[Haskell-cafe] GHC magic optimization ?

Luke Palmer lrpalmer at gmail.com
Fri Dec 4 05:43:27 EST 2009

On Fri, Dec 4, 2009 at 3:36 AM, Neil Brown <nccb2 at kent.ac.uk> wrote:
> But let's say you have:
> g x y = f x y * f x y
> Now the compiler (i.e. at compile-time) can do some magic.  It can spot the
> common expression and know the result of f x y must be the same both times,
> so it can convert to:
> g x y = let z = f x y in z * z

GHC does *not* do this by default, quite intentionally, even when
optimizations are enabled.  The reason is because it can cause major
changes in the space complexity of a program.  Eg.

x = sum [1..10^6] + product [1..10^6]
x' = let l = [1..10^6] in sum l + product l

x runs in constant space, but x' keeps the whole list in memory.  The
CSE here has actually wasted both time and space, since it is harder
to save [1..10^6] than to recompute it!  (Memory vs. arithmetic ops)

So GHC leaves it to the user to specify sharing.  If you want an
expression shared, let bind it and reuse.


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