Performance loss due to eta expansion
Given the attached file, at both -O2
and -O0
, GHC translates the function:
test1 [1,2,3,4,5,6,7,8,9,10] = \x -> x
test1 _ = \x -> negate x
To be equivalent to:
test0 [1,2,3,4,5,6,7,8,9,10] x = x
test0 _ x = negate x
When applied in a loop with something like:
map (test1 [1..]) [1..1000]
The eta-expanded variant is 3x slower. Adding a trace breaks that transformation, and then the code goes 3x faster. Specifically:
test2 [1,2,3,4,5,6,7,8,9,10] = \x -> x
test2 _ = trace "here" $ \x -> negate x
Timings, as reported by Criterion under O2 with GHC 7.10.2, are:
benchmarking test0 = 40.99 ns (40.96 ns .. 41.02 ns)
benchmarking test1 = 41.09 ns (41.06 ns .. 41.14 ns)
benchmarking test2 = 17.74 ns (17.68 ns .. 17.81 ns)