9 Declarations [dcl.dcl]

9.12 Attributes [dcl.attr]

9.12.6 Likelihood attributes [dcl.attr.likelihood]

1
#
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.
2
#
Recommended practice: 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.
[Note
:
Excessive usage of either of these attributes is liable to result in performance degradation.
— end note
]
3
#
[Example
: 
void g(int);
int f(int n) {
  if (n > 5) [[unlikely]] {     // n > 5 is considered to be arbitrarily unlikely
    g(0);
    return n * 2 + 1;
  }

  switch (n) {
  case 1:
    g(1);
    [[fallthrough]];

  [[likely]] case 2:            // n == 2 is considered to be arbitrarily more
    g(2);                       // likely than any other value of n
    break;
  }
  return 3;
}
— end example
]