5.13. Debugging the compiler

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

5.13.1. Dumping out compiler intermediate structures

-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-file-prefix=⟨str⟩

Set the prefix of the filenames used for debugging output. For example, -ddump-file-prefix=Foo will cause the output from -ddump-simpl to be dumped to Foo.dump-simpl.

-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.

-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 *** 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.

-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.

-ddump-timings

Show allocation and runtime statistics for various stages of compilation. Allocations are measured in bytes. Timings are measured in milliseconds.

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 -ddump-to-file.

5.13.1.1. Front-end

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

-ddump-parsed

Dump parser output

-ddump-parsed-ast

Dump parser output as a syntax tree

-ddump-if-trace

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

5.13.1.2. Type-checking and renaming

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

-ddump-tc-trace

Make the type checker 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.

-ddump-rn

Dump renamer output

-ddump-rn-ast

Dump renamer output as a syntax tree

-ddump-tc

Dump typechecker output. Note that this hides a great deal of detail by default; you might consider using this with -fprint-typechecker-elaboration.

-ddump-tc-ast

Dump typechecker output as a syntax tree

-ddump-hie

Dump the hie file syntax tree if we are generating extended interface files

-ddump-splices

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

-dth-dec-file

Dump expansions of all top-level Template Haskell splices into module.th.hs for each file module.hs.

-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

5.13.1.3. 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.

-ddump-core-stats

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

-ddump-ds
-ddump-ds-preopt

Dump desugarer output. -ddump-ds dumps the output after the very simple optimiser has run (which discards a lot of clutter and hence is a sensible default. -ddump-ds-preopt shows the output after desugaring but before the very simple optimiser.

-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).

-ddump-simpl-stats

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

-dverbose-core2core

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

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

-drule-check=⟨str⟩

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 fire 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.

-dinline-check=⟨str⟩

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.

-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-cpranal

Dump Constructed Product Result analysis output

-ddump-cpr-signatures

Dump Constructed Product Result 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

5.13.1.4. STG representation

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

-ddump-stg

Show the output of CoreToStg pass.

-dverbose-stg2stg

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

-ddump-stg-unarised

Show the output of the unarise pass.

-ddump-stg-final

Show the output of the last STG pass before we generate Cmm.

5.13.1.5. C-- representation

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

-ddump-cmm-verbose-by-proc

Dump output from main C-- pipeline stages. In case of .cmm compilation this also dumps the result of file parsing. Not included are passes run by the chosen backend. Currently only the NCG backends runs additional passes ( -ddump-opt-cmm ).

Cmm dumps don’t include unreachable blocks since we print blocks in reverse post-order.

-ddump-cmm-verbose

If used in conjunction with -ddump-to-file, writes dump output from main C-- pipeline stages to files (each stage per file).

-ddump-cmm-from-stg

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

-ddump-cmm-raw

Dump the “raw” C--.

-ddump-cmm-cfg

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

-ddump-cmm-cbe

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

-ddump-cmm-switch

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

-ddump-cmm-proc

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

-ddump-cmm-sp

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

-ddump-cmm-sink

Dump the results of the C-- sinking pass.

-ddump-cmm-caf

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

-ddump-cmm-procmap

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

-ddump-cmm-split

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

-ddump-cmm-info

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

-ddump-cmm-cps

Dump the results of the CPS pass.

-ddump-cmm

Dump the result of the C-- pipeline processing

-ddump-cfg-weights

Dumps the CFG with weights used by the new block layout code. Each CFG is dumped in dot format graph making it easy to visualize them.

5.13.1.6. LLVM code generator

-ddump-llvm
Implies:-fllvm

LLVM code from the LLVM code generator

5.13.1.7. Native code generator

These flags dump various stages of the native code generator’s pipeline, which starts with C-- and produces native assembler.

-ddump-cmm-opt

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

-ddump-opt-cmm

Alias for -ddump-cmm-opt

-ddump-asm-native

Dump the initial assembler output produced from C--.

-ddump-asm-liveness

Dump the result of the register liveness pass.

-ddump-asm-regalloc

Dump the result of the register allocation pass.

-ddump-asm-regalloc-stages

Dump the build/spill stages of the -fregs-graph register allocator.

-ddump-asm-stats

Dump statistics from the register allocator.

-ddump-asm-expanded

Dump the result of the synthetic instruction expansion pass.

-ddump-asm

Dump the final assembly produced by the native code generator.

5.13.1.8. Miscellaneous backend dumps

These flags dump various bits of information from other backends.

-ddump-bcos

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

-ddump-rtti

Trace runtime type inference done by various interpreter commands.

-ddump-foreign

Dump foreign export stubs.

-ddump-ticked

Dump the code instrumented by HPC (Observing Code Coverage).

-ddump-hpc

An alias for -ddump-ticked.

-ddump-mod-map

Dump a mapping of modules to where they come from, and how:

  • (hidden module): Module is hidden, and thus will never be available for import.
  • (unusable module): Module is unavailable because the package is unusable.
  • (hidden package): This module is in someone’s exported-modules list, but that package is hidden.
  • (exposed package): Module is available for import.
  • (reexport by <PACKAGES>): This module is available from a reexport of some set of exposed packages.
  • (hidden reexport by <PACKAGES>): This module is available from a reexport of some set of hidden packages.
  • (package flag): This module export comes from a package flag.

5.13.2. 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.

-dhex-word-literals

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.

-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.

5.13.3. 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-timestamps

Suppress the printing of timestamps. This makes it easier to diff dumps.

-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.

-dsuppress-var-kinds

Suppress the printing of variable kinds

-dsuppress-stg-free-vars

Suppress the printing of closure free variable lists in STG output

5.13.4. Checking for consistency

-dcore-lint

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

-dlinear-core-lint

Turn on linearity checking in GHC. Currently, some optimizations in GHC might not preserve linearity and they valid programs might fail Linear Core Lint. In the near future, this option will be removed and folded into normal Core Lint.

-dstg-lint

Ditto for STG level.

-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 #11487).

-falignment-sanitisation

Compile with alignment checks for all info table dereferences. This can be useful when finding pointer tagging issues.

-fproc-alignment

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.

-fcatch-bottoms

GHC generates case expressions without a default alternative in some cases:

  • When the demand analysis thinks that the scrutinee does not return (i.e. a bottoming expression)
  • When the scrutinee is a GADT and its type rules out some constructors, and others constructors are already handled by the case expression.

With this flag GHC generates a default alternative with error in these cases. This is helpful when debugging demand analysis or type checker bugs which can sometimes manifest as segmentation faults.

5.13.5. 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=⟨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

5.13.6. Other

-dno-typeable-binds

This avoid generating Typeable-related bindings for modules and types. This is useful when debugging because it gives smaller modules and dumps, but the compiler will panic if you try to use Typeable instances of things that you built with this flag.