[llvm] LoopVectorize: Set branch_weight for conditional branches (PR #72450)

[David Li via llvm-commits]

david-xl wrote:

> There is no measurable performance change from that on our end. I am merely hunting down conditional branches without `branch_weight` annotations which I think we can agree are never desirable in a function that has otherwise profile annotations.

Agree in general, but there might be undesirable outcome in some cases. There are 3 types of missing annotations: 1) real profile gets dropped; 2) real profile is unknown (due to lack of flow sensitivity) for a clone, but can be reasonablily guessed with scaling; and 3) new branches that require static heuristic to guess. It is the 3) category that can be problematic.  In the code, I added a question on the default values.

https://github.com/llvm/llvm-project/pull/72450