1 %************************************************************************
3 <sect1>Debugging the compiler
4 <label id="options-debugging">
6 <nidx>debugging options (for GHC)</nidx>
8 %************************************************************************
10 HACKER TERRITORY. HACKER TERRITORY.
13 %----------------------------------------------------------------------
14 <sect2>Replacing the program for one or more phases.
15 <label id="replacing-phases">
17 <nidx>GHC phases, changing</nidx>
18 <nidx>phases, changing GHC</nidx>
20 You may specify that a different program be used for one of the phases
21 of the compilation system, in place of whatever the driver @ghc@ has
22 wired into it. For example, you might want to try a different
24 @-pgm<phase-code><program-name>@<nidx>-pgm<phase><stuff>
25 option</nidx> option to @ghc@ will cause it to use @<program-name>@
26 for phase @<phase-code>@, where the codes to indicate the phases are:
29 <bf>code</bf> | <bf>phase</bf> @@
31 L | literate pre-processor @@
32 P | C pre-processor (if -cpp only) @@
33 C | Haskell compiler @@
37 dep | Makefile dependency generator @@
40 %----------------------------------------------------------------------
41 <sect2>Forcing options to a particular phase.
42 <label id="forcing-options-through">
44 <nidx>forcing GHC-phase options</nidx>
46 The preceding sections describe driver options that are mostly
47 applicable to one particular phase. You may also <em>force</em> a
48 specific option @<option>@ to be passed to a particular phase
49 @<phase-code>@ by feeding the driver the option
50 @-opt<phase-code><option>@.<nidx>-opt<phase><stuff>
51 option</nidx> The codes to indicate the phases are the same as in the
54 So, for example, to force an @-Ewurble@ option to the assembler, you
55 would tell the driver @-opta-Ewurble@ (the dash before the E is
58 Besides getting options to the Haskell compiler with @-optC<blah>@,
59 you can get options through to its runtime system with
60 @-optCrts<blah>@<nidx>-optCrts<blah> option</nidx>.
62 So, for example: when I want to use my normal driver but with my
63 profiled compiler binary, I use this script:
66 exec /local/grasp_tmp3/simonpj/ghc-BUILDS/working-alpha/ghc/driver/ghc \
67 -pgmC/local/grasp_tmp3/simonpj/ghc-BUILDS/working-hsc-prof/hsc \
73 %----------------------------------------------------------------------
74 <sect2>Dumping out compiler intermediate structures
75 <label id="dumping-output">
77 <nidx>dumping GHC intermediates</nidx>
78 <nidx>intermediate passes, output</nidx>
82 <nidx>-noC option</nidx>
83 Don't bother generating C output <em>or</em> an interface file. Usually
84 used in conjunction with one or more of the @-ddump-*@ options; for
85 example: @ghc -noC -ddump-simpl Foo.hs@
88 <nidx>-hi option</nidx>
89 <em>Do</em> generate an interface file. This would normally be used in
90 conjunction with @-noC@, which turns off interface generation;
93 <tag>@-dshow-passes@:</tag>
94 <nidx>-dshow-passes option</nidx>
95 Prints a message to stderr as each pass starts. Gives a warm but
96 undoubtedly misleading feeling that GHC is telling you what's
99 <tag>@-ddump-<pass>@:</tag>
100 <nidx>-ddump-<pass> options</nidx>
101 Make a debugging dump after pass @<pass>@ (may be common enough to
102 need a short form...). Some of the most useful ones are:
105 @-ddump-rdr@ | reader output (earliest stuff in the compiler) @@
106 @-ddump-rn@ | renamer output @@
107 @-ddump-tc@ | typechecker output @@
108 @-ddump-deriv@ | derived instances @@
109 @-ddump-ds@ | desugarer output @@
110 @-ddump-simpl@ | simplifer output (Core-to-Core passes) @@
111 @-ddump-stranal@ | strictness analyser output @@
112 @-ddump-occur-anal@ | `occurrence analysis' output @@
113 @-ddump-spec@ | dump specialisation info @@
114 @-ddump-stg@ | output of STG-to-STG passes @@
115 @-ddump-absC@ | <em>un</em>flattened Abstract~C @@
116 @-ddump-flatC@ | <em>flattened</em> Abstract~C @@
117 @-ddump-realC@ | same as what goes to the C compiler @@
118 @-ddump-asm@ | assembly language from the native-code generator @@
121 <nidx>-ddump-rdr option</nidx>%
122 <nidx>-ddump-rn option</nidx>%
123 <nidx>-ddump-tc option</nidx>%
124 <nidx>-ddump-deriv option</nidx>%
125 <nidx>-ddump-ds option</nidx>%
126 <nidx>-ddump-simpl option</nidx>%
127 <nidx>-ddump-stranal option</nidx>%
128 <nidx>-ddump-occur-anal option</nidx>%
129 <nidx>-ddump-spec option</nidx>%
130 <nidx>-ddump-stg option</nidx>%
131 <nidx>-ddump-absC option</nidx>%
132 <nidx>-ddump-flatC option</nidx>%
133 <nidx>-ddump-realC option</nidx>%
134 <nidx>-ddump-asm option</nidx>
136 %For any other @-ddump-*@ options: consult the source, notably
137 %@ghc/compiler/main/CmdLineOpts.lhs@.
139 <tag>@-dverbose-simpl@ and @-dverbose-stg@:</tag>
140 <nidx>-dverbose-simpl option</nidx>
141 <nidx>-dverbose-stg option</nidx>
142 Show the output of the intermediate Core-to-Core and STG-to-STG
143 passes, respectively. (<em>Lots</em> of output!) So: when we're
146 % ghc -noC -O -ddump-simpl -dverbose-simpl -dcore-lint Foo.hs
149 <tag>@-ddump-simpl-iterations@:</tag>
150 <nidx>-ddump-simpl-iterations option</nidx>
151 Show the output of each <em/iteration/ of the simplifier (each run of
152 the simplifier has a maximum number of iterations, normally 4). Used
153 when even @-dverbose-simpl@ doesn't cut it.
155 <tag>@-dppr-{user,debug,all@}:</tag>
156 <nidx>-dppr-user option</nidx>
157 <nidx>-dppr-debug option</nidx>
158 <nidx>-dppr-all option</nidx>
159 Debugging output is in one of several ``styles.'' Take the printing
160 of types, for example. In the ``user'' style, the compiler's internal
161 ideas about types are presented in Haskell source-level syntax,
162 insofar as possible. In the ``debug'' style (which is the default for
163 debugging output), the types are printed in the most-often-desired
164 form, with explicit foralls, etc. In the ``show all'' style, very
165 verbose information about the types (e.g., the Uniques on the
166 individual type variables) is displayed.
168 <tag>@-ddump-raw-asm@:</tag>
169 <nidx>-ddump-raw-asm option</nidx>
170 Dump out the assembly-language stuff, before the ``mangler'' gets it.
172 <tag>@-ddump-rn-trace@:</tag>
173 <nidx>-ddump-rn-trace</nidx>
174 Make the renamer be *real* chatty about what it is upto.
176 <tag>@-dshow-rn-stats@:</tag>
177 <nidx>-dshow-rn-stats</nidx>
178 Print out summary of what kind of information the renamer had to bring
180 <tag>@-dshow-unused-imports@:</tag>
181 <nidx>-dshow-unused-imports</nidx>
182 Have the renamer report what imports does not contribute.
185 %<tag>@-dgc-debug@:</tag>
186 %<nidx>-dgc-debug option</nidx>
187 %Enables some debugging code related to the garbage-collector.
190 %ToDo: -ddump-asm-insn-counts
191 %-ddump-asm-globals-info
193 %----------------------------------------------------------------------
194 <sect2>How to read Core syntax (from some @-ddump-*@ flags)
196 <nidx>reading Core syntax</nidx>
197 <nidx>Core syntax, how to read</nidx>
199 Let's do this by commenting an example. It's from doing
200 @-ddump-ds@ on this code:
202 skip2 m = m : skip2 (m+2)
205 Before we jump in, a word about names of things. Within GHC,
206 variables, type constructors, etc., are identified by their
207 ``Uniques.'' These are of the form `letter' plus `number' (both
208 loosely interpreted). The `letter' gives some idea of where the
209 Unique came from; e.g., @_@ means ``built-in type variable'';
210 @t@ means ``from the typechecker''; @s@ means ``from the
211 simplifier''; and so on. The `number' is printed fairly compactly in
212 a `base-62' format, which everyone hates except me (WDP).
214 Remember, everything has a ``Unique'' and it is usually printed out
215 when debugging, in some form or another. So here we go...
219 Main.skip2{-r1L6-} :: _forall_ a$_4 =>{{Num a$_4}} -> a$_4 -> [a$_4]
221 --# `r1L6' is the Unique for Main.skip2;
222 --# `_4' is the Unique for the type-variable (template) `a'
223 --# `{{Num a$_4}}' is a dictionary argument
227 --# `_NI_' means "no (pragmatic) information" yet; it will later
228 --# evolve into the GHC_PRAGMA info that goes into interface files.
231 /\ _4 -> \ d.Num.t4Gt ->
234 +.t4Hg :: _4 -> _4 -> _4
236 +.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt
238 fromInt.t4GS :: Int{-2i-} -> _4
240 fromInt.t4GS = (fromInt{-r3JX-} _4) d.Num.t4Gt
242 --# The `+' class method (Unique: r3JH) selects the addition code
243 --# from a `Num' dictionary (now an explicit lamba'd argument).
244 --# Because Core is 2nd-order lambda-calculus, type applications
245 --# and lambdas (/\) are explicit. So `+' is first applied to a
246 --# type (`_4'), then to a dictionary, yielding the actual addition
247 --# function that we will use subsequently...
249 --# We play the exact same game with the (non-standard) class method
250 --# `fromInt'. Unsurprisingly, the type `Int' is wired into the
260 } in fromInt.t4GS ds.d4Qz
262 --# `I# 2#' is just the literal Int `2'; it reflects the fact that
263 --# GHC defines `data Int = I# Int#', where Int# is the primitive
264 --# unboxed type. (see relevant info about unboxed types elsewhere...)
266 --# The `!' after `I#' indicates that this is a *saturated*
267 --# application of the `I#' data constructor (i.e., not partially
270 skip2.t3Ja :: _4 -> [_4]
274 let { ds.d4QQ :: [_4]
280 ds.d4QY = +.t4Hg m.r1H4 lit.t4Hb
281 } in skip2.t3Ja ds.d4QY
289 (``It's just a simple functional language'' is an unregisterised
290 trademark of Peyton Jones Enterprises, plc.)
292 %----------------------------------------------------------------------
293 <sect2>Command line options in source files
294 <label id="source-file-options">
296 <nidx>source-file options</nidx>
298 Sometimes it is useful to make the connection between a source file
299 and the command-line options it requires quite tight. For instance,
300 if a (Glasgow) Haskell source file uses @casm@s, the C back-end
301 often needs to be told about which header files to include. Rather than
302 maintaining the list of files the source depends on in a
303 @Makefile@ (using the @-#include@ command-line option), it is
304 possible to do this directly in the source file using the @OPTIONS@
305 pragma <nidx>OPTIONS pragma</nidx>:
308 {-# OPTIONS -#include "foo.h" #-}
314 @OPTIONS@ pragmas are only looked for at the top of your source
315 files, upto the first (non-literate,non-empty) line not containing
316 @OPTIONS@. Multiple @OPTIONS@ pragmas are recognised. Note
317 that your command shell does not get to the source file options, they
318 are just included literally in the array of command-line arguments
319 the compiler driver maintains internally, so you'll be desperately
320 disappointed if you try to glob etc. inside @OPTIONS@.
322 NOTE: the contents of OPTIONS are prepended to the command-line
323 options, so you *do* have the ability to override OPTIONS settings
324 via the command line.
326 It is not recommended to move all the contents of your Makefiles into
327 your source files, but in some circumstances, the @OPTIONS@ pragma
328 is the Right Thing. (If you use @-keep-hc-file-too@ and have OPTION
329 flags in your module, the OPTIONS will get put into the generated .hc
332 %----------------------------------------------------------------------