Perf regression in 7.8.2 relative to 7.6.3, possibly related to HashMap

$ git clone http://github.com/facebook/Haxl.git
Cloning into 'Haxl'...
remote: Counting objects: 77, done.
remote: Compressing objects: 100% (65/65), done.
remote: Total 77 (delta 17), reused 69 (delta 9)
Unpacking objects: 100% (77/77), done.
$ cd Haxl 
$ ghc-7.6.3 -O2 tests/Bench.hs -main-is Bench -o tests/Bench-76
[1 of 6] Compiling Haxl.Core.StateStore ( Haxl/Core/StateStore.hs, Haxl/Core/StateStore.o )
[2 of 6] Compiling Haxl.Core.Show1  ( Haxl/Core/Show1.hs, Haxl/Core/Show1.o )
[3 of 6] Compiling Haxl.Core.Util   ( Haxl/Core/Util.hs, Haxl/Core/Util.o )
[4 of 6] Compiling Haxl.Core.Types  ( Haxl/Core/Types.hs, Haxl/Core/Types.o )
[5 of 6] Compiling Haxl.Core.DataCache ( Haxl/Core/DataCache.hs, Haxl/Core/DataCache.o )
[6 of 6] Compiling Bench            ( tests/Bench.hs, tests/Bench.o )
Linking tests/Bench-76 ...
$ ./tests/Bench-76 500000                                      
Just (Right 0)
insert: 0.87s
500000
lookup: 0.24s
$ ./tests/Bench-76 500000
Just (Right 0)
insert: 0.87s
500000
lookup: 0.26s
$ ghc-7.8.2 -O2 tests/Bench.hs -main-is Bench -o tests/Bench-78
[1 of 5] Compiling Haxl.Core.StateStore ( Haxl/Core/StateStore.hs, Haxl/Core/StateStore.o )
[2 of 5] Compiling Haxl.Core.Show1  ( Haxl/Core/Show1.hs, Haxl/Core/Show1.o )
[3 of 5] Compiling Haxl.Core.Types  ( Haxl/Core/Types.hs, Haxl/Core/Types.o )
[4 of 5] Compiling Haxl.Core.DataCache ( Haxl/Core/DataCache.hs, Haxl/Core/DataCache.o )
[5 of 5] Compiling Bench            ( tests/Bench.hs, tests/Bench.o )
Linking tests/Bench-78 ...
$ ./tests/Bench-78 500000                                      
Just (Right 0)
insert: 1.09s
500000
lookup: 0.44s
$ ./tests/Bench-78 500000
Just (Right 0)
insert: 1.08s
500000
lookup: 0.44s

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture