Commits on Apr 13, 2022

1. Speed up Vec::clear(). …

   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 rust-lang#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.

   

   nnethercote committed Apr 13, 2022
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