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9c59d04d55
Currently it just calls `truncate(0)`. `truncate()` is (a) not marked as `#[inline]`, and (b) more general than needed for `clear()`. This commit changes `clear()` to do the work itself. This modest change was first proposed in rust-lang#74172, where the reviewer rejected it because there was insufficient evidence that `Vec::clear()`'s performance mattered enough to justify the change. Recent changes within rustc have made `Vec::clear()` hot within `macro_parser.rs`, so the change is now clearly worthwhile. Although it doesn't show wins on CI perf runs, this seems to be because they use PGO. But not all platforms currently use PGO. Also, local builds don't use PGO, and `truncate` sometimes shows up in an over-represented fashion in local profiles. So local profiling will be made easier by this change. Note that this will also benefit `String::clear()`, because it just calls `Vec::clear()`. Finally, the commit removes the `vec-clear.rs` codegen test. It was added in #52908. From before then until now, `Vec::clear()` just called `Vec::truncate()` with a zero length. The body of Vec::truncate() has changed a lot since then. Now that `Vec::clear()` is doing actual work itself, and not just calling `Vec::truncate()`, it's not surprising that its generated code includes a load and an icmp. I think it's reasonable to remove this test. |
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