6.13. Debugging the compiler

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

6.13.1. Dumping out compiler intermediate structures

-ddump- ⟨pass⟩

Make a debugging dump after pass <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-to-file

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 -ddump-simpl will end up in module.dump-simpl.

-ddump-parsed

Dump parser output

-ddump-parsed-ast

Dump parser output as a syntax tree

-ddump-rn

Dump renamer output

-ddump-rn-ast

Dump renamer output as a syntax tree

-ddump-tc

Dump typechecker output

-ddump-tc-ast

Dump typechecker output as a syntax tree

-ddump-splices

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

-dth-dec-file=⟨file⟩

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

-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

Dump derived instances

-ddump-ds

Dump desugarer output

-ddump-spec

Dump output of specialisation pass

-ddump-rules

Dumps all rewrite rules specified in this module; see Controlling what’s going on in rewrite rules.

-ddump-rule-firings

Dumps the names of all rules that fired in this module

-ddump-rule-rewrites

Dumps detailed information about all rules that fired in this module

-ddump-vect

Dumps the output of the vectoriser.

-ddump-simpl

Dump simplifier output (Core-to-Core passes)

-ddump-inlinings

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

-ddump-stranal

Dump strictness analyser output

-ddump-str-signatures

Dump strictness signatures

-ddump-cse

Dump common subexpression elimination (CSE) pass output

-ddump-worker-wrapper

Dump worker/wrapper split output

-ddump-occur-anal

Dump “occurrence analysis” output

-ddump-prep

Dump output of Core preparation pass

-ddump-stg

Dump output of STG-to-STG passes

-ddump-cmm

Dump the result of the C– pipeline processing

-ddump-cmm-from-stg

Dump the result of STG-to-C– conversion

-ddump-cmm-verbose

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

-ddump-opt-cmm

Dump the results of C– to C– optimising passes.

-ddump-asm

Dump assembly language produced by the native code generator

-ddump-llvm

Implies:-fllvm

LLVM code from the LLVM code generator

-ddump-bcos

Dump byte-code compiler output

-ddump-foreign

dump foreign export stubs

-ddump-json

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 -ddump-to-file.

-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

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

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

-ddump-tc-trace

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

-ddump-vt-trace

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

-ddump-rn-trace

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

-ddump-ec-trace

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

-ddump-rn-stats

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

-dverbose-core2core

-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

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 *** are 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.

-ddump-core-stats

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

-dfaststring-stats

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

-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.

6.13.2. Formatting dumps

-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-cols=N

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

-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

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.

6.13.3. Suppressing unwanted information

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

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-ticks

Suppress “ticks” in the pretty-printer output.

-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

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

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

-dsuppress-module-prefixes

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

-dsuppress-type-signatures

Suppress the printing of type signatures.

-dsuppress-type-applications

Suppress the printing of type applications.

-dsuppress-coercions

Suppress the printing of type coercions.

6.13.4. Checking for consistency

-dcore-lint

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

-dstg-lint

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

-dcmm-lint

Ditto for C– level.

-fllvm-fill-undef-with-garbage

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 Trac #11487).

6.13.5. Checking for determinism

-dinitial-unique=⟨s⟩

Start UniqSupply allocation from ⟨s⟩.

-dunique-increment=⟨i⟩

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