Compiler crash building regex-posix version 0.94.1 using ghc 7.0.1-rc1

[dsf]

Building regex-posix-0.94.1...
[1 of 6] Compiling Text.Regex.Posix.Wrap ( dist-ghc6/build/Text/Regex/Posix/Wrap.hs, dist-ghc6/build/Text/Regex/Posix/Wrap.o )
SpecConstr
    Function `lvl_s2Ay{v} [lid]'
      has three call patterns, but the limit is 0
    Use -fspec-constr-count=n to set the bound
    Use -dppr-debug to see specialisations
ghc: panic! (the 'impossible' happened)
  (GHC version 7.0.0 for i386-unknown-linux):
	initC: srt_lbl

[dsf]

This problem persists in regex-posix-0.94.4. It seems to be caused by the line

Just (whole:_) -> loop (succ acc) pos whole

in Text.Regex.Posix.Wrap.wrapCount.

[dsf]

This bug is a blocker for us, we have suspended testing of 7.0.1 rc1.

[simonmar]

There's a Core Lint error after the first simplification pass; I've attached a log from -dverbose-core2core -dcore-lint in case this helps Simon track down the problem (he was having trouble repro'ing because regex-posix doesn't build on Windows, which is a different problem).

[simonmar]

-dverbose-core2core -dcore-lint log

[igloo]

Standalone testcase attached.

$ ghc -c Wrap.hs -O   
ghc: panic! (the 'impossible' happened)
  (GHC version 7.0.0.20100924 for x86_64-unknown-linux):
        initC: srt_lbl

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

[simonpj]

Thanks for the test case! I've boiled it down a lot more.

module Wrap where

isNull :: IO Bool
isNull = error "urk"

wrapMatchAll :: IO (Maybe ())
wrapMatchAll = do
  nsub <- undefined
  let loop True = do atEnd <- isNull 
                     return Nothing
      loop False = loop False 
  result <- undefined
  loop undefined

It's a bug in the mighty simplifier. I'm on it.

Simon

[simonpj]

Done. Fixed by

Thu Oct  7 03:27:20 PDT 2010  simonpj@microsoft.com
  * Fix Trac #4345: simplifier bug
  
  This is another long-standing bug, in which there was a possibility
  that a loop-breaker could lose its loop-breaker-hood OccInfo, 
  and then the simplifer re-simplified the expression. Result, either
  non-termination or, in the case of #4345, an unbound identifier.
  
  The fix is very simple, in Id.transferPolyIdInfo. 
  See Note [transferPolyIdInfo].

    M ./compiler/basicTypes/Id.lhs -8 +25

[dsf]

Build of regex-posix now succeeds with this patch.

[igloo]

Merged.