#3138: Returning a known constructor: GHC generates terrible code for cmonad
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Reporter: simonpj | Owner:
Type: run-time performance bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.1
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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Lennart reports that GHC generates very poor code for his cmonad package.
If you want to look at a simple example, look at the Inf.hs example
included in package `cmonad-0.1.1`.
It's very simple, and ghc generates fantastically bad code for it.
It would be great if you could nail down why it's so amazingly unoptimal.
Even with everything inlined and no overloading left, ghc seems to
ignore the INLINE directives and use dictionaries left and right.
When I looked at it a year ago or so, it was a return of one
constructor in a sum. That is, a function always returns the same
constructor, so the case
analysis of the return value is not needed; it should be returned as
an unboxed tuple instead
Another unrelated problem, I think, is that ghc needs to promote
in-memory variables to registers when possible.
Perhaps the new code generator has such a transformation?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3138>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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