#5916: runST isn't free
------------------------------+---------------------------------------------
Reporter: tibbe | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
------------------------------+---------------------------------------------
While optimizing some code I discovered that `runST` isn't free. I had a
function on the form:
{{{
f ... = ...let x = runST in (f x)...
}}}
Manually transforming it into
{{{
f ... = runST (g ...)
where g ... = do
...
x <- ...
g x
...
}}}
(The real example is at https://github.com/tibbe/unordered-
containers/commit/58b7815a057b3575c58b746d5971d59d806b1333)
improved performance quite a bit on this, already highly tuned, function.
Unfortunately, combining all the calls to `runST` into one and pulling
them "up" is not all good. The code is now less modular, has a somewhat
over specified evaluation order, and generally looks more imperative.
The cause of the decrease in performance is that `runSTRep` cannot be
inlined, which causes allocation of closures inline in the code (where
none should be necessary.)
The comment next to `runSTRep` explains why it's implemented the way it
is, but I wonder if matters could be improved by making it a primop. If we
create a fresh state token every time, instead of reusing `realWorld#`, it
should be impossible for mutable structures to let-float and become CAFs
(which is what `runSTRep` tries to avoid.)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5916>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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