#10401: state hack-related regression
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Reporter: rwbarton | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.10.1
Keywords: | Operating System: Unknown/Multiple
Architecture: | Type of failure: Runtime
Unknown/Multiple | performance bug
Test Case: | Blocked By:
Blocking: | Related Tickets:
Differential Revisions: |
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Consider the following program `exp.hs`:
{{{
import Control.Monad
import Debug.Trace
expensive :: String -> String
expensive x = trace "$$$" x
{-# NOINLINE expensive #-}
main :: IO ()
main = do
str <- fmap expensive getLine
replicateM_ 3 $ print str
}}}
When run as `echo hi | ./exp`, it might print either
{{{
$$$
"hi"
"hi"
"hi"
}}}
or
{{{
$$$
"hi"
$$$
"hi"
$$$
"hi"
}}}
depending on the compiler version and optimization settings.
In 7.8.4, building with `-O2` produces the second (bad) output, while
building with `-O2 -fno-state-hack` produces the first output, not
surprisingly.
However in 7.10.1 and in HEAD both `-O2` and `-O2 -fno-state-hack` produce
the second output.
It seems this difference in behavior between 7.8.4 and 7.10.1 may be due
to the following difference in the unfolding for IO's fmap. In 7.8.4
{{{
ba867929df0910f70431843781ad014d
$fFunctorIO2 :: (a -> b)
-> GHC.Types.IO a
-> GHC.Prim.State# GHC.Prim.RealWorld
-> (# GHC.Prim.State# GHC.Prim.RealWorld, b #)
{- Arity: 3, HasNoCafRefs,
Strictness: