[GHC] #1600: Optimisation: CPR the results of IO
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Mon Oct 24 07:25:59 UTC 2016
#1600: Optimisation: CPR the results of IO
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Reporter: simonmar | Owner:
Type: task | Status: new
Priority: lowest | Milestone:
Component: Compiler | Version: 6.6.1
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: Runtime | Unknown/Multiple
performance bug | Test Case:
Blocked By: | Blocking:
Related Tickets: #8598 | Differential Rev(s):
Wiki Page: |
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Comment (by akio):
Regarding `wave4main`, the main problem seems to be destruction of a join
point.
The program contains an expression that looks like this:
{{{
#!haskell
runRW#
(\ s ->
letrec
go s1 =
let ...
in (# s2, Array a b c #)
in go s)
}}}
After a nested CPR transformation, it becomes:
{{{#!haskell
runRW#
(\ s ->
letrec
$wgo s1 =
let ...
in (# s2, a, b, c #)
in case $wgo s of
(# s3, a', b', c' #) -> (# s3, Array a' b' c' #))
}}}
destroying the join-point property of `go`. This is basically the same as
what @nomeata found 2 years ago.
I see two ways to move forward:
1. Wait for SequentCore to be implemented.
2. Have special cases for `runRW#` in the CPR analyzer and in the
simplifier, as discussed in
https://ghc.haskell.org/trac/ghc/ticket/10678#comment:10
I'm mildly interested in doing (2), but I don't think I fully understand
the idea. In particular, the above comment mentions an example expression
`case (runRW e) of BN# farr -> farr` that the simplifier would be able to
transform. However, since `runRW#` returns an unboxed pair rather than a
single boxed value, it seems that it would have to spot a much more
complicated pattern to accomplish this transformation.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1600#comment:56>
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