CAFs lose sharing due to implicit call stacks

The implicit call stack machinery adds a constraint to CAFs, which loses sharing in some cases. The regression is fixed by -O (actually -ffull-laziness), but it is surprising nonetheless, and might cause problems for people using GHCi or other places where -O is turned off.

For example:

{-# LANGUAGE NoMonomorphismRestriction #-}
module Main where

import System.Environment

fib :: Integer -> Integer
fib n = if n < 2 then 1 else fib (n-1) + fib (n-2)

x = if fib 3 > 20 then error "x" else fib 30

main = do
  [n] <- getArgs
  case n of
    "a" -> print (x + x)
    "b" -> print x

Try it as follows (requires 8.0+):

$ ghc imp.hs -fforce-recomp -O -fno-full-laziness
$ ./imp a +RTS -t
2692538
<<ghc: 430859568 bytes, 822 GCs, 36580/44384 avg/max bytes residency (2 samples), 1M in use, 0.000 INIT (0.000 elapsed), 0.156 MUT (0.203 elapsed), 0.018 GC (0.009 elapsed) :ghc>>
$ ./imp b +RTS -t
1346269
<<ghc: 215456192 bytes, 411 GCs, 36580/44384 avg/max bytes residency (2 samples), 1M in use, 0.000 INIT (0.000 elapsed), 0.073 MUT (0.119 elapsed), 0.008 GC (0.004 elapsed) :ghc>>

With GHC 7.10 and earlier both commands perform the same.

Note that this only uses error, and doesn't require ImplicitParams. It does require NoMonomorphismRestriction, however.

Trac metadata

Trac field	Value
Version	8.0.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonpj
Operating system
Architecture