7.13. Debugging the compiler — Glasgow Haskell Compiler 8.6.1 User's Guide

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# 7.13. Debugging the compiler

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

Dump flags

- Debugging the compiler
  - Dumping out compiler intermediate structures
    - Front-end
    - Type-checking and renaming
    - Core representation and simplification
    - STG representation
    - C-\- representation
    - LLVM code generator
    - Native code generator
    - Miscellaneous backend dumps
  - Formatting dumps
  - Suppressing unwanted information
  - Checking for consistency
  - Checking for determinism

## 7.13.1. 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-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.

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`.

### 7.13.1.1. 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.

### 7.13.1.2. 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

`-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`

  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

### 7.13.1.3. 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 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.

`-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-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

### 7.13.1.4. STG representation

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

`-ddump-stg`

  Dump output of STG-to-STG passes

`-dverbose-stg2stg`

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

### 7.13.1.5. C-\- representation

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

`-ddump-cmm-verbose`

  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.

`-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

### 7.13.1.6. LLVM code generator

`-ddump-llvm`

  Implies: | `-fllvm`

  LLVM code from the LLVM code generator

### 7.13.1.7. Native code generator

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

`-ddump-opt-cmm`

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

`-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 assembly language produced by the

### 7.13.1.8. 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-foreign`

  Dump foreign export stubs.

## 7.13.2. 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.

## 7.13.3. 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

## 7.13.4. 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.

`-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).

`-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`

  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.

## 7.13.5. 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

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