Cmm code shown in #8326 is suboptimal. For this Haskell code:

isTrue# :: Int# -> Bool
isTrue# 1# = True
isTrue# _  = False

f :: Int# -> Int
f x | isTrue# (x ># 0#) = I# x
    | otherwise         = -(I# x)

we get Cmm that contains dead code in a loop:

       cFg: // False branch
           Hp = Hp + 16;
           if (Hp > HpLim) goto cFy; else goto cFx;
       cFy: // not enough heap, call GC
           HpAlloc = 16;
           I64[Sp - 16] = cFf;
           R1 = _sEV::I64;
           I64[Sp - 8] = _sEU::I64;
           Sp = Sp - 16;
           call stg_gc_unbx_r1(R1) returns to cFf, args: 8, res: 8, upd: 8;
       cFf: // re-do the False branch
           _sEU::I64 = I64[Sp + 8];
           Sp = Sp + 16;
           _sEV::I64 = R1;
           goto cFg;
       cFx: // RHS of False branch
           I64[Hp - 8] = GHC.Types.I#_con_info;
           I64[Hp] = -_sEU::I64;
           R1 = Hp - 7;
           call (P64[Sp])(R1) args: 8, res: 0, upd: 8;

Notice that _sEV is dead - we should optimize it away. This is a known deficiency in the Cmm sinking pass (i.e. it does not work for loops). I'm putting this on Trac so we have this documented.