Re: [GHC] #7088: Float-in bug

[GHC]

#7088: Float-in bug
-+--
  Reporter:  simonpj |  Owner:  
  Type:  bug | Status:  closed  
  Priority:  normal  |  Milestone:  
 Component:  Compiler|Version:  7.4.2   
Resolution:  fixed   |   Keywords:  
Os:  Unknown/Multiple|   Architecture:  Unknown/Multiple
   Failure:  None/Unknown| Difficulty:  Unknown 
  Testcase:  simplCore/should_compile/T7088  |  Blockedby:  
  Blocking:  |Related:  
-+--
Changes (by simonpj):

  * status:  new => closed
  * testcase:  => simplCore/should_compile/T7088
  * resolution:  => fixed


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Re: [GHC] #7088: Float-in bug

[GHC]

#7088: Float-in bug
-+--
Reporter:  simonpj   |   Owner:  
Type:  bug   |  Status:  new 
Priority:  normal|   Milestone:  
   Component:  Compiler  | Version:  7.4.2   
Keywords:|  Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  | Failure:  None/Unknown
  Difficulty:  Unknown   |Testcase:  
   Blockedby:|Blocking:  
 Related:|  
-+--

Comment(by simonpj@…):

 commit b498b9983b75b8b7c3a365c1b44e85cbddd09648
 {{{
 Author: Simon Peyton Jones 
 Date:   Mon Jul 23 17:29:07 2012 +0100

 Fix a subtle float-in bug

 This patch makes
   fiExpr (AnnLam ...)
   noFloatIntoExpr (AnnLam ...)
 line up with each other.  Previously they behaved inconsistently
 with bad results.

 The details are in Trac #7088.There's a regression test
 in simplCore/should_compile/T7088

  compiler/simplCore/FloatIn.lhs |   24 +---
  1 files changed, 13 insertions(+), 11 deletions(-)
 }}}

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[GHC] #7088: Float-in bug

[GHC]

#7088: Float-in bug
-+--
Reporter:  simonpj   |   Owner:  
Type:  bug   |  Status:  new 
Priority:  normal|   Milestone:  
   Component:  Compiler  | Version:  7.4.2   
Keywords:|  Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  | Failure:  None/Unknown
  Difficulty:  Unknown   |Testcase:  
   Blockedby:|Blocking:  
 Related:|  
-+--
 Consider
 {{{
   let   m = e
foo = \(s:State) (x:Int). body
   in ...(foo s x)...
 }}}
 Then `loatIn.noFloatIntoRhs` will say `False`, because the top-level
 lambda is oneshot
 But the attempt to float into the lambda-group will fail, leaving
 {{{
   foo = let m = e in \s x. body
 }}}
 That changes foo’s arity, which is bad.

 Moreover, m might have been marked “demanded” in the original program, by
 the clever strictness analyser.  Then the Simplifier will not float the
 m-binding back out of ‘foo’, because it thinks that might make it lazier.

 So then the Simplifier leaves the m-binding in foo’s RHS, and then eta-
 expands it, giving
 {{{
foo = \s. let m = e in \x. body
 }}}
 which is very bad:
 {{{
 *** Core Lint errors : in result of Simplifier ***
 {-# LINE 11 "Float.hs #-}: Warning:
 [RHS of f_snv :: GHC.Prim.State# GHC.Prim.RealWorld
  -> GHC.Types.Int
  -> (# GHC.Types.Int, GHC.Prim.State#
 GHC.Prim.RealWorld #)]
 Demand type has  2  arguments, rhs has  1 arguments,  f_snv
 Binder's strictness signature: DmdType LL
 *** Offending Program ***
 }}}

 It's hard to trigger this bug but Geoff managed to, and this code does it:
 {{{
 {-# LANGUAGE MagicHash, UnboxedTuples #-}

 module Float where

 import GHC.Prim

 foo vs
   = let w = if length (reverse vs) > 10 then Just (length vs) else Nothing

 f :: State# RealWorld -> Int -> (# Int, State# RealWorld #)
 f s x | Just 0 <- w = case f s (x+1) of
 (# r, s' #) -> (# r, s' #)
   | otherwise = (# x, s #)

 in f realWorld# 1
 }}}

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