#7796: improve dead code elimination in CorePrep
-----------------------------+----------------------------------------------
Reporter: nfrisby | Owner: nfrisby
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.6.2 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Blockedby:
Blocking: | Related: #4962 #5433
-----------------------------+----------------------------------------------
#4962 is about generating code for (and allocating at runtime!) bindings
that are kept alive only by RULEs even though the RULEs are useless
downstream of .hi file generation.
Simon Marlow's patch for #5433 switched to a custom dead code eliminator,
since the "(case) binder swap" in OccurAnal was creating breaking some
code generator invariants.
However, that custom dead code generator doesn't do a dependency analysis,
so a letrec like this
{{{
let f = [g] \r [...] -> ...
g = [g] \r [...] -> ...
in ... g ...
}}}
is sent to the code generator ''without dropping'' `f`.
A patch to use a dependency analysis in CorePrep's dead code eliminator
improves allocation for several nofib programs (largest: cryptarithm2,
knights, fem).
{{{
Min -0.0% -4.2% -7.7% -7.7% -5.9%
Max +0.0% +0.0% +4.3% +4.3% +14.3%
Geometric Mean -0.0% -0.1% -0.2% -0.2% +0.2%
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7796>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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