P1774R8 Portable assumptions

Author: jensmaurer

source/declarations.tex

@@ -8529,6 +8529,43 @@
 \end{codeblock}
 \end{example}
 
+\rSec2[dcl.attr.assume]{Assumption attribute}
+
+The \grammarterm{attribute-token} \tcode{assume} may be applied to a null statement;
+such a statement is an \defn{assumption}.
+An \grammarterm{attribute-argument-clause} shall be present and
+shall have the form:
+\begin{ncsimplebnf}
+\terminal{(} conditional-expression \terminal{)}
+\end{ncsimplebnf}
+The expression is contextually converted to \tcode{bool}\iref{conv.general}.
+The expression is not evaluated.
+If the converted expression would evaluate to \tcode{true}
+at the point where the assumption appears,
+the assumption has no effect.
+Otherwise, the behavior is undefined.
+\begin{note}
+The expression is potentially evaluated\iref{basic.def.odr}.
+The use of assumptions is intended to allow implementations
+to analyze the form of the expression and
+deduce information used to optimize the program.
+Implementations are not required to deduce
+any information from any particular assumption.
+\end{note}
+\begin{example}
+\begin{codeblock}
+int divide_by_32(int x) {
+  [[assume(x >= 0)]];
+  return x/32;                  // The instructions produced for the division
+                                // may omit handling of negative values.
+}
+int f(int y) {
+  [[assume(++y == 43)]];        // \tcode{y} is not incremented
+  return y;                     // statement may be replaced with \tcode{return 42;}
+}
+\end{codeblock}
+\end{example}
+
 \rSec2[dcl.attr.depend]{Carries dependency attribute}%
 \indextext{attribute!carries dependency}
 

source/expressions.tex

@@ -7359,7 +7359,8 @@
 
 \item
 an operation that would have undefined behavior
-as specified in \ref{intro} through \ref{cpp};
+as specified in \ref{intro} through \ref{cpp},
+excluding \ref{dcl.attr.assume};
 \begin{footnote}
 This includes,
 for example, signed integer overflow\iref{expr.pre}, certain
@@ -7506,12 +7507,28 @@
 a \keyword{goto} statement\iref{stmt.goto}.
 \end{itemize}
 
-If $E$ satisfies the constraints of a core constant expression, but
-evaluation of $E$ would evaluate an operation that has undefined behavior
-as specified in \ref{library} through \ref{\lastlibchapter}, or
-an invocation of the \tcode{va_start} macro\iref{cstdarg.syn},
-it is unspecified whether $E$ is a core constant expression.
-
+It is unspecified whether $E$ is a core constant expression
+if $E$ satisfies the constraints of a core constant expression, but
+evaluation of $E$ would evaluate
+\begin{itemize}
+\item
+an operation that has undefined behavior
+as specified in \ref{library} through \ref{\lastlibchapter},
+\item
+an invocation of the \tcode{va_start} macro\iref{cstdarg.syn}, or
+\item
+a statement with an assumption\iref{dcl.attr.assume}
+whose converted \grammarterm{conditional-expression},
+if evaluated where the assumption appears,
+would not disqualify $E$ from being a core constant expression and
+would not evaluate to \tcode{true}.
+\begin{note}
+$E$ is not disqualified from being a core constant expression
+if the hypothetical evaluation of
+the converted \grammarterm{conditional-expression}
+would disqualify $E$ from being a core constant expression.
+\end{note}
+\end{itemize}
 \begin{example}
 \begin{codeblock}
 int x;                              // not constant