GHC doesn't normally do CSE. CSE can cause space leaks, so you can't
do it willy-nilly.
I'm sure there are some strict contexts where it could be done
safely, but I don't think ghc uses that information (yet).
-- Lennart
On Nov 27, 2006, at 08:34 , Christian Maeder wrote:
the following code does not run as fast as expected:
modexp a e n = if e <= 0 then 1 else
if even e
then mod (modexp a (div e 2) n * modexp a (div e 2) n) n
else mod (a * modexp a (e - 1) n) n
it gets only fast if written as:
modexp2 a e n = if e <= 0 then 1 else
if even e
then let d = modexp2 a (div e 2) n in mod (d * d) n
else mod (a * modexp2 a (e - 1) n) n
I wonder, why the common subexpression "modexp a (div e 2) n" is not
eliminated in the first case. Does CSE work at all?
For testing the exponent "e" should have at least 10 digits.
Cheers Christian
P.S. Other alternatives are slower, too
modexp1 a e n = if e <= 0 then 1 else
mod (a * modexp1 a (e - 1) n) n
modexp3 a e n = mod (a ^ e) n
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