Merge 2024-03 CWG Motion 6

P2809R3 Trivial infinite loops are not Undefined Behavior

Author: tkoeppe

source/basic.tex

@@ -6478,13 +6478,18 @@
 following:
 \begin{itemize}
 \item terminate,
+\item invoke the function \tcode{std::this_thread::yield}\iref{thread.thread.this},
 \item make a call to a library I/O function,
-\item perform an access through a volatile glvalue, or
-\item perform a synchronization operation or an atomic operation.
+\item perform an access through a volatile glvalue,
+\item perform a synchronization operation or an atomic operation, or
+\item continue execution of a trivial infinite loop\iref{stmt.iter.general}.
 \end{itemize}
 \begin{note}
-This is intended to allow compiler transformations such as removal of
-empty loops, even when termination cannot be proven.
+This is intended to allow compiler transformations
+such as removal, merging, and reordering of empty loops,
+even when termination cannot be proven.
+An affordance is made for trivial infinite loops,
+which cannot be removed nor reordered.
 \end{note}
 
 \pnum

source/statements.tex

@@ -546,6 +546,36 @@
 Thus after the \keyword{while} statement, \tcode{i} is no longer in scope.
 \end{example}
 
+\pnum
+A \defnadj{trivially empty}{iteration statement} is
+an iteration statement matching one of the following forms:
+\begin{itemize}
+\item \tcode{while (} \grammarterm{expression} \tcode{) ;}
+\item \tcode{while (} \grammarterm{expression} \tcode{) \{ \}}
+\item \tcode{do ; while (} \grammarterm{expression} \tcode{) ;}
+\item \tcode{do \{ \} while (} \grammarterm{expression} \tcode{) ;}
+\item \tcode{for (} \grammarterm{init-statement} \opt{\grammarterm{expression}} \tcode{; ) ;}
+\item \tcode{for (} \grammarterm{init-statement} \opt{\grammarterm{expression}} \tcode{; ) \{ \}}
+\end{itemize}
+The \defnadj{controlling}{expression} of a trivially empty iteration statement
+is the \grammarterm{expression} of
+a \tcode{while}, \tcode{do}, or \tcode{for} statement
+(or \tcode{true}, if the \tcode{for} statement has no \grammarterm{expression}).
+A \defnadj{trivial infinite}{loop} is a trivially empty iteration statement
+for which the converted controlling expression is a constant expression,
+when interpreted as a \grammarterm{constant-expression}\iref{expr.const}, and
+evaluates to \tcode{true}.
+The \grammarterm{statement} of a trivial infinite loop is replaced with
+a call to the function \tcode{std::this_thread::yield}\iref{thread.thread.this};
+it is implementation-defined whether this replacement occurs
+on freestanding implementations.
+\begin{note}
+In a freestanding environment,
+concurrent forward progress is not guaranteed;
+such systems therefore require explicit cooperation.
+A call to yield can add implicit cooperation where none is otherwise intended.
+\end{note}
+
 \rSec2[stmt.while]{The \keyword{while} statement}%
 \indextext{statement!\idxcode{while}}