From d91b10458885aede47ff17fa649b2c0fd9fdf3ca Mon Sep 17 00:00:00 2001
From: Ben Gamari <bgamari.foss@gmail.com>
Date: Mon, 6 Mar 2017 17:31:20 -0500
Subject: [PATCH] primops: Add comment describing type of atomicModifyMutVar#
This resolves #13130. It's not entirely clear to me why we don't use an
unboxed tuple here but this is at least better than the status quo.
[skip ci]
Test Plan: Read it
Reviewers: simonmar, austin, dfeuer
Reviewed By: dfeuer
Subscribers: dfeuer, rwbarton, thomie
Differential Revision: https://phabricator.haskell.org/D3288
---
compiler/prelude/primops.txt.pp | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/compiler/prelude/primops.txt.pp b/compiler/prelude/primops.txt.pp
index 855bdfcb62af..11928b6f69e0 100644
--- a/compiler/prelude/primops.txt.pp
+++ b/compiler/prelude/primops.txt.pp
@@ -1924,13 +1924,25 @@ primop WriteMutVarOp "writeMutVar#" GenPrimOp
primop SameMutVarOp "sameMutVar#" GenPrimOp
MutVar# s a -> MutVar# s a -> Int#
--- not really the right type, but we don't know about pairs here. The
--- correct type is
+-- Note [Why not an unboxed tuple in atomicModifyMutVar#?]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--
--- MutVar# s a -> (a -> (a,b)) -> State# s -> (# State# s, b #)
+-- Looking at the type of atomicModifyMutVar#, one might wonder why
+-- it doesn't return an unboxed tuple. e.g.,
--
+-- MutVar# s a -> (a -> (# a, b #)) -> State# s -> (# State# s, b #)
+--
+-- The reason is that atomicModifyMutVar# relies on laziness for its atomicity.
+-- Given a MutVar# containing x, atomicModifyMutVar# merely replaces the
+-- 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. }
with
out_of_line = True
has_side_effects = True
--
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