[GHC] #8327: Cmm sinking does not eliminate dead code in loops

[GHC]

#8327: Cmm sinking does not eliminate dead code in loops
------------------------------------+-------------------------------------
       Reporter:  jstolarek         |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 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.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8327>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler