It appears I get the same output wether I use `-fspec-constr` or not. I'm afraid so. Look in simplCore/SimplCore.lhs, function getCoreToDo. This builds the main optimisation pipeline. You'll see that it has a -O0 path and a -O1/-O2 path. The flag Opt_SpecConstr is consulted only in the latter. One could perhaps do it differently but that's the way it is right now. I test an isolated optimisation by switching it on and off with -O1. Simon From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Chris Heller Sent: 06 January 2014 02:44 To: ghc-devs@haskell.org Subject: Optimisation flags at -O0 I wanted to understand better what `-fspec-constr` does. So I compiled the User Guide example with `-O0 -fspec-constr` to isolate the effects of call-pattern specialization, and nothing else (I used ghc-core to pretty-print the resulting Core syntax). It appears I get the same output wether I use `-fspec-constr` or not. Does this mean that compiling with `-O0` even explicitly enabled optimizations are turned off? If that is the case, how does one test an isolated optimization? -Chris