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
37 likely to be much easier, and faster, than using
38 <command>make</command>.</para>
44 <command>ghc</command>
52 <indexterm><primary><option>--make</option></primary>
55 <para>This is the traditional batch-compiler mode, in which
56 GHC can compile source files one at a time, or link objects
57 together into an executable.</para>
63 <title>Options overview</title>
65 <para>GHC's behaviour is controlled by
66 <firstterm>options</firstterm>, which for historical reasons are
67 also sometimes referred to as command-line flags or arguments.
68 Options can be specified in three ways:</para>
71 <title>Command-line arguments</title>
73 <indexterm><primary>structure, command-line</primary></indexterm>
74 <indexterm><primary>command-line</primary><secondary>arguments</secondary></indexterm>
75 <indexterm><primary>arguments</primary><secondary>command-line</secondary></indexterm>
77 <para>An invocation of GHC takes the following form:</para>
83 <para>Command-line arguments are either options or file names.</para>
85 <para>Command-line options begin with <literal>-</literal>.
86 They may <emphasis>not</emphasis> be grouped:
87 <option>-vO</option> is different from <option>-v -O</option>.
88 Options need not precede filenames: e.g., <literal>ghc *.o -o
89 foo</literal>. All options are processed and then applied to
90 all files; you cannot, for example, invoke <literal>ghc -c -O1
91 Foo.hs -O2 Bar.hs</literal> to apply different optimisation
92 levels to the files <filename>Foo.hs</filename> and
93 <filename>Bar.hs</filename>.</para>
96 <Sect2 id="source-file-options">
97 <title>Command line options in source files</title>
99 <indexterm><primary>source-file options</primary></indexterm>
101 <para>Sometimes it is useful to make the connection between a
102 source file and the command-line options it requires quite
103 tight. For instance, if a Haskell source file uses GHC
104 extensions, it will always need to be compiled with the
105 <option>-fglasgow-exts</option> option. Rather than maintaining
106 the list of per-file options in a <filename>Makefile</filename>,
107 it is possible to do this directly in the source file using the
108 <literal>OPTIONS</literal> pragma <indexterm><primary>OPTIONS
109 pragma</primary></indexterm>:</para>
112 {-# OPTIONS -fglasgow-exts #-}
117 <para><literal>OPTIONS</literal> pragmas are only looked for at
118 the top of your source files, upto the first
119 (non-literate,non-empty) line not containing
120 <literal>OPTIONS</literal>. Multiple <literal>OPTIONS</literal>
121 pragmas are recognised. Note that your command shell does not
122 get to the source file options, they are just included literally
123 in the array of command-line arguments the compiler driver
124 maintains internally, so you'll be desperately disappointed if
125 you try to glob etc. inside <literal>OPTIONS</literal>.</para>
127 <para>NOTE: the contents of OPTIONS are prepended to the
128 command-line options, so you <emphasis>do</emphasis> have the
129 ability to override OPTIONS settings via the command
132 <para>It is not recommended to move all the contents of your
133 Makefiles into your source files, but in some circumstances, the
134 <literal>OPTIONS</literal> pragma is the Right Thing. (If you
135 use <option>-keep-hc-file-too</option> and have OPTION flags in
136 your module, the OPTIONS will get put into the generated .hc
141 <title>Setting options in GHCi</title>
143 <para>Options may also be modified from within GHCi, using the
144 <literal>:set</literal> command. See <xref linkend="ghci-set">
145 for more details.</para>
149 <sect1 id="static-dynamic-flags">
150 <title>Static vs. Dynamic options</title>
151 <indexterm><primary>static</primary><secondary>options</secondary>
153 <indexterm><primary>dynamic</primary><secondary>options</secondary>
156 <para>Each of GHC's command line options is classified as either
157 <firstterm>static</firstterm> or <firstterm>dynamic</firstterm>.
158 A static flag may only be specified on the command line, whereas a
159 dynamic flag may also be given in an <literal>OPTIONS</literal>
160 pragma in a source file or set from the GHCi command-line with
161 <literal>:set</literal>.</para>
163 <para>As a rule of thumb, all the language options are dynamic, as
164 are the warning options and the debugging options. The rest are
165 static, with the notable exceptions of <option>-v</option>,
166 <option>-cpp</option>, <option>-fasm</option>,
167 <option>-fvia-C</option>, and <option>-#include</option>.
169 The flag reference tables (<xref linkend="flag-reference">) lists
170 the status of each flag.</para>
173 <sect1 id="file-suffixes">
174 <title>Meaningful file suffixes</title>
176 <indexterm><primary>suffixes, file</primary></indexterm>
177 <indexterm><primary>file suffixes for GHC</primary></indexterm>
179 <para>File names with “meaningful” suffixes (e.g.,
180 <filename>.lhs</filename> or <filename>.o</filename>) cause the
181 “right thing” to happen to those files.</para>
186 <term><filename>.lhs</filename></term>
187 <indexterm><primary><literal>lhs</literal> suffix</primary></indexterm>
189 <para>A “literate Haskell” module.</para>
194 <term><filename>.hs</filename></term>
196 <para>A not-so-literate Haskell module.</para>
201 <term><filename>.hi</filename></term>
203 <para>A Haskell interface file, probably
204 compiler-generated.</para>
209 <term><filename>.hc</filename></term>
211 <para>Intermediate C file produced by the Haskell
217 <term><filename>.c</filename></term>
219 <para>A C file not produced by the Haskell
225 <term><filename>.s</filename></term>
227 <para>An assembly-language source file, usually produced by
233 <term><filename>.o</filename></term>
235 <para>An object file, produced by an assembler.</para>
240 <para>Files with other suffixes (or without suffixes) are passed
241 straight to the linker.</para>
245 <sect1 id="options-help">
246 <title>Help and verbosity options</title>
248 <IndexTerm><Primary>help options</Primary></IndexTerm>
249 <IndexTerm><Primary>verbosity options</Primary></IndexTerm>
253 <term><literal>-help</literal></term>
254 <term><literal>-?</literal></term>
255 <indexterm><primary><literal>-?</literal></primary></indexterm>
256 <indexterm><primary><literal>-help</literal></primary></indexterm>
258 <para>Cause GHC to spew a long usage message to standard
259 output and then exit.</para>
264 <term><literal>-v</literal></term>
265 <indexterm><primary><literal>-v</literal></primary></indexterm>
267 <para>The <option>-v</option> option makes GHC
268 <emphasis>verbose</emphasis>: it reports its version number
269 and shows (on stderr) exactly how it invokes each phase of
270 the compilation system. Moreover, it passes the
271 <option>-v</option> flag to most phases; each reports its
272 version number (and possibly some other information).</para>
274 <para>Please, oh please, use the <option>-v</option> option
275 when reporting bugs! Knowing that you ran the right bits in
276 the right order is always the first thing we want to
282 <term><literal>-v</literal><replaceable>n</replaceable></term>
283 <indexterm><primary><option>-v</option></primary></indexterm>
285 <para>To provide more control over the compiler's verbosity,
286 the <option>-v</option> flag takes an optional numeric
287 argument. Specifying <option>-v</option> on its own is
288 equivalent to <option>-v3</option>, and the other levels
289 have the following meanings:</para>
293 <term><literal>-v0</literal></term>
295 <para>Disable all non-essential messages (this is the
301 <term><literal>-v1</literal></term>
303 <para>Minimal verbosity: print one line per
304 compilation (this is the default when
305 <option>--make</option> or
306 <option>--interactive</option> is on).</para>
311 <term><literal>-v2</literal></term>
313 <para>Print the name of each compilation phase as it
314 is executed. (equivalent to
315 <option>-dshow-passes</option>).</para>
320 <term><literal>-v3</literal></term>
322 <para>The same as <option>-v2</option>, except that in
323 addition the full command line (if appropriate) for
324 each compilation phase is also printed.</para>
329 <term><literal>-v4</literal></term>
331 <para>The same as <option>-v3</option> except that the
332 intermediate program representation after each
333 compilation phase is also printed (excluding
334 preprocessed and C/assembly files).</para>
342 <term><literal>--version</literal></term>
343 <indexterm><primary><literal>--version</literal></primary></indexterm>
345 <para>Print a one-line string including GHC's version number.</para>
350 <term><literal>--numeric-version</literal></term>
351 <indexterm><primary><literal>--numeric-version</literal></primary></indexterm>
353 <para>Print GHC's numeric version number only.</para>
359 <sect1 id="make-mode">
360 <title>Using <command>ghc</command> <option>--make</option></title>
362 <indexterm><primary><option>--make</option></primary>
364 <indexterm><primary>separate compilation</primary>
367 <para>When given the <option>--make</option> option, GHC will
368 build a multi-module Haskell program by following dependencies
369 from a single root module (usually <literal>Main</literal>). For
370 example, if your <literal>Main</literal> module is in a file
371 called <filename>Main.hs</filename>, you could compile and link
372 the program like this:</para>
378 <para>The command line must contain one source file or module
379 name; GHC will figure out all the modules in the program by
380 following the imports from this initial module. It will then
381 attempt to compile each module which is out of date, and finally
382 if the top module is <literal>Main</literal>, the program
383 will also be linked into an executable.</para>
385 <para>The main advantages to using <literal>ghc --make</literal>
386 over traditional <literal>Makefile</literal>s are:</para>
390 <para>GHC doesn't have to be restarted for each compilation,
391 which means it can cache information between compilations.
392 Compiling a muli-module program with <literal>ghc
393 --make</literal> can be up to twice as fast as running
394 <literal>ghc</literal> individually on each source
398 <para>You don't have to write a
399 <literal>Makefile</literal>.</para>
401 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary>
404 <para>GHC re-calculates the dependencies each time it is
405 invoked, so the dependencies never get out of sync with the
410 <para>Any of the command-line options described in the rest of
411 this chapter can be used with <option>--make</option>, but note
412 that any options you give on the command line will apply to all
413 the source files compiled, so if you want any options to apply to
414 a single source file only, you'll need to use an
415 <literal>OPTIONS</literal> pragma (see <xref
416 linkend="source-file-options">).</para>
418 <para>If the program needs to be linked with additional objects
419 (say, some auxilliary C code), these can be specified on the
420 command line as usual.</para>
423 <Sect1 id="options-order">
424 <title>GHC without <option>--make</option></title>
426 <para>Without <option>--make</option>, GHC will compile one or
427 more source files given on the command line.</para>
429 <para>The first phase to run is determined by each input-file
430 suffix, and the last phase is determined by a flag. If no
431 relevant flag is present, then go all the way through linking.
432 This table summarises:</para>
436 <colspec align="left">
437 <colspec align="left">
438 <colspec align="left">
439 <colspec align="left">
443 <entry>Phase of the compilation system</entry>
444 <entry>Suffix saying “start here”</entry>
445 <entry>Flag saying “stop after”</entry>
446 <entry>(suffix of) output file</entry>
451 <entry>literate pre-processor</entry>
452 <entry><literal>.lhs</literal></entry>
454 <entry><literal>.hs</literal></entry>
458 <entry>C pre-processor (opt.)
460 <entry><literal>.hs</literal> (with
461 <option>-cpp</option>)</entry>
462 <entry><option>-E</option></entry>
463 <entry><literal>.hspp</literal></entry>
467 <entry>Haskell compiler</entry>
468 <entry><literal>.hs</literal></entry>
469 <entry><option>-C</option>, <option>-S</option></entry>
470 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
474 <entry>C compiler (opt.)</entry>
475 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
476 <entry><option>-S</option></entry>
477 <entry><literal>.s</literal></entry>
481 <entry>assembler</entry>
482 <entry><literal>.s</literal></entry>
483 <entry><option>-c</option></entry>
484 <entry><literal>.o</literal></entry>
488 <entry>linker</entry>
489 <entry><replaceable>other</replaceable></entry>
491 <entry><filename>a.out</filename></entry>
497 <indexterm><primary><option>-C</option></primary></indexterm>
498 <indexterm><primary><option>-E</option></primary></indexterm>
499 <indexterm><primary><option>-S</option></primary></indexterm>
500 <indexterm><primary><option>-c</option></primary></indexterm>
502 <para>Thus, a common invocation would be: <literal>ghc -c
503 Foo.hs</literal></para>
505 <para>Note: What the Haskell compiler proper produces depends on
506 whether a native-code generator<indexterm><primary>native-code
507 generator</primary></indexterm> is used (producing assembly
508 language) or not (producing C). See <xref
509 linkend="options-codegen"> for more details.</para>
511 <para>Note: C pre-processing is optional, the
512 <option>-ccp</option><indexterm><primary><option>-cpp</option></primary>
513 </indexterm>flag turns it on. See <xref
514 linkend="c-pre-processor"> for more details.</para>
516 <para>Note: The option <option>-E</option><IndexTerm><Primary>-E
517 option</Primary></IndexTerm> runs just the pre-processing passes
518 of the compiler, dumping the result in a file. Note that this
519 differs from the previous behaviour of dumping the file to
520 standard output.</para>
523 <sect1 id="options-output">
524 <title>Re-directing the compilation output(s)</title>
526 <indexterm><primary>output-directing options</primary></indexterm>
527 <indexterm><primary>redirecting compilation output</primary></indexterm>
532 <term><literal>-o</literal></term>
533 <indexterm><primary><literal>-o</literal></primary></indexterm>
535 <para>GHC's compiled output normally goes into a
536 <filename>.hc</filename>, <filename>.o</filename>, etc.,
537 file, depending on the last-run compilation phase. The
538 option <option>-o foo</option><IndexTerm><Primary>-o
539 option</Primary></IndexTerm> re-directs the output of that
540 last-run phase to file <filename>foo</filename>.</para>
542 <para>Note: this “feature” can be
543 counterintuitive: <command>ghc -C -o foo.o foo.hs</command>
544 will put the intermediate C code in the file
545 <filename>foo.o</filename>, name notwithstanding!</para>
550 <term><literal>-odir</literal></term>
551 <indexterm><primary><literal>-odir</literal></primary></indexterm>
553 <para>The <option>-o</option> option isn't of much use if
554 you have <emphasis>several</emphasis> input files…
555 Non-interface output files are normally put in the same
556 directory as their corresponding input file came from. You
557 may specify that they be put in another directory using the
558 <option>-odir <dir></option><IndexTerm><Primary>-odir
559 <dir> option</Primary></IndexTerm> (the “Oh,
560 dear” option). For example:</para>
563 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
566 <para>The output files, <filename>Foo.o</filename>,
567 <filename>Bar.o</filename>, and
568 <filename>Bumble.o</filename> would be put into a
569 subdirectory named after the architecture of the executing
570 machine (<filename>sun4</filename>,
571 <filename>mips</filename>, etc). The directory must already
572 exist; it won't be created.</para>
574 <para>Note that the <option>-odir</option> option does
575 <emphasis>not</emphasis> affect where the interface files
576 are put. In the above example, they would still be put in
577 <filename>parse/Foo.hi</filename>,
578 <filename>parse/Bar.hi</filename>, and
579 <filename>gurgle/Bumble.hi</filename>.</para>
584 <term><option>-ohi</option> <replaceable>file</replaceable></term>
585 <indexterm><primary><option>-ohi</option></primary>
588 <para>The interface output may be directed to another file
589 <filename>bar2/Wurble.iface</filename> with the option
590 <option>-ohi bar2/Wurble.iface</option> (not
593 <para>WARNING: if you redirect the interface file somewhere
594 that GHC can't find it, then the recompilation checker may
595 get confused (at the least, you won't get any recompilation
596 avoidance). We recommend using a combination of
597 <option>-hidir</option> and <option>-hisuf</option> options
598 instead, if possible.</para>
600 <para>To avoid generating an interface at all, you could use
601 this option to redirect the interface into the bit bucket:
602 <literal>-ohi /dev/null</literal>, for example.</para>
607 <term><option>-hidir</option> <replaceable>directory</replaceable></term>
608 <indexterm><primary><option>-hidir</option></primary>
611 <para>Redirects all generated interface files into
612 <replaceable>directory</replaceable>, instead of the default
613 which is to place the interface file in the same directory
614 as the source file.</para>
619 <term><option>-osuf</option> <replaceable>suffix</replaceable></term>
620 <term><option>-hisuf</option> <replaceable>suffix</replaceable></term>
621 <term><option>-hcsuf</option> <replaceable>suffix</replaceable></term>
622 <indexterm><primary><option>-osuf</option></primary></indexterm>
623 <indexterm><primary><option>-hisuf</option></primary></indexterm>
624 <indexterm><primary><option>-hcsuf</option></primary></indexterm>
626 <para>EXOTICA: The <option>-osuf</option>
627 <replaceable>suffix</replaceable> will change the
628 <literal>.o</literal> file suffix for object files to
629 whatever you specify. We use this when compiling libraries,
630 so that objects for the profiling versions of the libraries
631 don't clobber the normal ones.</para>
633 <para>Similarly, the <option>-hisuf</option>
634 <replaceable>suffix</replaceable> will change the
635 <literal>.hi</literal> file suffix for non-system interface
636 files (see <XRef LinkEnd="hi-options">).</para>
638 <para>Finally, the option <option>-hcsuf</option>
639 <replaceable>suffix</replaceable> will change the
640 <literal>.hc</literal> file suffix for compiler-generated
641 intermediate C files.</para>
643 <para>The <option>-hisuf</option>/<option>-osuf</option>
644 game is useful if you want to compile a program with both
645 GHC and HBC (say) in the same directory. Let HBC use the
646 standard <filename>.hi</filename>/<filename>.o</filename>
647 suffixes; add <option>-hisuf g_hi -osuf
648 g_o</option> to your <command>make</command> rule for
649 GHC compiling…</para>
654 <sect2 id="keeping-intermediates">
655 <title>Keeping Intermediate Files</title>
656 <indexterm><primary>intermediate files, saving</primary>
658 <indexterm><primary><literal>.hc</literal> files, saving</primary>
660 <indexterm><primary><literal>.s</literal> files, saving</primary>
664 <para>The following options are useful for keeping certain
665 intermediate files around, when normally GHC would throw these
666 away after compilation:</para>
670 <term><literal>-keep-hc-files</literal></term>
672 <primary><literal>-keep-hc-files</literal></primary>
675 <para>Keep intermediate <literal>.hc</literal> files when
676 doing <literal>.hs</literal>-to-<literal>.o</literal>
677 compilations via C (NOTE: <literal>.hc</literal> files
678 aren't generated when using the native code generator, you
679 may need to use <literal>-fvia-C</literal> to force them
680 to be produced).</para>
685 <term><literal>-keep-s-files</literal></term>
687 <primary><literal>-keep-s-files</literal></primary>
690 <para>Keep intermediate <literal>.s</literal> files.</para>
695 <term><literal>-keep-raw-s-files</literal></term>
697 <primary><literal>-keep-raw-s-files</literal></primary>
700 <para>Keep intermediate <literal>.raw-s</literal> files.
701 These are the direct output from the C compiler, before
702 GHC does “assembly mangling” to produce the
703 <literal>.s</literal> file. Again, these are not produced
704 when using the native code generator.</para>
709 <term><literal>-keep-tmp-files</literal></term>
711 <primary><literal>-keep-tmp-files</literal></primary>
714 <primary>temporary files</primary>
715 <secondary>keeping</secondary>
718 <para>Instructs the GHC driver not to delete any of its
719 temporary files, which it normally keeps in
720 <literal>/tmp</literal> (or possibly elsewhere; see <xref
721 linkend="temp-files">). Running GHC with
722 <literal>-v</literal> will show you what temporary files
723 were generated along the way.</para>
729 <sect2 id="temp-files">
730 <title>Redirecting temporary files</title>
733 <primary>temporary files</primary>
734 <secondary>redirecting</secondary>
739 <term><literal>-tmpdir</literal></term>
740 <indexterm><primary><literal>-tmpdir</literal></primary></indexterm>
742 <para>If you have trouble because of running out of space
743 in <filename>/tmp</filename> (or wherever your
744 installation thinks temporary files should go), you may
745 use the <option>-tmpdir
746 <dir></option><IndexTerm><Primary>-tmpdir
747 <dir> option</Primary></IndexTerm> option to specify
748 an alternate directory. For example, <option>-tmpdir
749 .</option> says to put temporary files in the current
750 working directory.</para>
752 <para>Alternatively, use your <Constant>TMPDIR</Constant>
753 environment variable.<IndexTerm><Primary>TMPDIR
754 environment variable</Primary></IndexTerm> Set it to the
755 name of the directory where temporary files should be put.
756 GCC and other programs will honour the
757 <Constant>TMPDIR</Constant> variable as well.</para>
759 <para>Even better idea: Set the
760 <Constant>DEFAULT_TMPDIR</Constant> make variable when
761 building GHC, and never worry about
762 <Constant>TMPDIR</Constant> again. (see the build
763 documentation).</para>
771 <sect1 id="options-sanity">
772 <title>Warnings and sanity-checking</title>
774 <indexterm><primary>sanity-checking options</primary></indexterm>
775 <indexterm><primary>warnings</primary></indexterm>
778 <para>GHC has a number of options that select which types of
779 non-fatal error messages, otherwise known as warnings, can be
780 generated during compilation. By default, you get a standard set
781 of warnings which are generally likely to indicate bugs in your
783 <option>-fwarn-overlpapping-patterns</option>,
784 <option>-fwarn-deprecations</option>,
785 <option>-fwarn-duplicate-exports</option>,
786 <option>-fwarn-missing-fields</option>, and
787 <option>-fwarn-missing-methods</option>. The following flags are
788 simple ways to select standard “packages” of warnings:
794 <term><option>-W</option>:</term>
796 <IndexTerm><Primary>-W option</Primary></IndexTerm>
797 <para>Provides the standard warnings plus
798 <option>-fwarn-incomplete-patterns</option>,
799 <option>-fwarn-unused-matches</option>,
800 <option>-fwarn-unused-imports</option>,
801 <option>-fwarn-misc</option>, and
802 <option>-fwarn-unused-binds</option>.</para>
807 <term><option>-w</option>:</term>
809 <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
810 <para>Turns off all warnings, including the standard ones.</para>
815 <term><option>-Wall</option>:</term>
817 <indexterm><primary><option>-Wall</option></primary></indexterm>
818 <para>Turns on all warning options.</para>
824 <para>The full set of warning options is described below. To turn
825 off any warning, simply give the corresponding
826 <option>-fno-warn-...</option> option on the command line.</para>
831 <term><option>-fwarn-deprecations</option>:</term>
833 <indexterm><primary><option>-fwarn-deprecations</option></primary>
835 <indexterm><primary>deprecations</primary></indexterm>
836 <para>Causes a warning to be emitted when a deprecated
837 function or type is used. Entities can be marked as
838 deprecated using a pragma, see <xref
839 linkend="deprecated-pragma">.</para>
844 <term><option>-fwarn-duplicate-exports</option>:</term>
846 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
847 <indexterm><primary>duplicate exports, warning</primary></indexterm>
848 <indexterm><primary>export lists, duplicates</primary></indexterm>
850 <para>Have the compiler warn about duplicate entries in
851 export lists. This is useful information if you maintain
852 large export lists, and want to avoid the continued export
853 of a definition after you've deleted (one) mention of it in
854 the export list.</para>
856 <para>This option is on by default.</para>
861 <term><option>-fwarn-hi-shadowing</option>:</term>
863 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
864 <indexterm><primary>shadowing</primary>
865 <secondary>interface files</secondary></indexterm>
867 <para>Causes the compiler to emit a warning when a module or
868 interface file in the current directory is shadowing one
869 with the same module name in a library or other
875 <term><option>-fwarn-incomplete-patterns</option>:</term>
877 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
878 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
879 <indexterm><primary>patterns, incomplete</primary></indexterm>
881 <para>Similarly for incomplete patterns, the function
882 <function>g</function> below will fail when applied to
883 non-empty lists, so the compiler will emit a warning about
884 this when <option>-fwarn-incomplete-patterns</option> is
891 <para>This option isn't enabled be default because it can be
892 a bit noisy, and it doesn't always indicate a bug in the
893 program. However, it's generally considered good practice
894 to cover all the cases in your functions.</para>
899 <term><option>-fwarn-misc</option>:</term>
900 <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
902 <para>Turns on warnings for various harmless but untidy
903 things. This currently includes: importing a type with
904 <literal>(..)</literal> when the export is abstract, and
905 listing duplicate class assertions in a qualified type.</para>
910 <term><option>-fwarn-missing-fields</option>:</term>
912 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
913 <indexterm><primary>missing fields, warning</primary></indexterm>
914 <indexterm><primary>fields, missing</primary></indexterm>
916 <para>This option is on by default, and warns you whenever
917 the construction of a labelled field constructor isn't
918 complete, missing initializers for one or more fields. While
919 not an error (the missing fields are initialised with
920 bottoms), it is often an indication of a programmer error.</para>
925 <term><option>-fwarn-missing-methods</option>:</term>
927 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
928 <indexterm><primary>missing methods, warning</primary></indexterm>
929 <indexterm><primary>methods, missing</primary></indexterm>
931 <para>This option is on by default, and warns you whenever
932 an instance declaration is missing one or more methods, and
933 the corresponding class declaration has no default
934 declaration for them.</para>
939 <term><option>-fwarn-missing-signatures</option>:</term>
941 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
942 <indexterm><primary>type signatures, missing</primary></indexterm>
944 <para>If you would like GHC to check that every top-level
945 function/value has a type signature, use the
946 <option>-fwarn-missing-signatures</option> option. This
947 option is off by default.</para>
952 <term><option>-fwarn-name-shadowing</option>:</term>
954 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
955 <indexterm><primary>shadowing, warning</primary></indexterm>
957 <para>This option causes a warning to be emitted whenever an
958 inner-scope value has the same name as an outer-scope value,
959 i.e. the inner value shadows the outer one. This can catch
960 typographical errors that turn into hard-to-find bugs, e.g.,
961 in the inadvertent cyclic definition <literal>let x = ... x
962 ... in</literal>.</para>
964 <para>Consequently, this option does
965 <emphasis>will</emphasis> complain about cyclic recursive
971 <term><option>-fwarn-overlapping-patterns</option>:</term>
972 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
973 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
974 <indexterm><primary>patterns, overlapping</primary></indexterm>
976 <para>By default, the compiler will warn you if a set of
977 patterns are overlapping, i.e.,</para>
980 f :: String -> Int
986 <para>where the last pattern match in <Function>f</Function>
987 won't ever be reached, as the second pattern overlaps
988 it. More often than not, redundant patterns is a programmer
989 mistake/error, so this option is enabled by default.</para>
994 <term><option>-fwarn-simple-patterns</option>:</term>
996 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
998 <para>Causes the compiler to warn about lambda-bound
999 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1000 Normally, these aren't treated as incomplete patterns by
1001 <option>-fwarn-incomplete-patterns</option>.</para>
1006 <term><option>-fwarn-type-defaults</option>:</term>
1008 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1009 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1010 <para>Have the compiler warn/inform you where in your source
1011 the Haskell defaulting mechanism for numeric types kicks
1012 in. This is useful information when converting code from a
1013 context that assumed one default into one with another,
1014 e.g., the `default default' for Haskell 1.4 caused the
1015 otherwise unconstrained value <Constant>1</Constant> to be
1016 given the type <literal>Int</literal>, whereas Haskell 98
1017 defaults it to <literal>Integer</literal>. This may lead to
1018 differences in performance and behaviour, hence the
1019 usefulness of being non-silent about this.</para>
1021 <para>This warning is off by default.</para>
1026 <term><option>-fwarn-unused-binds</option>:</term>
1028 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1029 <indexterm><primary>unused binds, warning</primary></indexterm>
1030 <indexterm><primary>binds, unused</primary></indexterm>
1031 <para>Report any function definitions (and local bindings)
1032 which are unused. For top-level functions, the warning is
1033 only given if the binding is not exported.</para>
1038 <term><option>-fwarn-unused-imports</option>:</term>
1040 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1041 <indexterm><primary>unused imports, warning</primary></indexterm>
1042 <indexterm><primary>imports, unused</primary></indexterm>
1044 <para>Report any objects that are explicitly imported but
1050 <term><option>-fwarn-unused-matches</option>:</term>
1052 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1053 <indexterm><primary>unused matches, warning</primary></indexterm>
1054 <indexterm><primary>matches, unused</primary></indexterm>
1056 <para>Report all unused variables which arise from pattern
1057 matches, including patterns consisting of a single variable.
1058 For instance <literal>f x y = []</literal> would report
1059 <VarName>x</VarName> and <VarName>y</VarName> as unused. To
1060 eliminate the warning, all unused variables can be replaced
1061 with wildcards.</para>
1067 <para>If you're feeling really paranoid, the
1068 <option>-dcore-lint</option>
1069 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1070 is a good choice. It turns on heavyweight intra-pass
1071 sanity-checking within GHC. (It checks GHC's sanity, not
1079 <sect1 id="options-optimise">
1080 <title>Optimisation (code improvement)</title>
1082 <indexterm><primary>optimisation</primary></indexterm>
1083 <indexterm><primary>improvement, code</primary></indexterm>
1085 <para>The <option>-O*</option> options specify convenient
1086 “packages” of optimisation flags; the
1087 <option>-f*</option> options described later on specify
1088 <emphasis>individual</emphasis> optimisations to be turned on/off;
1089 the <option>-m*</option> options specify
1090 <emphasis>machine-specific</emphasis> optimisations to be turned
1093 <sect2 id="optimise-pkgs">
1094 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1096 <para>There are <emphasis>many</emphasis> options that affect
1097 the quality of code produced by GHC. Most people only have a
1098 general goal, something like “Compile quickly” or
1099 “Make my program run like greased lightning.” The
1100 following “packages” of optimisations (or lack
1101 thereof) should suffice.</para>
1103 <para>Once you choose a <option>-O*</option>
1104 “package,” stick with it—don't chop and
1105 change. Modules' interfaces <emphasis>will</emphasis> change
1106 with a shift to a new <option>-O*</option> option, and you may
1107 have to recompile a large chunk of all importing modules before
1108 your program can again be run safely (see <XRef
1109 LinkEnd="recomp">).</para>
1114 <term>No <option>-O*</option>-type option specified:</term>
1115 <indexterm><primary>-O* not specified</primary></indexterm>
1117 <para>This is taken to mean: “Please compile
1118 quickly; I'm not over-bothered about compiled-code
1119 quality.” So, for example: <command>ghc -c
1120 Foo.hs</command></para>
1125 <term><option>-O0</option>:</term>
1126 <indexterm><primary><option>-O0</option></primary></indexterm>
1128 <para>Means “turn off all optimisation”,
1129 reverting to the same settings as if no
1130 <option>-O</option> options had been specified. Saying
1131 <option>-O0</option> can be useful if
1132 eg. <command>make</command> has inserted a
1133 <option>-O</option> on the command line already.</para>
1138 <term><option>-O</option> or <option>-O1</option>:</term>
1139 <indexterm><primary>-O option</primary></indexterm>
1140 <indexterm><primary>-O1 option</primary></indexterm>
1141 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1143 <para>Means: “Generate good-quality code without
1144 taking too long about it.” Thus, for example:
1145 <command>ghc -c -O Main.lhs</command></para>
1150 <term><option>-O2</option>:</term>
1151 <indexterm><primary>-O2 option</primary></indexterm>
1152 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1154 <para>Means: “Apply every non-dangerous
1155 optimisation, even if it means significantly longer
1156 compile times.”</para>
1158 <para>The avoided “dangerous” optimisations
1159 are those that can make runtime or space
1160 <emphasis>worse</emphasis> if you're unlucky. They are
1161 normally turned on or off individually.</para>
1163 <para>At the moment, <option>-O2</option> is
1164 <emphasis>unlikely</emphasis> to produce better code than
1165 <option>-O</option>.</para>
1170 <term><option>-O2-for-C</option>:</term>
1171 <indexterm><primary>-O2-for-C option</primary></indexterm>
1172 <indexterm><primary>gcc, invoking with -O2</primary></indexterm>
1174 <para>Says to run GCC with <option>-O2</option>, which may
1175 be worth a few percent in execution speed. Don't forget
1176 <option>-fvia-C</option>, lest you use the native-code
1177 generator and bypass GCC altogether!</para>
1182 <term><option>-Ofile <file></option>:</term>
1183 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1184 <indexterm><primary>optimising, customised</primary></indexterm>
1186 <para>(NOTE: not supported yet in GHC 5.x. Please ask if
1187 you're interested in this.)</para>
1189 <para>For those who need <emphasis>absolute</emphasis>
1190 control over <emphasis>exactly</emphasis> what options are
1191 used (e.g., compiler writers, sometimes :-), a list of
1192 options can be put in a file and then slurped in with
1193 <option>-Ofile</option>.</para>
1195 <para>In that file, comments are of the
1196 <literal>#</literal>-to-end-of-line variety; blank
1197 lines and most whitespace is ignored.</para>
1199 <para>Please ask if you are baffled and would like an
1200 example of <option>-Ofile</option>!</para>
1205 <para>We don't use a <option>-O*</option> flag for day-to-day
1206 work. We use <option>-O</option> to get respectable speed;
1207 e.g., when we want to measure something. When we want to go for
1208 broke, we tend to use <option>-O -fvia-C -O2-for-C</option> (and
1209 we go for lots of coffee breaks).</para>
1211 <para>The easiest way to see what <option>-O</option> (etc.)
1212 “really mean” is to run with <option>-v</option>,
1213 then stand back in amazement.</para>
1216 <sect2 id="options-f">
1217 <title><option>-f*</option>: platform-independent flags</title>
1219 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1220 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1222 <para>These flags turn on and off individual optimisations.
1223 They are normally set via the <option>-O</option> options
1224 described above, and as such, you shouldn't need to set any of
1225 them explicitly (indeed, doing so could lead to unexpected
1226 results). However, there are one or two that may be of
1231 <term><option>-fexcess-precision</option>:</term>
1233 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1234 <para>When this option is given, intermediate floating
1235 point values can have a <emphasis>greater</emphasis>
1236 precision/range than the final type. Generally this is a
1237 good thing, but some programs may rely on the exact
1239 <literal>Float</literal>/<literal>Double</literal> values
1240 and should not use this option for their compilation.</para>
1245 <term><option>-fignore-asserts</option>:</term>
1247 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1248 <para>Causes GHC to ignore uses of the function
1249 <literal>Exception.assert</literal> in source code (in
1250 other words, rewriting <literal>Exception.assert p
1251 e</literal> to <literal>e</literal> (see <xref
1252 linkend="sec-assertions">). This flag is turned on by
1253 <option>-O</option>.
1259 <term><option>-fno-strictness</option></term>
1260 <indexterm><primary><option>-fno-strictness</option></primary>
1263 <para>Turns off the strictness analyser; sometimes it eats
1264 too many cycles.</para>
1269 <term><option>-fno-cpr-analyse</option></term>
1270 <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1273 <para>Turns off the CPR (constructed product result)
1274 analysis; it is somewhat experimental.</para>
1279 <term><option>-funbox-strict-fields</option>:</term>
1281 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1282 <indexterm><primary>strict constructor fields</primary></indexterm>
1283 <indexterm><primary>constructor fields, strict</primary></indexterm>
1285 <para>This option causes all constructor fields which are
1286 marked strict (i.e. “!”) to be unboxed or
1287 unpacked if possible. For example:</para>
1290 data T = T !Float !Float
1293 <para>will create a constructor <literal>T</literal>
1294 containing two unboxed floats if the
1295 <option>-funbox-strict-fields</option> flag is given.
1296 This may not always be an optimisation: if the
1297 <Function>T</Function> constructor is scrutinised and the
1298 floats passed to a non-strict function for example, they
1299 will have to be reboxed (this is done automatically by the
1302 <para>This option should only be used in conjunction with
1303 <option>-O</option>, in order to expose unfoldings to the
1304 compiler so the reboxing can be removed as often as
1305 possible. For example:</para>
1309 f (T f1 f2) = f1 + f2
1312 <para>The compiler will avoid reboxing
1313 <Function>f1</Function> and <Function>f2</Function> by
1314 inlining <Function>+</Function> on floats, but only when
1315 <option>-O</option> is on.</para>
1317 <para>Any single-constructor data is eligible for
1318 unpacking; for example</para>
1321 data T = T !(Int,Int)
1324 <para>will store the two <literal>Int</literal>s directly
1325 in the <Function>T</Function> constructor, by flattening
1326 the pair. Multi-level unpacking is also supported:</para>
1330 data S = S !Int !Int
1333 <para>will store two unboxed <literal>Int#</literal>s
1334 directly in the <Function>T</Function> constructor.</para>
1339 <term><option>-funfolding-update-in-place<n></option></term>
1340 <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1342 <para>Switches on an experimental "optimisation".
1343 Switching it on makes the compiler a little keener to
1344 inline a function that returns a constructor, if the
1345 context is that of a thunk.
1349 If we inlined plusInt we might get an opportunity to use
1350 update-in-place for the thunk 'x'.</para>
1355 <term><option>-funfolding-creation-threshold<n></option>:</term>
1357 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1358 <indexterm><primary>inlining, controlling</primary></indexterm>
1359 <indexterm><primary>unfolding, controlling</primary></indexterm>
1361 <para>(Default: 45) Governs the maximum size that GHC will
1362 allow a function unfolding to be. (An unfolding has a
1363 “size” that reflects the cost in terms of
1364 “code bloat” of expanding that unfolding at
1365 at a call site. A bigger function would be assigned a
1366 bigger cost.) </para>
1368 <para> Consequences: (a) nothing larger than this will be
1369 inlined (unless it has an INLINE pragma); (b) nothing
1370 larger than this will be spewed into an interface
1374 <para> Increasing this figure is more likely to result in longer
1375 compile times than faster code. The next option is more
1381 <term><option>-funfolding-use-threshold<n></option>:</term>
1383 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1384 <indexterm><primary>inlining, controlling</primary></indexterm>
1385 <indexterm><primary>unfolding, controlling</primary></indexterm>
1387 <para>(Default: 8) This is the magic cut-off figure for
1388 unfolding: below this size, a function definition will be
1389 unfolded at the call-site, any bigger and it won't. The
1390 size computed for a function depends on two things: the
1391 actual size of the expression minus any discounts that
1392 apply (see <option>-funfolding-con-discount</option>).</para>
1403 <Sect1 id="sec-using-concurrent">
1404 <title>Using Concurrent Haskell</title>
1407 <indexterm><primary>Concurrent Haskell—use</primary></indexterm>
1411 GHC (as of version 4.00) supports Concurrent Haskell by default,
1412 without requiring a special option or libraries compiled in a certain
1413 way. To get access to the support libraries for Concurrent Haskell
1414 (i.e. <literal>Concurrent</literal> and friends), use the
1415 <option>-package concurrent</option> option.
1419 Three RTS options are provided for modifying the behaviour of the
1420 threaded runtime system. See the descriptions of
1421 <option>-C[<us>]</option>, <option>-q</option>, and
1422 <option>-t<num></option> in <XRef LinkEnd="parallel-rts-opts">.
1426 Concurrent Haskell is described in more detail in <XRef
1427 LinkEnd="sec-Concurrent">.
1432 <Sect1 id="sec-using-parallel">
1433 <title>Using Parallel Haskell</title>
1436 <indexterm><primary>Parallel Haskell—use</primary></indexterm>
1440 [You won't be able to execute parallel Haskell programs unless PVM3
1441 (Parallel Virtual Machine, version 3) is installed at your site.]
1445 To compile a Haskell program for parallel execution under PVM, use the
1446 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
1447 option</Primary></IndexTerm> both when compiling <Emphasis>and
1448 linking</Emphasis>. You will probably want to <Literal>import
1449 Parallel</Literal> into your Haskell modules.
1453 To run your parallel program, once PVM is going, just invoke it
1454 “as normal”. The main extra RTS option is
1455 <Option>-qp<n></Option>, to say how many PVM
1456 “processors” your program to run on. (For more details of
1457 all relevant RTS options, please see <XRef
1458 LinkEnd="parallel-rts-opts">.)
1462 In truth, running Parallel Haskell programs and getting information
1463 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1464 PVM, detailed in the following sections.
1467 <Sect2 id="pvm-dummies">
1468 <Title>Dummy's guide to using PVM</Title>
1471 <indexterm><primary>PVM, how to use</primary></indexterm>
1472 <indexterm><primary>Parallel Haskell—PVM use</primary></indexterm>
1473 Before you can run a parallel program under PVM, you must set the
1474 required environment variables (PVM's idea, not ours); something like,
1475 probably in your <filename>.cshrc</filename> or equivalent:
1478 setenv PVM_ROOT /wherever/you/put/it
1479 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1480 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1486 Creating and/or controlling your “parallel machine” is a purely-PVM
1487 business; nothing specific to Parallel Haskell. The following paragraphs
1488 describe how to configure your parallel machine interactively.
1492 If you use parallel Haskell regularly on the same machine configuration it
1493 is a good idea to maintain a file with all machine names and to make the
1494 environment variable PVM_HOST_FILE point to this file. Then you can avoid
1495 the interactive operations described below by just saying
1503 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
1504 machine. You can then do various things to control/monitor your
1505 “parallel machine;” the most useful being:
1511 <ColSpec Align="Left">
1515 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1516 <entry>exit <command>pvm</command>, leaving it running</entry>
1520 <entry><command>halt</command></entry>
1521 <entry>kill off this “parallel machine” & exit</entry>
1525 <entry><command>add <host></command></entry>
1526 <entry>add <command><host></command> as a processor</entry>
1530 <entry><command>delete <host></command></entry>
1531 <entry>delete <command><host></command></entry>
1535 <entry><command>reset</command></entry>
1536 <entry>kill what's going, but leave PVM up</entry>
1540 <entry><command>conf</command></entry>
1541 <entry>list the current configuration</entry>
1545 <entry><command>ps</command></entry>
1546 <entry>report processes' status</entry>
1550 <entry><command>pstat <pid></command></entry>
1551 <entry>status of a particular process</entry>
1560 The PVM documentation can tell you much, much more about <command>pvm</command>!
1565 <Sect2 id="par-profiles">
1566 <Title>Parallelism profiles</Title>
1569 <indexterm><primary>parallelism profiles</primary></indexterm>
1570 <indexterm><primary>profiles, parallelism</primary></indexterm>
1571 <indexterm><primary>visualisation tools</primary></indexterm>
1575 With Parallel Haskell programs, we usually don't care about the
1576 results—only with “how parallel” it was! We want pretty pictures.
1580 Parallelism profiles (à la <Command>hbcpp</Command>) can be generated with the
1581 <Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option. The
1582 per-processor profiling info is dumped into files named
1583 <Filename><full-path><program>.gr</Filename>. These are then munged into a PostScript picture,
1584 which you can then display. For example, to run your program
1585 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
1591 <prompt>$</prompt> ./a.out +RTS -qP -qp8
1592 <prompt>$</prompt> grs2gr *.???.gr > temp.gr # combine the 8 .gr files into one
1593 <prompt>$</prompt> gr2ps -O temp.gr # cvt to .ps; output in temp.ps
1594 <prompt>$</prompt> ghostview -seascape temp.ps # look at it!
1600 The scripts for processing the parallelism profiles are distributed
1601 in <filename>ghc/utils/parallel/</filename>.
1607 <Title>Other useful info about running parallel programs</Title>
1610 The “garbage-collection statistics” RTS options can be useful for
1611 seeing what parallel programs are doing. If you do either
1612 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
1613 you'll get mutator, garbage-collection, etc., times on standard
1614 error. The standard error of all PE's other than the `main thread'
1615 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1619 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1620 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1625 <Sect2 id="parallel-rts-opts">
1626 <title>RTS options for Concurrent/Parallel Haskell
1630 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1631 <indexterm><primary>RTS options, parallel</primary></indexterm>
1632 <indexterm><primary>Concurrent Haskell—RTS options</primary></indexterm>
1633 <indexterm><primary>Parallel Haskell—RTS options</primary></indexterm>
1637 Besides the usual runtime system (RTS) options
1638 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1639 for concurrent/parallel execution.
1646 <Term><Option>-qp<N></Option>:</Term>
1649 <IndexTerm><Primary>-qp<N> RTS option</Primary></IndexTerm>
1650 (PARALLEL ONLY) Use <Literal><N></Literal> PVM processors to run this program;
1656 <term><option>-C[<us>]</option>:</term>
1659 <indexterm><primary>-C<us> RTS option</primary></indexterm> Sets
1660 the context switch interval to <literal><s></literal> seconds.
1661 A context switch will occur at the next heap block allocation after
1662 the timer expires (a heap block allocation occurs every 4k of
1663 allocation). With <option>-C0</option> or <option>-C</option>,
1664 context switches will occur as often as possible (at every heap block
1665 allocation). By default, context switches occur every 20ms
1666 milliseconds. Note that GHC's internal timer ticks every 20ms, and
1667 the context switch timer is always a multiple of this timer, so 20ms
1668 is the maximum granularity available for timed context switches.
1673 <term><option>-q[v]</option>:</term>
1676 <indexterm><primary>-q RTS option</primary></indexterm>
1677 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1678 in the file <FIlename><program>.qp</FIlename>. In the style of <command>hbcpp</command>, this profile
1679 records the movement of threads between the green (runnable) and red
1680 (blocked) queues. If you specify the verbose suboption (<option>-qv</option>), the
1681 green queue is split into green (for the currently running thread
1682 only) and amber (for other runnable threads). We do not recommend
1683 that you use the verbose suboption if you are planning to use the
1684 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
1685 check (with <Option>-C</Option>).
1691 <Term><Option>-qt<num></Option>:</Term>
1694 <IndexTerm><Primary>-qt<num> RTS option</Primary></IndexTerm>
1695 (PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
1696 threads per processor to <Literal><num></Literal>. The default is
1697 32. Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
1698 the heap for thread state and stack objects. (For 32-bit machines, this
1699 translates to 4K bytes, and for 64-bit machines, 8K bytes.)
1705 <Term><Option>-d</Option>:</Term>
1708 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
1709 (PARALLEL ONLY) Turn on debugging. It pops up one xterm (or GDB, or
1710 something…) per PVM processor. We use the standard <Command>debugger</Command>
1711 script that comes with PVM3, but we sometimes meddle with the
1712 <Command>debugger2</Command> script. We include ours in the GHC distribution,
1713 in <Filename>ghc/utils/pvm/</Filename>.
1719 <Term><Option>-qe<num></Option>:</Term>
1722 <IndexTerm><Primary>-qe<num> RTS option
1723 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
1724 i.e. the number of pending sparks per processor to
1725 <Literal><num></Literal>. The default is 100. A larger number may be
1726 appropriate if your program generates large amounts of parallelism
1732 <Term><Option>-qQ<num></Option>:</Term>
1735 <IndexTerm><Primary>-qQ<num> RTS option (parallel)</Primary></IndexTerm>
1736 (PARALLEL ONLY) Set the size of packets transmitted between processors
1737 to <Literal><num></Literal>. The default is 1024 words. A larger number may be
1738 appropriate if your machine has a high communication cost relative to
1744 <Term><Option>-qh<num></Option>:</Term>
1747 <IndexTerm><Primary>-qh<num> RTS option (parallel)</Primary></IndexTerm>
1748 (PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
1749 <num>-1 (0 means infinity). By default GUM uses full-subgraph
1750 packing, i.e. the entire subgraph with the requested closure as root is
1751 transmitted (provided it fits into one packet). Choosing a smaller value
1752 reduces the amount of pre-fetching of work done in GUM. This can be
1753 advantageous for improving data locality but it can also worsen the balance
1754 of the load in the system.
1759 <Term><Option>-qg<num></Option>:</Term>
1762 <IndexTerm><Primary>-qg<num> RTS option
1763 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
1764 scheme. This option affects the
1765 generation of global addresses when transferring data. Global addresses are
1766 globally unique identifiers required to maintain sharing in the distributed
1767 graph structure. Currently this is a binary option. With <num>=0 full globalisation is used
1768 (default). This means a global address is generated for every closure that
1769 is transmitted. With <num>=1 a thunk-only globalisation scheme is
1770 used, which generated global address only for thunks. The latter case may
1771 lose sharing of data but has a reduced overhead in packing graph structures
1772 and maintaining internal tables of global addresses.
1783 <sect1 id="options-platform">
1784 <title>Platform-specific Flags</title>
1786 <indexterm><primary>-m* options</primary></indexterm>
1787 <indexterm><primary>platform-specific options</primary></indexterm>
1788 <indexterm><primary>machine-specific options</primary></indexterm>
1790 <para>Some flags only make sense for particular target
1796 <term><option>-mv8</option>:</term>
1798 <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1799 only)</primary></indexterm> Means to pass the like-named
1800 option to GCC; it says to use the Version 8 SPARC
1801 instructions, notably integer multiply and divide. The
1802 similiar <option>-m*</option> GCC options for SPARC also
1803 work, actually.</para>
1808 <term><option>-monly-[32]-regs</option>:</term>
1810 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1811 option (iX86 only)</primary></indexterm> GHC tries to
1812 “steal” four registers from GCC, for performance
1813 reasons; it almost always works. However, when GCC is
1814 compiling some modules with four stolen registers, it will
1815 crash, probably saying:
1818 Foo.hc:533: fixed or forbidden register was spilled.
1819 This may be due to a compiler bug or to impossible asm
1820 statements or clauses.
1823 Just give some registers back with
1824 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1825 If `2' doesn't work, please report the bug to us.</para>
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