Should GHC default to -O1 ?

David Fox ddssff at
Tue Nov 8 17:49:26 CET 2011

On Tue, Nov 8, 2011 at 8:16 AM, Simon Marlow <marlowsd at> wrote:
> On 08/11/2011 14:31, Daniel Fischer wrote:
>> On the haskell-cafe as well as the beginners mailing lists, there
>> frequently (for some value of frequent) are posts where the author
>> inquires
>> about a badly performing programme, in the form of stack overflows, space
>> leaks or slowness.
>> Often this is because they compiled their programme without optimisations,
>> simply recompiling with -O or -O2 yields a decently performing programme.
>> So I wonder, should ghc compile with -O1 by default?
>> What would be the downsides?
> I understand the problem.  However, -O has a couple of serious downsides:
>  1. it costs about 2x compile time and memory usage
>  2. it forces a lot more recompilation to happen after changes,
>     due to inter-module optimisations.
> most people know about 1, but I think 2 is probably less well-known. When in
> the edit-compile-debug cycle it really helps to have -O off, because your
> compiles will be so much quicker due to both factors 1 & 2.
> So the default -O setting is a careful compromise, trying to hit a good
> compile-time/runtime tradeoff.  Perhaps we're more sensitive in Haskell
> because -O can easily give you an order of magnitude or more speedup,
> whereas in C you're likely to get a pretty consistent 30% or so.  The
> difference between -O and -O2 is another careful tradeoff.
> Also bear in mind that using GHCi gives you another 2x speedup in
> compilation (approx), but 30x slowdown in runtime (varies wildly from
> program to program though).  And subsequent recompiles are much faster
> because GHCi has cached a lot of interfaces.
> I suppose we should really run an up to date set of benchmarks on some real
> Haskell programs (i.e. not nofib) and reconsider how we set these defaults.
>  I really doubt that we'll want to turn on -O by default, though.

We should remember that we are only talking about which default leads
to the best outcome when, due to inexperience, someone fails to set
the option the way they want it.

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