Optimisation flags at -O0

[Edward Z. Yang]

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