Debugging GHC

This page is about strategies that may help you find a bug in GHC or its runtime system.

Consistency checking flags

At compile time (see also the relevant User Manual section):

• 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!

At link time:

• Add -debug to the GHC command line when linking. This links the program against a special version of the runtime system that does lots of extra internal consistency checking. Overall performance is significantly reduced. Simon: any flags?

At run time (see also the relevant User Manual section):

• Use +RTS -sstderr or +RTS -Sstderr to watch garbage collection activity.

Ticky-ticky profiling

Ticky-ticky profiling adds counters to every STG function. It's very low-level, but it really tells you what is going on:

• Add the -ticky flag when compiling a Haskell module to enable "ticky-ticky" profiling of that module. This makes GHC emit performance-counting instructions in every STG function.

• Add -ticky to the command line when linking, so that you link against a version of the runtime system that allows you to display the results.

• Add +RTS -rfoo.ticky to the run-time command line, to put the ticky-ticky profile in the file foo.ticky.

It's very low level stuff. You need to use -ddump-simpl -ddump-prep when compiling the source files to see the functions that correspond to the performance counter report.

You can mix modules compiled with -ticky and modules compiled without.

To really see everything you need to compile all the libraries with -ticky. To do that in a standard build tree, here are some flag settings in build.mk that work:

# Build all libraries with -ticky
GhcLibOpts += -ticky

# Build the RTS in the ticky way
GhcRTSWays += t

# Currently ticky is incompatible with threading
GhcThreaded = NO

# When linking stage2 you need -ticky, else you get unresolved symols
GhcStage2HcOpts += -ticky

# Ditto Haddock
utils/haddock_dist_EXTRA_HC_OPTS += -ticky

But see #3439, which would allow you to drop the last two.