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Diffstat (limited to 'gcc/doc/extend.texi')
| -rw-r--r-- | gcc/doc/extend.texi | 49 |
1 files changed, 36 insertions, 13 deletions
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index a5afb46..9ddfcf7 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -9187,6 +9187,20 @@ available for functions (@pxref{Function Attributes}), variables (@pxref{Variable Attributes}), labels (@pxref{Label Attributes}), enumerators (@pxref{Enumerator Attributes}), and for types (@pxref{Type Attributes}). +@table @code +@item fallthrough +@cindex @code{fallthrough} statement attribute +The @code{fallthrough} attribute with a null statement serves as a +fallthrough statement. It hints to the compiler that a statement +that falls through to another case label, or user-defined label +in a switch statement is intentional and thus the +@option{-Wimplicit-fallthrough} warning must not trigger. The +fallthrough attribute may appear at most once in each attribute +list, and may not be mixed with other attributes. It can only +be used in a switch statement (the compiler will issue an error +otherwise), after a preceding statement and before a logically +succeeding case label, or user-defined label. + This example uses the @code{fallthrough} statement attribute to indicate that the @option{-Wimplicit-fallthrough} warning should not be emitted: @@ -9201,19 +9215,28 @@ switch (cond) @} @end smallexample -@table @code -@item fallthrough -@cindex @code{fallthrough} statement attribute -The @code{fallthrough} attribute with a null statement serves as a -fallthrough statement. It hints to the compiler that a statement -that falls through to another case label, or user-defined label -in a switch statement is intentional and thus the -@option{-Wimplicit-fallthrough} warning must not trigger. The -fallthrough attribute may appear at most once in each attribute -list, and may not be mixed with other attributes. It can only -be used in a switch statement (the compiler will issue an error -otherwise), after a preceding statement and before a logically -succeeding case label, or user-defined label. +@item assume +@cindex @code{assume} statement attribute +The @code{assume} attribute with a null statement serves as portable +assumption. It should have a single argument, a conditional expression, +which is not evaluated. If the argument would evaluate to true +at the point where it appears, it has no effect, otherwise there +is undefined behavior. This is a GNU variant of the ISO C++23 +standard @code{assume} attribute, but it can be used in any version of +both C and C++. + +@smallexample +int +foo (int x, int y) +@{ + __attribute__((assume(x == 42))); + __attribute__((assume(++y == 43))); + return x + y; +@} +@end smallexample + +@code{y} is not actually incremented and the compiler can but does not +have to optimize it to just @code{return 42 + 42;}. @end table |