X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Fprelude%2FPrimOp.lhs;h=6338941662520431a85142045ab1e300da32c2dc;hp=7c30d9eb6419c4fffe8234ccdddd9d75b47cf6be;hb=8a3ed3364fbc74b1f1b87b049737da2b251f92df;hpb=9448411acfcaae8109c31be5828e75619f2cad9b

diff --git a/compiler/prelude/PrimOp.lhs b/compiler/prelude/PrimOp.lhs
index 7c30d9e..6338941 100644
--- a/compiler/prelude/PrimOp.lhs
+++ b/compiler/prelude/PrimOp.lhs
@@ -286,12 +286,6 @@ These primops are pretty wierd.
 The constraints aren't currently checked by the front end, but the
 code generator will fall over if they aren't satisfied.
 
-\begin{code}
-#ifdef DEBUG
-primOpInfo op = pprPanic "primOpInfo:" (ppr op)
-#endif
-\end{code}
-
 %************************************************************************
 %*									*
 \subsubsection[PrimOp-ool]{Which PrimOps are out-of-line}
@@ -386,6 +380,38 @@ 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"