.. _options-debugging: Debugging the compiler ====================== .. index:: single: debugging options (for GHC) HACKER TERRITORY. HACKER TERRITORY. (You were warned.) .. _dumping-output: Dumping out compiler intermediate structures -------------------------------------------- .. index:: single: dumping GHC intermediates single: intermediate passes, output ``-ddump-`` ⟨pass⟩ .. index:: single: -ddump options Make a debugging dump after pass ```` (may be common enough to need a short form…). You can get all of these at once (*lots* of output) by using ``-v5``, or most of them with ``-v4``. You can prevent them from clogging up your standard output by passing ``-ddump-to-file``. Some of the most useful ones are: ``-ddump-parsed`` .. index:: single: -ddump-parsed Dump parser output ``-ddump-rn`` .. index:: single: -ddump-rn Dump renamer output ``-ddump-tc`` .. index:: single: -ddump-tc Dump typechecker output ``-ddump-splices`` .. index:: single: -ddump-splices Dump Template Haskell expressions that we splice in, and what Haskell code the expression evaluates to. ``-ddump-types`` .. index:: single: -ddump-types Dump a type signature for each value defined at the top level of the module. The list is sorted alphabetically. Using ``-dppr-debug`` dumps a type signature for all the imported and system-defined things as well; useful for debugging the compiler. ``-ddump-deriv`` .. index:: single: -ddump-deriv Dump derived instances ``-ddump-ds`` .. index:: single: -ddump-ds Dump desugarer output ``-ddump-spec`` .. index:: single: -ddump-spec Dump output of specialisation pass ``-ddump-rules`` .. index:: single: -ddump-rules Dumps all rewrite rules specified in this module; see :ref:`controlling-rules`. ``-ddump-rule-firings`` .. index:: single: -ddump-rule-firings Dumps the names of all rules that fired in this module ``-ddump-rule-rewrites`` .. index:: single: -ddump-rule-rewrites Dumps detailed information about all rules that fired in this module ``-ddump-vect`` .. index:: single: -ddump-vect Dumps the output of the vectoriser. ``-ddump-simpl`` .. index:: single: -ddump-simpl Dump simplifier output (Core-to-Core passes) ``-ddump-inlinings`` .. index:: single: -ddump-inlinings Dumps inlining info from the simplifier ``-ddump-stranal`` .. index:: single: -ddump-stranal Dump strictness analyser output ``-ddump-strsigs`` .. index:: single: -ddump-strsigs Dump strictness signatures ``-ddump-cse`` .. index:: single: -ddump-cse Dump common subexpression elimination (CSE) pass output ``-ddump-worker-wrapper`` .. index:: single: -ddump-worker-wrapper Dump worker/wrapper split output ``-ddump-occur-anal`` .. index:: single: -ddump-occur-anal Dump "occurrence analysis" output ``-ddump-prep`` .. index:: single: -ddump-prep Dump output of Core preparation pass ``-ddump-stg`` .. index:: single: -ddump-stg Dump output of STG-to-STG passes ``-ddump-cmm`` .. index:: single: -ddump-cmm Print the C-- code out. ``-ddump-opt-cmm`` .. index:: single: -ddump-opt-cmm Dump the results of C-- to C-- optimising passes. ``-ddump-asm`` .. index:: single: -ddump-asm Dump assembly language produced by the :ref:`native code generator ` ``-ddump-llvm`` .. index:: single: -ddump-llvm LLVM code from the :ref:`LLVM code generator ` ``-ddump-bcos`` .. index:: single: -ddump-bcos Dump byte-code compiler output ``-ddump-foreign`` .. index:: single: -ddump-foreign dump foreign export stubs ``-ddump-simpl-iterations`` .. index:: single: -ddump-simpl-iterations 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``. ``-ddump-simpl-stats`` .. index:: single: -ddump-simpl-stats option Dump statistics about how many of each kind of transformation too place. If you add ``-dppr-debug`` you get more detailed information. ``-ddump-if-trace`` .. index:: single: -ddump-if-trace Make the interface loader be *real* chatty about what it is up to. ``-ddump-tc-trace`` .. index:: single: -ddump-tc-trace Make the type checker be *real* chatty about what it is up to. ``-ddump-vt-trace`` .. index:: single: -ddump-tv-trace Make the vectoriser be *real* chatty about what it is up to. ``-ddump-rn-trace`` .. index:: single: -ddump-rn-trace Make the renamer be *real* chatty about what it is up to. ``-ddump-rn-stats`` .. index:: single: -dshow-rn-stats Print out summary of what kind of information the renamer had to bring in. ``-dverbose-core2core``, ``-dverbose-stg2stg`` .. index:: single: -dverbose-core2core single: -dverbose-stg2stg Show the output of the intermediate Core-to-Core and STG-to-STG passes, respectively. (*lots* of output!) So: when we're really desperate: :: % ghc -noC -O -ddump-simpl -dverbose-core2core -dcore-lint Foo.hs ``-dshow-passes`` .. index:: single: -dshow-passes Print out each pass name as it happens. ``-ddump-core-stats`` .. index:: single: -ddump-core-stats Print a one-line summary of the size of the Core program at the end of the optimisation pipeline. ``-dfaststring-stats`` .. index:: single: -dfaststring-stats Show statistics on the usage of fast strings by the compiler. ``-dppr-debug`` .. index:: single: -dppr-debug 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. .. _formatting dumps: Formatting dumps ---------------- .. index:: single: formatting dumps ``-dppr-user-length`` .. index:: single: -dppr-user-length 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. ``-dppr-colsNNN`` .. index:: single: -dppr-colsNNN Set the width of debugging output. Use this if your code is wrapping too much. For example: ``-dppr-cols200``. ``-dppr-case-as-let`` .. index:: single: -dppr-case-as-let Print single alternative case expressions as though they were strict let expressions. This is helpful when your code does a lot of unboxing. ``-dno-debug-output`` .. index:: single: -dno-debug-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. ``-dsuppress-all`` .. index:: single: -dsuppress-all 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. ``-dsuppress-uniques`` .. index:: single: -dsuppress-uniques 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 ``-dsuppress-uniques`` ``-dsuppress-idinfo`` .. index:: single: -dsuppress-idinfo 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 ``-dsuppress-unfoldings`` .. index:: single: -dsuppress-unfoldings Suppress the printing of the stable unfolding of a variable at its binding site. ``-dsuppress-module-prefixes`` .. index:: single: -dsuppress-module-prefixes Suppress the printing of module qualification prefixes. This is the ``Data.List`` in ``Data.List.length``. ``-dsuppress-type-signatures`` .. index:: single: -dsuppress-type-signatures Suppress the printing of type signatures. ``-dsuppress-type-applications`` .. index:: single: -dsuppress-type-applications Suppress the printing of type applications. ``-dsuppress-coercions`` .. index:: single: -dsuppress-coercions Suppress the printing of type coercions. .. _checking-consistency: Checking for consistency ------------------------ .. index:: single: consistency checks single: lint ``-dcore-lint`` .. index:: single: -dcore-lint Turn on heavyweight intra-pass sanity-checking within GHC, at Core level. (It checks GHC's sanity, not yours.) ``-dstg-lint`` .. index:: single: -dstg-lint Ditto for STG level. (note: currently doesn't work). ``-dcmm-lint`` .. index:: single: -dcmm-lint Ditto for C-- level. .. _checking-determinism: Checking for determinism ------------------------ .. index:: single: deterministic builds ``-dinitial-unique=⟨s⟩`` .. index:: single: -dinitial-unique Start ``UniqSupply`` allocation from ⟨s⟩. ``-dunique-increment=⟨i⟩`` .. index:: single: -dunique-increment Set the increment for the generated ``Unique``'s to ⟨i⟩. This is useful in combination with ``-dinitial-unique`` 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