How to use C++20's likely/unlikely attribute in if-else statement

Question

This question is about C++20's [[likely]]/[[unlikely]] feature, not compiler-defined macros.

This documents (cppreference) only gave an example on applying them to a switch-case statement. This switch-case example compiles perfectly with my compiler (g++-7.2) so I assume the compiler has implemented this feature, though it's not yet officially introduced in current C++ standards.

But when I use them like this: if (condition) [[likely]] { ... } else { ... }, I got a warning:

"warning: attributes at the beginning of statement are ignored [-Wattributes]".

So how should I use these attributes in an if-else statement?

Answer 1

Based on example from Jacksonville’18 ISO C++ Report the syntax is correct, but it seems that it is not implemented yet:

if (a>b) [[likely]] {

See Likelihood attributes [dcl.attr.likelihood]:

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;
}