RFC: omit write barrier instructions unless '-threaded'

[Simon Marlow]

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
>>>
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>>>
>>
>>