1 <Sect1 id="options-debugging">
2 <Title>Debugging the compiler
6 <IndexTerm><Primary>debugging options (for GHC)</Primary></IndexTerm>
10 HACKER TERRITORY. HACKER TERRITORY.
14 <Sect2 id="replacing-phases">
15 <Title>Replacing the program for one or more phases.
19 <IndexTerm><Primary>GHC phases, changing</Primary></IndexTerm>
20 <IndexTerm><Primary>phases, changing GHC</Primary></IndexTerm>
21 You may specify that a different program be used for one of the phases
22 of the compilation system, in place of whatever the driver <Command>ghc</Command> has
23 wired into it. For example, you might want to try a different
25 <Option>-pgm<phase-code><program-name></Option><IndexTerm><Primary>-pgm<phase><stuff>
26 option</Primary></IndexTerm> option to <Command>ghc</Command> will cause it to use <Literal><program-name></Literal>
27 for phase <Literal><phase-code></Literal>, where the codes to indicate the phases are:
33 <ColSpec Align="Left" Colsep="0">
34 <ColSpec Align="Left" Colsep="0">
37 <Entry><Emphasis>code</Emphasis> </Entry>
38 <Entry><Emphasis>phase</Emphasis> </Entry>
44 <Entry> literate pre-processor </Entry>
49 <Entry> C pre-processor (if -cpp only) </Entry>
54 <Entry> Haskell compiler </Entry>
59 <Entry> C compiler</Entry>
64 <Entry> assembler </Entry>
69 <Entry> linker </Entry>
74 <Entry> Makefile dependency generator </Entry>
85 <Sect2 id="forcing-options-through">
86 <Title>Forcing options to a particular phase.
90 <IndexTerm><Primary>forcing GHC-phase options</Primary></IndexTerm>
94 The preceding sections describe driver options that are mostly
95 applicable to one particular phase. You may also <Emphasis>force</Emphasis> a
96 specific option <Option><option></Option> to be passed to a particular phase
97 <Literal><phase-code></Literal> by feeding the driver the option
98 <Option>-opt<phase-code><option></Option>.<IndexTerm><Primary>-opt<phase><stuff>
99 option</Primary></IndexTerm> The codes to indicate the phases are the same as in the
104 So, for example, to force an <Option>-Ewurble</Option> option to the assembler, you
105 would tell the driver <Option>-opta-Ewurble</Option> (the dash before the E is
110 Besides getting options to the Haskell compiler with <Option>-optC<blah></Option>,
111 you can get options through to its runtime system with
112 <Option>-optCrts<blah></Option><IndexTerm><Primary>-optCrts<blah> option</Primary></IndexTerm>.
116 So, for example: when I want to use my normal driver but with my
117 profiled compiler binary, I use this script:
121 exec /local/grasp_tmp3/simonpj/ghc-BUILDS/working-alpha/ghc/driver/ghc \
122 -pgmC/local/grasp_tmp3/simonpj/ghc-BUILDS/working-hsc-prof/hsc \
132 <Sect2 id="dumping-output">
133 <Title>Dumping out compiler intermediate structures
137 <IndexTerm><Primary>dumping GHC intermediates</Primary></IndexTerm>
138 <IndexTerm><Primary>intermediate passes, output</Primary></IndexTerm>
145 <Term><Option>-noC</Option>:</Term>
148 <IndexTerm><Primary>-noC option</Primary></IndexTerm>
149 Don't bother generating C output <Emphasis>or</Emphasis> an interface file. Usually
150 used in conjunction with one or more of the <Option>-ddump-*</Option> options; for
151 example: <Command>ghc -noC -ddump-simpl Foo.hs</Command>
156 <Term><Option>-hi</Option>:</Term>
159 <IndexTerm><Primary>-hi option</Primary></IndexTerm>
160 <Emphasis>Do</Emphasis> generate an interface file. This would normally be used in
161 conjunction with <Option>-noC</Option>, which turns off interface generation;
162 thus: <Option>-noC -hi</Option>.
167 <Term><Option>-dshow-passes</Option>:</Term>
170 <IndexTerm><Primary>-dshow-passes option</Primary></IndexTerm>
171 Prints a message to stderr as each pass starts. Gives a warm but
172 undoubtedly misleading feeling that GHC is telling you what's
178 <Term><Option>-ddump-<pass></Option>:</Term>
181 <IndexTerm><Primary>-ddump-<pass> options</Primary></IndexTerm>
182 Make a debugging dump after pass <Literal><pass></Literal> (may be common enough to
183 need a short form…). You can get all of these at once (<Emphasis>lots</Emphasis> of
184 output) by using <Option>-ddump-all</Option>, or most of them with <Option>-ddump-most</Option>.
185 Some of the most useful ones are:
192 <Term><Option>-ddump-parsed</Option>:</Term>
200 <Term><Option>-ddump-rn</Option>:</Term>
208 <Term><Option>-ddump-tc</Option>:</Term>
216 <Term><Option>-ddump-deriv</Option>:</Term>
224 <Term><Option>-ddump-ds</Option>:</Term>
232 <Term><Option>-ddump-spec</Option>:</Term>
235 output of specialisation pass
240 <Term><Option>-ddump-rules</Option>:</Term>
243 dumps all rewrite rules (including those generated by the specialisation pass)
248 <Term><Option>-ddump-simpl</Option>:</Term>
251 simplifer output (Core-to-Core passes)
256 <Term><Option>-ddump-usagesp</Option>:</Term>
259 UsageSP inference pre-inf and output
264 <Term><Option>-ddump-cpranal</Option>:</Term>
272 <Term><Option>-ddump-stranal</Option>:</Term>
275 strictness analyser output
280 <Term><Option>-ddump-workwrap</Option>:</Term>
283 worker/wrapper split output
288 <Term><Option>-ddump-occur-anal</Option>:</Term>
291 `occurrence analysis' output
296 <Term><Option>-ddump-stg</Option>:</Term>
299 output of STG-to-STG passes
304 <Term><Option>-ddump-absC</Option>:</Term>
307 <Emphasis>un</Emphasis>flattened Abstract C
312 <Term><Option>-ddump-flatC</Option>:</Term>
315 <Emphasis>flattened</Emphasis> Abstract C
320 <Term><Option>-ddump-realC</Option>:</Term>
323 same as what goes to the C compiler
328 <Term><Option>-ddump-asm</Option>:</Term>
331 assembly language from the native-code generator
336 <IndexTerm><Primary>-ddump-all option</Primary></IndexTerm>
337 <IndexTerm><Primary>-ddump-most option</Primary></IndexTerm>
338 <IndexTerm><Primary>-ddump-parsed option</Primary></IndexTerm>
339 <IndexTerm><Primary>-ddump-rn option</Primary></IndexTerm>
340 <IndexTerm><Primary>-ddump-tc option</Primary></IndexTerm>
341 <IndexTerm><Primary>-ddump-deriv option</Primary></IndexTerm>
342 <IndexTerm><Primary>-ddump-ds option</Primary></IndexTerm>
343 <IndexTerm><Primary>-ddump-simpl option</Primary></IndexTerm>
344 <IndexTerm><Primary>-ddump-cpranal option</Primary></IndexTerm>
345 <IndexTerm><Primary>-ddump-workwrap option</Primary></IndexTerm>
346 <IndexTerm><Primary>-ddump-rules option</Primary></IndexTerm>
347 <IndexTerm><Primary>-ddump-usagesp option</Primary></IndexTerm>
348 <IndexTerm><Primary>-ddump-stranal option</Primary></IndexTerm>
349 <IndexTerm><Primary>-ddump-occur-anal option</Primary></IndexTerm>
350 <IndexTerm><Primary>-ddump-spec option</Primary></IndexTerm>
351 <IndexTerm><Primary>-ddump-stg option</Primary></IndexTerm>
352 <IndexTerm><Primary>-ddump-absC option</Primary></IndexTerm>
353 <IndexTerm><Primary>-ddump-flatC option</Primary></IndexTerm>
354 <IndexTerm><Primary>-ddump-realC option</Primary></IndexTerm>
355 <IndexTerm><Primary>-ddump-asm option</Primary></IndexTerm>
360 <Term><Option>-dverbose-simpl</Option> and <Option>-dverbose-stg</Option>:</Term>
363 <IndexTerm><Primary>-dverbose-simpl option</Primary></IndexTerm>
364 <IndexTerm><Primary>-dverbose-stg option</Primary></IndexTerm>
365 Show the output of the intermediate Core-to-Core and STG-to-STG
366 passes, respectively. (<Emphasis>Lots</Emphasis> of output!) So: when we're
370 % ghc -noC -O -ddump-simpl -dverbose-simpl -dcore-lint Foo.hs
377 <Term><Option>-ddump-simpl-iterations</Option>:</Term>
380 <IndexTerm><Primary>-ddump-simpl-iterations option</Primary></IndexTerm>
381 Show the output of each <Emphasis>iteration</Emphasis> of the simplifier (each run of
382 the simplifier has a maximum number of iterations, normally 4). Used
383 when even <Option>-dverbose-simpl</Option> doesn't cut it.
388 <Term><Option>-dppr-{user,debug</Option>}:</Term>
391 <IndexTerm><Primary>-dppr-user option</Primary></IndexTerm>
392 <IndexTerm><Primary>-dppr-debug option</Primary></IndexTerm>
393 Debugging output is in one of several “styles.” Take the printing
394 of types, for example. In the “user” style, the compiler's internal
395 ideas about types are presented in Haskell source-level syntax,
396 insofar as possible. In the “debug” style (which is the default for
397 debugging output), the types are printed in with
398 explicit foralls, and variables have their unique-id attached (so you
399 can check for things that look the same but aren't).
404 <Term><Option>-ddump-simpl-stats</Option>:</Term>
407 <IndexTerm><Primary>-ddump-simpl-stats option</Primary></IndexTerm>
408 Dump statistics about how many of each kind
409 of transformation too place. If you add <Option>-dppr-debug</Option> you get more detailed information.
414 <Term><Option>-ddump-raw-asm</Option>:</Term>
417 <IndexTerm><Primary>-ddump-raw-asm option</Primary></IndexTerm>
418 Dump out the assembly-language stuff, before the “mangler” gets it.
423 <Term><Option>-ddump-rn-trace</Option>:</Term>
426 <IndexTerm><Primary>-ddump-rn-trace</Primary></IndexTerm>
427 Make the renamer be *real* chatty about what it is upto.
432 <Term><Option>-dshow-rn-stats</Option>:</Term>
435 <IndexTerm><Primary>-dshow-rn-stats</Primary></IndexTerm>
436 Print out summary of what kind of information the renamer had to bring
442 <Term><Option>-dshow-unused-imports</Option>:</Term>
445 <IndexTerm><Primary>-dshow-unused-imports</Primary></IndexTerm>
446 Have the renamer report what imports does not contribute.
455 <Sect2 id="checking-consistency">
456 <Title>Checking for consistency
460 <IndexTerm><Primary>consistency checks</Primary></IndexTerm>
461 <IndexTerm><Primary>lint</Primary></IndexTerm>
468 <Term><Option>-dcore-lint</Option>:</Term>
471 <IndexTerm><Primary>-dcore-lint option</Primary></IndexTerm>
472 Turn on heavyweight intra-pass sanity-checking within GHC, at Core
473 level. (It checks GHC's sanity, not yours.)
478 <Term><Option>-dstg-lint</Option>:</Term>
481 <IndexTerm><Primary>-dstg-lint option</Primary></IndexTerm>
487 <Term><Option>-dusagesp-lint</Option>:</Term>
490 <IndexTerm><Primary>-dstg-lint option</Primary></IndexTerm>
491 Turn on checks around UsageSP inference (<Option>-fusagesp</Option>). This verifies
492 various simple properties of the results of the inference, and also
493 warns if any identifier with a used-once annotation before the
494 inference has a used-many annotation afterwards; this could indicate a
495 non-worksafe transformation is being applied.
505 <Title>How to read Core syntax (from some <Option>-ddump-*</Option> flags)</Title>
508 <IndexTerm><Primary>reading Core syntax</Primary></IndexTerm>
509 <IndexTerm><Primary>Core syntax, how to read</Primary></IndexTerm>
513 Let's do this by commenting an example. It's from doing
514 <Option>-ddump-ds</Option> on this code:
517 skip2 m = m : skip2 (m+2)
523 Before we jump in, a word about names of things. Within GHC,
524 variables, type constructors, etc., are identified by their
525 “Uniques.” These are of the form `letter' plus `number' (both
526 loosely interpreted). The `letter' gives some idea of where the
527 Unique came from; e.g., <Literal>_</Literal> means “built-in type variable”;
528 <Literal>t</Literal> means “from the typechecker”; <Literal>s</Literal> means “from the
529 simplifier”; and so on. The `number' is printed fairly compactly in
530 a `base-62' format, which everyone hates except me (WDP).
534 Remember, everything has a “Unique” and it is usually printed out
535 when debugging, in some form or another. So here we go…
541 Main.skip2{-r1L6-} :: _forall_ a$_4 =>{{Num a$_4}} -> a$_4 -> [a$_4]
543 --# `r1L6' is the Unique for Main.skip2;
544 --# `_4' is the Unique for the type-variable (template) `a'
545 --# `{{Num a$_4}}' is a dictionary argument
549 --# `_NI_' means "no (pragmatic) information" yet; it will later
550 --# evolve into the GHC_PRAGMA info that goes into interface files.
553 /\ _4 -> \ d.Num.t4Gt ->
556 +.t4Hg :: _4 -> _4 -> _4
558 +.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt
560 fromInt.t4GS :: Int{-2i-} -> _4
562 fromInt.t4GS = (fromInt{-r3JX-} _4) d.Num.t4Gt
564 --# The `+' class method (Unique: r3JH) selects the addition code
565 --# from a `Num' dictionary (now an explicit lamba'd argument).
566 --# Because Core is 2nd-order lambda-calculus, type applications
567 --# and lambdas (/\) are explicit. So `+' is first applied to a
568 --# type (`_4'), then to a dictionary, yielding the actual addition
569 --# function that we will use subsequently...
571 --# We play the exact same game with the (non-standard) class method
572 --# `fromInt'. Unsurprisingly, the type `Int' is wired into the
582 } in fromInt.t4GS ds.d4Qz
584 --# `I# 2#' is just the literal Int `2'; it reflects the fact that
585 --# GHC defines `data Int = I# Int#', where Int# is the primitive
586 --# unboxed type. (see relevant info about unboxed types elsewhere...)
588 --# The `!' after `I#' indicates that this is a *saturated*
589 --# application of the `I#' data constructor (i.e., not partially
592 skip2.t3Ja :: _4 -> [_4]
596 let { ds.d4QQ :: [_4]
602 ds.d4QY = +.t4Hg m.r1H4 lit.t4Hb
603 } in skip2.t3Ja ds.d4QY
613 (“It's just a simple functional language” is an unregisterised
614 trademark of Peyton Jones Enterprises, plc.)
619 <Sect2 id="source-file-options">
620 <Title>Command line options in source files
624 <IndexTerm><Primary>source-file options</Primary></IndexTerm>
628 Sometimes it is useful to make the connection between a source file
629 and the command-line options it requires quite tight. For instance,
630 if a (Glasgow) Haskell source file uses <Literal>casm</Literal>s, the C back-end
631 often needs to be told about which header files to include. Rather than
632 maintaining the list of files the source depends on in a
633 <Filename>Makefile</Filename> (using the <Option>-#include</Option> command-line option), it is
634 possible to do this directly in the source file using the <Literal>OPTIONS</Literal>
635 pragma <IndexTerm><Primary>OPTIONS pragma</Primary></IndexTerm>:
640 {-# OPTIONS -#include "foo.h" #-}
648 <Literal>OPTIONS</Literal> pragmas are only looked for at the top of your source
649 files, upto the first (non-literate,non-empty) line not containing
650 <Literal>OPTIONS</Literal>. Multiple <Literal>OPTIONS</Literal> pragmas are recognised. Note
651 that your command shell does not get to the source file options, they
652 are just included literally in the array of command-line arguments
653 the compiler driver maintains internally, so you'll be desperately
654 disappointed if you try to glob etc. inside <Literal>OPTIONS</Literal>.
658 NOTE: the contents of OPTIONS are prepended to the command-line
659 options, so you *do* have the ability to override OPTIONS settings
660 via the command line.
664 It is not recommended to move all the contents of your Makefiles into
665 your source files, but in some circumstances, the <Literal>OPTIONS</Literal> pragma
666 is the Right Thing. (If you use <Option>-keep-hc-file-too</Option> and have OPTION
667 flags in your module, the OPTIONS will get put into the generated .hc