Keep Touch'd variables live through the back end

When we used derived pointers into the middle of an object,
we need to keep the pointer to the start of the object live.
We use a "fat machine instruction" with the primitive MO_Touch
to propagate this information through the back end.

compiler/cmm/Cmm.hs

@@ -384,6 +384,7 @@ data CallishMachOp
   | MO_F32_Exp
   | MO_F32_Sqrt
   | MO_WriteBarrier
+  | MO_Touch         -- Keep variables live (when using interior pointers)
   deriving (Eq, Show)
 
 pprCallishMachOp :: CallishMachOp -> SDoc

compiler/cmm/CmmCvt.hs

@@ -115,6 +115,7 @@ ofZgraph g = ListGraph $ swallow blocks
           mid (MidComment s)  = CmmComment s
           mid (MidAssign l r) = CmmAssign l r
           mid (MidStore  l r) = CmmStore  l r
+          mid (MidForeignCall _ (PrimTarget MO_Touch) _ _) = CmmNop
           mid (MidForeignCall _ target ress args)
 		= CmmCall (cmm_target target)
 			  (add_hints conv Results   ress) 

compiler/cmm/ZipCfgCmmRep.hs

@@ -114,13 +114,13 @@ data Last
 	  -- the call goes into a loop.
 	}
 
-data MidCallTarget	-- The target of a MidUnsafeCall
-  = ForeignTarget 	-- A foreign procedure
-	CmmExpr			-- Its address
-	ForeignConvention	-- Its calling convention
+data MidCallTarget        -- The target of a MidUnsafeCall
+  = ForeignTarget         -- A foreign procedure
+        CmmExpr                  -- Its address
+        ForeignConvention        -- Its calling convention
 
-  | PrimTarget		-- A possibly-side-effecting machine operation
-	CallishMachOp		-- Which one
+  | PrimTarget            -- A possibly-side-effecting machine operation
+        CallishMachOp            -- Which one
   deriving Eq
 
 data Convention
@@ -277,8 +277,8 @@ instance UserOfLocalRegs MidCallTarget where
   foldRegsUsed f  z (ForeignTarget e _) = foldRegsUsed f z e
 
 instance UserOfSlots MidCallTarget where
+  foldSlotsUsed  f z (ForeignTarget e _) = foldSlotsUsed f z e
   foldSlotsUsed _f z (PrimTarget _)      = z
-  foldSlotsUsed f  z (ForeignTarget e _) = foldSlotsUsed f z e
 
 instance (UserOfLocalRegs a) => UserOfLocalRegs (Maybe a) where
   foldRegsUsed f z (Just x) = foldRegsUsed f z x

compiler/codeGen/StgCmmForeign.hs

@@ -97,9 +97,9 @@ emitCCall hinted_results fn hinted_args
     fc = ForeignConvention CCallConv arg_hints result_hints
     
 
-emitPrimCall :: CmmFormal -> CallishMachOp -> CmmActuals -> FCode ()
+emitPrimCall :: CmmFormals -> CallishMachOp -> CmmActuals -> FCode ()
 emitPrimCall res op args
-  = emitForeignCall PlayRisky [res] (PrimTarget op) args NoC_SRT CmmMayReturn
+  = emitForeignCall PlayRisky res (PrimTarget op) args NoC_SRT CmmMayReturn
 
 -- alternative entry point, used by CmmParse
 emitForeignCall

compiler/codeGen/StgCmmPrim.hs

@@ -232,8 +232,8 @@ emitPrimOp [res] SizeofMutableByteArrayOp [arg]
 
 
 --  #define touchzh(o)                  /* nothing */
-emitPrimOp [] TouchOp [_arg]
-   = nopC
+emitPrimOp res@[] TouchOp args@[_arg]
+   = do emitPrimCall res MO_Touch args
 
 --  #define byteArrayContentszh(r,a) r = BYTE_ARR_CTS(a)
 emitPrimOp [res] ByteArrayContents_Char [arg]
@@ -413,9 +413,9 @@ emitPrimOp [res] op [arg]
    = emit (mkAssign (CmmLocal res) $
 	   CmmMachOp (mop rep wordWidth) [CmmMachOp (mop wordWidth rep) [arg]])
 
-emitPrimOp [res] op args
+emitPrimOp r@[res] op args
    | Just prim <- callishOp op
-   = do emitPrimCall res prim args
+   = do emitPrimCall r prim args
 
    | Just mop <- translateOp op
    = let stmt = mkAssign (CmmLocal res) (CmmMachOp mop args) in