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3b022d8cee
improve cold_path() #120370 added a new instrinsic `cold_path()` and used it to fix `likely` and `unlikely` However, in order to limit scope, the information about cold code paths is only used in 2-target switch instructions. This is sufficient for `likely` and `unlikely`, but limits usefulness of `cold_path` for idiomatic rust. For example, code like this: ``` if let Some(x) = y { ... } ``` may generate 3-target switch: ``` switch y.discriminator: 0 => true branch 1 = > false branch _ => unreachable ``` and therefore marking a branch as cold will have no effect. This PR improves `cold_path()` to work with arbitrary switch instructions. Note that for 2-target switches, we can use `llvm.expect`, but for multiple targets we need to manually emit branch weights. I checked Clang and it also emits weights in this situation. The Clang's weight calculation is more complex that this PR, which I believe is mainly because `switch` in `C/C++` can have multiple cases going to the same target. |
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Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.