The relation to strictness is that you have much tighter control over
when things are evaluated (typically) for something strict, so it is
less likely to leak.
Take the expression 'e + e' at some base type. It's harmless to CSE
this to 'let x = e in x+x' because + is strict in x. Whereas '(e,e)'
can't be CSE:ed to 'let x = e in (x,x)' without risking a space leak.
It's not exactly strictness, it's more about knowing when a value is
going to be consumed.
Also, things that are "small" can be CSE:ed, since the evaulated form
doesn't take more space than the unevaluated.
-- Lennart
On Nov 28, 2006, at 09:50 , Bertram Felgenhauer wrote:
Dinko Tenev wrote:
On 11/27/06, Lennart Augustsson <[EMAIL PROTECTED]> wrote:
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
My apologies in advance for asking possibly stupid questions, but
I don't
understand this.
How exactly can CSE cause space leaks, and what does this have to
do with
strictness?
Combining two expressions means that they're represented by the same
memory location. In particular when you start evaluating the first,
the second reference to the value will keep all of it alive even if
parts of it could otherwise be freed. This is especially problematic
for infinite lists.
http://hackage.haskell.org/trac/ghc/ticket/947
demonstrates this problem, caused by the little CSE that ghc does.
(Note: This is not of the form "let x = term in ... term ...", but
it will be once it's desugared and the simplifier has floated out
the constant expressions from the "primes0" and "primes" functions)
I'm not sure how it relates to strictness. I'd be more worried about
about the size of the data that's being kept alive. Numbers are
more likely to be ok than lists.
Bertram
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