path: root/docs/users_guide/debugging.xml

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<sect1 id="options-debugging">
  <title>Debugging the compiler</title>

  <indexterm><primary>debugging options (for GHC)</primary></indexterm>

  <para>HACKER TERRITORY. HACKER TERRITORY.  (You were warned.)</para>

  <sect2 id="dumping-output">
    <title>Dumping out compiler intermediate structures</title>
    
    <indexterm><primary>dumping GHC intermediates</primary></indexterm>
    <indexterm><primary>intermediate passes, output</primary></indexterm>
    
    <variablelist>
      <varlistentry>
	<term>
          <option>-ddump-</option><replaceable>pass</replaceable>
          <indexterm><primary><option>-ddump</option> options</primary></indexterm>
        </term>
	<listitem>
	  <para>Make a debugging dump after pass
        <literal>&lt;pass&gt;</literal> (may be common enough to need
        a short form&hellip;).  You can get all of these at once
        (<emphasis>lots</emphasis> of output) by using
        <option>-v5</option>, or most of them with
        <option>-v4</option>.  Some of the most useful ones
        are:</para>

	  <variablelist>
	    <varlistentry>
	      <term>
	        <option>-ddump-parsed</option>:
                <indexterm><primary><option>-ddump-parsed</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>parser output</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-rn</option>:
                <indexterm><primary><option>-ddump-rn</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>renamer output</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-tc</option>:
                <indexterm><primary><option>-ddump-tc</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>typechecker output</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-splices</option>:
                <indexterm><primary><option>-ddump-splices</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>Dump Template Haskell expressions that we splice in,
                and what Haskell code the expression evaluates to.</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-types</option>:
                <indexterm><primary><option>-ddump-types</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>Dump a type signature for each value defined at
              the top level of the module.  The list is sorted
              alphabetically.  Using <option>-dppr-debug</option>
              dumps a type signature for all the imported and
              system-defined things as well; useful for debugging the
              compiler.</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-deriv</option>:
                <indexterm><primary><option>-ddump-deriv</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>derived instances</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-ds</option>:
                <indexterm><primary><option>-ddump-ds</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>desugarer output</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-spec</option>:
                <indexterm><primary><option>-ddump-spec</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>output of specialisation pass</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-rules</option>:
                <indexterm><primary><option>-ddump-rules</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>dumps all rewrite rules (including those generated
	      by the specialisation pass)</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-simpl</option>:
                <indexterm><primary><option>-ddump-simpl</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>simplifier output (Core-to-Core passes)</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-inlinings</option>:
                <indexterm><primary><option>-ddump-inlinings</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>inlining info from the simplifier</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-cpranal</option>:
                <indexterm><primary><option>-ddump-cpranal</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>CPR analyser output</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-stranal</option>:
                <indexterm><primary><option>-ddump-stranal</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>strictness analyser output</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-cse</option>:
                <indexterm><primary><option>-ddump-cse</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>CSE pass output</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-workwrap</option>:
                <indexterm><primary><option>-ddump-workwrap</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>worker/wrapper split output</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-occur-anal</option>:
                <indexterm><primary><option>-ddump-occur-anal</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>`occurrence analysis' output</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-prep</option>:
                <indexterm><primary><option>-ddump-prep</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>output of core preparation pass</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-stg</option>:
                <indexterm><primary><option>-ddump-stg</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>output of STG-to-STG passes</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-flatC</option>:
                <indexterm><primary><option>-ddump-flatC</option></primary></indexterm>
	      </term>
	      <listitem>
		<para><emphasis>flattened</emphasis> Abstract&nbsp;C</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-cmm</option>:
                <indexterm><primary><option>-ddump-cmm</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>Print the C-- code out.</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-opt-cmm</option>:
                <indexterm><primary><option>-ddump-opt-cmm</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>Dump the results of C-- to C-- optimising passes.</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-asm</option>:
                <indexterm><primary><option>-ddump-asm</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>assembly language from the native-code generator</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-bcos</option>:
                <indexterm><primary><option>-ddump-bcos</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>byte code compiler output</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term>
	        <option>-ddump-foreign</option>:
                <indexterm><primary><option>-ddump-foreign</option></primary></indexterm>
	      </term>
	      <listitem>
		<para>dump foreign export stubs</para>
	      </listitem>
	    </varlistentry>
	  </variablelist>
	</listitem>
      </varlistentry>
      
      <varlistentry>
	<term>
          <option>-ddump-simpl-phases</option>:
          <indexterm><primary><option>-ddump-simpl-phases</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Show the output of each run of the simplifier.  Used when even
        <option>-dverbose-simpl</option> doesn't cut it.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-ddump-simpl-iterations</option>:
          <indexterm><primary><option>-ddump-simpl-iterations</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Show the output of each <emphasis>iteration</emphasis>
        of the simplifier (each run of the simplifier has a maximum
        number of iterations, normally 4).  This outputs even more information
        than <option>-ddump-simpl-phases</option>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-ddump-simpl-stats</option>
          <indexterm><primary><option>-ddump-simpl-stats option</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Dump statistics about how many of each kind of
        transformation too place.  If you add
        <option>-dppr-debug</option> you get more detailed
        information.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-ddump-if-trace</option>
          <indexterm><primary><option>-ddump-if-trace</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Make the interface loader be *real* chatty about what it is
	upto.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-ddump-tc-trace</option>
          <indexterm><primary><option>-ddump-tc-trace</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Make the type checker be *real* chatty about what it is
	upto.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-ddump-rn-trace</option>
          <indexterm><primary><option>-ddump-rn-trace</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Make the renamer be *real* chatty about what it is
	upto.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-ddump-rn-stats</option>
          <indexterm><primary><option>-dshow-rn-stats</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Print out summary of what kind of information the renamer
        had to bring in.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-dverbose-core2core</option>
          <indexterm><primary><option>-dverbose-core2core</option></primary></indexterm>
        </term>
	<term>
          <option>-dverbose-stg2stg</option>
          <indexterm><primary><option>-dverbose-stg2stg</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Show the output of the intermediate Core-to-Core and
        STG-to-STG passes, respectively.  (<emphasis>Lots</emphasis>
        of output!) So: when we're really desperate:</para>

	  <screen>
% ghc -noC -O -ddump-simpl -dverbose-simpl -dcore-lint Foo.hs
</screen>

	</listitem>
      </varlistentry>
      
      <varlistentry>
	<term>
          <option>-dshow-passes</option>
          <indexterm><primary><option>-dshow-passes</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Print out each pass name as it happens.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-dfaststring-stats</option>
          <indexterm><primary><option>-dfaststring-stats</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Show statistics for the usage of fast strings by the
          compiler.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-dppr-debug</option>
          <indexterm><primary><option>-dppr-debug</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Debugging output is in one of several
          &ldquo;styles.&rdquo; Take the printing of types, for
          example.  In the &ldquo;user&rdquo; style (the default), the
          compiler's internal ideas about types are presented in
          Haskell source-level syntax, insofar as possible.  In the
          &ldquo;debug&rdquo; 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).  This
          flag makes debugging output appear in the more verbose debug
          style.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-dsuppress-uniques</option>
          <indexterm><primary><option>-dsuppress-uniques</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Suppress the printing of uniques in debugging output. This may make 
	  the printout ambiguous (e.g. unclear where an occurrence of 'x' is bound), but
	  it makes the output of two compiler runs have many fewer gratuitous differences,
	    so you can realistically apply <command>diff</command>.  Once <command>diff</command>
	  has shown you where to look, you can try again without <option>-dsuppress-uniques</option></para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-dsuppress-coercions</option>
          <indexterm><primary><option>-dsuppress-coercions</option></primary></indexterm>
        </term>
	<listitem>
          <para>Suppress the printing of coercions in Core dumps to make them
shorter.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-dppr-user-length</option>
          <indexterm><primary><option>-dppr-user-length</option></primary></indexterm>
        </term>
	<listitem>
	  <para>In error messages, expressions are printed to a
	  certain &ldquo;depth&rdquo;, with subexpressions beyond the
	  depth replaced by ellipses.  This flag sets the
	  depth.  Its default value is 5.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-dno-debug-output</option>
          <indexterm><primary><option>-dno-debug-output</option></primary></indexterm>
        </term>
        <listitem>
          <para>Suppress any unsolicited debugging output.  When GHC
            has been built with the <literal>DEBUG</literal> option it
            occasionally emits debug output of interest to developers.
            The extra output can confuse the testing framework and
            cause bogus test failures, so this flag is provided to
            turn it off.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </sect2>

  <sect2 id="checking-consistency">
    <title>Checking for consistency</title>

    <indexterm><primary>consistency checks</primary></indexterm>
    <indexterm><primary>lint</primary></indexterm>

    <variablelist>

      <varlistentry>
	<term>
          <option>-dcore-lint</option>
          <indexterm><primary><option>-dcore-lint</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Turn on heavyweight intra-pass sanity-checking within
          GHC, at Core level.  (It checks GHC's sanity, not yours.)</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-dstg-lint</option>:
          <indexterm><primary><option>-dstg-lint</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Ditto for STG level. (NOTE: currently doesn't work).</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-dcmm-lint</option>:
          <indexterm><primary><option>-dcmm-lint</option></primary></indexterm>
        </term>
	<listitem>
	  <para>Ditto for C-- level.</para>
	</listitem>
      </varlistentry>

    </variablelist>
  </sect2>

  <sect2>
    <title>How to read Core syntax (from some <option>-ddump</option>
    flags)</title>

    <indexterm><primary>reading Core syntax</primary></indexterm>
    <indexterm><primary>Core syntax, how to read</primary></indexterm>

    <para>Let's do this by commenting an example.  It's from doing
    <option>-ddump-ds</option> on this code:

<programlisting>
skip2 m = m : skip2 (m+2)
</programlisting>

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

    <para>Remember, everything has a &ldquo;Unique&rdquo; and it is
    usually printed out when debugging, in some form or another.  So
    here we go&hellip;</para>

<programlisting>
Desugared:
Main.skip2{-r1L6-} :: _forall_ a$_4 =&#62;{{Num a$_4}} -&#62; a$_4 -&#62; [a$_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 -&#62; \ d.Num.t4Gt -&#62;
        let {
          {- CoRec -}
          +.t4Hg :: _4 -&#62; _4 -&#62; _4
          _NI_
          +.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt

          fromInt.t4GS :: Int{-2i-} -&#62; _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 lambda'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 -&#62; [_4]
          _NI_
          skip2.t3Ja =
              \ m.r1H4 -&#62;
                  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
</programlisting>

    <para>(&ldquo;It's just a simple functional language&rdquo; is an
    unregisterised trademark of Peyton Jones Enterprises, plc.)</para>

  </sect2>

  <sect2 id="unreg">
    <title>Unregisterised compilation</title>
    <indexterm><primary>unregisterised compilation</primary></indexterm>

    <para>The term "unregisterised" really means "compile via vanilla
    C", disabling some of the platform-specific tricks that GHC
    normally uses to make programs go faster.  When compiling
    unregisterised, GHC simply generates a C file which is compiled
    via gcc.</para>

    <para>Unregisterised compilation can be useful when porting GHC to
    a new machine, since it reduces the prerequisite tools to
    <command>gcc</command>, <command>as</command>, and
    <command>ld</command> and nothing more, and furthermore the amount
    of platform-specific code that needs to be written in order to get
    unregisterised compilation going is usually fairly small.</para>

   <para>Unregisterised compilation cannot be selected at
   compile-time; you have to build GHC with the appropriate options
   set.  Consult the GHC Building Guide for details.</para>
  </sect2>

</sect1>

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