path: root/docs/users_guide/debugging.rst

.. _options-debugging:

Debugging the compiler
======================

.. index::
   single: debugging options (for GHC)

..
    It is not necessary to provide :category: tags for ``ghc-flag:``s defined in
    this file; a default is specified in ``flags.py``.

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

.. contents:: Dump flags

.. _dumping-output:

Dumping out compiler intermediate structures
--------------------------------------------

.. index::
   single: dumping GHC intermediates
   single: intermediate passes, output

.. ghc-flag:: -ddump-to-file
    :shortdesc: Dump to files instead of stdout
    :type: dynamic

    Causes the output from all of the flags listed below to be dumped
    to a file. The file name depends upon the output produced; for instance,
    output from :ghc-flag:`-ddump-simpl` will end up in
    :file:`{module}.dump-simpl`.

.. ghc-flag:: -ddump-json
    :shortdesc: Dump error messages as JSON documents
    :type: dynamic

    Dump error messages as JSON documents. This is intended to be consumed
    by external tooling. A good way to use it is in conjunction with
    :ghc-flag:`-ddump-to-file`.

.. ghc-flag:: -dshow-passes
    :shortdesc: Print out each pass name as it happens
    :type: dynamic

    Print out each pass name, its runtime and heap allocations as it happens.
    Note that this may come at a slight performance cost as the compiler will
    be a bit more eager in forcing pass results to more accurately account for
    their costs.

    Two types of messages are produced: Those beginning with ``***`` do
    denote the beginning of a compilation phase whereas those starting with
    ``!!!`` mark the end of a pass and are accompanied by allocation and
    runtime statistics.

.. ghc-flag:: -dfaststring-stats
    :shortdesc: Show statistics for fast string usage when finished
    :type: dynamic

    Show statistics on the usage of fast strings by the compiler.

.. ghc-flag:: -dppr-debug
    :shortdesc: Turn on debug printing (more verbose)
    :type: dynamic

    Debugging output is in one of several "styles." Take the printing of
    types, for example. In the "user" style (the default), 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). This flag makes debugging output appear in the more verbose
    debug style.


GHC is a large program consisting of a number of stages. You can tell GHC to
dump information from various stages of compilation using the ``-ddump-⟨pass⟩``
flags listed below. Note that some of these tend to produce a lot of output.
You can prevent them from clogging up your standard output by passing
:ghc-flag:`-ddump-to-file`.

Front-end
~~~~~~~~~

These flags dump various information from GHC's frontend. This includes the
parser and interface file reader.

.. ghc-flag:: -ddump-parsed
    :shortdesc: Dump parse tree
    :type: dynamic

    Dump parser output

.. ghc-flag:: -ddump-parsed-ast
    :shortdesc: Dump parser output as a syntax tree
    :type: dynamic

    Dump parser output as a syntax tree

.. ghc-flag:: -ddump-if-trace
    :shortdesc: Trace interface files
    :type: dynamic

    Make the interface loader be *real* chatty about what it is up to.


Type-checking and renaming
~~~~~~~~~~~~~~~~~~~~~~~~~~

These flags dump various information from GHC's typechecker and renamer.

.. ghc-flag:: -ddump-tc-trace
    :shortdesc: Trace typechecker
    :type: dynamic

    Make the type checker be *real* chatty about what it is up to.

.. ghc-flag:: -ddump-rn-trace
    :shortdesc: Trace renamer
    :type: dynamic

    Make the renamer be *real* chatty about what it is up to.

.. ghc-flag:: -ddump-ec-trace
    :shortdesc: Trace exhaustiveness checker
    :type: dynamic

    Make the pattern match exhaustiveness checker be *real* chatty about
    what it is up to.

.. ghc-flag:: -ddump-rn-stats
    :shortdesc: Renamer stats
    :type: dynamic

    Print out summary of what kind of information the renamer had to
    bring in.

.. ghc-flag:: -ddump-rn
    :shortdesc: Dump renamer output
    :type: dynamic

    Dump renamer output

.. ghc-flag:: -ddump-rn-ast
    :shortdesc: Dump renamer output as a syntax tree
    :type: dynamic

    Dump renamer output as a syntax tree

.. ghc-flag:: -ddump-tc
    :shortdesc: Dump typechecker output
    :type: dynamic

    Dump typechecker output

.. ghc-flag:: -ddump-tc-ast
    :shortdesc: Dump typechecker output as a syntax tree
    :type: dynamic

    Dump typechecker output as a syntax tree

.. ghc-flag:: -ddump-splices
    :shortdesc: Dump TH spliced expressions, and what they evaluate to
    :type: dynamic

    Dump Template Haskell expressions that we splice in, and what
    Haskell code the expression evaluates to.

.. ghc-flag:: -dth-dec-file=⟨file⟩
    :shortdesc: Show evaluated TH declarations in a .th.hs file
    :type: dynamic

    Dump expansions of all top-level Template Haskell splices into ⟨file⟩.

.. ghc-flag:: -ddump-types
    :shortdesc: Dump type signatures
    :type: dynamic

    Dump a type signature for each value defined at the top level of
    the module. The list is sorted alphabetically. Using
    :ghc-flag:`-dppr-debug` dumps a type signature for all the imported and
    system-defined things as well; useful for debugging the
    compiler.

.. ghc-flag:: -ddump-deriv
    :shortdesc: Dump deriving output
    :type: dynamic

    Dump derived instances


Core representation and simplification
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

These flags dump various phases of GHC's Core-to-Core pipeline. This begins with
the desugarer and includes the simplifier, worker-wrapper transformation, the
rule engine, the specialiser, the strictness/occurrence analyser, and a common
subexpression elimination pass.

.. ghc-flag:: -ddump-core-stats
    :shortdesc: Print a one-line summary of the size of the Core program at the
        end of the optimisation pipeline
    :type: dynamic

    Print a one-line summary of the size of the Core program at the end
    of the optimisation pipeline.

.. ghc-flag:: -ddump-ds
    :shortdesc: Dump desugarer output
    :type: dynamic

    Dump desugarer output

.. ghc-flag:: -ddump-simpl-iterations
    :shortdesc: Dump output from each simplifier iteration
    :type: dynamic

    Show the output of each *iteration* of the simplifier (each run of
    the simplifier has a maximum number of iterations, normally 4). This
    outputs even more information than ``-ddump-simpl-phases``.

.. ghc-flag:: -ddump-simpl-stats
    :shortdesc: Dump simplifier stats
    :type: dynamic

    Dump statistics about how many of each kind of transformation too
    place. If you add ``-dppr-debug`` you get more detailed information.

.. ghc-flag:: -dverbose-core2core
    :shortdesc: Show output from each core-to-core pass
    :type: dynamic

    Show the output of the intermediate Core-to-Core pass. (*lots* of output!)
    So: when we're really desperate:

    .. code-block:: sh

        % ghc -noC -O -ddump-simpl -dverbose-core2core -dcore-lint Foo.hs

.. ghc-flag:: -ddump-spec
    :shortdesc: Dump specialiser output
    :type: dynamic

    Dump output of specialisation pass

.. ghc-flag:: -ddump-rules
    :shortdesc: Dump rewrite rules
    :type: dynamic

    Dumps all rewrite rules specified in this module; see
    :ref:`controlling-rules`.

.. ghc-flag:: -ddump-rule-firings
    :shortdesc: Dump rule firing info
    :type: dynamic

    Dumps the names of all rules that fired in this module

.. ghc-flag:: -ddump-rule-rewrites
    :shortdesc: Dump detailed rule firing info
    :type: dynamic

    Dumps detailed information about all rules that fired in this
    module

.. ghc-flag:: -ddump-vect
    :shortdesc: Dump vectoriser input and output
    :type: dynamic

    Dumps the output of the vectoriser.

.. ghc-flag:: -ddump-simpl
    :shortdesc: Dump final simplifier output
    :type: dynamic

    Dump simplifier output (Core-to-Core passes)

.. ghc-flag:: -ddump-inlinings
    :shortdesc: Dump inlining info
    :type: dynamic

    Dumps inlining info from the simplifier. Note that if used in
    conjunction with :ghc-flag:`-dverbose-core2core` the compiler will
    also dump the inlinings that it considers but passes up, along with
    its rationale.

.. ghc-flag:: -ddump-stranal
    :shortdesc: Dump strictness analyser output
    :type: dynamic

    Dump strictness analyser output

.. ghc-flag:: -ddump-str-signatures
    :shortdesc: Dump strictness signatures
    :type: dynamic

    Dump strictness signatures

.. ghc-flag:: -ddump-cse
    :shortdesc: Dump CSE output
    :type: dynamic

    Dump common subexpression elimination (CSE) pass output

.. ghc-flag:: -ddump-worker-wrapper
    :shortdesc: Dump worker-wrapper output
    :type: dynamic

    Dump worker/wrapper split output

.. ghc-flag:: -ddump-occur-anal
    :shortdesc: Dump occurrence analysis output
    :type: dynamic

    Dump "occurrence analysis" output

.. ghc-flag:: -ddump-vt-trace
    :shortdesc: Trace vectoriser
    :type: dynamic

    Make the vectoriser be *real* chatty about what it is up to.

.. ghc-flag:: -ddump-prep
    :shortdesc: Dump prepared core
    :type: dynamic

    Dump output of Core preparation pass


STG representation
~~~~~~~~~~~~~~~~~~

These flags dump various phases of GHC's STG pipeline.

.. ghc-flag:: -ddump-stg
    :shortdesc: Dump final STG
    :type: dynamic

    Dump output of STG-to-STG passes

.. ghc-flag:: -dverbose-stg2stg
    :shortdesc: Show output from each STG-to-STG pass
    :type: dynamic

    Show the output of the intermediate STG-to-STG pass. (*lots* of output!)


C-- representation
~~~~~~~~~~~~~~~~~~

These flags dump various phases of GHC's C-- pipeline.

.. ghc-flag:: -ddump-cmm-verbose
    :shortdesc: Show output from each C-- pipeline pass
    :type: dynamic

    Dump output from all C-- pipeline stages. In case of
    ``.cmm`` compilation this also dumps the result of
    file parsing.

.. ghc-flag:: -ddump-cmm-from-stg
    :shortdesc: Dump STG-to-C-- output
    :type: dynamic

    Dump the result of STG-to-C-- conversion

.. ghc-flag:: -ddump-cmm-raw
    :shortdesc: Dump raw C--
    :type: dynamic

    Dump the “raw” C--.

.. ghc-flag:: -ddump-cmm-cfg
    :shortdesc: Dump the results of the C-- control flow optimisation pass.
    :type: dynamic

    Dump the results of the C-- control flow optimisation pass.

.. ghc-flag:: -ddump-cmm-cbe
    :shortdesc: Dump the results of common block elimination
    :type: dynamic

    Dump the results of the C-- Common Block Elimination (CBE) pass.

.. ghc-flag:: -ddump-cmm-switch
    :shortdesc: Dump the results of switch lowering passes
    :type: dynamic

    Dump the results of the C-- switch lowering pass.

.. ghc-flag:: -ddump-cmm-proc
    :shortdesc: Dump the results of proc-point analysis
    :type: dynamic

    Dump the results of the C-- proc-point analysis pass.

.. ghc-flag:: -ddump-cmm-sp
    :shortdesc: Dump the results of the C-- stack layout pass.
    :type: dynamic

    Dump the results of the C-- stack layout pass.

.. ghc-flag:: -ddump-cmm-sink
    :shortdesc: Dump the results of the C-- sinking pass.
    :type: dynamic

    Dump the results of the C-- sinking pass.

.. ghc-flag:: -ddump-cmm-caf
    :shortdesc: Dump the results of the C-- CAF analysis pass.
    :type: dynamic

    Dump the results of the C-- CAF analysis pass.

.. ghc-flag:: -ddump-cmm-procmap
    :shortdesc: Dump the results of the C-- proc-point map pass.
    :type: dynamic

    Dump the results of the C-- proc-point map pass.

.. ghc-flag:: -ddump-cmm-split
    :shortdesc: Dump the results of the C-- proc-point splitting pass.
    :type: dynamic

    Dump the results of the C-- proc-point splitting pass.

.. ghc-flag:: -ddump-cmm-info
    :shortdesc: Dump the results of the C-- info table augmentation pass.
    :type: dynamic

    Dump the results of the C-- info table augmentation pass.

.. ghc-flag:: -ddump-cmm-cps
    :shortdesc: Dump the results of the CPS pass
    :type: dynamic

    Dump the results of the CPS pass.

.. ghc-flag:: -ddump-cmm
    :shortdesc: Dump the final C-- output
    :type: dynamic

    Dump the result of the C-- pipeline processing



LLVM code generator
~~~~~~~~~~~~~~~~~~~~~~

.. ghc-flag:: -ddump-llvm
    :shortdesc: Dump LLVM intermediate code.
    :type: dynamic

    :implies: :ghc-flag:`-fllvm`

    LLVM code from the :ref:`LLVM code generator <llvm-code-gen>`

Native code generator
~~~~~~~~~~~~~~~~~~~~~

These flags dump various stages of the :ref:`native code generator's
<native-code-gen>` pipeline, which starts with C-- and produces native
assembler.

.. ghc-flag:: -ddump-opt-cmm
    :shortdesc: Dump the results of C-- to C-- optimising passes
    :type: dynamic

    Dump the results of C-- to C-- optimising passes performed by the NCG.

.. ghc-flag:: -ddump-asm-native
    :shortdesc: Dump initial assembly
    :type: dynamic

    Dump the initial assembler output produced from C--.

.. ghc-flag:: -ddump-asm-liveness
    :shortdesc: Dump assembly augmented with register liveness
    :type: dynamic

    Dump the result of the register liveness pass.

.. ghc-flag:: -ddump-asm-regalloc
    :shortdesc: Dump the result of register allocation
    :type: dynamic

    Dump the result of the register allocation pass.

.. ghc-flag:: -ddump-asm-regalloc-stages
    :shortdesc: Dump the build/spill stages of the :ghc-flag:`-fregs-graph`
                register allocator.
    :type: dynamic

    Dump the build/spill stages of the :ghc-flag:`-fregs-graph` register
    allocator.

.. ghc-flag:: -ddump-asm-stats
    :shortdesc: Dump statistics from the register allocator.
    :type: dynamic

    Dump statistics from the register allocator.

.. ghc-flag:: -ddump-asm-expanded
    :shortdesc: Dump the result of the synthetic instruction expansion pass.
    :type: dynamic

    Dump the result of the synthetic instruction expansion pass.

.. ghc-flag:: -ddump-asm
    :shortdesc: Dump final assembly
    :type: dynamic

    Dump assembly language produced by the


Miscellaneous backend dumps
~~~~~~~~~~~~~~~~~~~~~~~~~~~

These flags dump various bits of information from other backends.

.. ghc-flag:: -ddump-bcos
    :shortdesc: Dump interpreter byte code
    :type: dynamic

    Dump byte-code objects (BCOs) produced for the GHC's byte-code interpreter.

.. ghc-flag:: -ddump-foreign
    :shortdesc: Dump ``foreign export`` stubs
    :type: dynamic

    Dump foreign export stubs.



.. _formatting dumps:

Formatting dumps
----------------

.. index::
   single: formatting dumps

.. ghc-flag:: -dppr-user-length
    :shortdesc: Set the depth for printing expressions in error msgs
    :type: dynamic

    In error messages, expressions are printed to a certain "depth",
    with subexpressions beyond the depth replaced by ellipses. This flag
    sets the depth. Its default value is 5.

.. ghc-flag:: -dppr-cols=⟨n⟩
    :shortdesc: Set the width of debugging output. For example ``-dppr-cols200``
    :type: dynamic

    Set the width of debugging output. Use this if your code is wrapping
    too much. For example: ``-dppr-cols=200``.

.. ghc-flag:: -dppr-case-as-let
    :shortdesc: Print single alternative case expressions as strict lets.
    :type: dynamic

    Print single alternative case expressions as though they were strict
    let expressions. This is helpful when your code does a lot of
    unboxing.

.. ghc-flag:: -dno-debug-output
    :shortdesc: Suppress unsolicited debugging output
    :type: dynamic
    :reverse: -ddebug-output

    Suppress any unsolicited debugging output. When GHC has been built
    with the ``DEBUG`` 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.

.. _suppression:

Suppressing unwanted information
--------------------------------

.. index::
   single: suppression; of unwanted dump output

Core dumps contain a large amount of information. Depending on what you
are doing, not all of it will be useful. Use these flags to suppress the
parts that you are not interested in.

.. ghc-flag:: -dsuppress-all
    :shortdesc: In core dumps, suppress everything (except for uniques) that is
        suppressible.
    :type: dynamic

    Suppress everything that can be suppressed, except for unique ids as
    this often makes the printout ambiguous. If you just want to see the
    overall structure of the code, then start here.

.. ghc-flag:: -dsuppress-ticks
    :shortdesc: Suppress "ticks" in the pretty-printer output.
    :type: dynamic

    Suppress "ticks" in the pretty-printer output.

.. ghc-flag:: -dsuppress-uniques
    :shortdesc: Suppress the printing of uniques in debug output (easier to use
        ``diff``)
    :type: dynamic

    Suppress the printing of uniques. 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 ``diff``. Once ``diff``
    has shown you where to look, you can try again without
    :ghc-flag:`-dsuppress-uniques`

.. ghc-flag:: -dsuppress-idinfo
    :shortdesc: Suppress extended information about identifiers where they
        are bound
    :type: dynamic

    Suppress extended information about identifiers where they are
    bound. This includes strictness information and inliner templates.
    Using this flag can cut the size of the core dump in half, due to
    the lack of inliner templates

.. ghc-flag:: -dsuppress-unfoldings
    :shortdesc: Suppress the printing of the stable unfolding of a variable at
        its binding site
    :type: dynamic

    Suppress the printing of the stable unfolding of a variable at its
    binding site.

.. ghc-flag:: -dsuppress-module-prefixes
    :shortdesc: Suppress the printing of module qualification prefixes
    :type: dynamic

    Suppress the printing of module qualification prefixes. This is the
    ``Data.List`` in ``Data.List.length``.

.. ghc-flag:: -dsuppress-type-signatures
    :shortdesc: Suppress type signatures
    :type: dynamic

    Suppress the printing of type signatures.

.. ghc-flag:: -dsuppress-type-applications
    :shortdesc: Suppress type applications
    :type: dynamic

    Suppress the printing of type applications.

.. ghc-flag:: -dsuppress-coercions
    :shortdesc: Suppress the printing of coercions in Core dumps to make them
        shorter
    :type: dynamic

    Suppress the printing of type coercions.


.. _checking-consistency:

Checking for consistency
------------------------

.. index::
   single: consistency checks
   single: lint

.. ghc-flag:: -dcore-lint
    :shortdesc: Turn on internal sanity checking
    :type: dynamic

    Turn on heavyweight intra-pass sanity-checking within GHC, at Core
    level. (It checks GHC's sanity, not yours.)

.. ghc-flag:: -dstg-lint
    :shortdesc: STG pass sanity checking
    :type: dynamic

    Ditto for STG level. (note: currently doesn't work).

.. ghc-flag:: -dcmm-lint
    :shortdesc: C-- pass sanity checking
    :type: dynamic

    Ditto for C-- level.

.. ghc-flag:: -fllvm-fill-undef-with-garbage
    :shortdesc: Intruct LLVM to fill dead STG registers with garbage
    :type: dynamic

    Instructs the LLVM code generator to fill dead STG registers with garbage
    instead of ``undef`` in calls. This makes it easier to catch subtle
    code generator and runtime system bugs (e.g. see :ghc-ticket:`11487`).

.. ghc-flag:: -fcatch-bottoms
    :shortdesc: Insert ``error`` expressions after bottoming expressions; useful
        when debugging the compiler.
    :type: dynamic

    Instructs the simplifier to emit ``error`` expressions in the continuation
    of empty case analyses (which should bottom and consequently not return).
    This is helpful when debugging demand analysis bugs which can sometimes
    manifest as segmentation faults.

.. _checking-determinism:

Checking for determinism
------------------------

.. index::
   single: deterministic builds

.. ghc-flag:: -dinitial-unique=⟨s⟩
    :shortdesc: Start ``UniqSupply`` allocation from ⟨s⟩.
    :type: dynamic

    Start ``UniqSupply`` allocation from ⟨s⟩.

.. ghc-flag:: -dunique-increment=⟨i⟩
    :shortdesc: Set the increment for the generated ``Unique``'s to ⟨i⟩.
    :type: dynamic

    Set the increment for the generated ``Unique``'s to ⟨i⟩.

    This is useful in combination with :ghc-flag:`-dinitial-unique=⟨s⟩` to test
    if the generated files depend on the order of ``Unique``'s.

    Some interesting values:

    * ``-dinitial-unique=0 -dunique-increment=1`` - current sequential
      ``UniqSupply``
    * ``-dinitial-unique=16777215 -dunique-increment=-1`` - ``UniqSupply`` that
      generates in decreasing order
    * ``-dinitial-unique=1 -dunique-increment=PRIME`` - where PRIME big enough
      to overflow often - nonsequential order