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. In fact, in the link phase -ticky implies -debug, so you get the debug version of the runtime system too.

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

You need to use -ddump-simpl -ddump-prep when compiling the source files to see the functions that correspond to the performance counter report.

It's very low level stuff, but in exchange:

• It's guaranteed that adding -ticky doesn't affect optimisation or transformation. It just adds the overhead of performance counters to the final code.

• 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
GhcLibHcOpts += -ticky

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

# Currently ticky is incompatible with threading
GhcThreaded = NO