Dear Cafe, I'm currently looking at the optimization GHC is doing and I cannot find the rewrite rules it fires. When I run my test code with ghc -O2 -ddump-simpl-stats -ddump-rule-firings Main.hs GHC shows the rules which are fired: ... Rule fired: Class op + ... Rule fired: +## ... and so on. Nothing new, nothing special. However, where do I find the definitions of these rules ? I grepped[1] the GHC code base and found nothing so far. I didn't find any documentation on it either. Can anyone point me to some place where I can find further information ? Thank you folks and have a nice day Dominik PS.: Since I'm working on numerical stable code with directed rounding I'm only interested in these two particular rules. I suspect them to break parts of my code. [1] http://jamie-wong.com/2013/07/12/grep-test