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

On 25/02/13 22:07, Gabor Greif wrote:

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Thanks, this helped!

Something like this?

$ git show c3f5312 commit c3f53126fb0835cb21fc085e9b7cb5acb1ea1fba Author: Gabor Greif 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

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Cheers,

Gabor

On 2/25/13, Simon Marlow wrote:

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On 25/02/13 14:53, Gabor Greif wrote:

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On 2/25/13, Simon Marlow wrote:

...

On 24/02/13 20:40, Gabor Greif wrote:

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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!

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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

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This one is creating a blackhole (IISC) by means of StgCmmBind.hs:614.

How can we go on from here?

Cheers,

Gabor

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Cheers, Simon

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Cheers,

Gabor

$ git show c0d682fb98f32e4ce5d27ff3a30f43b6cd70733e commit c0d682fb98f32e4ce5d27ff3a30f43b6cd70733e Author: Gabor Greif 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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