Debugging/Compiler

Debugging the compiler

Basic strategies

When compiling GHC:

  • add -DDEBUG to your GhcStage1HcOpts and/or GhcStage2HcOpts in mk/build.mk. This enables assertions and extra debug code.

When compiling the program (see also the relevant User Manual section):

  • Use -v3 or -v4 to get an idea about what GHC is doing when the problem occurs.
  • Add -dcore-lint the GHC command line when compiling each Haskell module. This makes GHC type-check the intermediate program after every optimisation pass, which often nails a fault.
  • Add -ddump-simpl to see the optimised Core output. There are a number of other -ddump-x flags; see the user manual.
  • The flag -dppr-debug makes the -ddump-x flags print much more verbose output. Use this if you are getting desperate!

Adding debugging code to the compiler

  • Outputable.pprTrace is a nice way to print trace messages from the compiler
  • ASSERT(p), ASSERT2(p,msg), WARN(p,msg) are assertions and warning enabled only when the compiler is compiled with -DDEBUG. There are also variants of these that work better in a monad setting; see compiler/HsVersions.h.