Investigate performance of CoreTidy

Considering the following profile output (compiling https://ghc.haskell.org/trac/ghc/attachment/ticket/14683/Grammar.hs):

	Thu Jan 25 13:11 2018 Time and Allocation Profiling Report  (Final)

	   ghc-stage2 +RTS -p -RTS -B/home/tobias/well-typed/devel/ghc/inplace/lib Grammar.hs -ddump-stg -ddump-simpl -ddump-to-file -fforce-recomp

	total time  =       20.99 secs   (20989 ticks @ 1000 us, 1 processor)
	total alloc = 29,250,375,256 bytes  (excludes profiling overheads)

COST CENTRE           MODULE     SRC                                                 %time %alloc

CoreTidy              HscMain    compiler/main/HscMain.hs:1253:27-67                  24.2   28.5
Stg2Stg               HscMain    compiler/main/HscMain.hs:1489:12-44                  20.3   24.2
simplCast             Simplify   compiler/simplCore/Simplify.hs:871:62-87             18.7   15.9
tc_rn_src_decls       TcRnDriver compiler/typecheck/TcRnDriver.hs:(494,4)-(556,7)      9.0    6.9
addCoerce-pushCoTyArg Simplify   compiler/simplCore/Simplify.hs:(1236,12)-(1237,72)    7.4    6.4
subst_ty              TyCoRep    compiler/types/TyCoRep.hs:2237:28-32                  4.3    5.1
zonkTopDecls          TcRnDriver compiler/typecheck/TcRnDriver.hs:(445,16)-(446,43)    1.4    1.1
coercionKind          Coercion   compiler/types/Coercion.hs:1725:3-7                   1.3    3.0
simplExprF1-Lam       Simplify   compiler/simplCore/Simplify.hs:896:5-39               1.0    1.1

1. ..the amount of time spent in CoreTidy is striking, and should be investigated.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture