Is this a known bug? (GHC 6.10.x)
It's known to happen when optimising shares what shouldn't be shared. Try compiling with -O2 -fno-cse (if that doesn't help, it doesn't necessarily mean it's not unwanted sharing, though). And, please, let us see some code to identify the problem.
I tried -O2 -fno-cse. No difference. I also tried -O2 -fno-full-laziness. BIG DIFFERENCE. The program now runs in constant space (like with -O0), but it also runs about 2x faster than -O0. I have no idea what these switches do, but clearly one of these optimisations is actually pessimal for this particular program. I still want to try compiling with a newer version of GHC to see what difference that makes. (And yes, if I had the code on this PC, I could post it. It's kinda long though... In essence, it just calls randomRIO a bazillion times and writes the results into a file using hPutChar.)