The following program goes 25% slower with HEAD compared to 6.12.3:

module Main (main) where

import DeepSeq

main :: IO ()
main = do
  rnf [ mk x | x <- [ 1 .. 1024 ] ] `seq` return ()
  where
    mk :: Float -> [(Float,Float)]
    mk x = [ (x+i,x+i+1) | i <- [ 1 .. 2048] ]

using the attached DeepSeq module, or indeed the standard Control.DeepSeq.

Simon and I diagnosed the problem to be the following dictionary for NFData (Float,Float) (this is HEAD):

Main.main6 :: DeepSeq.NFData (GHC.Types.Float, GHC.Types.Float)
Main.main6 =
  DeepSeq.$fNFData(,)
    @ GHC.Types.Float
    @ GHC.Types.Float
    DeepSeq.$fNFDataFloat
    DeepSeq.$fNFDataFloat

GHC has not inlined the dictionary function or the arguments here, even though this class is in fact just a single-method dictionary. With 6.12 we got:

Main.main6 =
  \ (ds_dBc :: (GHC.Types.Float, GHC.Types.Float)) ->
    case ds_dBc of _ { (x_awr, y_aws) ->
    case x_awr of _ { GHC.Types.F# _ ->
    case y_aws of _ { GHC.Types.F# _ -> GHC.Unit.() }
    }
    }

i.e. everything fully inlined and a nice efficient definition.

This is currently affecting parallel programs where we typically use rnf quite a lot.