From 66ce6410dbfd11220c7204b0ea09a695894bd37b Mon Sep 17 00:00:00 2001
From: Florian Schmaus <flow@cs.fau.de>
Date: Wed, 19 Feb 2020 15:57:48 +0100
Subject: [PATCH] Add code comment why bottom is modified in work-stealing
queues
---
emper/lib/adt/WsClQueue.hpp | 5 +++++
emper/lib/adt/WsClV2Queue.hpp | 6 +++++-
2 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/emper/lib/adt/WsClQueue.hpp b/emper/lib/adt/WsClQueue.hpp
index 881fff37..ec90f919 100644
--- a/emper/lib/adt/WsClQueue.hpp
+++ b/emper/lib/adt/WsClQueue.hpp
@@ -109,6 +109,11 @@ bool WsClQueue<_PAYLOAD, _CAPACITY>::popBottom(_PAYLOAD *item) {
// bool res = top.compare_exchange_weak(localTop, localTop + 1, std::memory_order_release, std::memory_order_relaxed);
bool res = top.compare_exchange_weak(localTop, localTop + 1);
+ // Either a popTop() removed the element ('res' is false) or we
+ // removed the element ('res' is true), but we need to increment
+ // the 'bottom' value, since the element bottom pointed at is now
+ // gone. N.B. bottom does point to the next free slot, the actual
+ // element we remove is bottom-1.
bottom = localBottom + 1;
return res;
}
diff --git a/emper/lib/adt/WsClV2Queue.hpp b/emper/lib/adt/WsClV2Queue.hpp
index ebb4343b..102f8867 100644
--- a/emper/lib/adt/WsClV2Queue.hpp
+++ b/emper/lib/adt/WsClV2Queue.hpp
@@ -100,7 +100,11 @@ bool WsClV2Queue<ITEM_TYPE, CAPACITY>::popBottom(ITEM_TYPE *item) {
if (localBottom > localTop) return true;
bool res = top.compare_exchange_weak(localTop, localTop + 1, std::memory_order_release, std::memory_order_relaxed);
- // TODO: Why do we reset bottom here?
+ // Either a popTop() removed the element ('res' is false) or we
+ // removed the element ('res' is true), but we need to increment
+ // the 'bottom' value, since the element bottom pointed at is now
+ // gone. N.B. bottom does point to the next free slot, the actual
+ // element we remove is bottom-1.
bottom = localBottom + 1;
return res;
}
--
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