CWG1953 Data races and common initial sequence

Author: burblebee

source/basic.tex

@@ -3115,7 +3115,9 @@
 \end{note}
 
 \pnum
-A \defn{memory location} is either an object of scalar type that is not a bit-field
+A \defn{memory location} is
+the storage occupied by the object representation of
+either an object of scalar type that is not a bit-field
 or a maximal sequence of adjacent bit-fields all having nonzero width.
 \begin{note}
 Various
@@ -6037,13 +6039,30 @@
 \end{note}
 The value computations of the operands of an
 operator are sequenced before the value computation of the result of the
-operator. If a
+operator.
+The behavior is undefined if
+\begin{itemize}
+\item
 \indextext{side effects}%
-side effect on a memory location\iref{intro.memory} is unsequenced
-relative to either another side effect on the same memory location or
+a side effect on a memory location\iref{intro.memory} or
+\item
+starting or ending the lifetime of an object in a memory location
+\end{itemize}
+is unsequenced relative to
+\begin{itemize}
+\item
+another side effect on the same memory location,
+\item
+starting or ending the lifetime of an object occupying storage that
+overlaps with the memory location, or
+\item
 a value computation using the value of any object in the same memory location,
-and they are not potentially concurrent\iref{intro.multithread},
-the behavior is undefined.
+\end{itemize}
+and the two evaluations are not potentially concurrent\iref{intro.multithread}.
+\begin{note}
+Starting the lifetime of an object in a memory location can end the lifetime of
+objects in other memory locations\iref{basic.life}.
+\end{note}
 \begin{note}
 The next subclause imposes similar, but more complex restrictions on
 potentially concurrent computations.
@@ -6057,6 +6076,9 @@
   i = i++ + 1;                  // the value of \tcode{i} is incremented
   i = i++ + i;                  // undefined behavior
   i = i + 1;                    // the value of \tcode{i} is incremented
+
+  union U { int x, y; } u;
+  (u.x = 1, 0) + (u.y = 2, 0);  // undefined behavior
 }
 \end{codeblock}
 \end{example}
@@ -6167,9 +6189,21 @@
 \end{note}
 
 \pnum
-Two expression evaluations \defn{conflict} if one of them modifies\iref{defns.access} a memory
-location\iref{intro.memory} and the other one reads or modifies the same
-memory location.
+Two expression evaluations \defn{conflict} if one of them
+\begin{itemize}
+\item
+modifies\iref{defns.access} a memory location\iref{intro.memory} or
+\item
+starts or ends the lifetime of an object in a memory location
+\end{itemize}
+and the other one
+\begin{itemize}
+\item
+reads or modifies the same memory location or
+\item
+starts or ends the lifetime of an object occupying storage that
+overlaps with the memory location.
+\end{itemize}
 \begin{note}
 A modification can still conflict
 even if it does not alter the value of any bits.