X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Fprelude%2FPrimOp.lhs;h=a9a8fa277e588030d03d8e0c715d64faa898f66f;hp=61ccc8dd4adfd339e423cb188daa82e3ed0a39e8;hb=cbbee4e8727c583daf32d9bf17f00afaa839ef10;hpb=36f7a29252e805a669ffb5437b6df6a90c1bc8dd

diff --git a/compiler/prelude/PrimOp.lhs b/compiler/prelude/PrimOp.lhs
index 61ccc8d..a9a8fa2 100644
--- a/compiler/prelude/PrimOp.lhs
+++ b/compiler/prelude/PrimOp.lhs
@@ -21,7 +21,9 @@ module PrimOp (
 	primOpOutOfLine, primOpNeedsWrapper, 
 	primOpOkForSpeculation, primOpIsCheap, primOpIsDupable,
 
-	getPrimOpResultInfo,  PrimOpResultInfo(..)
+	getPrimOpResultInfo,  PrimOpResultInfo(..),
+
+        PrimCall(..)
     ) where
 
 #include "HsVersions.h"
@@ -36,6 +38,7 @@ import TyCon		( TyCon, isPrimTyCon, tyConPrimRep, PrimRep(..) )
 import Type		( Type, mkForAllTys, mkFunTy, mkFunTys, tyConAppTyCon,
 			  typePrimRep )
 import BasicTypes	( Arity, Boxity(..) )
+import ForeignCall	( CLabelString )
 import Unique		( Unique, mkPrimOpIdUnique )
 import Outputable
 import FastTypes
@@ -380,6 +383,38 @@ primOpCanFail :: PrimOp -> Bool
 And some primops have side-effects and so, for example, must not be
 duplicated.
 
+This predicate means a little more than just "modifies the state of
+the world".  What it really means is "it cosumes the state on its
+input".  To see what this means, consider
+
+ let
+     t = case readMutVar# v s0 of (# s1, x #) -> (S# s1, x)
+     y = case t of (s,x) -> x
+ in
+     ... y ... y ...
+
+Now, this is part of an ST or IO thread, so we are guaranteed by
+construction that the program uses the state in a single-threaded way.
+Whenever the state resulting from the readMutVar# is demanded, the
+readMutVar# will be performed, and it will be ordered correctly with
+respect to other operations in the monad.
+
+But there's another way this could go wrong: GHC can inline t into y,
+and inline y.  Then although the original readMutVar# will still be
+correctly ordered with respect to the other operations, there will be
+one or more extra readMutVar#s performed later, possibly out-of-order.
+This really happened; see #3207.
+
+The property we need to capture about readMutVar# is that it consumes
+the State# value on its input.  We must retain the linearity of the
+State#.
+
+Our fix for this is to declare any primop that must be used linearly
+as having side-effects.  When primOpHasSideEffects is True,
+primOpOkForSpeculation will be False, and hence primOpIsCheap will
+also be False, and applications of the primop will never be
+duplicated.
+
 \begin{code}
 primOpHasSideEffects :: PrimOp -> Bool
 #include "primop-has-side-effects.hs-incl"
@@ -474,3 +509,17 @@ pprPrimOp  :: PrimOp -> SDoc
 pprPrimOp other_op = pprOccName (primOpOcc other_op)
 \end{code}
 
+
+%************************************************************************
+%*									*
+\subsubsection[PrimCall]{User-imported primitive calls}
+%*									*
+%************************************************************************
+
+\begin{code}
+newtype PrimCall = PrimCall CLabelString
+
+instance Outputable PrimCall where
+  ppr (PrimCall lbl) = ppr lbl
+
+\end{code}