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 must contain one source file or module
397 name; GHC will figure out all the modules in the program by
398 following the imports from this initial module. It will then
399 attempt to compile each module which is out of date, and finally
400 if the top module is <literal>Main</literal>, 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), these can be specified on the
438 command line as usual.</para>
440 <para>Note that GHC can only follow dependencies if it has the
441 source file available, so if your program includes a module for
442 which there is no source file, even if you have an object and an
443 interface file for the module, then GHC will complain. The
444 exception to this rule is for package modules, which may or may
445 not have source files.</para>
447 <para>The source files for the program don't all need to be in the
448 same directory; the <option>-i</option> option can be used to add
449 directories to the search path (see <xref
450 linkend="options-finding-imports">).</para>
454 <Sect1 id="options-order">
455 <title>GHC without <option>––make</option></title>
457 <para>Without <option>––make</option>, GHC will compile one or
458 more source files given on the command line.</para>
460 <para>The first phase to run is determined by each input-file
461 suffix, and the last phase is determined by a flag. If no
462 relevant flag is present, then go all the way through linking.
463 This table summarises:</para>
467 <colspec align="left">
468 <colspec align="left">
469 <colspec align="left">
470 <colspec align="left">
474 <entry>Phase of the compilation system</entry>
475 <entry>Suffix saying “start here”</entry>
476 <entry>Flag saying “stop after”</entry>
477 <entry>(suffix of) output file</entry>
482 <entry>literate pre-processor</entry>
483 <entry><literal>.lhs</literal></entry>
485 <entry><literal>.hs</literal></entry>
489 <entry>C pre-processor (opt.)
491 <entry><literal>.hs</literal> (with
492 <option>-cpp</option>)</entry>
493 <entry><option>-E</option></entry>
494 <entry><literal>.hspp</literal></entry>
498 <entry>Haskell compiler</entry>
499 <entry><literal>.hs</literal></entry>
500 <entry><option>-C</option>, <option>-S</option></entry>
501 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
505 <entry>C compiler (opt.)</entry>
506 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
507 <entry><option>-S</option></entry>
508 <entry><literal>.s</literal></entry>
512 <entry>assembler</entry>
513 <entry><literal>.s</literal></entry>
514 <entry><option>-c</option></entry>
515 <entry><literal>.o</literal></entry>
519 <entry>linker</entry>
520 <entry><replaceable>other</replaceable></entry>
522 <entry><filename>a.out</filename></entry>
528 <indexterm><primary><option>-C</option></primary></indexterm>
529 <indexterm><primary><option>-E</option></primary></indexterm>
530 <indexterm><primary><option>-S</option></primary></indexterm>
531 <indexterm><primary><option>-c</option></primary></indexterm>
533 <para>Thus, a common invocation would be: <literal>ghc -c
534 Foo.hs</literal></para>
536 <para>Note: What the Haskell compiler proper produces depends on
537 whether a native-code generator<indexterm><primary>native-code
538 generator</primary></indexterm> is used (producing assembly
539 language) or not (producing C). See <xref
540 linkend="options-codegen"> for more details.</para>
542 <para>Note: C pre-processing is optional, the
543 <option>-ccp</option><indexterm><primary><option>-cpp</option></primary>
544 </indexterm>flag turns it on. See <xref
545 linkend="c-pre-processor"> for more details.</para>
547 <para>Note: The option <option>-E</option><IndexTerm><Primary>-E
548 option</Primary></IndexTerm> runs just the pre-processing passes
549 of the compiler, dumping the result in a file. Note that this
550 differs from the previous behaviour of dumping the file to
551 standard output.</para>
554 <sect1 id="options-output">
555 <title>Re-directing the compilation output(s)</title>
557 <indexterm><primary>output-directing options</primary></indexterm>
558 <indexterm><primary>redirecting compilation output</primary></indexterm>
563 <term><option>-o</option></term>
564 <indexterm><primary><option>-o</option></primary></indexterm>
566 <para>GHC's compiled output normally goes into a
567 <filename>.hc</filename>, <filename>.o</filename>, etc.,
568 file, depending on the last-run compilation phase. The
569 option <option>-o foo</option><IndexTerm><Primary>-o
570 option</Primary></IndexTerm> re-directs the output of that
571 last-run phase to file <filename>foo</filename>.</para>
573 <para>Note: this “feature” can be
574 counterintuitive: <command>ghc -C -o foo.o foo.hs</command>
575 will put the intermediate C code in the file
576 <filename>foo.o</filename>, name notwithstanding!</para>
578 <para>Note: on Windows, if the result is an executable file, the
579 extension "<filename>.exe</filename>" is added if the specified filename
580 does not already have an extension. Thus
584 will compile and link the module <filename>Main.hs</filename>, and put the
585 resulting executable in <filename>foo.exe</filename> (not <filename>foo</filename>).
591 <term><option>-odir</option></term>
592 <indexterm><primary><option>-odir</option></primary></indexterm>
594 <para>The <option>-o</option> option isn't of much use if
595 you have <emphasis>several</emphasis> input files…
596 Non-interface output files are normally put in the same
597 directory as their corresponding input file came from. You
598 may specify that they be put in another directory using the
599 <option>-odir <dir></option><IndexTerm><Primary>-odir
600 <dir> option</Primary></IndexTerm> (the “Oh,
601 dear” option). For example:</para>
604 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
607 <para>The output files, <filename>Foo.o</filename>,
608 <filename>Bar.o</filename>, and
609 <filename>Bumble.o</filename> would be put into a
610 subdirectory named after the architecture of the executing
611 machine (<filename>sun4</filename>,
612 <filename>mips</filename>, etc). The directory must already
613 exist; it won't be created.</para>
615 <para>Note that the <option>-odir</option> option does
616 <emphasis>not</emphasis> affect where the interface files
617 are put. In the above example, they would still be put in
618 <filename>parse/Foo.hi</filename>,
619 <filename>parse/Bar.hi</filename>, and
620 <filename>gurgle/Bumble.hi</filename>.</para>
625 <term><option>-ohi</option> <replaceable>file</replaceable></term>
626 <indexterm><primary><option>-ohi</option></primary>
629 <para>The interface output may be directed to another file
630 <filename>bar2/Wurble.iface</filename> with the option
631 <option>-ohi bar2/Wurble.iface</option> (not
634 <para>WARNING: if you redirect the interface file somewhere
635 that GHC can't find it, then the recompilation checker may
636 get confused (at the least, you won't get any recompilation
637 avoidance). We recommend using a combination of
638 <option>-hidir</option> and <option>-hisuf</option> options
639 instead, if possible.</para>
641 <para>To avoid generating an interface at all, you could use
642 this option to redirect the interface into the bit bucket:
643 <literal>-ohi /dev/null</literal>, for example.</para>
648 <term><option>-hidir</option> <replaceable>directory</replaceable></term>
649 <indexterm><primary><option>-hidir</option></primary>
652 <para>Redirects all generated interface files into
653 <replaceable>directory</replaceable>, instead of the default
654 which is to place the interface file in the same directory
655 as the source file.</para>
660 <term><option>-osuf</option> <replaceable>suffix</replaceable></term>
661 <term><option>-hisuf</option> <replaceable>suffix</replaceable></term>
662 <term><option>-hcsuf</option> <replaceable>suffix</replaceable></term>
663 <indexterm><primary><option>-osuf</option></primary></indexterm>
664 <indexterm><primary><option>-hisuf</option></primary></indexterm>
665 <indexterm><primary><option>-hcsuf</option></primary></indexterm>
667 <para>EXOTICA: The <option>-osuf</option>
668 <replaceable>suffix</replaceable> will change the
669 <literal>.o</literal> file suffix for object files to
670 whatever you specify. We use this when compiling libraries,
671 so that objects for the profiling versions of the libraries
672 don't clobber the normal ones.</para>
674 <para>Similarly, the <option>-hisuf</option>
675 <replaceable>suffix</replaceable> will change the
676 <literal>.hi</literal> file suffix for non-system interface
677 files (see <XRef LinkEnd="hi-options">).</para>
679 <para>Finally, the option <option>-hcsuf</option>
680 <replaceable>suffix</replaceable> will change the
681 <literal>.hc</literal> file suffix for compiler-generated
682 intermediate C files.</para>
684 <para>The <option>-hisuf</option>/<option>-osuf</option>
685 game is useful if you want to compile a program with both
686 GHC and HBC (say) in the same directory. Let HBC use the
687 standard <filename>.hi</filename>/<filename>.o</filename>
688 suffixes; add <option>-hisuf g_hi -osuf
689 g_o</option> to your <command>make</command> rule for
690 GHC compiling…</para>
695 <sect2 id="keeping-intermediates">
696 <title>Keeping Intermediate Files</title>
697 <indexterm><primary>intermediate files, saving</primary>
699 <indexterm><primary><literal>.hc</literal> files, saving</primary>
701 <indexterm><primary><literal>.s</literal> files, saving</primary>
705 <para>The following options are useful for keeping certain
706 intermediate files around, when normally GHC would throw these
707 away after compilation:</para>
711 <term><option>-keep-hc-files</option></term>
713 <primary><option>-keep-hc-files</option></primary>
716 <para>Keep intermediate <literal>.hc</literal> files when
717 doing <literal>.hs</literal>-to-<literal>.o</literal>
718 compilations via C (NOTE: <literal>.hc</literal> files
719 aren't generated when using the native code generator, you
720 may need to use <option>-fvia-C</option> to force them
721 to be produced).</para>
726 <term><option>-keep-s-files</option></term>
728 <primary><option>-keep-s-files</option></primary>
731 <para>Keep intermediate <literal>.s</literal> files.</para>
736 <term><option>-keep-raw-s-files</option></term>
738 <primary><option>-keep-raw-s-files</option></primary>
741 <para>Keep intermediate <literal>.raw-s</literal> files.
742 These are the direct output from the C compiler, before
743 GHC does “assembly mangling” to produce the
744 <literal>.s</literal> file. Again, these are not produced
745 when using the native code generator.</para>
750 <term><option>-keep-tmp-files</option></term>
752 <primary><option>-keep-tmp-files</option></primary>
755 <primary>temporary files</primary>
756 <secondary>keeping</secondary>
759 <para>Instructs the GHC driver not to delete any of its
760 temporary files, which it normally keeps in
761 <literal>/tmp</literal> (or possibly elsewhere; see <xref
762 linkend="temp-files">). Running GHC with
763 <option>-v</option> will show you what temporary files
764 were generated along the way.</para>
770 <sect2 id="temp-files">
771 <title>Redirecting temporary files</title>
774 <primary>temporary files</primary>
775 <secondary>redirecting</secondary>
780 <term><option>-tmpdir</option></term>
781 <indexterm><primary><option>-tmpdir</option></primary></indexterm>
783 <para>If you have trouble because of running out of space
784 in <filename>/tmp</filename> (or wherever your
785 installation thinks temporary files should go), you may
786 use the <option>-tmpdir
787 <dir></option><IndexTerm><Primary>-tmpdir
788 <dir> option</Primary></IndexTerm> option to specify
789 an alternate directory. For example, <option>-tmpdir
790 .</option> says to put temporary files in the current
791 working directory.</para>
793 <para>Alternatively, use your <Constant>TMPDIR</Constant>
794 environment variable.<IndexTerm><Primary>TMPDIR
795 environment variable</Primary></IndexTerm> Set it to the
796 name of the directory where temporary files should be put.
797 GCC and other programs will honour the
798 <Constant>TMPDIR</Constant> variable as well.</para>
800 <para>Even better idea: Set the
801 <Constant>DEFAULT_TMPDIR</Constant> make variable when
802 building GHC, and never worry about
803 <Constant>TMPDIR</Constant> again. (see the build
804 documentation).</para>
812 <sect1 id="options-sanity">
813 <title>Warnings and sanity-checking</title>
815 <indexterm><primary>sanity-checking options</primary></indexterm>
816 <indexterm><primary>warnings</primary></indexterm>
819 <para>GHC has a number of options that select which types of
820 non-fatal error messages, otherwise known as warnings, can be
821 generated during compilation. By default, you get a standard set
822 of warnings which are generally likely to indicate bugs in your
824 <option>-fwarn-overlpapping-patterns</option>,
825 <option>-fwarn-deprecations</option>,
826 <option>-fwarn-duplicate-exports</option>,
827 <option>-fwarn-missing-fields</option>, and
828 <option>-fwarn-missing-methods</option>. The following flags are
829 simple ways to select standard “packages” of warnings:
835 <term><option>-W</option>:</term>
837 <IndexTerm><Primary>-W option</Primary></IndexTerm>
838 <para>Provides the standard warnings plus
839 <option>-fwarn-incomplete-patterns</option>,
840 <option>-fwarn-unused-matches</option>,
841 <option>-fwarn-unused-imports</option>,
842 <option>-fwarn-misc</option>, and
843 <option>-fwarn-unused-binds</option>.</para>
848 <term><option>-w</option>:</term>
850 <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
851 <para>Turns off all warnings, including the standard ones.</para>
856 <term><option>-Wall</option>:</term>
858 <indexterm><primary><option>-Wall</option></primary></indexterm>
859 <para>Turns on all warning options.</para>
865 <para>The full set of warning options is described below. To turn
866 off any warning, simply give the corresponding
867 <option>-fno-warn-...</option> option on the command line.</para>
872 <term><option>-fwarn-deprecations</option>:</term>
874 <indexterm><primary><option>-fwarn-deprecations</option></primary>
876 <indexterm><primary>deprecations</primary></indexterm>
877 <para>Causes a warning to be emitted when a deprecated
878 function or type is used. Entities can be marked as
879 deprecated using a pragma, see <xref
880 linkend="deprecated-pragma">.</para>
885 <term><option>-fwarn-duplicate-exports</option>:</term>
887 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
888 <indexterm><primary>duplicate exports, warning</primary></indexterm>
889 <indexterm><primary>export lists, duplicates</primary></indexterm>
891 <para>Have the compiler warn about duplicate entries in
892 export lists. This is useful information if you maintain
893 large export lists, and want to avoid the continued export
894 of a definition after you've deleted (one) mention of it in
895 the export list.</para>
897 <para>This option is on by default.</para>
902 <term><option>-fwarn-hi-shadowing</option>:</term>
904 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
905 <indexterm><primary>shadowing</primary>
906 <secondary>interface files</secondary></indexterm>
908 <para>Causes the compiler to emit a warning when a module or
909 interface file in the current directory is shadowing one
910 with the same module name in a library or other
916 <term><option>-fwarn-incomplete-patterns</option>:</term>
918 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
919 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
920 <indexterm><primary>patterns, incomplete</primary></indexterm>
922 <para>Similarly for incomplete patterns, the function
923 <function>g</function> below will fail when applied to
924 non-empty lists, so the compiler will emit a warning about
925 this when <option>-fwarn-incomplete-patterns</option> is
932 <para>This option isn't enabled be default because it can be
933 a bit noisy, and it doesn't always indicate a bug in the
934 program. However, it's generally considered good practice
935 to cover all the cases in your functions.</para>
940 <term><option>-fwarn-misc</option>:</term>
941 <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
943 <para>Turns on warnings for various harmless but untidy
944 things. This currently includes: importing a type with
945 <literal>(..)</literal> when the export is abstract, and
946 listing duplicate class assertions in a qualified type.</para>
951 <term><option>-fwarn-missing-fields</option>:</term>
953 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
954 <indexterm><primary>missing fields, warning</primary></indexterm>
955 <indexterm><primary>fields, missing</primary></indexterm>
957 <para>This option is on by default, and warns you whenever
958 the construction of a labelled field constructor isn't
959 complete, missing initializers for one or more fields. While
960 not an error (the missing fields are initialised with
961 bottoms), it is often an indication of a programmer error.</para>
966 <term><option>-fwarn-missing-methods</option>:</term>
968 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
969 <indexterm><primary>missing methods, warning</primary></indexterm>
970 <indexterm><primary>methods, missing</primary></indexterm>
972 <para>This option is on by default, and warns you whenever
973 an instance declaration is missing one or more methods, and
974 the corresponding class declaration has no default
975 declaration for them.</para>
980 <term><option>-fwarn-missing-signatures</option>:</term>
982 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
983 <indexterm><primary>type signatures, missing</primary></indexterm>
985 <para>If you would like GHC to check that every top-level
986 function/value has a type signature, use the
987 <option>-fwarn-missing-signatures</option> option. This
988 option is off by default.</para>
993 <term><option>-fwarn-name-shadowing</option>:</term>
995 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
996 <indexterm><primary>shadowing, warning</primary></indexterm>
998 <para>This option causes a warning to be emitted whenever an
999 inner-scope value has the same name as an outer-scope value,
1000 i.e. the inner value shadows the outer one. This can catch
1001 typographical errors that turn into hard-to-find bugs, e.g.,
1002 in the inadvertent cyclic definition <literal>let x = ... x
1003 ... in</literal>.</para>
1005 <para>Consequently, this option does
1006 <emphasis>will</emphasis> complain about cyclic recursive
1012 <term><option>-fwarn-overlapping-patterns</option>:</term>
1013 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
1014 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
1015 <indexterm><primary>patterns, overlapping</primary></indexterm>
1017 <para>By default, the compiler will warn you if a set of
1018 patterns are overlapping, i.e.,</para>
1021 f :: String -> Int
1027 <para>where the last pattern match in <Function>f</Function>
1028 won't ever be reached, as the second pattern overlaps
1029 it. More often than not, redundant patterns is a programmer
1030 mistake/error, so this option is enabled by default.</para>
1035 <term><option>-fwarn-simple-patterns</option>:</term>
1037 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1039 <para>Causes the compiler to warn about lambda-bound
1040 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1041 Normally, these aren't treated as incomplete patterns by
1042 <option>-fwarn-incomplete-patterns</option>.</para>
1047 <term><option>-fwarn-type-defaults</option>:</term>
1049 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1050 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1051 <para>Have the compiler warn/inform you where in your source
1052 the Haskell defaulting mechanism for numeric types kicks
1053 in. This is useful information when converting code from a
1054 context that assumed one default into one with another,
1055 e.g., the `default default' for Haskell 1.4 caused the
1056 otherwise unconstrained value <Constant>1</Constant> to be
1057 given the type <literal>Int</literal>, whereas Haskell 98
1058 defaults it to <literal>Integer</literal>. This may lead to
1059 differences in performance and behaviour, hence the
1060 usefulness of being non-silent about this.</para>
1062 <para>This warning is off by default.</para>
1067 <term><option>-fwarn-unused-binds</option>:</term>
1069 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1070 <indexterm><primary>unused binds, warning</primary></indexterm>
1071 <indexterm><primary>binds, unused</primary></indexterm>
1072 <para>Report any function definitions (and local bindings)
1073 which are unused. For top-level functions, the warning is
1074 only given if the binding is not exported.</para>
1079 <term><option>-fwarn-unused-imports</option>:</term>
1081 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1082 <indexterm><primary>unused imports, warning</primary></indexterm>
1083 <indexterm><primary>imports, unused</primary></indexterm>
1085 <para>Report any objects that are explicitly imported but
1091 <term><option>-fwarn-unused-matches</option>:</term>
1093 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1094 <indexterm><primary>unused matches, warning</primary></indexterm>
1095 <indexterm><primary>matches, unused</primary></indexterm>
1097 <para>Report all unused variables which arise from pattern
1098 matches, including patterns consisting of a single variable.
1099 For instance <literal>f x y = []</literal> would report
1100 <VarName>x</VarName> and <VarName>y</VarName> as unused. To
1101 eliminate the warning, all unused variables can be replaced
1102 with wildcards.</para>
1108 <para>If you're feeling really paranoid, the
1109 <option>-dcore-lint</option>
1110 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1111 is a good choice. It turns on heavyweight intra-pass
1112 sanity-checking within GHC. (It checks GHC's sanity, not
1120 <sect1 id="options-optimise">
1121 <title>Optimisation (code improvement)</title>
1123 <indexterm><primary>optimisation</primary></indexterm>
1124 <indexterm><primary>improvement, code</primary></indexterm>
1126 <para>The <option>-O*</option> options specify convenient
1127 “packages” of optimisation flags; the
1128 <option>-f*</option> options described later on specify
1129 <emphasis>individual</emphasis> optimisations to be turned on/off;
1130 the <option>-m*</option> options specify
1131 <emphasis>machine-specific</emphasis> optimisations to be turned
1134 <sect2 id="optimise-pkgs">
1135 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1137 <para>There are <emphasis>many</emphasis> options that affect
1138 the quality of code produced by GHC. Most people only have a
1139 general goal, something like “Compile quickly” or
1140 “Make my program run like greased lightning.” The
1141 following “packages” of optimisations (or lack
1142 thereof) should suffice.</para>
1144 <para>Once you choose a <option>-O*</option>
1145 “package,” stick with it—don't chop and
1146 change. Modules' interfaces <emphasis>will</emphasis> change
1147 with a shift to a new <option>-O*</option> option, and you may
1148 have to recompile a large chunk of all importing modules before
1149 your program can again be run safely (see <XRef
1150 LinkEnd="recomp">).</para>
1155 <term>No <option>-O*</option>-type option specified:</term>
1156 <indexterm><primary>-O* not specified</primary></indexterm>
1158 <para>This is taken to mean: “Please compile
1159 quickly; I'm not over-bothered about compiled-code
1160 quality.” So, for example: <command>ghc -c
1161 Foo.hs</command></para>
1166 <term><option>-O0</option>:</term>
1167 <indexterm><primary><option>-O0</option></primary></indexterm>
1169 <para>Means “turn off all optimisation”,
1170 reverting to the same settings as if no
1171 <option>-O</option> options had been specified. Saying
1172 <option>-O0</option> can be useful if
1173 eg. <command>make</command> has inserted a
1174 <option>-O</option> on the command line already.</para>
1179 <term><option>-O</option> or <option>-O1</option>:</term>
1180 <indexterm><primary>-O option</primary></indexterm>
1181 <indexterm><primary>-O1 option</primary></indexterm>
1182 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1184 <para>Means: “Generate good-quality code without
1185 taking too long about it.” Thus, for example:
1186 <command>ghc -c -O Main.lhs</command></para>
1191 <term><option>-O2</option>:</term>
1192 <indexterm><primary>-O2 option</primary></indexterm>
1193 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1195 <para>Means: “Apply every non-dangerous
1196 optimisation, even if it means significantly longer
1197 compile times.”</para>
1199 <para>The avoided “dangerous” optimisations
1200 are those that can make runtime or space
1201 <emphasis>worse</emphasis> if you're unlucky. They are
1202 normally turned on or off individually.</para>
1204 <para>At the moment, <option>-O2</option> is
1205 <emphasis>unlikely</emphasis> to produce better code than
1206 <option>-O</option>.</para>
1211 <term><option>-Ofile <file></option>:</term>
1212 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1213 <indexterm><primary>optimising, customised</primary></indexterm>
1215 <para>(NOTE: not supported yet in GHC 5.x. Please ask if
1216 you're interested in this.)</para>
1218 <para>For those who need <emphasis>absolute</emphasis>
1219 control over <emphasis>exactly</emphasis> what options are
1220 used (e.g., compiler writers, sometimes :-), a list of
1221 options can be put in a file and then slurped in with
1222 <option>-Ofile</option>.</para>
1224 <para>In that file, comments are of the
1225 <literal>#</literal>-to-end-of-line variety; blank
1226 lines and most whitespace is ignored.</para>
1228 <para>Please ask if you are baffled and would like an
1229 example of <option>-Ofile</option>!</para>
1234 <para>We don't use a <option>-O*</option> flag for day-to-day
1235 work. We use <option>-O</option> to get respectable speed;
1236 e.g., when we want to measure something. When we want to go for
1237 broke, we tend to use <option>-O -fvia-C</option> (and we go for
1238 lots of coffee breaks).</para>
1240 <para>The easiest way to see what <option>-O</option> (etc.)
1241 “really mean” is to run with <option>-v</option>,
1242 then stand back in amazement.</para>
1245 <sect2 id="options-f">
1246 <title><option>-f*</option>: platform-independent flags</title>
1248 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1249 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1251 <para>These flags turn on and off individual optimisations.
1252 They are normally set via the <option>-O</option> options
1253 described above, and as such, you shouldn't need to set any of
1254 them explicitly (indeed, doing so could lead to unexpected
1255 results). However, there are one or two that may be of
1260 <term><option>-fexcess-precision</option>:</term>
1262 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1263 <para>When this option is given, intermediate floating
1264 point values can have a <emphasis>greater</emphasis>
1265 precision/range than the final type. Generally this is a
1266 good thing, but some programs may rely on the exact
1268 <literal>Float</literal>/<literal>Double</literal> values
1269 and should not use this option for their compilation.</para>
1274 <term><option>-fignore-asserts</option>:</term>
1276 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1277 <para>Causes GHC to ignore uses of the function
1278 <literal>Exception.assert</literal> in source code (in
1279 other words, rewriting <literal>Exception.assert p
1280 e</literal> to <literal>e</literal> (see <xref
1281 linkend="sec-assertions">). This flag is turned on by
1282 <option>-O</option>.
1288 <term><option>-fno-strictness</option></term>
1289 <indexterm><primary><option>-fno-strictness</option></primary>
1292 <para>Turns off the strictness analyser; sometimes it eats
1293 too many cycles.</para>
1298 <term><option>-fno-cpr-analyse</option></term>
1299 <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1302 <para>Turns off the CPR (constructed product result)
1303 analysis; it is somewhat experimental.</para>
1308 <term><option>-funbox-strict-fields</option>:</term>
1310 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1311 <indexterm><primary>strict constructor fields</primary></indexterm>
1312 <indexterm><primary>constructor fields, strict</primary></indexterm>
1314 <para>This option causes all constructor fields which are
1315 marked strict (i.e. “!”) to be unboxed or
1316 unpacked if possible. For example:</para>
1319 data T = T !Float !Float
1322 <para>will create a constructor <literal>T</literal>
1323 containing two unboxed floats if the
1324 <option>-funbox-strict-fields</option> flag is given.
1325 This may not always be an optimisation: if the
1326 <Function>T</Function> constructor is scrutinised and the
1327 floats passed to a non-strict function for example, they
1328 will have to be reboxed (this is done automatically by the
1331 <para>This option should only be used in conjunction with
1332 <option>-O</option>, in order to expose unfoldings to the
1333 compiler so the reboxing can be removed as often as
1334 possible. For example:</para>
1338 f (T f1 f2) = f1 + f2
1341 <para>The compiler will avoid reboxing
1342 <Function>f1</Function> and <Function>f2</Function> by
1343 inlining <Function>+</Function> on floats, but only when
1344 <option>-O</option> is on.</para>
1346 <para>Any single-constructor data is eligible for
1347 unpacking; for example</para>
1350 data T = T !(Int,Int)
1353 <para>will store the two <literal>Int</literal>s directly
1354 in the <Function>T</Function> constructor, by flattening
1355 the pair. Multi-level unpacking is also supported:</para>
1359 data S = S !Int !Int
1362 <para>will store two unboxed <literal>Int#</literal>s
1363 directly in the <Function>T</Function> constructor.</para>
1368 <term><option>-funfolding-update-in-place<n></option></term>
1369 <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1371 <para>Switches on an experimental "optimisation".
1372 Switching it on makes the compiler a little keener to
1373 inline a function that returns a constructor, if the
1374 context is that of a thunk.
1378 If we inlined plusInt we might get an opportunity to use
1379 update-in-place for the thunk 'x'.</para>
1384 <term><option>-funfolding-creation-threshold<n></option>:</term>
1386 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1387 <indexterm><primary>inlining, controlling</primary></indexterm>
1388 <indexterm><primary>unfolding, controlling</primary></indexterm>
1390 <para>(Default: 45) Governs the maximum size that GHC will
1391 allow a function unfolding to be. (An unfolding has a
1392 “size” that reflects the cost in terms of
1393 “code bloat” of expanding that unfolding at
1394 at a call site. A bigger function would be assigned a
1395 bigger cost.) </para>
1397 <para> Consequences: (a) nothing larger than this will be
1398 inlined (unless it has an INLINE pragma); (b) nothing
1399 larger than this will be spewed into an interface
1403 <para> Increasing this figure is more likely to result in longer
1404 compile times than faster code. The next option is more
1410 <term><option>-funfolding-use-threshold<n></option>:</term>
1412 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1413 <indexterm><primary>inlining, controlling</primary></indexterm>
1414 <indexterm><primary>unfolding, controlling</primary></indexterm>
1416 <para>(Default: 8) This is the magic cut-off figure for
1417 unfolding: below this size, a function definition will be
1418 unfolded at the call-site, any bigger and it won't. The
1419 size computed for a function depends on two things: the
1420 actual size of the expression minus any discounts that
1421 apply (see <option>-funfolding-con-discount</option>).</para>
1432 <sect1 id="sec-using-concurrent">
1433 <title>Using Concurrent Haskell</title>
1435 <indexterm><primary>Concurrent Haskell—use</primary></indexterm>
1438 GHC supports Concurrent Haskell by default, without requiring a
1439 special option or libraries compiled in a certain way. To get access
1440 to the support libraries for Concurrent Haskell, just import
1441 <literal>Control.Concurrent</literal> (details are in the accompanying
1442 library documentation).</para>
1445 RTS options are provided for modifying the behaviour of the threaded
1446 runtime system. See <XRef LinkEnd="parallel-rts-opts">.
1450 Concurrent Haskell is described in more detail in the documentation
1451 for the <literal>Control.Concurrent</literal> module.
1456 <Sect1 id="sec-using-parallel">
1457 <title>Using Parallel Haskell</title>
1460 <indexterm><primary>Parallel Haskell—use</primary></indexterm>
1464 [You won't be able to execute parallel Haskell programs unless PVM3
1465 (Parallel Virtual Machine, version 3) is installed at your site.]
1469 To compile a Haskell program for parallel execution under PVM, use the
1470 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
1471 option</Primary></IndexTerm> both when compiling <Emphasis>and
1472 linking</Emphasis>. You will probably want to <Literal>import
1473 Parallel</Literal> into your Haskell modules.
1477 To run your parallel program, once PVM is going, just invoke it
1478 “as normal”. The main extra RTS option is
1479 <Option>-qp<n></Option>, to say how many PVM
1480 “processors” your program to run on. (For more details of
1481 all relevant RTS options, please see <XRef
1482 LinkEnd="parallel-rts-opts">.)
1486 In truth, running Parallel Haskell programs and getting information
1487 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1488 PVM, detailed in the following sections.
1491 <Sect2 id="pvm-dummies">
1492 <Title>Dummy's guide to using PVM</Title>
1495 <indexterm><primary>PVM, how to use</primary></indexterm>
1496 <indexterm><primary>Parallel Haskell—PVM use</primary></indexterm>
1497 Before you can run a parallel program under PVM, you must set the
1498 required environment variables (PVM's idea, not ours); something like,
1499 probably in your <filename>.cshrc</filename> or equivalent:
1502 setenv PVM_ROOT /wherever/you/put/it
1503 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1504 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1510 Creating and/or controlling your “parallel machine” is a purely-PVM
1511 business; nothing specific to Parallel Haskell. The following paragraphs
1512 describe how to configure your parallel machine interactively.
1516 If you use parallel Haskell regularly on the same machine configuration it
1517 is a good idea to maintain a file with all machine names and to make the
1518 environment variable PVM_HOST_FILE point to this file. Then you can avoid
1519 the interactive operations described below by just saying
1527 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
1528 machine. You can then do various things to control/monitor your
1529 “parallel machine;” the most useful being:
1535 <ColSpec Align="Left">
1539 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1540 <entry>exit <command>pvm</command>, leaving it running</entry>
1544 <entry><command>halt</command></entry>
1545 <entry>kill off this “parallel machine” & exit</entry>
1549 <entry><command>add <host></command></entry>
1550 <entry>add <command><host></command> as a processor</entry>
1554 <entry><command>delete <host></command></entry>
1555 <entry>delete <command><host></command></entry>
1559 <entry><command>reset</command></entry>
1560 <entry>kill what's going, but leave PVM up</entry>
1564 <entry><command>conf</command></entry>
1565 <entry>list the current configuration</entry>
1569 <entry><command>ps</command></entry>
1570 <entry>report processes' status</entry>
1574 <entry><command>pstat <pid></command></entry>
1575 <entry>status of a particular process</entry>
1584 The PVM documentation can tell you much, much more about <command>pvm</command>!
1589 <Sect2 id="par-profiles">
1590 <Title>Parallelism profiles</Title>
1593 <indexterm><primary>parallelism profiles</primary></indexterm>
1594 <indexterm><primary>profiles, parallelism</primary></indexterm>
1595 <indexterm><primary>visualisation tools</primary></indexterm>
1599 With Parallel Haskell programs, we usually don't care about the
1600 results—only with “how parallel” it was! We want pretty pictures.
1604 Parallelism profiles (à la <Command>hbcpp</Command>) can be generated with the
1605 <Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option. The
1606 per-processor profiling info is dumped into files named
1607 <Filename><full-path><program>.gr</Filename>. These are then munged into a PostScript picture,
1608 which you can then display. For example, to run your program
1609 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
1615 <prompt>$</prompt> ./a.out +RTS -qP -qp8
1616 <prompt>$</prompt> grs2gr *.???.gr > temp.gr # combine the 8 .gr files into one
1617 <prompt>$</prompt> gr2ps -O temp.gr # cvt to .ps; output in temp.ps
1618 <prompt>$</prompt> ghostview -seascape temp.ps # look at it!
1624 The scripts for processing the parallelism profiles are distributed
1625 in <filename>ghc/utils/parallel/</filename>.
1631 <Title>Other useful info about running parallel programs</Title>
1634 The “garbage-collection statistics” RTS options can be useful for
1635 seeing what parallel programs are doing. If you do either
1636 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
1637 you'll get mutator, garbage-collection, etc., times on standard
1638 error. The standard error of all PE's other than the `main thread'
1639 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1643 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1644 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1649 <Sect2 id="parallel-rts-opts">
1650 <title>RTS options for Concurrent/Parallel Haskell
1654 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1655 <indexterm><primary>RTS options, parallel</primary></indexterm>
1656 <indexterm><primary>Concurrent Haskell—RTS options</primary></indexterm>
1657 <indexterm><primary>Parallel Haskell—RTS options</primary></indexterm>
1661 Besides the usual runtime system (RTS) options
1662 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1663 for concurrent/parallel execution.
1670 <Term><Option>-qp<N></Option>:</Term>
1673 <IndexTerm><Primary>-qp<N> RTS option</Primary></IndexTerm>
1674 (PARALLEL ONLY) Use <Literal><N></Literal> PVM processors to run this program;
1680 <term><option>-C[<us>]</option>:</term>
1683 <indexterm><primary>-C<us> RTS option</primary></indexterm> Sets
1684 the context switch interval to <literal><s></literal> seconds.
1685 A context switch will occur at the next heap block allocation after
1686 the timer expires (a heap block allocation occurs every 4k of
1687 allocation). With <option>-C0</option> or <option>-C</option>,
1688 context switches will occur as often as possible (at every heap block
1689 allocation). By default, context switches occur every 20ms
1690 milliseconds. Note that GHC's internal timer ticks every 20ms, and
1691 the context switch timer is always a multiple of this timer, so 20ms
1692 is the maximum granularity available for timed context switches.
1697 <term><option>-q[v]</option>:</term>
1700 <indexterm><primary>-q RTS option</primary></indexterm>
1701 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1702 in the file <FIlename><program>.qp</FIlename>. In the style of <command>hbcpp</command>, this profile
1703 records the movement of threads between the green (runnable) and red
1704 (blocked) queues. If you specify the verbose suboption (<option>-qv</option>), the
1705 green queue is split into green (for the currently running thread
1706 only) and amber (for other runnable threads). We do not recommend
1707 that you use the verbose suboption if you are planning to use the
1708 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
1709 check (with <Option>-C</Option>).
1715 <Term><Option>-qt<num></Option>:</Term>
1718 <IndexTerm><Primary>-qt<num> RTS option</Primary></IndexTerm>
1719 (PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
1720 threads per processor to <Literal><num></Literal>. The default is
1721 32. Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
1722 the heap for thread state and stack objects. (For 32-bit machines, this
1723 translates to 4K bytes, and for 64-bit machines, 8K bytes.)
1729 <Term><Option>-d</Option>:</Term>
1732 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
1733 (PARALLEL ONLY) Turn on debugging. It pops up one xterm (or GDB, or
1734 something…) per PVM processor. We use the standard <Command>debugger</Command>
1735 script that comes with PVM3, but we sometimes meddle with the
1736 <Command>debugger2</Command> script. We include ours in the GHC distribution,
1737 in <Filename>ghc/utils/pvm/</Filename>.
1743 <Term><Option>-qe<num></Option>:</Term>
1746 <IndexTerm><Primary>-qe<num> RTS option
1747 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
1748 i.e. the number of pending sparks per processor to
1749 <Literal><num></Literal>. The default is 100. A larger number may be
1750 appropriate if your program generates large amounts of parallelism
1756 <Term><Option>-qQ<num></Option>:</Term>
1759 <IndexTerm><Primary>-qQ<num> RTS option (parallel)</Primary></IndexTerm>
1760 (PARALLEL ONLY) Set the size of packets transmitted between processors
1761 to <Literal><num></Literal>. The default is 1024 words. A larger number may be
1762 appropriate if your machine has a high communication cost relative to
1768 <Term><Option>-qh<num></Option>:</Term>
1771 <IndexTerm><Primary>-qh<num> RTS option (parallel)</Primary></IndexTerm>
1772 (PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
1773 <num>-1 (0 means infinity). By default GUM uses full-subgraph
1774 packing, i.e. the entire subgraph with the requested closure as root is
1775 transmitted (provided it fits into one packet). Choosing a smaller value
1776 reduces the amount of pre-fetching of work done in GUM. This can be
1777 advantageous for improving data locality but it can also worsen the balance
1778 of the load in the system.
1783 <Term><Option>-qg<num></Option>:</Term>
1786 <IndexTerm><Primary>-qg<num> RTS option
1787 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
1788 scheme. This option affects the
1789 generation of global addresses when transferring data. Global addresses are
1790 globally unique identifiers required to maintain sharing in the distributed
1791 graph structure. Currently this is a binary option. With <num>=0 full globalisation is used
1792 (default). This means a global address is generated for every closure that
1793 is transmitted. With <num>=1 a thunk-only globalisation scheme is
1794 used, which generated global address only for thunks. The latter case may
1795 lose sharing of data but has a reduced overhead in packing graph structures
1796 and maintaining internal tables of global addresses.
1807 <sect1 id="options-platform">
1808 <title>Platform-specific Flags</title>
1810 <indexterm><primary>-m* options</primary></indexterm>
1811 <indexterm><primary>platform-specific options</primary></indexterm>
1812 <indexterm><primary>machine-specific options</primary></indexterm>
1814 <para>Some flags only make sense for particular target
1820 <term><option>-mv8</option>:</term>
1822 <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1823 only)</primary></indexterm> Means to pass the like-named
1824 option to GCC; it says to use the Version 8 SPARC
1825 instructions, notably integer multiply and divide. The
1826 similiar <option>-m*</option> GCC options for SPARC also
1827 work, actually.</para>
1832 <term><option>-monly-[32]-regs</option>:</term>
1834 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1835 option (iX86 only)</primary></indexterm> GHC tries to
1836 “steal” four registers from GCC, for performance
1837 reasons; it almost always works. However, when GCC is
1838 compiling some modules with four stolen registers, it will
1839 crash, probably saying:
1842 Foo.hc:533: fixed or forbidden register was spilled.
1843 This may be due to a compiler bug or to impossible asm
1844 statements or clauses.
1847 Just give some registers back with
1848 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1849 If `2' doesn't work, please report the bug to us.</para>
1858 <sect1 id="ext-core">
1859 <title>Generating External Core Files</title>
1861 <indexterm><primary>intermediate code generation</primary></indexterm>
1863 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1864 to a file as a side-effect of compilation. Core files, which are given the suffix
1865 <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1866 tools. The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz"
1867 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1868 and sample tools (in Haskell)
1869 for manipulating Core files are available in the GHC source distribution
1870 directory <literal>/fptools/ghc/utils/ext-core</literal>.
1871 Note that the format of <literal>.hcr</literal>
1872 files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1873 for debugging purposes (<xref linkend="options-debugging">).</para>
1878 <term><option>-fext-core</option></term>
1880 <primary><option>-fext-core</option></primary>
1883 <para>Generate <literal>.hcr</literal> files.</para>
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