Replace atomicModifyMutVar#

Reviewers: simonmar, hvr, bgamari, erikd, fryguybob, rrnewton

Reviewed By: simonmar

Subscribers: fryguybob, rwbarton, thomie, carter

GHC Trac Issues: #15364

Differential Revision: https://phabricator.haskell.org/D4884

1 files changed, 23 insertions, 11 deletions

diff --git a/compiler/prelude/primops.txt.pp b/compiler/prelude/primops.txt.pp
index 468299f5d2..b8c6811668 100644
--- a/compiler/prelude/primops.txt.pp
+++ b/compiler/prelude/primops.txt.pp
@@ -2224,25 +2224,37 @@ primop  WriteMutVarOp "writeMutVar#"  GenPrimOp
 primop  SameMutVarOp "sameMutVar#" GenPrimOp
    MutVar# s a -> MutVar# s a -> Int#
 
--- Note [Why not an unboxed tuple in atomicModifyMutVar#?]
+-- Note [Why not an unboxed tuple in atomicModifyMutVar2#?]
 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 --
--- Looking at the type of atomicModifyMutVar#, one might wonder why
+-- Looking at the type of atomicModifyMutVar2#, one might wonder why
 -- it doesn't return an unboxed tuple. e.g.,
 --
---   MutVar# s a -> (a -> (# a, b #)) -> State# s -> (# State# s, b #)
+--   MutVar# s a -> (a -> (# a, b #)) -> State# s -> (# State# s, a, (# a, b #) #)
 --
--- The reason is that atomicModifyMutVar# relies on laziness for its atomicity.
--- Given a MutVar# containing x, atomicModifyMutVar# merely replaces the
+-- The reason is that atomicModifyMutVar2# relies on laziness for its atomicity.
+-- Given a MutVar# containing x, atomicModifyMutVar2# merely replaces
 -- its contents with a thunk of the form (fst (f x)). This can be done using an
 -- atomic compare-and-swap as it is merely replacing a pointer.
 
-primop  AtomicModifyMutVarOp "atomicModifyMutVar#" GenPrimOp
-   MutVar# s a -> (a -> b) -> State# s -> (# State# s, c #)
-   { Modify the contents of a {\tt MutVar\#}. Note that this isn't strictly
-     speaking the correct type for this function, it should really be
-     {\tt MutVar# s a -> (a -> (a,b)) -> State# s -> (# State# s, b #)}, however
-     we don't know about pairs here. }
+primop  AtomicModifyMutVar2Op "atomicModifyMutVar2#" GenPrimOp
+   MutVar# s a -> (a -> c) -> State# s -> (# State# s, a, c #)
+   { Modify the contents of a {\tt MutVar\#}, returning the previous
+     contents and the result of applying the given function to the
+     previous contents. Note that this isn't strictly
+     speaking the correct type for this function; it should really be
+     {\tt MutVar# s a -> (a -> (a,b)) -> State# s -> (# State# s, a, (a, b) #)},
+     but we don't know about pairs here. }
+   with
+   out_of_line = True
+   has_side_effects = True
+   can_fail         = True
+
+primop  AtomicModifyMutVar_Op "atomicModifyMutVar_#" GenPrimOp
+   MutVar# s a -> (a -> a) -> State# s -> (# State# s, a, a #)
+   { Modify the contents of a {\tt MutVar\#}, returning the previous
+     contents and the result of applying the given function to the
+     previous contents. }
    with
    out_of_line = True
    has_side_effects = True