.. _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-flag:: -ddump-timings
:shortdesc: Dump per-pass timing and allocation statistics
:type: dynamic
Show allocation and runtime statistics for various stages of compilation.
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
:shortdesc: Dump evaluated TH declarations into `*.th.hs` files
:type: dynamic
Dump expansions of all top-level Template Haskell splices into
:file:`{module}.th.hs` for each file :file:`{module}.hs`.
.. 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 -ddump-ds-preopt
:shortdesc: Dump desugarer output.
:type: dynamic
Dump desugarer output. :ghc-flag:`-ddump-ds` dumps the output after the very
simple optimiser has run (which discards a lot of clutter and hence is a
sensible default. :ghc-flag:`-ddump-ds-preopt` shows the output after
desugaring but before the very simple optimiser.
.. 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).
.. ghc-flag:: -ddump-simpl-stats
:shortdesc: Dump simplifier stats
:type: dynamic
Dump statistics about how many of each kind of transformation took
place. If you add :ghc-flag:`-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:: -drule-check=⟨str⟩
:shortdesc: Dump information about potential rule application
:type: dynamic
This flag is useful for debugging why a rule you expect to be firing isn't.
Rules are filtered by the user provided string, a rule is kept if a prefix
of its name matches the string.
The pass then checks whether any of these rules could apply to
the program but which didn't file for some reason. For example, specifying
``-drule-check=SPEC`` will check whether there are any applications which
might be subject to a rule created by specialisation.
.. ghc-flag:: -dinline-check=⟨str⟩
:shortdesc: Dump information about inlining decisions
:type: dynamic
This flag is useful for debugging why a definition is not inlined.
When a string is passed to this flag we report information
about all functions whose name shares a prefix with the string.
For example, if you are inspecting the core of your program and you observe
that ``foo`` is not being inlined. You can pass ``-dinline-check foo`` and
you will see a report about why ``foo`` is not inlined.
.. 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-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.
Cmm dumps don't include unreachable blocks since we print
blocks in reverse post-order.
.. 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:: -dhex-word-literals
:shortdesc: Print values of type `Word#` in hexadecimal.
:type: dynamic
Print values of type `Word#` and `Word64#` (but not values of
type `Int#` and `Int64#`) in hexadecimal instead of decimal.
The hexadecimal is zero-padded to make the length of the
representation a power of two. For example: `0x0A0A##`,
`0x000FFFFF##`, `0xC##`. This flag may be helpful when you
are producing a bit pattern that to expect to work correctly on a 32-bit
or a 64-bit architecture. Dumping hexadecimal literals after
optimizations and constant folding makes it easier to confirm
that the generated bit pattern is correct.
.. 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 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-timestamps
:shortdesc: Suppress timestamps in dumps
:type: dynamic
Suppress the printing of timestamps.
This makes it easier to diff dumps.
.. 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.
.. ghc-flag:: -dsuppress-var-kinds
:shortdesc: Suppress the printing of variable kinds
:type: dynamic
Suppress the printing of variable kinds
.. ghc-flag:: -dsuppress-stg-free-vars
:shortdesc: Suppress the printing of closure free variable lists in STG output
:type: dynamic
Suppress the printing of closure free variable lists in STG output
.. _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.
.. 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:: -falignment-sanitisation
:shortdesc: Compile with alignment checks for all info table dereferences.
:type: dynamic
Compile with alignment checks for all info table dereferences. This can be
useful when finding pointer tagging issues.
.. ghc-flag:: -fproc-alignment
:shortdesc: Align functions at given boundary.
:type: dynamic
Align functions to multiples of the given value. Only valid values are powers
of two.
``-fproc-alignment=64`` can be used to limit alignment impact on performance
as each function will start at a cache line.
However forcing larger alignments in general reduces performance.
.. 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