#11318: Data.Text.length allocates one closure per character -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: #11284 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- In #11284 I noticed that `map Data.Text.length` applied to a list of `Text` values would result in code which would allocate one closure per character due to lack of lambda lifting. It turns out it's even worse than this. Consider this example (inspired by #11284), {{{#!hs module Hi2 (hello) where import qualified Data.Text as T hello :: Int hello = T.length hi hi :: T.Text hi = T.pack "hello" }}} When compiled with GHC 7.10.3 with `-O`, the following simplified Core is produced, {{{#!hs hello :: Int hello = case unpackCString# "hello"# of _ { Text dt_a33D dt1_a33E dt2_a33F -> let { a_a33C :: Int# a_a33C = +# dt1_a33E dt2_a33F } in letrec { $wloop_length_s3bt :: Int# -> Int# -> Int# $wloop_length_s3bt = \ (ww_s3bk :: Int#) (ww_s3bo :: Int#) -> case tagToEnum# @ Bool (>=# ww_s3bo a_a33C) of _ { False -> case indexWord16Array# dt_a33D ww_s3bo of r#_a35n { __DEFAULT -> case tagToEnum# @ Bool (geWord# r#_a35n (__word 55296)) of _ { False -> $wloop_length_s3bt (+# ww_s3bk 1) (+# ww_s3bo 1); True -> case tagToEnum# @ Bool (leWord# r#_a35n (__word 56319)) of _ { False -> $wloop_length_s3bt (+# ww_s3bk 1) (+# ww_s3bo 1); True -> $wloop_length_s3bt (+# ww_s3bk 1) (+# ww_s3bo 2) } } }; True -> ww_s3bk }; } in case $wloop_length_s3bt 0 dt1_a33E of ww_s3bs { __DEFAULT -> I# ww_s3bs } } }}} Even this simple example produces a `length` function which allocations -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11318> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler