Fwd: RFC: omit write barrier instructions unless '-threaded'
Gabor Greif
ggreif at gmail.com
Tue Feb 26 10:47:04 CET 2013
From: Gabor Greif <ggreif at gmail.com>
Date: Tue, 26 Feb 2013 10:45:52 +0100
Subject: Re: RFC: omit write barrier instructions unless '-threaded'
To: Simon Marlow <marlowsd at gmail.com>
Cool,
pushed
https://github.com/ghc/ghc/commit/b7ae3cd9273da044384de4a327a4a5d71190129e
Cheers,
Gabor
On 2/26/13, Simon Marlow <marlowsd at gmail.com> wrote:
> 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
>>>>>>
>>>>>
>>>>>
>>>
>>>
>
>
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