Merge 2025-02 CWG Motion 6

P3475R2 Defang and deprecate memory_order::consume

Author: tkoeppe

source/basic.tex

@@ -6319,7 +6319,7 @@
 operations on mutexes\iref{thread} that are specially identified as
 synchronization operations. These operations play a special role in making
 assignments in one thread visible to another. A synchronization operation on one
-or more memory locations is either a consume operation, an acquire operation, a
+or more memory locations is either an acquire operation, a
 release operation, or both an acquire and release operation. A synchronization
 operation without an associated memory location is a fence and can be either an
 acquire fence, a release fence, or both an acquire and release fence. In
@@ -6375,122 +6375,17 @@
 the value written'' by the last mutex release.
 \end{note}
 
-\pnum
-An evaluation $A$ \defn{carries a dependency} to an evaluation $B$ if
-\begin{itemize}
-\item
-the value of $A$ is used as an operand of $B$, unless:
-\begin{itemize}
-\item
-$B$ is an invocation of any specialization of
-\tcode{std::kill_dependency}\iref{atomics.order}, or
-\item
-$A$ is the left operand of a built-in logical \logop{and} (\tcode{\&\&},
-see~\ref{expr.log.and}) or logical \logop{or} (\tcode{||}, see~\ref{expr.log.or})
-operator, or
-\item
-$A$ is the left operand of a conditional (\tcode{?:}, see~\ref{expr.cond})
-operator, or
-\item
-$A$ is the left operand of the built-in comma (\tcode{,})
-operator\iref{expr.comma}; \end{itemize} or
-\item
-$A$ writes a scalar object or bit-field $M$, $B$ reads the value
-written by $A$ from $M$, and $A$ is sequenced before $B$, or
-\item
-for some evaluation $X$, $A$ carries a dependency to $X$, and
-$X$ carries a dependency to $B$.
-\end{itemize}
-\begin{note}
-``Carries a dependency to'' is a subset of ``is sequenced before'',
-and is similarly strictly intra-thread.
-\end{note}
-
-\pnum
-An evaluation $A$ is \defn{dependency-ordered before} an evaluation
-$B$ if
-\begin{itemize}
-\item
-$A$ performs a release operation on an atomic object $M$, and, in
-another thread, $B$ performs a consume operation on $M$ and reads
-the value written by $A$, or
-
-\item
-for some evaluation $X$, $A$ is dependency-ordered before $X$ and
-$X$ carries a dependency to $B$.
-
-\end{itemize}
-\begin{note}
-The relation ``is dependency-ordered before'' is analogous to
-``synchronizes with'', but uses release/consume in place of release/acquire.
-\end{note}
-
-\pnum
-An evaluation $A$ \defn{inter-thread happens before} an evaluation $B$
-if
-\begin{itemize}
-\item
-  $A$ synchronizes with $B$, or
-\item
-  $A$ is dependency-ordered before $B$, or
-\item
-  for some evaluation $X$
-  \begin{itemize}
-  \item
-    $A$ synchronizes with $X$ and $X$
-    is sequenced before $B$, or
-  \item
-    $A$ is sequenced before $X$ and $X$
-    inter-thread happens before $B$, or
-  \item
-    $A$ inter-thread happens before $X$ and $X$
-    inter-thread happens before $B$.
-  \end{itemize}
-\end{itemize}
-\begin{note}
-The ``inter-thread happens before'' relation describes arbitrary
-concatenations of ``sequenced before'', ``synchronizes with'' and
-``dependency-ordered before'' relationships, with two exceptions. The first
-exception is that a concatenation never ends with
-``dependency-ordered before'' followed by ``sequenced before''. The reason for
-this limitation is that a consume operation participating in a
-``dependency-ordered before'' relationship provides ordering only with respect
-to operations to which this consume operation actually carries a dependency. The
-reason that this limitation applies only to the end of such a concatenation is
-that any subsequent release operation will provide the required ordering for a
-prior consume operation. The second exception is that a concatenation never
-consist entirely of ``sequenced before''. The reasons for this
-limitation are (1) to permit ``inter-thread happens before'' to be transitively
-closed and (2) the ``happens before'' relation, defined below, provides for
-relationships consisting entirely of ``sequenced before''.
-\end{note}
-
 \pnum
 An evaluation $A$ \defn{happens before} an evaluation $B$
-(or, equivalently, $B$ \defn{happens after} $A$) if
-\begin{itemize}
-\item $A$ is sequenced before $B$, or
-\item $A$ inter-thread happens before $B$.
-\end{itemize}
-The implementation shall ensure that no program execution demonstrates a cycle
-in the ``happens before'' relation.
-\begin{note}
-This cycle would otherwise be
-possible only through the use of consume operations.
-\end{note}
-
-\pnum
-An evaluation $A$ \defn{simply happens before} an evaluation $B$
+(or, equivalently, $B$ happens after $A$)
 if either
 \begin{itemize}
 \item $A$ is sequenced before $B$, or
 \item $A$ synchronizes with $B$, or
-\item $A$ simply happens before $X$ and
-$X$ simply happens before $B$.
+\item $A$ happens before $X$ and $X$ happens before $B$.
 \end{itemize}
 \begin{note}
-In the absence of consume operations,
-the happens before and simply happens before relations are identical.
+An evaluation does not happen before itself.
 \end{note}
 
 \pnum
@@ -6503,7 +6398,7 @@
 sequentially consistent atomic operations\iref{atomics.order}, or
 \item there are evaluations $B$ and $C$
 such that $A$ is sequenced before $B$,
-$B$ simply happens before $C$, and
+$B$ happens before $C$, and
 $C$ is sequenced before $D$, or
 \item there is an evaluation $B$ such that
 $A$ strongly happens before $B$, and
@@ -6512,7 +6407,7 @@
 \begin{note}
 Informally, if $A$ strongly happens before $B$,
 then $A$ appears to be evaluated before $B$
-in all contexts. Strongly happens before excludes consume operations.
+in all contexts.
 \end{note}
 
 \pnum

source/compatibility.tex

@@ -2386,8 +2386,8 @@
 Avoid hard to diagnose or non-portable constructs.
 \effect
 Names of attribute identifiers may not be used as macro names. Valid \CppIII{}
-code that defines \tcode{override}, \tcode{final},
-\tcode{carries_dependency}, or \tcode{noreturn} as macros is invalid in this
+code that defines \tcode{override}, \tcode{final}, or
+\tcode{noreturn} as macros is invalid in this
 revision of \Cpp{}.
 
 \rSec2[diff.cpp03.language.support]{\ref{support}:

source/declarations.tex

@@ -9550,86 +9550,6 @@
 \end{codeblock}
 \end{example}
 
-\rSec2[dcl.attr.depend]{Carries dependency attribute}%
-\indextext{attribute!carries dependency}
-
-\pnum
-The \grammarterm{attribute-token} \tcode{carries_dependency} specifies
-dependency propagation into and out of functions.
-No
-\grammarterm{attribute-argument-clause} shall be present. The attribute may be
-applied to a parameter of a function or lambda, in
-which case it specifies that the initialization of the parameter carries a
-dependency to\iref{intro.multithread} each lvalue-to-rvalue
-conversion\iref{conv.lval} of that object. The attribute may also be applied
-to a function or a lambda call operator, in which case it
-specifies that the return value, if any, carries a dependency to the evaluation
-of the function call expression.
-
-\pnum
-The first declaration of a function shall specify the \tcode{carries_dependency} attribute for its
-\grammarterm{declarator-id} if any declaration of the function specifies the
-\tcode{carries_dependency} attribute. Furthermore, the first declaration of a function shall specify
-the \tcode{carries_dependency} attribute for a parameter if any declaration of that function
-specifies the \tcode{carries_dependency} attribute for that parameter. If a function or one of its
-parameters is declared with the \tcode{carries_dependency} attribute in its first declaration in one
-translation unit and the same function or one of its parameters is declared without the
-\tcode{carries_dependency} attribute in its first declaration in another translation unit, the
-program is ill-formed, no diagnostic required.
-
-\pnum
-\begin{note}
-The \tcode{carries_dependency} attribute does not change the meaning of the
-program, but might result in generation of more efficient code.
-\end{note}
-
-\pnum
-\begin{example}
-\begin{codeblock}
-/* Translation unit A. */
-
-struct foo { int* a; int* b; };
-std::atomic<struct foo *> foo_head[10];
-int foo_array[10][10];
-
-[[carries_dependency]] struct foo* f(int i) {
-  return foo_head[i].load(memory_order::consume);
-}
-
-int g(int* x, int* y [[carries_dependency]]) {
-  return kill_dependency(foo_array[*x][*y]);
-}
-
-/* Translation unit B. */
-
-[[carries_dependency]] struct foo* f(int i);
-int g(int* x, int* y [[carries_dependency]]);
-
-int c = 3;
-
-void h(int i) {
-  struct foo* p;
-
-  p = f(i);
-  do_something_with(g(&c, p->a));
-  do_something_with(g(p->a, &c));
-}
-\end{codeblock}
-
-The \tcode{carries_dependency} attribute on function \tcode{f} means that the
-return value carries a dependency out of \tcode{f}, so that the implementation
-need not constrain ordering upon return from \tcode{f}. Implementations of
-\tcode{f} and its caller may choose to preserve dependencies instead of emitting
-hardware memory ordering instructions (a.k.a.\ fences).
-Function \tcode{g}'s second parameter has a \tcode{carries_dependency} attribute,
-but its first parameter does not. Therefore, function \tcode{h}'s first call to
-\tcode{g} carries a dependency into \tcode{g}, but its second call does not. The
-implementation might need to insert a fence prior to the second call to
-\tcode{g}.
-\end{example}
-\indextext{attribute|)}%
-\indextext{declaration|)}
-
 \rSec2[dcl.attr.deprecated]{Deprecated attribute}%
 \indextext{attribute!deprecated}
 
@@ -10106,3 +10026,6 @@
 could have the same address as \tcode{buckets}
 if their respective types are all empty.
 \end{example}
+
+\indextext{attribute|)}%
+\indextext{declaration|)}

source/future.tex

@@ -854,6 +854,9 @@
     void atomic_init(volatile atomic<T>*, typename atomic<T>::value_type) noexcept;
   template<class T>
     void atomic_init(atomic<T>*, typename atomic<T>::value_type) noexcept;
+  template<class T>
+    constexpr T kill_dependency(T y) noexcept;                                  // freestanding
+  inline constexpr memory_order memory_order_consume = memory_order::consume;   // freestanding
 
   #define @\libmacro{ATOMIC_VAR_INIT}@(value) @\seebelow@
 }
@@ -924,3 +927,24 @@
 \end{codeblock}
 \end{example}
 \end{itemdescr}
+
+\rSec2[depr.atomics.order]{\tcode{memory_order::consume}}
+
+\indexlibrarymember{consume}{memory_order}%
+\pnum
+The memory_order enumeration contains an additional enumerator:
+\begin{codeblock}
+consume = 1
+\end{codeblock}
+The \tcode{memory_order::consume} enumerator is allowed wherever
+\tcode{memory_order::acquire} is allowed, and it has the same meaning.
+
+\begin{itemdecl}
+template<class T> constexpr T kill_dependency(T y) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+\tcode{y}.
+\end{itemdescr}

source/preprocessor.tex

@@ -400,7 +400,6 @@
 \topline
 \lhdr{Attribute} & \rhdr{Value} \\ \rowsep
 \tcode{assume}                & \tcode{202207L} \\
-\tcode{carries_dependency}    & \tcode{200809L} \\
 \tcode{deprecated}            & \tcode{201309L} \\
 \tcode{fallthrough}           & \tcode{201603L} \\
 \tcode{likely}                & \tcode{201803L} \\