All -f flags have a 'no' form, as in '-fno-spec-constr', so you can manually toggle a single optimization on/off. Some optimizations apply even at -O0, see optLevelFlags in compiler/main/DynFlags.hs Edward Excerpts from Chris Heller's message of 2014-01-05 18:43:46 -0800:
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