[Haskell-cafe] GHC magic optimization ?
Neil Brown
nccb2 at kent.ac.uk
Fri Dec 4 05:36:09 EST 2009
Emmanuel CHANTREAU wrote:
> I will take an example:
>
> f x y= x+y
>
> The program ask the user to enter two numbers and print the sum. If the
> user enter "1 2" "f 1 2=3" is stored and a gargage collector is used to
> remove this dandling expression later ?
> If the user enter again "1 2", ghc search in dandling results to
> try to find the result without computing it again ?
>
Hi,
I think what you're asking is how Haskell knows at run-time for which
expressions it can re-use the results. The answer is: it doesn't, it
works it out at compile-time. So if you have:
f x y = x + y
And at some point in your program you call f 1 2, and later on from a
totally separate function you call f 1 2, the function will be evaluated
twice (assuming 1 and 2 weren't known constants at compile-time). 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
Now, the Haskell run-time will evaluate f x y once, store the result in
z, and use it twice. That's how it can use commonalities in your code
and avoid multiple evaluations of the same function call, which I
*think* was your question.
Thanks,
Neil.
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