RFC: omit write barrier instructions unless '-threaded'

[Gabor Greif]

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?

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