id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,testcase,blockedby,blocking,related,differential,wikipage
947,ghc -O space leak: CSE between different CAFs,int-e@…,,"Consider the following program for generating the 1,000,000th prime:
{{{
module Main (main) where
sieve0 (n:ns) (p:ps)
| p*p == n = sieve0 (filter (\n -> n`mod`p /= 0) ns) ps
| otherwise = n:sieve0 ns (p:ps)
primes0 :: [Int]
primes0 = 3:sieve0 [5,7..] primes0
primes :: [Int]
primes = 2:3:sieve0 [5,7..] primes0
main = print $ primes !! 1000000
}}}
The intention of the separate primes0 function is to limit the number of primes that need to be held in memory. Unfortunately, ghc -O combines the common subexpressions in primes0 and primes so this effort is wasted. The resulting program needs noticably more memory than required, as can be seen by replacing the definition of {{{primes}}} by
{{{
primes = 2:3:sieve0 (iterate (2+) 5) primes0
}}}
which prevents the CSE from happening.
Excerpt of {{{+RTS -s}}} output, original version:
{{{
12,099,221,160 bytes allocated in the heap
279,720,116 bytes copied during GC
15,834,912 bytes maximum residency (6 sample(s))
}}}
modified version that prevents CSE:
{{{
127,736,408 bytes copied during GC (scavenged)
233,388 bytes copied during GC (not scavenged)
30,624 bytes maximum residency (1 sample(s))
}}}
Tested with ghc 6.4.2 and a current (as of 2006-10-17) darcs ghc 6.5.",bug,new,normal,⊥,Compiler,6.5,,,hackage.haskell.org@… jwlato@…,Linux,x86,None/Unknown,,,,,,