#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 -----------------------------+---------------------------------------------- Comment(by nicolas.frisby@…): commit c7d80c6524390551b64e9c1d651e1a03ed3c7617 {{{ Author: Nicolas Frisby Date: Wed Mar 27 20:25:28 2013 +0000 improve dead code elimination in CorePrep (fixes #7796) compiler/coreSyn/CorePrep.lhs | 72 ++++++-------------------------------- compiler/simplCore/OccurAnal.lhs | 38 +++++++++++++------- 2 files changed, 36 insertions(+), 74 deletions(-) }}} -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7796#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#7796: improve dead code elimination in CorePrep ---------------------------------------------+------------------------------ Reporter: nfrisby | Owner: nfrisby Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.6.2 Resolution: fixed | Keywords: Os: Unknown/Multiple | Architecture: Unknown/Multiple Failure: None/Unknown | Difficulty: Unknown Testcase: simplCore/should_compile/T7796 | Blockedby: Blocking: | Related: #4962 #5433 ---------------------------------------------+------------------------------ Changes (by simonpj): * testcase: => simplCore/should_compile/T7796 * difficulty: => Unknown Old description:

#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% }}}

New description: #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#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler