|Version 3 (modified by simonpj, 6 years ago) (diff)|
This page is about strategies that may help you find a bug in GHC or its runtime system.
At compile time
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
At run time (see also the relevant User Manual section):
- Use +RTS -sstderr or +RTS -Sstderr to watch garbage collection activity.
At crash time
If your GHC-compiled program actually crashes, you may have to roll up your sleeves and wake up gdb. There's a whole Wiki page describing what to do.
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.