LLVM doesn't discard obviously incorrect annotations

[philnik777]

LLVM currently doesn't discard likelihood information even when a condition is known to be true in certain cases. e.g.

if (condition) [[likely]]
  do_something();
else
  do_something_else();

will avoid inlining do_something_else even if condition is known to be false.

[nikic]

Can you please provide a complete example?

If condition is just a constant then the branch will get folded away and the branch weight information is also lost at that point. You clearly have something more than that in mind here.

[philnik777]

#include <vector>

static auto test(std::vector<int>& v, size_t new_cap) {
  if (v.capacity() < new_cap) [[unlikely]]
    v.reserve(new_cap);
}

auto func() {
  std::vector<int> vec;
  test(vec, 2);
}

Godbolt
Our original case is very close to the above code. the v.reserve() call doesn't get inlined, even though it's known in func that the branch will be taken unconditionally. This in turn causes a bunch of code not to be constant folded away. Does that help?

[nikic]

I think the problem here is just that the condition only becomes known after inlining has already been performed. It requires test() to be inlined and then non-trivially simplified before the reserve call becomes unconditional. There isn't another attempt to inline it after that has happened.