compiler: implement @branchHint
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mlugg
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mlugg: this is cherry-picked from Andrew's nosanitize branch (with Jacob's fixes squashed in) since I needed this for `unpredictable` and `prof` metadata. The nosanitize-specific changes are reverted in the next commit. Co-authored-by: Jacob Young <[email protected]>
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Implements the accepted proposal to introduce `@branchHint`. This builtin is permitted as the first statement of a block if that block is the direct body of any of the following: * a function (*not* a `test`) * either branch of an `if` * the RHS of a `catch` or `orelse` * a `switch` prong * an `or` or `and` expression It lowers to the ZIR instruction `extended(branch_hint(...))`. When Sema encounters this instruction, it sets `sema.branch_hint` appropriately, and `zirCondBr` etc are expected to reset this value as necessary. The state is on `Sema` rather than `Block` to make it automatically propagate up non-conditional blocks without special handling. If `@panic` is reached, the branch hint is set to `.cold` if none was already set; similarly, error branches get a hint of `.unlikely` if no hint is explicitly provided. If a condition is comptime-known, `cold` hints from the taken branch are allowed to propagate up, but other hints are discarded. This is because a `likely`/`unlikely` hint just indicates the direction this branch is likely to go, which is redundant information when the branch is known at comptime; but `cold` hints indicate that control flow is unlikely to ever reach this branch, meaning if the branch is always taken from its parent, then the parent is also unlikely to ever be reached. This branch information is stored in AIR `cond_br` and `switch_br`. In addition, `try` and `try_ptr` instructions have variants `try_cold` and `try_ptr_cold` which indicate that the error case is cold (rather than just unlikely); this is reachable through e.g. `errdefer unreachable` or `errdefer @Panic("")`. A new API `unwrapSwitch` is introduced to `Air` to make it more convenient to access `switch_br` instructions. In time, I plan to update all AIR instructions to be accessed via an `unwrap` method which returns a convenient tagged union a la `InternPool.indexToKey`. The LLVM backend lowers branch hints for conditional branches and switches as follows: * If any branch is marked `unpredictable`, the instruction is marked `!unpredictable`. * Any branch which is marked as `cold` gets a `llvm.assume(i1 true) [ "cold"() ]` call to mark the code path cold. * If any branch is marked `likely` or `unlikely`, branch weight metadata is attached with `!prof`. Likely branches get a weight of 2000, and unlikely branches a weight of 1. In `switch` statements, un-annotated branches get a weight of 1000 as a "middle ground" hint, since there could be likely *and* unlikely *and* un-annotated branches. For functions, a `cold` hint corresponds to the `cold` function attribute, and other hints are currently ignored -- as far as I can tell LLVM doesn't really have a way to lower them. (Ideally, we would want the branch hint given in the function to propagate to call sites.) The compiler and standard library do not yet use this new builtin. Resolves: ziglang#21148
Signed-off-by: mlugg <[email protected]>
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@Rexicon226 confirmed that this seems to be getting the desired codegen. |
andrewrk
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Aug 26, 2024
And remove the now-invalid test for the return value of `@branchHint`.
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Superceded by #21214. |
maximousblk
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updated in ziglang/zig#21191 tested on `[email protected]+7b8fc18c6`
judofyr
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updated in ziglang/zig#21191 tested on `[email protected]+7b8fc18c6`
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