Re: [GHC] #8885: Add inline versions of clone array primops

13 Mar 2014

      #8885: Add inline versions of clone array primops
-------------------------------------+------------------------------------
        Reporter:  tibbe             |            Owner:  simonmar
            Type:  feature request   |           Status:  patch
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.9
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+------------------------------------

Comment (by tibbe):

 I did some investigation to figure out why the old implementation holds on
 to all memory (assuming that it's not an accounting error). First, here's
 what the inner loop looks like for the old implementation:

 {{{
 $wa_entry() //  [R3, R2]
         { [(c3i2,
             $wa_info:
                 const 12884901901;
                 const 0;
                 const 15;)]
         }
     {offset
       c3i2:
           if (R2 != 0) goto c3i0; else goto c3i1;
       c3i0:
           _s3f9::P64 = R3;
           (_c3hS::I64) = call "ccall" arg hints:  [PtrHint,]  result
 hints:  [PtrHint] allocate(BaseReg - 24, 20);
           I64[_c3hS::I64] = I64[PicBaseReg +
 stg_MUT_ARR_PTRS_FROZEN0_info@GOTPCREL];
           I64[_c3hS::I64 + 8] = 16;
           I64[_c3hS::I64 + 16] = 17;
           _c3i8::I64 = _c3hS::I64 + 24;
           call MO_Memcpy(_c3i8::I64, _s3f9::P64 + 24, 128, 8);
           call MO_Memset(_c3i8::I64 + 128, 1, 1, 8);
           R3 = _s3f9::P64;
           R2 = R2 - 1;
           call $wa_info(R3, R2) args: 8, res: 0, upd: 8;
       c3i1:
           R1 = PicBaseReg + (()_closure+1);
           call (P64[Sp])(R1) args: 8, res: 0, upd: 8;
     }
 }
 }}}

 The first thing I did was to use the correct info table (`FROZEN`, not
 `FROZEN0`). That didn't help.

 The second thing I did was to use  `-fno-omit-yields` to try to make sure
 the GC was run, as there's no heap check in this loop (and I don't know if
 `allocate` ever invokes the GC). That didn't have any effect.

 For reference, here's what the code with `-fno-omit-yields` looks like:

 {{{
 $wa_entry() //  [R3, R2]
         { [(c3i7,
             $wa_info:
                 const 12884901901;
                 const 0;
                 const 15;)]
         }
     {offset
       c3i7:
           if (I64[BaseReg + 856] == 0) goto c3i8; else goto c3ia;
       c3i8:
           // nop
           // nop
           R1 = PicBaseReg + $wa_closure;
           call (I64[BaseReg - 8])(R3, R2, R1) args: 8, res: 0, upd: 8;
       c3ia:
           if (R2 != 0) goto c3i5; else goto c3i6;
       c3i5:
           _s3f9::P64 = R3;
           (_c3hX::I64) = call "ccall" arg hints:  [PtrHint,]  result
 hints:  [PtrHint] allocate(BaseReg - 24, 20);
           I64[_c3hX::I64] = I64[PicBaseReg +
 stg_MUT_ARR_PTRS_FROZEN_info@GOTPCREL];
           I64[_c3hX::I64 + 8] = 16;
           I64[_c3hX::I64 + 16] = 17;
           _c3ie::I64 = _c3hX::I64 + 24;
           call MO_Memcpy(_c3ie::I64, _s3f9::P64 + 24, 128, 8);
           call MO_Memset(_c3ie::I64 + 128, 1, 1, 8);
           R3 = _s3f9::P64;
           R2 = R2 - 1;
           call $wa_info(R3, R2) args: 8, res: 0, upd: 8;
       c3i6:
           R1 = PicBaseReg + (()_closure+1);
           call (P64[Sp])(R1) args: 8, res: 0, upd: 8;
     }
 }
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8885#comment:2
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler