[cpeterso]

> This is standardizing vendor-specific attributes

Except the standard’s likely and unlikely attributes invented new syntax that is not drop-in compatible with clang’s and gcc’s attributes.

Where clang and gcc would use:

  if (__builtin_expect(x > 0, 1)) { … }

the standard uses:

  if (x > 0) [[likely]] { … }

[ack_complete]

It's definitely an unconventional syntax. In addition to the above, OpenMP and shader languages annotate the branch statement for parallelism or branch/predication hints. I can't think of precedent for C++ putting the hints in the branch targets. It does have some advantages, but it's not very intuitive. The first time I tried to use the new hints I did [[likely]] if(), which of course did nothing.