id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
4945	Another SpecConstr infelicity	batterseapower		"I'm beginning to sound like a broken record, but SpecConstr still doesn't seem to be right! The last problem has been fixed, but I've found a new one.

Please observe the output of compiling the attached code with:

{{{
./ghc -fforce-recomp -c -dverbose-core2core -O2 -fno-liberate-case STUArray-Rewrite2.hs
}}}

In the output of SpecConstr we have a local letrec:

{{{
(letrec {
   $wa_s1G7 [Occ=LoopBreaker]
     :: forall s_aJm.
        Data.Array.Base.STUArray
          s_aJm GHC.Types.Int GHC.Types.Int
        -> GHC.Prim.Int#
        -> GHC.Prim.State# s_aJm
        -> (# GHC.Prim.State# s_aJm, () #)
   [LclId, Arity=3, Str=DmdType LLL]
   $wa_s1G7 =
     \ (@ s_aJm)
       (w_s1FS
          :: Data.Array.Base.STUArray
               s_aJm GHC.Types.Int GHC.Types.Int)
       (ww_s1FV :: GHC.Prim.Int#)
       (w_s1FX :: GHC.Prim.State# s_aJm) ->
       case GHC.Prim.># ww_s1FV ww_s1FN
       of wild_Xj [Dmd=Just A] {
         GHC.Types.False ->
           case w_s1FS
           of wild_aTj [Dmd=Just L]
           { Data.Array.Base.STUArray ds1_aTl [Dmd=Just U]
                                      ds2_aTm [Dmd=Just U]
                                      n_aTn [Dmd=Just U(L)]
                                      ds3_aTo [Dmd=Just A] ->
           case n_aTn
           of wild_aTs [Dmd=Just A]
           { GHC.Types.I# x_aTu [Dmd=Just L] ->
           case $wa_s1G0
                  @ s_aJm
                  w_s1FS
                  (GHC.Types.I# ww_s1FV)
                  0
                  (GHC.Prim.-# x_aTu 1)
                  w_s1FX
           of wild_XUw [Dmd=Just A]
           { (# new_s_XUB [Dmd=Just L], r_XUD [Dmd=Just A] #) ->
           $wa_s1G7
             @ s_aJm w_s1FS (GHC.Prim.+# ww_s1FV 1) new_s_XUB
           }
           }
           };
         GHC.Types.True -> (# w_s1FX, GHC.Unit.() #)
       }; } in
 $wa_s1G7)
}}}

This is a local recursive loop with an invariant first argument (w_s1FS) that is recrutinised every time! This seems deeply uncool.

This is with HEAD (7.1.20110203, incorporating the patch ""Fix typo in SpecConstr that made it not work at all"")"	bug	closed	normal	7.0.2	Compiler	7.0.1	fixed			Unknown/Multiple	Unknown/Multiple	Runtime performance bug		simplCore/should_compile/T4945