RFC: omit write barrier instructions unless '-threaded'

[Simon Marlow]

On 25/02/13 22:07, Gabor Greif wrote:
> Thanks, this helped!
>
>
> Something like this?
>
> $ git show c3f5312
> commit c3f53126fb0835cb21fc085e9b7cb5acb1ea1fba
> Author: Gabor Greif <ggreif at gmail.com>
> Date:   Mon Feb 25 23:03:23 2013 +0100
>
>      Only emit %write_barrier primitive for THREADED_RTS
>
> diff --git a/includes/Cmm.h b/includes/Cmm.h
> index 1505b1c..bc5702c 100644
> --- a/includes/Cmm.h
> +++ b/includes/Cmm.h
> @@ -583,6 +583,12 @@
>   #define OVERWRITING_CLOSURE(c) /* nothing */
>   #endif
>
> +#ifdef THREADED_RTS
> +#define prim_write_barrier prim %write_barrier()
> +#else
> +#define prim_write_barrier /* nothing */
> +#endif
> +
>   /* -----------------------------------------------------------------------------
>      Ticky macros
>      --------------------------------------------------------------------------
> */
> diff --git a/rts/Updates.h b/rts/Updates.h
> index b4ff7d1..e6a2eb9 100644
> --- a/rts/Updates.h
> +++ b/rts/Updates.h
> @@ -46,7 +46,7 @@
>                                                                  \
>       OVERWRITING_CLOSURE(p1);                                    \
>       StgInd_indirectee(p1) = p2;                                 \
> -    prim %write_barrier();                                      \
> +    prim_write_barrier;                                         \
>       SET_INFO(p1, stg_BLACKHOLE_info);                           \
>       LDV_RECORD_CREATE(p1);                                      \
>       bd = Bdescr(p1);                                           \
>
>
> Okay to commit?

Yes, ok.

Cheers,
	Simon



> Cheers,
>
>      Gabor
>
> On 2/25/13, Simon Marlow <marlowsd at gmail.com> wrote:
>> On 25/02/13 14:53, Gabor Greif wrote:
>>> On 2/25/13, Simon Marlow <marlowsd at gmail.com> wrote:
>>>> On 24/02/13 20:40, Gabor Greif wrote:
>>>>> Hi all,
>>>>>
>>>>> from what I gathered so far no emission of write barriers is needed
>>>>> when
>>>>>     - running on a uniprocessor (-threaded or not)
>>>>>     - running on a multiprocessor sans having linked with -threaded.
>>>>>
>>>>> Below patch suppresses the emission of 'lwsync' when no '-threaded' is
>>>>> specified on PPC only. So it does not cover both criteria above.
>>>>>
>>>>> It helps me a lot since I have a uniprocessor target that does not
>>>>> understand the 'lwsync' instruction (instruction is newer than core).
>>>>>
>>>>> Still, I have some doubts:
>>>>>     o do we want to extend this approach to other archs?
>>>>>     o possibly suppress the emission of MO_WriteBarrier in
>>>>> compiler/codeGen/StgCmmBind.hs
>>>>>        (more care is needed to also cover compiler/cmm/CmmParse.y).
>>>>>
>>>>> Anyway this should be a safe first step, and I'd like to push it. When
>>>>> we find a general solution be can back this commit out.
>>>>>
>>>>> What do you think?
>>>>
>>>> I don't think this is the right way to do it.  The -threaded flag is not
>>>> supposed to affect code generation, it only changes the RTS that gets
>>>> linked in.  That is, you can compile all your code without -threaded and
>>>> then just add -threaded at the link step, and it will work.
>>>>
>>>
>>> Hi Simon,
>>>
>>> thanks for your review!
>>>
>>>> Instead, use conditional compilation in the RTS so that the
>>>> write_barrer() calls are only present when THREADED_RTS is on.
>>>
>>> This is already done the way you suggest:
>>>
>>> #define write_barrier()      /* nothing */
>>>
>>> in includes/stg/SMP.h:368
>>>
>>> Works perfectly, 'lwsync' only appears in .thr_*.o files.
>>>
>>> My problem stems from another call:
>>>
>>>       emitPrimCall [] MO_WriteBarrier []
>>>
>>> in compiler/codeGen/StgCmmBind.hs:614
>>>
>>> or possibly
>>>
>>> callishMachOps = listToUFM $
>>>           map (\(x, y) -> (mkFastString x, y)) [
>>>           ( "write_barrier", MO_WriteBarrier ),
>>>           ( "memcpy", MO_Memcpy ),
>>>           ( "memset", MO_Memset ),
>>>           ( "memmove", MO_Memmove )
>>>           -- ToDo: the rest, maybe
>>>       ]
>>>
>>> in compiler/cmm/CmmParse.y:920
>>>
>>> I doubt these files should be compiled with a dependency on
>>> THREADED_RTS, and indeed this symbol is only ever defined
>>> (-optc-DTHREADED_RTS) for the C compiler, and thus not available when
>>> compiling haskell source.
>>>
>>> So I am still stumped.
>>>
>>> The function where "lwsync" surfaces is:
>>>
>>> stg_marked_upd_frame_info which is rts/Updates.cmm:46.
>>
>> These come from rts/Updates.h:updateWithIndirection(), which expands to
>> some Cmm code that uses "prim %write_barrier()".  So you want to use
>> some conditional compilation to make that prim %write_barrier()
>> disappear when THREADED_RTS is off.  I suggest replacing it with
>> write_barrier(), and then #define write_barrier() in includes/Cmm.h.
>>
>> Cheers,
>> 	Simon
>>
>>
>>> This one is creating a blackhole (IISC) by means of StgCmmBind.hs:614.
>>>
>>> How can we go on from here?
>>>
>>> Cheers,
>>>
>>>       Gabor
>>>
>>>
>>>
>>>>
>>>> Cheers,
>>>> 	Simon
>>>>
>>>>
>>>>> Cheers,
>>>>>
>>>>>        Gabor
>>>>>
>>>>> $ git show c0d682fb98f32e4ce5d27ff3a30f43b6cd70733e
>>>>> commit c0d682fb98f32e4ce5d27ff3a30f43b6cd70733e
>>>>> Author: Gabor Greif <ggreif at gmail.com>
>>>>> Date:   Fri Feb 22 18:47:00 2013 +0100
>>>>>
>>>>>        Do not emit barriers on PPC unless we go the threaded way
>>>>>
>>>>> diff --git a/compiler/nativeGen/PPC/CodeGen.hs
>>>>> b/compiler/nativeGen/PPC/CodeGen.hs
>>>>> index 92eff36..6c33cca 100644
>>>>> --- a/compiler/nativeGen/PPC/CodeGen.hs
>>>>> +++ b/compiler/nativeGen/PPC/CodeGen.hs
>>>>> @@ -1,9 +1,8 @@
>>>>> -
>>>>>
>>>>> -----------------------------------------------------------------------------
>>>>>     --
>>>>>     -- Generating machine code (instruction selection)
>>>>>     --
>>>>> --- (c) The University of Glasgow 1996-2004
>>>>> +-- (c) The University of Glasgow 1996-2013
>>>>>     --
>>>>>
>>>>> -----------------------------------------------------------------------------
>>>>>
>>>>> @@ -906,8 +905,10 @@ genCCall'
>>>>>     -}
>>>>>
>>>>>
>>>>> -genCCall' _ _ (PrimTarget MO_WriteBarrier) _ _
>>>>> - = return $ unitOL LWSYNC
>>>>> +genCCall' dflags _ (PrimTarget MO_WriteBarrier) _ _
>>>>> + = return (if WayThreaded `elem` ways dflags
>>>>> +           then unitOL LWSYNC
>>>>> +           else nilOL)
>>>>>
>>>>>     genCCall' _ _ (PrimTarget MO_Touch) _ _
>>>>>      = return $ nilOL
>>>>>
>>>>> _______________________________________________
>>>>> ghc-devs mailing list
>>>>> ghc-devs at haskell.org
>>>>> http://www.haskell.org/mailman/listinfo/ghc-devs
>>>>>
>>>>
>>>>
>>
>>