[PATCH] D86559: [Sema, CodeGen] Allow [[likely]] and [[unlikely]] on labels

[Mark de Wever via Phabricator via cfe-commits]

Mordante added a comment.

In D86559#2243583 <https://reviews.llvm.org/D86559#2243583>, @staffantj wrote:

> The use case for this becomes clearer when considering that the attribute that will be used 95% of the time is [[unlikely]]. You may have a constructor that you wish to ask the compiler to please, please, do not inline this, in a particular function, even if the rest of that function is either likely or neutral.

At the moment the likelihood is used only for branch prediction and not for inlining decisions. I'm not sure it would be a good idea to use this attribute for that duality. If it's wanted to control the inlining of calls at the callers side I feel a separate attribute would make more sense.

  void foo()
  {
    [[unlikely]] expensive_class q{};
    ...
  }

This looks odd to me and the attribute isn't allowed on the declaration. I think this looks better:

  void foo()
  {
    [[noinline]] expensive_class q{};
    ...
  }



>   if (a)
>     [[unlikely]] expensive_class q{};
>
> This could be the case if expensive_class held large static members, for instance.

This can be rewritten to be an expression and not a declaration, then the attribute can be used:

  if(a)
    [[unlikely]] expensive_class{};




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https://reviews.llvm.org/D86559