Document number: P0479R5
Date: 2018-03-16
Audience: Core Working Group, SG14
Reply-to: Clay Trychta <clay.trychta@gmail.com>

Proposed wording for likely and unlikely attributes (Revision 5)

Wording

10.6.N Likelihood attributes [dcl.attr.likelihood]

The attribute-tokens likely and unlikely may be applied to labels or statements. The attribute-tokens likely and unlikely shall appear at most once in each attribute-list and no attribute-argument-clause shall be present. The attribute-token likely shall not appear in an attribute-specifier-seq that contains the attribute-token unlikely.

[Note: The use of the likely attribute is intended to allow implementations to optimize for the case where paths of execution including it are arbitrarily more likely than any alternative path of execution that does not include such an attribute on a statement or label. The use of the unlikely attribute is intended to allow implementations to optimize for the case where paths of execution including it are arbitrarily more unlikely than any alternative path of execution that does not include such an attribute on a statement or label. A path of execution includes a label if and only if it contains a jump to that label. Excessive usage of either of these attributes is liable to result in performance degradation. — end note]

Acknowledgements

Thank you to Jens Maurer, Chandler Carruth, Andrew Pardoe, Davis Herring, John McFarlane, Bartosz Bielecki, Carl Cook, Matt Dziubinski, Mathias Gaunard, Paul Hampson, and others in SG14 for sharing their insights and suggestions.