path: root/ghc/docs/users_guide/debugging.vsgml

%************************************************************************
%*                                                                      *
<sect1>Debugging the compiler
<label id="options-debugging">
<p>
<nidx>debugging options (for GHC)</nidx>
%*                                                                      *
%************************************************************************

HACKER TERRITORY. HACKER TERRITORY.
(You were warned.)

%----------------------------------------------------------------------
<sect2>Replacing the program for one or more phases.
<label id="replacing-phases">
<p>
<nidx>GHC phases, changing</nidx>
<nidx>phases, changing GHC</nidx>

You may specify that a different program be used for one of the phases
of the compilation system, in place of whatever the driver @ghc@ has
wired into it.  For example, you might want to try a different
assembler.  The
@-pgm<phase-code><program-name>@<nidx>-pgm&lt;phase&gt;&lt;stuff&gt;
option</nidx> option to @ghc@ will cause it to use @<program-name>@
for phase @<phase-code>@, where the codes to indicate the phases are:

<tabular ca="ll">
<bf>code</bf> | <bf>phase</bf> @@
@@
L    | literate pre-processor @@
P    | C pre-processor (if -cpp only) @@
C    | Haskell compiler @@
c    | C compiler@@
a    | assembler @@
l    | linker @@
dep  | Makefile dependency generator @@
</tabular>

%----------------------------------------------------------------------
<sect2>Forcing options to a particular phase.
<label id="forcing-options-through">
<p>
<nidx>forcing GHC-phase options</nidx>

The preceding sections describe driver options that are mostly
applicable to one particular phase.  You may also <em>force</em> a
specific option @<option>@ to be passed to a particular phase
@<phase-code>@ by feeding the driver the option
@-opt<phase-code><option>@.<nidx>-opt&lt;phase&gt;&lt;stuff&gt;
option</nidx> The codes to indicate the phases are the same as in the
previous section.

So, for example, to force an @-Ewurble@ option to the assembler, you
would tell the driver @-opta-Ewurble@ (the dash before the E is
required).

Besides getting options to the Haskell compiler with @-optC<blah>@,
you can get options through to its runtime system with
@-optCrts<blah>@<nidx>-optCrts&lt;blah&gt; option</nidx>.

So, for example: when I want to use my normal driver but with my
profiled compiler binary, I use this script:
<tscreen><verb>
#! /bin/sh
exec /local/grasp_tmp3/simonpj/ghc-BUILDS/working-alpha/ghc/driver/ghc \
     -pgmC/local/grasp_tmp3/simonpj/ghc-BUILDS/working-hsc-prof/hsc \
     -optCrts-i0.5 \
     -optCrts-PT \
     "$@"
</verb></tscreen>

%----------------------------------------------------------------------
<sect2>Dumping out compiler intermediate structures
<label id="dumping-output">
<p>
<nidx>dumping GHC intermediates</nidx>
<nidx>intermediate passes, output</nidx>

<descrip>
<tag>@-noC@:</tag>
<nidx>-noC option</nidx>
Don't bother generating C output <em>or</em> an interface file.  Usually
used in conjunction with one or more of the @-ddump-*@ options; for
example: @ghc -noC -ddump-simpl Foo.hs@

<tag>@-hi@:</tag>
<nidx>-hi option</nidx>
<em>Do</em> generate an interface file.  This would normally be used in
conjunction with @-noC@, which turns off interface generation;
thus: @-noC -hi@.

<tag>@-dshow-passes@:</tag>
<nidx>-dshow-passes option</nidx>
Prints a message to stderr as each pass starts.  Gives a warm but
undoubtedly misleading feeling that GHC is telling you what's
happening.

<tag>@-ddump-<pass>@:</tag>
<nidx>-ddump-&lt;pass&gt; options</nidx>
Make a debugging dump after pass @<pass>@ (may be common enough to
need a short form...).  You can get all of these at once (<em/lots/ of
output) by using @-ddump-all@, or most of them with @-ddump-most@.
Some of the most useful ones are:

<descrip>
<tag>@-ddump-parsed@:</tag> oarser output
<tag>@-ddump-rn@:</tag>  renamer output
<tag>@-ddump-tc@:</tag>  typechecker output
<tag>@-ddump-deriv@:</tag>  derived instances
<tag>@-ddump-ds@:</tag>  desugarer output
<tag>@-ddump-spec@:</tag>  output of specialisation pass
<tag>@-ddump-rules@:</tag>  dumps all rewrite rules (including those generated by the specialisation pass)
<tag>@-ddump-simpl@:</tag>  simplifer output (Core-to-Core passes)
<tag>@-ddump-usagesp@:</tag> UsageSP inference pre-inf and output
<tag>@-ddump-cpranal@:</tag>  CPR analyser output
<tag>@-ddump-stranal@:</tag>  strictness analyser output
<tag>@-ddump-workwrap@:</tag>  worker/wrapper split output
<tag>@-ddump-occur-anal@:</tag>  `occurrence analysis' output
<tag>@-ddump-stg@:</tag>  output of STG-to-STG passes
<tag>@-ddump-absC@:</tag>  <em>un</em>flattened Abstract~C
<tag>@-ddump-flatC@:</tag>  <em>flattened</em> Abstract~C
<tag>@-ddump-realC@:</tag>  same as what goes to the C compiler
<tag>@-ddump-asm@:</tag>  assembly language from the native-code generator
</descrip>

<nidx>-ddump-all option</nidx>%
<nidx>-ddump-most option</nidx>%
<nidx>-ddump-parsed option</nidx>%
<nidx>-ddump-rn option</nidx>%
<nidx>-ddump-tc option</nidx>%
<nidx>-ddump-deriv option</nidx>%
<nidx>-ddump-ds option</nidx>%
<nidx>-ddump-simpl option</nidx>%
<nidx>-ddump-cpranal option</nidx>%
<nidx>-ddump-workwrap option</nidx>%
<nidx>-ddump-rules option</nidx>%
<nidx>-ddump-usagesp option</nidx>%
<nidx>-ddump-stranal option</nidx>%
<nidx>-ddump-occur-anal option</nidx>%
<nidx>-ddump-spec option</nidx>%
<nidx>-ddump-stg option</nidx>%
<nidx>-ddump-absC option</nidx>%
<nidx>-ddump-flatC option</nidx>%
<nidx>-ddump-realC option</nidx>%
<nidx>-ddump-asm option</nidx>

%For any other @-ddump-*@ options: consult the source, notably
%@ghc/compiler/main/CmdLineOpts.lhs@.

<tag>@-dverbose-simpl@ and @-dverbose-stg@:</tag>
<nidx>-dverbose-simpl option</nidx>
<nidx>-dverbose-stg option</nidx>
Show the output of the intermediate Core-to-Core and STG-to-STG
passes, respectively.  (<em>Lots</em> of output!) So: when we're 
really desperate:
<tscreen><verb>
% ghc -noC -O -ddump-simpl -dverbose-simpl -dcore-lint Foo.hs
</verb></tscreen>

<tag>@-ddump-simpl-iterations@:</tag>
<nidx>-ddump-simpl-iterations option</nidx>
Show the output of each <em/iteration/ of the simplifier (each run of
the simplifier has a maximum number of iterations, normally 4).  Used
when even @-dverbose-simpl@ doesn't cut it.

<tag>@-dppr-{user,debug@}:</tag>
<nidx>-dppr-user option</nidx>
<nidx>-dppr-debug option</nidx>
Debugging output is in one of several ``styles.''  Take the printing
of types, for example.  In the ``user'' style, the compiler's internal
ideas about types are presented in Haskell source-level syntax,
insofar as possible.  In the ``debug'' style (which is the default for
debugging output), the types are printed in with
explicit foralls, and variables have their unique-id attached (so you
can check for things that look the same but aren't).

<tag>@-ddump-simpl-stats@:</tag>
<nidx>-ddump-simpl-stats option</nidx>
Dump statistics about how many of each kind
of transformation too place.  If you add @-dppr-debug@ you get more detailed information.

<tag>@-ddump-raw-asm@:</tag>
<nidx>-ddump-raw-asm option</nidx>
Dump out the assembly-language stuff, before the ``mangler'' gets it.

<tag>@-ddump-rn-trace@:</tag>
<nidx>-ddump-rn-trace</nidx>
Make the renamer be *real* chatty about what it is upto.

<tag>@-dshow-rn-stats@:</tag>
<nidx>-dshow-rn-stats</nidx>
Print out summary of what kind of information the renamer had to bring
in.
<tag>@-dshow-unused-imports@:</tag>
<nidx>-dshow-unused-imports</nidx>
Have the renamer report what imports does not contribute.

%
%<tag>@-dgc-debug@:</tag>
%<nidx>-dgc-debug option</nidx>
%Enables some debugging code related to the garbage-collector.
</descrip>

%ToDo: -ddump-asm-insn-counts
%-ddump-asm-globals-info

%----------------------------------------------------------------------
<sect2>Checking for consistency
<label id="checking-consistency">
<p>
<nidx>consistency checks</nidx>
<nidx>lint</nidx>

<descrip>
<tag>@-dcore-lint@:</tag>
<nidx>-dcore-lint option</nidx>
Turn on heavyweight intra-pass sanity-checking within GHC, at Core
level.  (It checks GHC's sanity, not yours.)

<tag>@-dstg-lint@:</tag>
<nidx>-dstg-lint option</nidx>
Ditto for STG level.

<tag>@-dusagesp-lint@:</tag>
<nidx>-dstg-lint option</nidx>
Turn on checks around UsageSP inference (@-fusagesp@).  This verifies
various simple properties of the results of the inference, and also
warns if any identifier with a used-once annotation before the
inference has a used-many annotation afterwards; this could indicate a
non-worksafe transformation is being applied.
</descrip>

%----------------------------------------------------------------------
<sect2>How to read Core syntax (from some @-ddump-*@ flags)
<p>
<nidx>reading Core syntax</nidx>
<nidx>Core syntax, how to read</nidx>

Let's do this by commenting an example.  It's from doing
@-ddump-ds@ on this code:
<tscreen><verb>
skip2 m = m : skip2 (m+2)
</verb></tscreen>

Before we jump in, a word about names of things.  Within GHC,
variables, type constructors, etc., are identified by their
``Uniques.''  These are of the form `letter' plus `number' (both
loosely interpreted).  The `letter' gives some idea of where the
Unique came from; e.g., @_@ means ``built-in type variable'';
@t@ means ``from the typechecker''; @s@ means ``from the
simplifier''; and so on.  The `number' is printed fairly compactly in
a `base-62' format, which everyone hates except me (WDP).

Remember, everything has a ``Unique'' and it is usually printed out
when debugging, in some form or another.  So here we go...

<tscreen><verb>
Desugared:
Main.skip2{-r1L6-} :: _forall_ a$_4 =>{{Num a$_4}} -> a$_4 -> [a$_4]

--# `r1L6' is the Unique for Main.skip2;
--# `_4' is the Unique for the type-variable (template) `a'
--# `{{Num a$_4}}' is a dictionary argument

_NI_

--# `_NI_' means "no (pragmatic) information" yet; it will later
--# evolve into the GHC_PRAGMA info that goes into interface files.

Main.skip2{-r1L6-} =
    /\ _4 -> \ d.Num.t4Gt ->
	let {
	  {- CoRec -}
	  +.t4Hg :: _4 -> _4 -> _4
	  _NI_
	  +.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt

	  fromInt.t4GS :: Int{-2i-} -> _4
	  _NI_
	  fromInt.t4GS = (fromInt{-r3JX-} _4) d.Num.t4Gt

--# The `+' class method (Unique: r3JH) selects the addition code
--# from a `Num' dictionary (now an explicit lamba'd argument).
--# Because Core is 2nd-order lambda-calculus, type applications
--# and lambdas (/\) are explicit.  So `+' is first applied to a
--# type (`_4'), then to a dictionary, yielding the actual addition
--# function that we will use subsequently...

--# We play the exact same game with the (non-standard) class method
--# `fromInt'.  Unsurprisingly, the type `Int' is wired into the
--# compiler.

	  lit.t4Hb :: _4
	  _NI_
	  lit.t4Hb =
	      let {
		ds.d4Qz :: Int{-2i-}
		_NI_
		ds.d4Qz = I#! 2#
	      } in  fromInt.t4GS ds.d4Qz

--# `I# 2#' is just the literal Int `2'; it reflects the fact that
--# GHC defines `data Int = I# Int#', where Int# is the primitive
--# unboxed type.  (see relevant info about unboxed types elsewhere...)

--# The `!' after `I#' indicates that this is a *saturated*
--# application of the `I#' data constructor (i.e., not partially
--# applied).

	  skip2.t3Ja :: _4 -> [_4]
	  _NI_
	  skip2.t3Ja =
	      \ m.r1H4 ->
		  let { ds.d4QQ :: [_4]
			_NI_
			ds.d4QQ =
		    let {
		      ds.d4QY :: _4
		      _NI_
		      ds.d4QY = +.t4Hg m.r1H4 lit.t4Hb
		    } in  skip2.t3Ja ds.d4QY
		  } in
		  :! _4 m.r1H4 ds.d4QQ

	  {- end CoRec -}
	} in  skip2.t3Ja
</verb></tscreen>

(``It's just a simple functional language'' is an unregisterised
trademark of Peyton Jones Enterprises, plc.)

%----------------------------------------------------------------------
<sect2>Command line options in source files
<label id="source-file-options">
<p>
<nidx>source-file options</nidx>

Sometimes it is useful to make the connection between a source file
and the command-line options it requires quite tight. For instance,
if a (Glasgow) Haskell source file uses @casm@s, the C back-end
often needs to be told about which header files to include. Rather than
maintaining the list of files the source depends on in a
@Makefile@ (using the @-#include@ command-line option), it is
possible to do this directly in the source file using the @OPTIONS@
pragma <nidx>OPTIONS pragma</nidx>: 

<tscreen><verb>
{-# OPTIONS -#include "foo.h" #-}
module X where

...
</verb></tscreen>

@OPTIONS@ pragmas are only looked for at the top of your source
files, upto the first (non-literate,non-empty) line not containing
@OPTIONS@. Multiple @OPTIONS@ pragmas are recognised. Note
that your command shell does not get to the source file options, they
are just included literally in the array of command-line arguments
the compiler driver maintains internally, so you'll be desperately
disappointed if you try to glob etc. inside @OPTIONS@.

NOTE: the contents of OPTIONS are prepended to the command-line
options, so you *do* have the ability to override OPTIONS settings
via the command line.

It is not recommended to move all the contents of your Makefiles into
your source files, but in some circumstances, the @OPTIONS@ pragma
is the Right Thing. (If you use @-keep-hc-file-too@ and have OPTION
flags in your module, the OPTIONS will get put into the generated .hc
file).

%----------------------------------------------------------------------