Re: RFC: omit write barrier instructions unless '-threaded'

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. Instead, use conditional compilation in the RTS so that the write_barrer() calls are only present when THREADED_RTS is on. Cheers, Simon

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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