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><literal>--help</literal></term>
256 <term><literal>-?</literal></term>
257 <indexterm><primary><literal>-?</literal></primary></indexterm>
258 <indexterm><primary><literal>--help</literal></primary></indexterm>
260 <para>Cause GHC to spew a long usage message to standard
261 output and then exit.</para>
266 <term><literal>-v</literal></term>
267 <indexterm><primary><literal>-v</literal></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><literal>-v</literal><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><literal>-v0</literal></term>
297 <para>Disable all non-essential messages (this is the
303 <term><literal>-v1</literal></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><literal>-v2</literal></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><literal>-v3</literal></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><literal>-v4</literal></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><literal>--version</literal></term>
345 <indexterm><primary><literal>--version</literal></primary></indexterm>
347 <para>Print a one-line string including GHC's version number.</para>
352 <term><literal>--numeric-version</literal></term>
353 <indexterm><primary><literal>--numeric-version</literal></primary></indexterm>
355 <para>Print GHC's numeric version number only.</para>
361 <sect1 id="make-mode">
362 <title>Using <command>ghc</command> <option>--make</option></title>
364 <indexterm><primary><option>--make</option></primary>
366 <indexterm><primary>separate compilation</primary>
369 <para>When given the <option>--make</option> option, GHC will
370 build a multi-module Haskell program by following dependencies
371 from a single root module (usually <literal>Main</literal>). For
372 example, if your <literal>Main</literal> module is in a file
373 called <filename>Main.hs</filename>, you could compile and link
374 the program like this:</para>
380 <para>The command line must contain one source file or module
381 name; GHC will figure out all the modules in the program by
382 following the imports from this initial module. It will then
383 attempt to compile each module which is out of date, and finally
384 if the top module is <literal>Main</literal>, the program
385 will also be linked into an executable.</para>
387 <para>The main advantages to using <literal>ghc --make</literal>
388 over traditional <literal>Makefile</literal>s are:</para>
392 <para>GHC doesn't have to be restarted for each compilation,
393 which means it can cache information between compilations.
394 Compiling a muli-module program with <literal>ghc
395 --make</literal> can be up to twice as fast as running
396 <literal>ghc</literal> individually on each source
400 <para>You don't have to write a
401 <literal>Makefile</literal>.</para>
403 <indexterm><primary><literal>Makefile</literal>s</primary><secondary>avoiding</secondary>
406 <para>GHC re-calculates the dependencies each time it is
407 invoked, so the dependencies never get out of sync with the
412 <para>Any of the command-line options described in the rest of
413 this chapter can be used with <option>--make</option>, but note
414 that any options you give on the command line will apply to all
415 the source files compiled, so if you want any options to apply to
416 a single source file only, you'll need to use an
417 <literal>OPTIONS</literal> pragma (see <xref
418 linkend="source-file-options">).</para>
420 <para>If the program needs to be linked with additional objects
421 (say, some auxilliary C code), these can be specified on the
422 command line as usual.</para>
424 <para>Note that GHC can only follow dependencies if it has the
425 source file available, so if your program includes a module for
426 which there is no source file, even if you have an object and an
427 interface file for the module, then GHC will complain. The
428 exception to this rule is for package modules, which may or may
429 not have source files.</para>
432 <Sect1 id="options-order">
433 <title>GHC without <option>--make</option></title>
435 <para>Without <option>--make</option>, GHC will compile one or
436 more source files given on the command line.</para>
438 <para>The first phase to run is determined by each input-file
439 suffix, and the last phase is determined by a flag. If no
440 relevant flag is present, then go all the way through linking.
441 This table summarises:</para>
445 <colspec align="left">
446 <colspec align="left">
447 <colspec align="left">
448 <colspec align="left">
452 <entry>Phase of the compilation system</entry>
453 <entry>Suffix saying “start here”</entry>
454 <entry>Flag saying “stop after”</entry>
455 <entry>(suffix of) output file</entry>
460 <entry>literate pre-processor</entry>
461 <entry><literal>.lhs</literal></entry>
463 <entry><literal>.hs</literal></entry>
467 <entry>C pre-processor (opt.)
469 <entry><literal>.hs</literal> (with
470 <option>-cpp</option>)</entry>
471 <entry><option>-E</option></entry>
472 <entry><literal>.hspp</literal></entry>
476 <entry>Haskell compiler</entry>
477 <entry><literal>.hs</literal></entry>
478 <entry><option>-C</option>, <option>-S</option></entry>
479 <entry><literal>.hc</literal>, <literal>.s</literal></entry>
483 <entry>C compiler (opt.)</entry>
484 <entry><literal>.hc</literal> or <literal>.c</literal></entry>
485 <entry><option>-S</option></entry>
486 <entry><literal>.s</literal></entry>
490 <entry>assembler</entry>
491 <entry><literal>.s</literal></entry>
492 <entry><option>-c</option></entry>
493 <entry><literal>.o</literal></entry>
497 <entry>linker</entry>
498 <entry><replaceable>other</replaceable></entry>
500 <entry><filename>a.out</filename></entry>
506 <indexterm><primary><option>-C</option></primary></indexterm>
507 <indexterm><primary><option>-E</option></primary></indexterm>
508 <indexterm><primary><option>-S</option></primary></indexterm>
509 <indexterm><primary><option>-c</option></primary></indexterm>
511 <para>Thus, a common invocation would be: <literal>ghc -c
512 Foo.hs</literal></para>
514 <para>Note: What the Haskell compiler proper produces depends on
515 whether a native-code generator<indexterm><primary>native-code
516 generator</primary></indexterm> is used (producing assembly
517 language) or not (producing C). See <xref
518 linkend="options-codegen"> for more details.</para>
520 <para>Note: C pre-processing is optional, the
521 <option>-ccp</option><indexterm><primary><option>-cpp</option></primary>
522 </indexterm>flag turns it on. See <xref
523 linkend="c-pre-processor"> for more details.</para>
525 <para>Note: The option <option>-E</option><IndexTerm><Primary>-E
526 option</Primary></IndexTerm> runs just the pre-processing passes
527 of the compiler, dumping the result in a file. Note that this
528 differs from the previous behaviour of dumping the file to
529 standard output.</para>
532 <sect1 id="options-output">
533 <title>Re-directing the compilation output(s)</title>
535 <indexterm><primary>output-directing options</primary></indexterm>
536 <indexterm><primary>redirecting compilation output</primary></indexterm>
541 <term><literal>-o</literal></term>
542 <indexterm><primary><literal>-o</literal></primary></indexterm>
544 <para>GHC's compiled output normally goes into a
545 <filename>.hc</filename>, <filename>.o</filename>, etc.,
546 file, depending on the last-run compilation phase. The
547 option <option>-o foo</option><IndexTerm><Primary>-o
548 option</Primary></IndexTerm> re-directs the output of that
549 last-run phase to file <filename>foo</filename>.</para>
551 <para>Note: this “feature” can be
552 counterintuitive: <command>ghc -C -o foo.o foo.hs</command>
553 will put the intermediate C code in the file
554 <filename>foo.o</filename>, name notwithstanding!</para>
559 <term><literal>-odir</literal></term>
560 <indexterm><primary><literal>-odir</literal></primary></indexterm>
562 <para>The <option>-o</option> option isn't of much use if
563 you have <emphasis>several</emphasis> input files…
564 Non-interface output files are normally put in the same
565 directory as their corresponding input file came from. You
566 may specify that they be put in another directory using the
567 <option>-odir <dir></option><IndexTerm><Primary>-odir
568 <dir> option</Primary></IndexTerm> (the “Oh,
569 dear” option). For example:</para>
572 % ghc -c parse/Foo.hs parse/Bar.hs gurgle/Bumble.hs -odir `arch`
575 <para>The output files, <filename>Foo.o</filename>,
576 <filename>Bar.o</filename>, and
577 <filename>Bumble.o</filename> would be put into a
578 subdirectory named after the architecture of the executing
579 machine (<filename>sun4</filename>,
580 <filename>mips</filename>, etc). The directory must already
581 exist; it won't be created.</para>
583 <para>Note that the <option>-odir</option> option does
584 <emphasis>not</emphasis> affect where the interface files
585 are put. In the above example, they would still be put in
586 <filename>parse/Foo.hi</filename>,
587 <filename>parse/Bar.hi</filename>, and
588 <filename>gurgle/Bumble.hi</filename>.</para>
593 <term><option>-ohi</option> <replaceable>file</replaceable></term>
594 <indexterm><primary><option>-ohi</option></primary>
597 <para>The interface output may be directed to another file
598 <filename>bar2/Wurble.iface</filename> with the option
599 <option>-ohi bar2/Wurble.iface</option> (not
602 <para>WARNING: if you redirect the interface file somewhere
603 that GHC can't find it, then the recompilation checker may
604 get confused (at the least, you won't get any recompilation
605 avoidance). We recommend using a combination of
606 <option>-hidir</option> and <option>-hisuf</option> options
607 instead, if possible.</para>
609 <para>To avoid generating an interface at all, you could use
610 this option to redirect the interface into the bit bucket:
611 <literal>-ohi /dev/null</literal>, for example.</para>
616 <term><option>-hidir</option> <replaceable>directory</replaceable></term>
617 <indexterm><primary><option>-hidir</option></primary>
620 <para>Redirects all generated interface files into
621 <replaceable>directory</replaceable>, instead of the default
622 which is to place the interface file in the same directory
623 as the source file.</para>
628 <term><option>-osuf</option> <replaceable>suffix</replaceable></term>
629 <term><option>-hisuf</option> <replaceable>suffix</replaceable></term>
630 <term><option>-hcsuf</option> <replaceable>suffix</replaceable></term>
631 <indexterm><primary><option>-osuf</option></primary></indexterm>
632 <indexterm><primary><option>-hisuf</option></primary></indexterm>
633 <indexterm><primary><option>-hcsuf</option></primary></indexterm>
635 <para>EXOTICA: The <option>-osuf</option>
636 <replaceable>suffix</replaceable> will change the
637 <literal>.o</literal> file suffix for object files to
638 whatever you specify. We use this when compiling libraries,
639 so that objects for the profiling versions of the libraries
640 don't clobber the normal ones.</para>
642 <para>Similarly, the <option>-hisuf</option>
643 <replaceable>suffix</replaceable> will change the
644 <literal>.hi</literal> file suffix for non-system interface
645 files (see <XRef LinkEnd="hi-options">).</para>
647 <para>Finally, the option <option>-hcsuf</option>
648 <replaceable>suffix</replaceable> will change the
649 <literal>.hc</literal> file suffix for compiler-generated
650 intermediate C files.</para>
652 <para>The <option>-hisuf</option>/<option>-osuf</option>
653 game is useful if you want to compile a program with both
654 GHC and HBC (say) in the same directory. Let HBC use the
655 standard <filename>.hi</filename>/<filename>.o</filename>
656 suffixes; add <option>-hisuf g_hi -osuf
657 g_o</option> to your <command>make</command> rule for
658 GHC compiling…</para>
663 <sect2 id="keeping-intermediates">
664 <title>Keeping Intermediate Files</title>
665 <indexterm><primary>intermediate files, saving</primary>
667 <indexterm><primary><literal>.hc</literal> files, saving</primary>
669 <indexterm><primary><literal>.s</literal> files, saving</primary>
673 <para>The following options are useful for keeping certain
674 intermediate files around, when normally GHC would throw these
675 away after compilation:</para>
679 <term><literal>-keep-hc-files</literal></term>
681 <primary><literal>-keep-hc-files</literal></primary>
684 <para>Keep intermediate <literal>.hc</literal> files when
685 doing <literal>.hs</literal>-to-<literal>.o</literal>
686 compilations via C (NOTE: <literal>.hc</literal> files
687 aren't generated when using the native code generator, you
688 may need to use <literal>-fvia-C</literal> to force them
689 to be produced).</para>
694 <term><literal>-keep-s-files</literal></term>
696 <primary><literal>-keep-s-files</literal></primary>
699 <para>Keep intermediate <literal>.s</literal> files.</para>
704 <term><literal>-keep-raw-s-files</literal></term>
706 <primary><literal>-keep-raw-s-files</literal></primary>
709 <para>Keep intermediate <literal>.raw-s</literal> files.
710 These are the direct output from the C compiler, before
711 GHC does “assembly mangling” to produce the
712 <literal>.s</literal> file. Again, these are not produced
713 when using the native code generator.</para>
718 <term><literal>-keep-tmp-files</literal></term>
720 <primary><literal>-keep-tmp-files</literal></primary>
723 <primary>temporary files</primary>
724 <secondary>keeping</secondary>
727 <para>Instructs the GHC driver not to delete any of its
728 temporary files, which it normally keeps in
729 <literal>/tmp</literal> (or possibly elsewhere; see <xref
730 linkend="temp-files">). Running GHC with
731 <literal>-v</literal> will show you what temporary files
732 were generated along the way.</para>
738 <sect2 id="temp-files">
739 <title>Redirecting temporary files</title>
742 <primary>temporary files</primary>
743 <secondary>redirecting</secondary>
748 <term><literal>-tmpdir</literal></term>
749 <indexterm><primary><literal>-tmpdir</literal></primary></indexterm>
751 <para>If you have trouble because of running out of space
752 in <filename>/tmp</filename> (or wherever your
753 installation thinks temporary files should go), you may
754 use the <option>-tmpdir
755 <dir></option><IndexTerm><Primary>-tmpdir
756 <dir> option</Primary></IndexTerm> option to specify
757 an alternate directory. For example, <option>-tmpdir
758 .</option> says to put temporary files in the current
759 working directory.</para>
761 <para>Alternatively, use your <Constant>TMPDIR</Constant>
762 environment variable.<IndexTerm><Primary>TMPDIR
763 environment variable</Primary></IndexTerm> Set it to the
764 name of the directory where temporary files should be put.
765 GCC and other programs will honour the
766 <Constant>TMPDIR</Constant> variable as well.</para>
768 <para>Even better idea: Set the
769 <Constant>DEFAULT_TMPDIR</Constant> make variable when
770 building GHC, and never worry about
771 <Constant>TMPDIR</Constant> again. (see the build
772 documentation).</para>
780 <sect1 id="options-sanity">
781 <title>Warnings and sanity-checking</title>
783 <indexterm><primary>sanity-checking options</primary></indexterm>
784 <indexterm><primary>warnings</primary></indexterm>
787 <para>GHC has a number of options that select which types of
788 non-fatal error messages, otherwise known as warnings, can be
789 generated during compilation. By default, you get a standard set
790 of warnings which are generally likely to indicate bugs in your
792 <option>-fwarn-overlpapping-patterns</option>,
793 <option>-fwarn-deprecations</option>,
794 <option>-fwarn-duplicate-exports</option>,
795 <option>-fwarn-missing-fields</option>, and
796 <option>-fwarn-missing-methods</option>. The following flags are
797 simple ways to select standard “packages” of warnings:
803 <term><option>-W</option>:</term>
805 <IndexTerm><Primary>-W option</Primary></IndexTerm>
806 <para>Provides the standard warnings plus
807 <option>-fwarn-incomplete-patterns</option>,
808 <option>-fwarn-unused-matches</option>,
809 <option>-fwarn-unused-imports</option>,
810 <option>-fwarn-misc</option>, and
811 <option>-fwarn-unused-binds</option>.</para>
816 <term><option>-w</option>:</term>
818 <IndexTerm><Primary><option>-w</option></Primary></IndexTerm>
819 <para>Turns off all warnings, including the standard ones.</para>
824 <term><option>-Wall</option>:</term>
826 <indexterm><primary><option>-Wall</option></primary></indexterm>
827 <para>Turns on all warning options.</para>
833 <para>The full set of warning options is described below. To turn
834 off any warning, simply give the corresponding
835 <option>-fno-warn-...</option> option on the command line.</para>
840 <term><option>-fwarn-deprecations</option>:</term>
842 <indexterm><primary><option>-fwarn-deprecations</option></primary>
844 <indexterm><primary>deprecations</primary></indexterm>
845 <para>Causes a warning to be emitted when a deprecated
846 function or type is used. Entities can be marked as
847 deprecated using a pragma, see <xref
848 linkend="deprecated-pragma">.</para>
853 <term><option>-fwarn-duplicate-exports</option>:</term>
855 <indexterm><primary><option>-fwarn-duplicate-exports</option></primary></indexterm>
856 <indexterm><primary>duplicate exports, warning</primary></indexterm>
857 <indexterm><primary>export lists, duplicates</primary></indexterm>
859 <para>Have the compiler warn about duplicate entries in
860 export lists. This is useful information if you maintain
861 large export lists, and want to avoid the continued export
862 of a definition after you've deleted (one) mention of it in
863 the export list.</para>
865 <para>This option is on by default.</para>
870 <term><option>-fwarn-hi-shadowing</option>:</term>
872 <indexterm><primary><option>-fwarn-hi-shadowing</option></primary></indexterm>
873 <indexterm><primary>shadowing</primary>
874 <secondary>interface files</secondary></indexterm>
876 <para>Causes the compiler to emit a warning when a module or
877 interface file in the current directory is shadowing one
878 with the same module name in a library or other
884 <term><option>-fwarn-incomplete-patterns</option>:</term>
886 <indexterm><primary><option>-fwarn-incomplete-patterns</option></primary></indexterm>
887 <indexterm><primary>incomplete patterns, warning</primary></indexterm>
888 <indexterm><primary>patterns, incomplete</primary></indexterm>
890 <para>Similarly for incomplete patterns, the function
891 <function>g</function> below will fail when applied to
892 non-empty lists, so the compiler will emit a warning about
893 this when <option>-fwarn-incomplete-patterns</option> is
900 <para>This option isn't enabled be default because it can be
901 a bit noisy, and it doesn't always indicate a bug in the
902 program. However, it's generally considered good practice
903 to cover all the cases in your functions.</para>
908 <term><option>-fwarn-misc</option>:</term>
909 <indexterm><primary><option>-fwarn-misc</option></primary></indexterm>
911 <para>Turns on warnings for various harmless but untidy
912 things. This currently includes: importing a type with
913 <literal>(..)</literal> when the export is abstract, and
914 listing duplicate class assertions in a qualified type.</para>
919 <term><option>-fwarn-missing-fields</option>:</term>
921 <indexterm><primary><option>-fwarn-missing-fields</option></primary></indexterm>
922 <indexterm><primary>missing fields, warning</primary></indexterm>
923 <indexterm><primary>fields, missing</primary></indexterm>
925 <para>This option is on by default, and warns you whenever
926 the construction of a labelled field constructor isn't
927 complete, missing initializers for one or more fields. While
928 not an error (the missing fields are initialised with
929 bottoms), it is often an indication of a programmer error.</para>
934 <term><option>-fwarn-missing-methods</option>:</term>
936 <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>
937 <indexterm><primary>missing methods, warning</primary></indexterm>
938 <indexterm><primary>methods, missing</primary></indexterm>
940 <para>This option is on by default, and warns you whenever
941 an instance declaration is missing one or more methods, and
942 the corresponding class declaration has no default
943 declaration for them.</para>
948 <term><option>-fwarn-missing-signatures</option>:</term>
950 <indexterm><primary><option>-fwarn-missing-signatures</option></primary></indexterm>
951 <indexterm><primary>type signatures, missing</primary></indexterm>
953 <para>If you would like GHC to check that every top-level
954 function/value has a type signature, use the
955 <option>-fwarn-missing-signatures</option> option. This
956 option is off by default.</para>
961 <term><option>-fwarn-name-shadowing</option>:</term>
963 <indexterm><primary><option>-fwarn-name-shadowing</option></primary></indexterm>
964 <indexterm><primary>shadowing, warning</primary></indexterm>
966 <para>This option causes a warning to be emitted whenever an
967 inner-scope value has the same name as an outer-scope value,
968 i.e. the inner value shadows the outer one. This can catch
969 typographical errors that turn into hard-to-find bugs, e.g.,
970 in the inadvertent cyclic definition <literal>let x = ... x
971 ... in</literal>.</para>
973 <para>Consequently, this option does
974 <emphasis>will</emphasis> complain about cyclic recursive
980 <term><option>-fwarn-overlapping-patterns</option>:</term>
981 <indexterm><primary><option>-fwarn-overlapping-patterns</option></primary></indexterm>
982 <indexterm><primary>overlapping patterns, warning</primary></indexterm>
983 <indexterm><primary>patterns, overlapping</primary></indexterm>
985 <para>By default, the compiler will warn you if a set of
986 patterns are overlapping, i.e.,</para>
989 f :: String -> Int
995 <para>where the last pattern match in <Function>f</Function>
996 won't ever be reached, as the second pattern overlaps
997 it. More often than not, redundant patterns is a programmer
998 mistake/error, so this option is enabled by default.</para>
1003 <term><option>-fwarn-simple-patterns</option>:</term>
1005 <indexterm><primary><option>-fwarn-simple-patterns</option></primary>
1007 <para>Causes the compiler to warn about lambda-bound
1008 patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
1009 Normally, these aren't treated as incomplete patterns by
1010 <option>-fwarn-incomplete-patterns</option>.</para>
1015 <term><option>-fwarn-type-defaults</option>:</term>
1017 <indexterm><primary><option>-fwarn-type-defaults</option></primary></indexterm>
1018 <indexterm><primary>defaulting mechanism, warning</primary></indexterm>
1019 <para>Have the compiler warn/inform you where in your source
1020 the Haskell defaulting mechanism for numeric types kicks
1021 in. This is useful information when converting code from a
1022 context that assumed one default into one with another,
1023 e.g., the `default default' for Haskell 1.4 caused the
1024 otherwise unconstrained value <Constant>1</Constant> to be
1025 given the type <literal>Int</literal>, whereas Haskell 98
1026 defaults it to <literal>Integer</literal>. This may lead to
1027 differences in performance and behaviour, hence the
1028 usefulness of being non-silent about this.</para>
1030 <para>This warning is off by default.</para>
1035 <term><option>-fwarn-unused-binds</option>:</term>
1037 <indexterm><primary><option>-fwarn-unused-binds</option></primary></indexterm>
1038 <indexterm><primary>unused binds, warning</primary></indexterm>
1039 <indexterm><primary>binds, unused</primary></indexterm>
1040 <para>Report any function definitions (and local bindings)
1041 which are unused. For top-level functions, the warning is
1042 only given if the binding is not exported.</para>
1047 <term><option>-fwarn-unused-imports</option>:</term>
1049 <indexterm><primary><option>-fwarn-unused-imports</option></primary></indexterm>
1050 <indexterm><primary>unused imports, warning</primary></indexterm>
1051 <indexterm><primary>imports, unused</primary></indexterm>
1053 <para>Report any objects that are explicitly imported but
1059 <term><option>-fwarn-unused-matches</option>:</term>
1061 <indexterm><primary><option>-fwarn-unused-matches</option></primary></indexterm>
1062 <indexterm><primary>unused matches, warning</primary></indexterm>
1063 <indexterm><primary>matches, unused</primary></indexterm>
1065 <para>Report all unused variables which arise from pattern
1066 matches, including patterns consisting of a single variable.
1067 For instance <literal>f x y = []</literal> would report
1068 <VarName>x</VarName> and <VarName>y</VarName> as unused. To
1069 eliminate the warning, all unused variables can be replaced
1070 with wildcards.</para>
1076 <para>If you're feeling really paranoid, the
1077 <option>-dcore-lint</option>
1078 option<indexterm><primary><option>-dcore-lint</option></primary></indexterm>
1079 is a good choice. It turns on heavyweight intra-pass
1080 sanity-checking within GHC. (It checks GHC's sanity, not
1088 <sect1 id="options-optimise">
1089 <title>Optimisation (code improvement)</title>
1091 <indexterm><primary>optimisation</primary></indexterm>
1092 <indexterm><primary>improvement, code</primary></indexterm>
1094 <para>The <option>-O*</option> options specify convenient
1095 “packages” of optimisation flags; the
1096 <option>-f*</option> options described later on specify
1097 <emphasis>individual</emphasis> optimisations to be turned on/off;
1098 the <option>-m*</option> options specify
1099 <emphasis>machine-specific</emphasis> optimisations to be turned
1102 <sect2 id="optimise-pkgs">
1103 <title><option>-O*</option>: convenient “packages” of optimisation flags.</title>
1105 <para>There are <emphasis>many</emphasis> options that affect
1106 the quality of code produced by GHC. Most people only have a
1107 general goal, something like “Compile quickly” or
1108 “Make my program run like greased lightning.” The
1109 following “packages” of optimisations (or lack
1110 thereof) should suffice.</para>
1112 <para>Once you choose a <option>-O*</option>
1113 “package,” stick with it—don't chop and
1114 change. Modules' interfaces <emphasis>will</emphasis> change
1115 with a shift to a new <option>-O*</option> option, and you may
1116 have to recompile a large chunk of all importing modules before
1117 your program can again be run safely (see <XRef
1118 LinkEnd="recomp">).</para>
1123 <term>No <option>-O*</option>-type option specified:</term>
1124 <indexterm><primary>-O* not specified</primary></indexterm>
1126 <para>This is taken to mean: “Please compile
1127 quickly; I'm not over-bothered about compiled-code
1128 quality.” So, for example: <command>ghc -c
1129 Foo.hs</command></para>
1134 <term><option>-O0</option>:</term>
1135 <indexterm><primary><option>-O0</option></primary></indexterm>
1137 <para>Means “turn off all optimisation”,
1138 reverting to the same settings as if no
1139 <option>-O</option> options had been specified. Saying
1140 <option>-O0</option> can be useful if
1141 eg. <command>make</command> has inserted a
1142 <option>-O</option> on the command line already.</para>
1147 <term><option>-O</option> or <option>-O1</option>:</term>
1148 <indexterm><primary>-O option</primary></indexterm>
1149 <indexterm><primary>-O1 option</primary></indexterm>
1150 <indexterm><primary>optimise</primary><secondary>normally</secondary></indexterm>
1152 <para>Means: “Generate good-quality code without
1153 taking too long about it.” Thus, for example:
1154 <command>ghc -c -O Main.lhs</command></para>
1159 <term><option>-O2</option>:</term>
1160 <indexterm><primary>-O2 option</primary></indexterm>
1161 <indexterm><primary>optimise</primary><secondary>aggressively</secondary></indexterm>
1163 <para>Means: “Apply every non-dangerous
1164 optimisation, even if it means significantly longer
1165 compile times.”</para>
1167 <para>The avoided “dangerous” optimisations
1168 are those that can make runtime or space
1169 <emphasis>worse</emphasis> if you're unlucky. They are
1170 normally turned on or off individually.</para>
1172 <para>At the moment, <option>-O2</option> is
1173 <emphasis>unlikely</emphasis> to produce better code than
1174 <option>-O</option>.</para>
1179 <term><option>-O2-for-C</option>:</term>
1180 <indexterm><primary>-O2-for-C option</primary></indexterm>
1181 <indexterm><primary>gcc, invoking with -O2</primary></indexterm>
1183 <para>Says to run GCC with <option>-O2</option>, which may
1184 be worth a few percent in execution speed. Don't forget
1185 <option>-fvia-C</option>, lest you use the native-code
1186 generator and bypass GCC altogether!</para>
1188 <para><emphasis>Note: some versions of gcc are known to
1189 have code generation bugs with <option>-O2</option>. Use
1190 this option at your own risk! But we'd be keen to here
1191 any reports of whether (a) it works or (b) it improves
1192 performance at all.</emphasis></para>
1197 <term><option>-Ofile <file></option>:</term>
1198 <indexterm><primary>-Ofile <file> option</primary></indexterm>
1199 <indexterm><primary>optimising, customised</primary></indexterm>
1201 <para>(NOTE: not supported yet in GHC 5.x. Please ask if
1202 you're interested in this.)</para>
1204 <para>For those who need <emphasis>absolute</emphasis>
1205 control over <emphasis>exactly</emphasis> what options are
1206 used (e.g., compiler writers, sometimes :-), a list of
1207 options can be put in a file and then slurped in with
1208 <option>-Ofile</option>.</para>
1210 <para>In that file, comments are of the
1211 <literal>#</literal>-to-end-of-line variety; blank
1212 lines and most whitespace is ignored.</para>
1214 <para>Please ask if you are baffled and would like an
1215 example of <option>-Ofile</option>!</para>
1220 <para>We don't use a <option>-O*</option> flag for day-to-day
1221 work. We use <option>-O</option> to get respectable speed;
1222 e.g., when we want to measure something. When we want to go for
1223 broke, we tend to use <option>-O -fvia-C</option> (and we go for
1224 lots of coffee breaks).</para>
1226 <para>The easiest way to see what <option>-O</option> (etc.)
1227 “really mean” is to run with <option>-v</option>,
1228 then stand back in amazement.</para>
1231 <sect2 id="options-f">
1232 <title><option>-f*</option>: platform-independent flags</title>
1234 <indexterm><primary>-f* options (GHC)</primary></indexterm>
1235 <indexterm><primary>-fno-* options (GHC)</primary></indexterm>
1237 <para>These flags turn on and off individual optimisations.
1238 They are normally set via the <option>-O</option> options
1239 described above, and as such, you shouldn't need to set any of
1240 them explicitly (indeed, doing so could lead to unexpected
1241 results). However, there are one or two that may be of
1246 <term><option>-fexcess-precision</option>:</term>
1248 <indexterm><primary><option>-fexcess-precision</option></primary></indexterm>
1249 <para>When this option is given, intermediate floating
1250 point values can have a <emphasis>greater</emphasis>
1251 precision/range than the final type. Generally this is a
1252 good thing, but some programs may rely on the exact
1254 <literal>Float</literal>/<literal>Double</literal> values
1255 and should not use this option for their compilation.</para>
1260 <term><option>-fignore-asserts</option>:</term>
1262 <indexterm><primary><option>-fignore-asserts</option></primary></indexterm>
1263 <para>Causes GHC to ignore uses of the function
1264 <literal>Exception.assert</literal> in source code (in
1265 other words, rewriting <literal>Exception.assert p
1266 e</literal> to <literal>e</literal> (see <xref
1267 linkend="sec-assertions">). This flag is turned on by
1268 <option>-O</option>.
1274 <term><option>-fno-strictness</option></term>
1275 <indexterm><primary><option>-fno-strictness</option></primary>
1278 <para>Turns off the strictness analyser; sometimes it eats
1279 too many cycles.</para>
1284 <term><option>-fno-cpr-analyse</option></term>
1285 <indexterm><primary><option>-fno-cpr-analyse</option></primary>
1288 <para>Turns off the CPR (constructed product result)
1289 analysis; it is somewhat experimental.</para>
1294 <term><option>-funbox-strict-fields</option>:</term>
1296 <indexterm><primary><option>-funbox-strict-fields</option></primary></indexterm>
1297 <indexterm><primary>strict constructor fields</primary></indexterm>
1298 <indexterm><primary>constructor fields, strict</primary></indexterm>
1300 <para>This option causes all constructor fields which are
1301 marked strict (i.e. “!”) to be unboxed or
1302 unpacked if possible. For example:</para>
1305 data T = T !Float !Float
1308 <para>will create a constructor <literal>T</literal>
1309 containing two unboxed floats if the
1310 <option>-funbox-strict-fields</option> flag is given.
1311 This may not always be an optimisation: if the
1312 <Function>T</Function> constructor is scrutinised and the
1313 floats passed to a non-strict function for example, they
1314 will have to be reboxed (this is done automatically by the
1317 <para>This option should only be used in conjunction with
1318 <option>-O</option>, in order to expose unfoldings to the
1319 compiler so the reboxing can be removed as often as
1320 possible. For example:</para>
1324 f (T f1 f2) = f1 + f2
1327 <para>The compiler will avoid reboxing
1328 <Function>f1</Function> and <Function>f2</Function> by
1329 inlining <Function>+</Function> on floats, but only when
1330 <option>-O</option> is on.</para>
1332 <para>Any single-constructor data is eligible for
1333 unpacking; for example</para>
1336 data T = T !(Int,Int)
1339 <para>will store the two <literal>Int</literal>s directly
1340 in the <Function>T</Function> constructor, by flattening
1341 the pair. Multi-level unpacking is also supported:</para>
1345 data S = S !Int !Int
1348 <para>will store two unboxed <literal>Int#</literal>s
1349 directly in the <Function>T</Function> constructor.</para>
1354 <term><option>-funfolding-update-in-place<n></option></term>
1355 <indexterm><primary><option>-funfolding-update-in-place</option></primary></indexterm>
1357 <para>Switches on an experimental "optimisation".
1358 Switching it on makes the compiler a little keener to
1359 inline a function that returns a constructor, if the
1360 context is that of a thunk.
1364 If we inlined plusInt we might get an opportunity to use
1365 update-in-place for the thunk 'x'.</para>
1370 <term><option>-funfolding-creation-threshold<n></option>:</term>
1372 <indexterm><primary><option>-funfolding-creation-threshold</option></primary></indexterm>
1373 <indexterm><primary>inlining, controlling</primary></indexterm>
1374 <indexterm><primary>unfolding, controlling</primary></indexterm>
1376 <para>(Default: 45) Governs the maximum size that GHC will
1377 allow a function unfolding to be. (An unfolding has a
1378 “size” that reflects the cost in terms of
1379 “code bloat” of expanding that unfolding at
1380 at a call site. A bigger function would be assigned a
1381 bigger cost.) </para>
1383 <para> Consequences: (a) nothing larger than this will be
1384 inlined (unless it has an INLINE pragma); (b) nothing
1385 larger than this will be spewed into an interface
1389 <para> Increasing this figure is more likely to result in longer
1390 compile times than faster code. The next option is more
1396 <term><option>-funfolding-use-threshold<n></option>:</term>
1398 <indexterm><primary><option>-funfolding-use-threshold</option></primary></indexterm>
1399 <indexterm><primary>inlining, controlling</primary></indexterm>
1400 <indexterm><primary>unfolding, controlling</primary></indexterm>
1402 <para>(Default: 8) This is the magic cut-off figure for
1403 unfolding: below this size, a function definition will be
1404 unfolded at the call-site, any bigger and it won't. The
1405 size computed for a function depends on two things: the
1406 actual size of the expression minus any discounts that
1407 apply (see <option>-funfolding-con-discount</option>).</para>
1418 <Sect1 id="sec-using-concurrent">
1419 <title>Using Concurrent Haskell</title>
1422 <indexterm><primary>Concurrent Haskell—use</primary></indexterm>
1426 GHC (as of version 4.00) supports Concurrent Haskell by default,
1427 without requiring a special option or libraries compiled in a certain
1428 way. To get access to the support libraries for Concurrent Haskell
1429 (i.e. <literal>Concurrent</literal> and friends), use the
1430 <option>-package concurrent</option> option.
1434 Three RTS options are provided for modifying the behaviour of the
1435 threaded runtime system. See the descriptions of
1436 <option>-C[<us>]</option>, <option>-q</option>, and
1437 <option>-t<num></option> in <XRef LinkEnd="parallel-rts-opts">.
1441 Concurrent Haskell is described in more detail in <XRef
1442 LinkEnd="sec-Concurrent">.
1447 <Sect1 id="sec-using-parallel">
1448 <title>Using Parallel Haskell</title>
1451 <indexterm><primary>Parallel Haskell—use</primary></indexterm>
1455 [You won't be able to execute parallel Haskell programs unless PVM3
1456 (Parallel Virtual Machine, version 3) is installed at your site.]
1460 To compile a Haskell program for parallel execution under PVM, use the
1461 <Option>-parallel</Option> option,<IndexTerm><Primary>-parallel
1462 option</Primary></IndexTerm> both when compiling <Emphasis>and
1463 linking</Emphasis>. You will probably want to <Literal>import
1464 Parallel</Literal> into your Haskell modules.
1468 To run your parallel program, once PVM is going, just invoke it
1469 “as normal”. The main extra RTS option is
1470 <Option>-qp<n></Option>, to say how many PVM
1471 “processors” your program to run on. (For more details of
1472 all relevant RTS options, please see <XRef
1473 LinkEnd="parallel-rts-opts">.)
1477 In truth, running Parallel Haskell programs and getting information
1478 out of them (e.g., parallelism profiles) is a battle with the vagaries of
1479 PVM, detailed in the following sections.
1482 <Sect2 id="pvm-dummies">
1483 <Title>Dummy's guide to using PVM</Title>
1486 <indexterm><primary>PVM, how to use</primary></indexterm>
1487 <indexterm><primary>Parallel Haskell—PVM use</primary></indexterm>
1488 Before you can run a parallel program under PVM, you must set the
1489 required environment variables (PVM's idea, not ours); something like,
1490 probably in your <filename>.cshrc</filename> or equivalent:
1493 setenv PVM_ROOT /wherever/you/put/it
1494 setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
1495 setenv PVM_DPATH $PVM_ROOT/lib/pvmd
1501 Creating and/or controlling your “parallel machine” is a purely-PVM
1502 business; nothing specific to Parallel Haskell. The following paragraphs
1503 describe how to configure your parallel machine interactively.
1507 If you use parallel Haskell regularly on the same machine configuration it
1508 is a good idea to maintain a file with all machine names and to make the
1509 environment variable PVM_HOST_FILE point to this file. Then you can avoid
1510 the interactive operations described below by just saying
1518 You use the <Command>pvm</Command><IndexTerm><Primary>pvm command</Primary></IndexTerm> command to start PVM on your
1519 machine. You can then do various things to control/monitor your
1520 “parallel machine;” the most useful being:
1526 <ColSpec Align="Left">
1530 <entry><KeyCombo><KeyCap>Control</KeyCap><KeyCap>D</KeyCap></KeyCombo></entry>
1531 <entry>exit <command>pvm</command>, leaving it running</entry>
1535 <entry><command>halt</command></entry>
1536 <entry>kill off this “parallel machine” & exit</entry>
1540 <entry><command>add <host></command></entry>
1541 <entry>add <command><host></command> as a processor</entry>
1545 <entry><command>delete <host></command></entry>
1546 <entry>delete <command><host></command></entry>
1550 <entry><command>reset</command></entry>
1551 <entry>kill what's going, but leave PVM up</entry>
1555 <entry><command>conf</command></entry>
1556 <entry>list the current configuration</entry>
1560 <entry><command>ps</command></entry>
1561 <entry>report processes' status</entry>
1565 <entry><command>pstat <pid></command></entry>
1566 <entry>status of a particular process</entry>
1575 The PVM documentation can tell you much, much more about <command>pvm</command>!
1580 <Sect2 id="par-profiles">
1581 <Title>Parallelism profiles</Title>
1584 <indexterm><primary>parallelism profiles</primary></indexterm>
1585 <indexterm><primary>profiles, parallelism</primary></indexterm>
1586 <indexterm><primary>visualisation tools</primary></indexterm>
1590 With Parallel Haskell programs, we usually don't care about the
1591 results—only with “how parallel” it was! We want pretty pictures.
1595 Parallelism profiles (à la <Command>hbcpp</Command>) can be generated with the
1596 <Option>-qP</Option><IndexTerm><Primary>-qP RTS option (concurrent, parallel)</Primary></IndexTerm> RTS option. The
1597 per-processor profiling info is dumped into files named
1598 <Filename><full-path><program>.gr</Filename>. These are then munged into a PostScript picture,
1599 which you can then display. For example, to run your program
1600 <Filename>a.out</Filename> on 8 processors, then view the parallelism profile, do:
1606 <prompt>$</prompt> ./a.out +RTS -qP -qp8
1607 <prompt>$</prompt> grs2gr *.???.gr > temp.gr # combine the 8 .gr files into one
1608 <prompt>$</prompt> gr2ps -O temp.gr # cvt to .ps; output in temp.ps
1609 <prompt>$</prompt> ghostview -seascape temp.ps # look at it!
1615 The scripts for processing the parallelism profiles are distributed
1616 in <filename>ghc/utils/parallel/</filename>.
1622 <Title>Other useful info about running parallel programs</Title>
1625 The “garbage-collection statistics” RTS options can be useful for
1626 seeing what parallel programs are doing. If you do either
1627 <Option>+RTS -Sstderr</Option><IndexTerm><Primary>-Sstderr RTS option</Primary></IndexTerm> or <Option>+RTS -sstderr</Option>, then
1628 you'll get mutator, garbage-collection, etc., times on standard
1629 error. The standard error of all PE's other than the `main thread'
1630 appears in <filename>/tmp/pvml.nnn</filename>, courtesy of PVM.
1634 Whether doing <option>+RTS -Sstderr</option> or not, a handy way to watch
1635 what's happening overall is: <command>tail -f /tmp/pvml.nnn</command>.
1640 <Sect2 id="parallel-rts-opts">
1641 <title>RTS options for Concurrent/Parallel Haskell
1645 <indexterm><primary>RTS options, concurrent</primary></indexterm>
1646 <indexterm><primary>RTS options, parallel</primary></indexterm>
1647 <indexterm><primary>Concurrent Haskell—RTS options</primary></indexterm>
1648 <indexterm><primary>Parallel Haskell—RTS options</primary></indexterm>
1652 Besides the usual runtime system (RTS) options
1653 (<XRef LinkEnd="runtime-control">), there are a few options particularly
1654 for concurrent/parallel execution.
1661 <Term><Option>-qp<N></Option>:</Term>
1664 <IndexTerm><Primary>-qp<N> RTS option</Primary></IndexTerm>
1665 (PARALLEL ONLY) Use <Literal><N></Literal> PVM processors to run this program;
1671 <term><option>-C[<us>]</option>:</term>
1674 <indexterm><primary>-C<us> RTS option</primary></indexterm> Sets
1675 the context switch interval to <literal><s></literal> seconds.
1676 A context switch will occur at the next heap block allocation after
1677 the timer expires (a heap block allocation occurs every 4k of
1678 allocation). With <option>-C0</option> or <option>-C</option>,
1679 context switches will occur as often as possible (at every heap block
1680 allocation). By default, context switches occur every 20ms
1681 milliseconds. Note that GHC's internal timer ticks every 20ms, and
1682 the context switch timer is always a multiple of this timer, so 20ms
1683 is the maximum granularity available for timed context switches.
1688 <term><option>-q[v]</option>:</term>
1691 <indexterm><primary>-q RTS option</primary></indexterm>
1692 (PARALLEL ONLY) Produce a quasi-parallel profile of thread activity,
1693 in the file <FIlename><program>.qp</FIlename>. In the style of <command>hbcpp</command>, this profile
1694 records the movement of threads between the green (runnable) and red
1695 (blocked) queues. If you specify the verbose suboption (<option>-qv</option>), the
1696 green queue is split into green (for the currently running thread
1697 only) and amber (for other runnable threads). We do not recommend
1698 that you use the verbose suboption if you are planning to use the
1699 <Command>hbcpp</Command> profiling tools or if you are context switching at every heap
1700 check (with <Option>-C</Option>).
1706 <Term><Option>-qt<num></Option>:</Term>
1709 <IndexTerm><Primary>-qt<num> RTS option</Primary></IndexTerm>
1710 (PARALLEL ONLY) Limit the thread pool size, i.e. the number of concurrent
1711 threads per processor to <Literal><num></Literal>. The default is
1712 32. Each thread requires slightly over 1K <Emphasis>words</Emphasis> in
1713 the heap for thread state and stack objects. (For 32-bit machines, this
1714 translates to 4K bytes, and for 64-bit machines, 8K bytes.)
1720 <Term><Option>-d</Option>:</Term>
1723 <IndexTerm><Primary>-d RTS option (parallel)</Primary></IndexTerm>
1724 (PARALLEL ONLY) Turn on debugging. It pops up one xterm (or GDB, or
1725 something…) per PVM processor. We use the standard <Command>debugger</Command>
1726 script that comes with PVM3, but we sometimes meddle with the
1727 <Command>debugger2</Command> script. We include ours in the GHC distribution,
1728 in <Filename>ghc/utils/pvm/</Filename>.
1734 <Term><Option>-qe<num></Option>:</Term>
1737 <IndexTerm><Primary>-qe<num> RTS option
1738 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Limit the spark pool size
1739 i.e. the number of pending sparks per processor to
1740 <Literal><num></Literal>. The default is 100. A larger number may be
1741 appropriate if your program generates large amounts of parallelism
1747 <Term><Option>-qQ<num></Option>:</Term>
1750 <IndexTerm><Primary>-qQ<num> RTS option (parallel)</Primary></IndexTerm>
1751 (PARALLEL ONLY) Set the size of packets transmitted between processors
1752 to <Literal><num></Literal>. The default is 1024 words. A larger number may be
1753 appropriate if your machine has a high communication cost relative to
1759 <Term><Option>-qh<num></Option>:</Term>
1762 <IndexTerm><Primary>-qh<num> RTS option (parallel)</Primary></IndexTerm>
1763 (PARALLEL ONLY) Select a packing scheme. Set the number of non-root thunks to pack in one packet to
1764 <num>-1 (0 means infinity). By default GUM uses full-subgraph
1765 packing, i.e. the entire subgraph with the requested closure as root is
1766 transmitted (provided it fits into one packet). Choosing a smaller value
1767 reduces the amount of pre-fetching of work done in GUM. This can be
1768 advantageous for improving data locality but it can also worsen the balance
1769 of the load in the system.
1774 <Term><Option>-qg<num></Option>:</Term>
1777 <IndexTerm><Primary>-qg<num> RTS option
1778 (parallel)</Primary></IndexTerm> (PARALLEL ONLY) Select a globalisation
1779 scheme. This option affects the
1780 generation of global addresses when transferring data. Global addresses are
1781 globally unique identifiers required to maintain sharing in the distributed
1782 graph structure. Currently this is a binary option. With <num>=0 full globalisation is used
1783 (default). This means a global address is generated for every closure that
1784 is transmitted. With <num>=1 a thunk-only globalisation scheme is
1785 used, which generated global address only for thunks. The latter case may
1786 lose sharing of data but has a reduced overhead in packing graph structures
1787 and maintaining internal tables of global addresses.
1798 <sect1 id="options-platform">
1799 <title>Platform-specific Flags</title>
1801 <indexterm><primary>-m* options</primary></indexterm>
1802 <indexterm><primary>platform-specific options</primary></indexterm>
1803 <indexterm><primary>machine-specific options</primary></indexterm>
1805 <para>Some flags only make sense for particular target
1811 <term><option>-mv8</option>:</term>
1813 <para>(SPARC machines)<indexterm><primary>-mv8 option (SPARC
1814 only)</primary></indexterm> Means to pass the like-named
1815 option to GCC; it says to use the Version 8 SPARC
1816 instructions, notably integer multiply and divide. The
1817 similiar <option>-m*</option> GCC options for SPARC also
1818 work, actually.</para>
1823 <term><option>-monly-[32]-regs</option>:</term>
1825 <para>(iX86 machines)<indexterm><primary>-monly-N-regs
1826 option (iX86 only)</primary></indexterm> GHC tries to
1827 “steal” four registers from GCC, for performance
1828 reasons; it almost always works. However, when GCC is
1829 compiling some modules with four stolen registers, it will
1830 crash, probably saying:
1833 Foo.hc:533: fixed or forbidden register was spilled.
1834 This may be due to a compiler bug or to impossible asm
1835 statements or clauses.
1838 Just give some registers back with
1839 <option>-monly-N-regs</option>. Try `3' first, then `2'.
1840 If `2' doesn't work, please report the bug to us.</para>
1849 <sect1 id="ext-core">
1850 <title>Generating External Core Files</title>
1852 <indexterm><primary>intermediate code generation</primary></indexterm>
1854 <para>GHC can dump its optimized intermediate code (said to be in “Core” format)
1855 to a file as a side-effect of compilation. Core files, which are given the suffix
1856 <filename>.hcr</filename>, can be read and processed by non-GHC back-end
1857 tools. The Core format is formally described in <ulink url="http://www.haskell.org/ghc/docs/papers/core.ps.gz"
1858 <citetitle>An External Representation for the GHC Core Language</citetitle></ulink>,
1859 and sample tools (in Haskell)
1860 for manipulating Core files are available in the GHC source distribution
1861 directory <literal>/fptools/ghc/utils/ext-core</literal>.
1862 Note that the format of <literal>.hcr</literal>
1863 files is <emphasis>different</emphasis> (though similar) to the Core output format generated
1864 for debugging purposes (<xref linkend="options-debugging">).</para>
1869 <term><literal>-fext-core</literal></term>
1871 <primary><literal>-fext-core</literal></primary>
1874 <para>Generate <literal>.hcr</literal> files.</para>
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