From: Rob Pike <r@golang.org>
Date: Thu, 3 Oct 2013 17:40:42 +0000 (-0700)
Subject: sync/atomic: explain how to subtract an unsigned constant
X-Git-Tag: go1.2rc2~77
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=c4579635cf6aafdb5db231058dd0aedee14fe0ee;p=gostls13.git

sync/atomic: explain how to subtract an unsigned constant
Explain for those unfamiliar with twos-complement arithmetic how to
implement negation of signed positive constant.
Fixes #6408.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/14267044
---

diff --git a/src/pkg/sync/atomic/doc.go b/src/pkg/sync/atomic/doc.go
index 34a2bc1390..17ba72fa17 100644
--- a/src/pkg/sync/atomic/doc.go
+++ b/src/pkg/sync/atomic/doc.go
@@ -94,12 +94,16 @@ func CompareAndSwapPointer(addr *unsafe.Pointer, old, new unsafe.Pointer) (swapp
 func AddInt32(addr *int32, delta int32) (new int32)
 
 // AddUint32 atomically adds delta to *addr and returns the new value.
+// To subtract a signed positive constant value c from x, do AddUint32(&x, ^uint32(c-1)).
+// In particular, to decrement x, do AddUint32(&x, ^uint32(0)).
 func AddUint32(addr *uint32, delta uint32) (new uint32)
 
 // AddInt64 atomically adds delta to *addr and returns the new value.
 func AddInt64(addr *int64, delta int64) (new int64)
 
 // AddUint64 atomically adds delta to *addr and returns the new value.
+// To subtract a signed positive constant value c from x, do AddUint64(&x, ^uint64(c-1)).
+// In particular, to decrement x, do AddUint64(&x, ^uint64(0)).
 func AddUint64(addr *uint64, delta uint64) (new uint64)
 
 // AddUintptr atomically adds delta to *addr and returns the new value.