Performance regression (involving sum, map, enumFromThenTo)

This program's performance suffers when compiled with 7.10, compared to 7.8:

main = print $ sum $ map bitcount [0, 4 .. 2^24-1 ]

bitcount :: Int -> Int
bitcount x =
  if x > 0 then let (d,m) = divMod x 2 in bitcount d + m else 0

See discussion in this thread: https://mail.haskell.org/pipermail/haskell-cafe/2015-December/122485.html

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

No time to look deeply, but: Does the problem go away if the list is not statically known and thus prevented from floated out, e.g.:

main = print $ foo (2^24-1)

foo n = sum $ map bitcount [0, 4 .. n]
{-# NOINLINE foo #-}

bitcount :: Int -> Int
bitcount x =
  if x > 0 then let (d,m) = divMod x 2 in bitcount d + m else 0

@nomeata: no, does not go away

Trac metadata

Trac field	Value
CC	nomeata → -

I found that this works fine with 7.10:

main = print $ sum $ map bitcount $ takeWhile (< 2^24) [0, 4 .. ]

Trac metadata

Trac field	Value
CC	- → nomeata

possibly related: #10992

Trac metadata

Trac field	Value
TypeOfFailure	OtherFailure → RuntimePerformance

added Pnormal label