</option> options had been specified. Saying
1201 <option>-O0</option> can be useful if
1202 eg. <command>make</command> has inserted a
1203 <option>-O</option> on the command line already.</para>
1208 <term><option>-O</option> or <option>-O1</option>:</term>
1209 <indexterm><primary>-O option</primary></indexterm>
1210 <indexterm><primary>-O1 option</primary></indexterm>
1211 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1213 <para>Means: “Generate good-quality code without
1214 taking too long about it.” Thus, for example:
1215 <command>ghc -c -O Main.lhs</command></para>
1220 <term><option>-O2</option>:</term>
1221 <indexterm><primary>-O2 option</primary></indexterm>
1222 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1224 <para>Means: “Apply every non-dangerous
1225 optimisation, even if it means significantly longer
1226 compile times.”</para>
1228 <para>The avoided “dangerous” optimisations
1229 are those that can make runtime or space
1230 <emphasis>worse</emphasis> if you're unlucky. They are
1231 normally turned on or off individually.</para>
1233 <para>At the moment, <option>-O2</option> is
1234 <emphasis>unlikely</emphasis> to produce better code than
1235 <option>-O</option>.</para>
1240 <term><option>-Ofile <file></option>:</term>
1241 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1242 <indexterm><primary>optimising, customised</primary></indexterm>
1244 <para>(NOTE: not supported yet in GHC 5.x. Please ask if
1245 you're interested in this.)</para>
1247 <para>For those who need <emphasis>absolute</emphasis>
1248 control over <emphasis>exactly</emphasis> what options are
1249 used (e.g., compiler writers, sometimes :-), a list of
1250 options can be put in a file and then slurped in with
1251 <option>-Ofile</option>.</para>
1253 <para>In that file, comments are of the
1254 <literal>#</literal>-to-end-of-line variety; blank
1255 lines and most whitespace is ignored.</para>
1257 <para>Please ask if you are baffled and would like an
1258 example of <option>-Ofile</option>!</para>
1263 <para>We don't use a <option>-O*</option> flag for day-to-day
1264 work. We use <option>-O</option> to get respectable speed;
1265 e.g., when we want to measure something. When we want to go for
1266 broke, we tend to use <option>-O -fvia-C</option> (and we go for
1267 lots of coffee breaks).</para>
1269 <para>The easiest way to see what <option>-O</option> (etc.)
1270 “really mean” is to run with <option>-v</option>,
1271 then stand back in amazement.</para>
1274 <sect2 id="options-f">
1275 <title><option>-f*</option>: platform-independent flags</title>
1277 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1278 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1280 <para>These flags turn on and off individual optimisations.
1281 They are normally set via the <option>-O</option> options
1282 described above, and as such, you shouldn't need to set any of
1283 them explicitly (indeed, doing so could lead to unexpected
1284 results). However, there are one or two that may be of
1289 <term><option>-fexcess-precision</option>:</term>
1291 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1292 <para>When this option is given, intermediate floating
1293 point values can have a <emphasis>greater</emphasis>
1294 precision/range than the final type. Generally this is a
1295 good thing, but some programs may rely on the exact
1297 <literal>Float</literal>/<literal>Double</literal> values
1298 and should not use this option for their compilation.</para>
1303 <term><option>-fignore-asserts</option>:</term>
1305 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1306 <para>Causes GHC to ignore uses of the function
1307 <literal>Exception.assert</literal> in source code (in
1308 other words, rewriting <literal>Exception.assert p
1309 e</literal> to <literal>e</literal> (see <xref
1310 linkend="sec-assertions">). This flag is turned on by
1311 <option>-O</option>.
1317 <term><option>-fno-strictness</option></term>
1318 <indexterm><primary><option>-fno-strictness</option></primary>
1321 <para>Turns off the strictness analyser; sometimes it eats
1322 too many cycles.</para>
1327 <term><option>-fno-cpr-analyse</option></term>
1328 <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1331 <para>Turns off the CPR (constructed product result)
1332 analysis; it is somewhat experimental.</para>
1337 <term><option>-funbox-strict-fields</option>:</term>
1339 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1340 <indexterm><primary>strict constructor fields</primary></indexterm>
1341 <indexterm><primary>constructor fields, strict</primary></indexterm>
1343 <para>This option causes all constructor fields which are
1344 marked strict (i.e. “!”) to be unboxed or
1345 unpacked if possible. For example:</para>
1348 data T = T !Float !Float
1351 <para>will create a constructor <literal>T</literal>
1352 containing two unboxed floats if the
1353 <option>-funbox-strict-fields</option> flag is given.
1354 This may not always be an optimisation: if the
1355 <Function>T</Function> constructor is scrutinised and the
1356 floats passed to a non-strict function for example, they
1357 will have to be reboxed (this is done automatically by the
1360 <para>This option should only be used in conjunction with
1361 <option>-O</option>, in order to expose unfoldings to the
1362 compiler so the reboxing can be removed as often as
1363 possible. For example:</para>
1367 f (T f1 f2) = f1 + f2
1370 <para>The compiler will avoid reboxing
1371 <Function>f1</Function> and <Function>f2</Function> by
1372 inlining <Function>+</Function> on floats, but only when
1373 <option>-O</option> is on.</para>
1375 <para>Any single-constructor data is eligible for
1376 unpacking; for example</para>
1379 data T = T !(Int,Int)
1382 <para>will store the two <literal>Int</literal>s directly
1383 in the <Function>T</Function> constructor, by flattening
1384 the pair. Multi-level unpacking is also supported:</para>
1388 data S = S !Int !Int
1391 <para>will store two unboxed <literal>Int#</literal>s
1392 directly in the <Function>T</Function> constructor.</para>
1397 <term><option>-funfolding-update-in-place<n></option></term>
1398 <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1400 <para>Switches on an experimental "optimisation".
1401 Switching it on makes the compiler a little keener to
1402 inline a function that returns a constructor, if the
1403 context is that of a thunk.
1407 If we inlined plusInt we might get an opportunity to use
1408 update-in-place for the thunk 'x'.</para>
1413 <term><option>-funfolding-creation-threshold<n></option>:</term>
1415 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1416 <indexterm><primary>inlining, controlling</primary></indexterm>
1417 <indexterm><primary>unfolding, controlling</primary></indexterm>
1419 <para>(Default: 45) Governs the maximum size that GHC will
1420 allow a function unfolding to be. (An unfolding has a
1421 “size” that reflects the cost in terms of
1422 “code bloat” of expanding that unfolding at
1423 at a call site. A bigger function would be assigned a
1424 bigger cost.) </para>
1426 <para> Consequences: (a) nothing larger than this will be
1427 inlined (unless it has an INLINE pragma); (b) nothing
1428 larger than this will be spewed into an interface
1432 <para> Increasing this figure is more likely to result in longer
1433 compile times than faster code. The next option is more
1439 <term><option>-funfolding-use-threshold<n></option>:</term>
1441 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1442 <indexterm><primary>inlining, controlling</primary></indexterm>
1443 <indexterm><primary>unfolding, controlling</primary></indexterm>
1445 <para>(Default: 8) This is the magic cut-off figure for
1446 unfolding: below this size, a function definition will be
1447 unfolded at the call-site, any bigger and it won't. The
1448 size computed for a function depends on two things: the
1449 actual size of the expression minus any discounts that
1450 apply (see <option>-funfolding-con-discount</option>).</para>
1461 <sect1 id="sec-using-concurrent">
1462 <title>Using Concurrent Haskell</title>
1464 <indexterm><primary>Concurrent Haskell—use</primary></indexterm>
1467 GHC supports Concurrent Haskell by default, without requiring a
1468 special option or libraries compiled in a certain way. To get access
1469 to the support libraries for Concurrent Haskell, just import
1470 <literal>Control.Concurrent</literal> (details are in the accompanying
1471 library documentation).</para>
1474 RTS options are provided for modifying the behaviour of the threaded
1475 runtime system. See <XRef LinkEnd="parallel-rts-opts">.
1479 Concurrent Haskell is described in more detail in the documentation
1480 for the <literal>Control.Concurrent</literal> module.
1485 <Sect1 id="sec-using-parallel">
1486 <title>Using Parallel Haskell</title>
1489 <indexterm><primary>Parallel Haskell—use</primary></indexterm>
1493 [You won't be able to execute parallel Haskell programs unless PVM3
1494 (Parallel Virtual Machine, version 3) is installed at your site.]
1498 To compile a Haskell program for parallel execution under PVM, use the
1499 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
1500 option</Primary></IndexTerm> both when compiling <Emphasis>and
1501 linking</Emphasis>. You will probably want to <Literal>import
1502 Parallel</Literal> into your Haskell modules.
1506 To run your parallel program, once PVM is going, just invoke it
1507 “as normal”. The main extra RTS option is
1508 <Option>-qp<n></Option>, to say how many PVM
1509 “processors” your program to run on. (For more details of
1510 all relevant RTS options, please see <XRef
1511 LinkEnd="parallel-rts-opts">.)
1515 In truth, running Parallel Haskell programs and getting information
1516 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1517 PVM, detailed in the following sections.
1520 <Sect2 id="pvm-dummies">
1521 <Title>Dummy's guide to using PVM</Title>
1524 <indexterm><primary>PVM, how to use</primary></indexterm>
1525 <indexterm><primary>Parallel Haskell—PVM use</primary></indexterm>
1526 Before you can run a parallel program under PVM, you must set the
1527 required environment variables (PVM's idea, not ours); something like,
1528 probably in your <filename>.cshrc</filename> or equivalent:
1531 setenv PVM_ROOT /wherever/you/put/it
1532 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1533 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1539 Creating and/or controlling your “parallel machine” is a purely-PVM
1540 business; nothing specific to Parallel Haskell. The following paragraphs
1541 describe how to configure your parallel machine interactively.
1545 If you use parallel Haskell regularly on the same machine configuration it
1546 is a good idea to maintain a file with all machine names and to make the
1547 environment variable PVM_HOST_FILE point to this file. Then you can avoid
1548 the interactive operations described below by just saying
1556 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
1557 machine. You can then do various things to control/monitor your
1558 “parallel machine;” the most useful being:
1564 <ColSpec Align="Left">
1568 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1569 <entry>exit <command>pvm</command>, leaving it running</entry>
1573 <entry><command>halt</command></entry>
1574 <entry>kill off this “parallel machine” & exit</entry>
1578 <entry><command>add <host></command></entry>
1579 <entry>add <command><host></command> as a processor</entry>
1583 <entry><command>delete <host></command></entry>
1584 <entry>delete <command><host></command></entry>
1588 <entry><command>reset</command></entry>
1589 <entry>kill what's going, but leave PVM up</entry>
1593 <entry><command>conf</command></entry>
1594 <entry>list the current configuration</entry>
1598 <entry><command>ps</command></entry>
1599 <entry>report processes' status</entry>
1603 <entry><command>pstat <pid></command></entry>
1604 <entry>status of a particular process</entry>
1613 The PVM documentation can tell you much, much more about <command>pvm</command>!
1618 <Sect2 id="par-profiles">
1619 <Title>Parallelism profiles</Title>
1622 <indexterm><primary>parallelism profiles</primary></indexterm>
1623 <indexterm><primary>profiles, parallelism</primary></indexterm>
1624 <indexterm><primary>visualisation tools</primary></indexterm>
1628 With Parallel Haskell programs, we usually don't care about the
1629 results—only with “how parallel” it was! We want pretty pictures.
1633 Parallelism profiles (à la <Command>hbcpp</Command>) can be generated with the
1634 <Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option. The
1635 per-processor profiling info is dumped into files named
1636 <Filename><full-path><program>.gr</Filename>. These are then munged into a PostScript picture,
1637 which you can then display. For example, to run your program
1638 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
1644 <prompt>$</prompt> ./a.out +RTS -qP -qp8
1645 <prompt>$</prompt> grs2gr *.???.gr > temp.gr # combine the 8 .gr files into one
1646 <prompt>$</prompt> gr2ps -O temp.gr # cvt to .ps; output in temp.ps
1647 <prompt>$</prompt> ghostview -seascape temp.ps # look at it!
1653 The scripts for processing the parallelism profiles are distributed
1654 in <filename>ghc/utils/parallel/</filename>.
1660 <Title>Other useful info about running parallel programs</Title>
1663 The “garbage-collection statistics” RTS options can be useful for
1664 seeing what parallel programs are doing. If you do either
1665 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
1666 you'll get mutator, garbage-collection, etc., times on standard
1667 error. The standard error of all PE's other than the `main thread'
1668 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1672 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1673 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1678 <Sect2 id="parallel-rts-opts">
1679 <title>RTS options for Concurrent/Parallel Haskell
1683 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1684 <indexterm><primary>RTS options, parallel</primary></indexterm>
1685 <indexterm><primary>Concurrent Haskell—RTS options</primary></indexterm>
1686 <indexterm><primary>Parallel Haskell—RTS options</primary></indexterm>
1690 Besides the usual runtime system (RTS) options
1691 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1692 for concurrent/parallel execution.
1699 <Term><Option>-qp<N></Option>:</Term>
1702 <IndexTerm><Primary>-qp<N> RTS option</Primary></IndexTerm>
1703 (PARALLEL ONLY) Use <Literal><N></Literal> PVM processors to run this program;
1709 <term><option>-C[<us>]</option>:</term>
1712 <indexterm><primary>-C<us> RTS option</primary></indexterm> Sets
1713 the context switch interval to <literal><s></literal> seconds.
1714 A context switch will occur at the next heap block allocation after
1715 the timer expires (a heap block allocation occurs every 4k of
1716 allocation). With <option>-C0</option> or <option>-C</option>,
1717 context switches will occur as often as possible (at every heap block
1718 allocation). By default, context switches occur every 20ms
1719 milliseconds. Note that GHC's internal timer ticks every 20ms, and
1720 the context switch timer is always a multiple of this timer, so 20ms
1721 is the maximum granularity available for timed context switches.
1726 <term><option>-q[v]</option>:</term>
1729 <indexterm><primary>-q RTS option</primary></indexterm>
1730 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1731 in the file <FIlename><program>.qp</FIlename>. In the style of <command>hbcpp</command>, this profile
1732 records the movement of threads between the green (runnable) and red
1733 (blocked) queues. If you specify the verbose suboption (<option>-qv</option>), the
1734 green queue is split into green (for the currently running thread
1735 only) and amber (for other runnable threads). We do not recommend
1736 that you use the verbose suboption if you are planning to use the
1737 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
1738 check (with <Option>-C</Option>).
1744 <Term><Option>-qt<num></Option>:</Term>
1747 <IndexTerm><Primary>-qt<num> RTS option</Primary></IndexTerm>
1748 (PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
1749 threads per processor to <Literal><num></Literal>. The default is
1750 32. Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
1751 the heap for thread state and stack objects. (For 32-bit machines, this
1752 translates to 4K bytes, and for 64-bit machines, 8K bytes.)
1758 <Term><Option>-d</Option>:</Term>
1761 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
1762 (PARALLEL ONLY) Turn on debugging. It pops up one xterm (or GDB, or
1763 something…) per PVM processor. We use the standard <Command>debugger</Command>
1764 script that comes with PVM3, but we sometimes meddle with the
1765 <Command>debugger2</Command> script. We include ours in the GHC distribution,
1766 in <Filename>ghc/utils/pvm/</Filename>.
1772 <Term><Option>-qe<num></Option>:</Term>
1775 <IndexTerm><Primary>-qe<num> RTS option
1776 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
1777 i.e. the number of pending sparks per processor to
1778 <Literal><num></Literal>. The default is 100. A larger number may be
1779 appropriate if your program generates large amounts of parallelism
1785 <Term><Option>-qQ<num></Option>:</Term>
1788 <IndexTerm><Primary>-qQ<num> RTS option (parallel)</Primary></IndexTerm>
1789 (PARALLEL ONLY) Set the size of packets transmitted between processors
1790 to <Literal><num></Literal>. The default is 1024 words. A larger number may be
1791 appropriate if your machine has a high communication cost relative to
1797 <Term><Option>-qh<num></Option>:</Term>
1800 <IndexTerm><Primary>-qh<num> RTS option (parallel)</Primary></IndexTerm>
1801 (PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
1802 <num>-1 (0 means infinity). By default GUM uses full-subgraph
1803 packing, i.e. the entire subgraph with the requested closure as root is
1804 transmitted (provided it fits into one packet). Choosing a smaller value
1805 reduces the amount of pre-fetching of work done in GUM. This can be
1806 advantageous for improving data locality but it can also worsen the balance
1807 of the load in the system.
1812 <Term><Option>-qg<num></Option>:</Term>
1815 <IndexTerm><Primary>-qg<num> RTS option
1816 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
1817 scheme. This option affects the
1818 generation of global addresses when transferring data. Global addresses are
1819 globally unique identifiers required to maintain sharing in the distributed
1820 graph structure. Currently this is a binary option. With <num>=0 full globalisation is used
1821 (default). This means a global address is generated for every closure that
1822 is transmitted. With <num>=1 a thunk-only globalisation scheme is
1823 used, which generated global address only for thunks. The latter case may
1824 lose sharing of data but has a reduced overhead in packing graph structures
1825 and maintaining internal tables of global addresses.
1836 <sect1 id="options-platform">
1837 <title>Platform-specific Flags</title>
1839 <indexterm><primary>-m* options</primary></indexterm>
1840 <indexterm><primary>platform-specific options</primary></indexterm>
1841 <indexterm><primary>machine-specific options</primary></indexterm>
1843 <para>Some flags only make sense for particular target
1849 <term><option>-mv8</option>:</term>
1851 <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1852 only)</primary></indexterm> Means to pass the like-named
1853 option to GCC; it says to use the Version 8 SPARC
1854 instructions, notably integer multiply and divide. The
1855 similiar <option>-m*</option> GCC options for SPARC also
1856 work, actually.</para>
1861 <term><option>-monly-[32]-regs</option>:</term>
1863 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1864 option (iX86 only)</primary></indexterm> GHC tries to
1865 “steal” four registers from GCC, for performance
1866 reasons; it almost always works. However, when GCC is
1867 compiling some modules with four stolen registers, it will
1868 crash, probably saying:
1871 Foo.hc:533: fixed or forbidden register was spilled.
1872 This may be due to a compiler bug or to impossible asm
1873 statements or clauses.
1876 Just give some registers back with
1877 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1878 If `2' doesn't work, please report the bug to us.</para>
1887 <sect1 id="ext-core">
1888 <title>Generating External Core Files</title>
1890 <indexterm><primary>intermediate code generation</primary></indexterm>
1892 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1893 to a file as a side-effect of compilation. Core files, which are given the suffix
1894 <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1895 tools. The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz"
1896 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1897 and sample tools (in Haskell)
1898 for manipulating Core files are available in the GHC source distribution
1899 directory <literal>/fptools/ghc/utils/ext-core</literal>.
1900 Note that the format of <literal>.hcr</literal>
1901 files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1902 for debugging purposes (<xref linkend="options-debugging">).</para>
1907 <term><option>-fext-core</option></term>
1909 <primary><option>-fext-core</option></primary>
1912 <para>Generate <literal>.hcr</literal> files.</para>
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