ghc-dump: A tool for analysing GHC Core

ghc-dump is a library, GHC plugin, and set of tools for recording and analysing GHC's Core representation. The plugin is compatible with GHC 7.10 through 8.10, exporting a consistent (albeit somewhat lossy) representation across these versions. The AST is encoded as CBOR, which is small and easy to deserialise.

Dumping Core from compilation

The GHC plugin GhcDump.Plugin provides a Core-to-Core plugin which dumps a representation of the Core AST to a file after every Core-to-Core pass. To use it, first install the ghc-dump-core package,

$ cabal install ghc-dump-core

And then invoke GHC with the -fplugin GhcDump.Plugin flag,

$ ghc -fplugin GhcDump.Plugin -dumpdir=dump -O Test.hs
[1 of 1] Compiling Main             ( Test.hs, Test.o )
Linking Test ...
$ cd dump
$ ls
Test                 Test.pass-0011.cbor  Test.pass-0017.cbor  Test.pass-0006.cbor
Test.hi              Test.pass-0012.cbor  Test.pass-0001.cbor  Test.pass-0007.cbor
Test.hs              Test.pass-0013.cbor  Test.pass-0002.cbor  Test.pass-0008.cbor
Test.o               Test.pass-0014.cbor  Test.pass-0003.cbor  Test.pass-0009.cbor
Test.pass-0000.cbor  Test.pass-0015.cbor  Test.pass-0004.cbor
Test.pass-0010.cbor  Test.pass-0016.cbor  Test.pass-0005.cbor

Here we see a pass-N.cbor file was produced for each Core-to-Core pass.

Analysis in GHCi

One can then load this into ghci for analysis,

$ ghci
GHCi, version 8.3.20170413: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/ben/.ghci
λ> import GhcDump.Repl as Dump
λ> mod <- readDump "Test.pass-0.cbor"
λ> pretty mod
module Main where

nsoln :: Int-> Int
{- Core Size{terms=98 types=66 cos=0 vbinds=0 jbinds=0} -}
nsoln =
  λ nq →
    let safe =
          λ x d ds →
            case ds of wild {
              [] → GHC.Types.True
              : q l →
                GHC.Classes.&&
...

Analysis with CLI tool

Alternatively, the ghc-dump utility can be used to render the representation in human-readable form. For instance, we can filter the dump to include only top-level binders containing main in its name,

$ ghc-dump show --filter='.*main.*' Test.pass-0.cbor

You can conveniently summarize the top-level bindings of the program,

$ ghc-dump list-bindings --sort=terms Test.pass-0.cbor
Name                 Terms  Types  Coerc. Type
nsoln                98     66     0      Int-> Int
main                 30     28     0      IO ()
$trModule            5      0      0      Module
main                 2      1      0      IO ()
...