From 43ee7cd57c1706b949c92d6b29a7034b40dff533 Mon Sep 17 00:00:00 2001 From: Josh Triplett Date: Thu, 5 Jun 2025 12:02:04 -0700 Subject: Simplify and optimize `SlotIndex::from_index` Break out bucket 0 (containing `idx < 4096`) as an early return, which simplifies the remainder of the function, and allows optimizing the `checked_ilog2` since it can no longer return `None`. This reduces the runtime of `vec_cache::tests::slot_index_exhaustive` (which calls `SlotIndex::from_index` for every `u32`, twice) from ~15.5s to ~13.3s. --- compiler/rustc_data_structures/src/vec_cache.rs | 21 ++++++--------------- 1 file changed, 6 insertions(+), 15 deletions(-) (limited to 'compiler/rustc_data_structures/src') diff --git a/compiler/rustc_data_structures/src/vec_cache.rs b/compiler/rustc_data_structures/src/vec_cache.rs index 2ff60ab7f36..cc6973fb42e 100644 --- a/compiler/rustc_data_structures/src/vec_cache.rs +++ b/compiler/rustc_data_structures/src/vec_cache.rs @@ -68,22 +68,13 @@ impl SlotIndex { // slots (see `slot_index_exhaustive` in tests). #[inline] const fn from_index(idx: u32) -> Self { - let mut bucket = match idx.checked_ilog2() { - Some(x) => x as usize, - None => 0, - }; - let entries; - let running_sum; - if bucket <= 11 { - entries = 1 << 12; - running_sum = 0; - bucket = 0; - } else { - entries = 1 << bucket; - running_sum = entries; - bucket = bucket - 11; + if idx < 4096 { + return SlotIndex { bucket_idx: 0, entries: 4096, index_in_bucket: idx as usize }; } - SlotIndex { bucket_idx: bucket, entries, index_in_bucket: idx as usize - running_sum } + // SAFETY: We already ruled out idx 0, so `checked_ilog2` can't return `None`. + let bucket = unsafe { idx.checked_ilog2().unwrap_unchecked() as usize }; + let entries = 1 << bucket; + SlotIndex { bucket_idx: bucket - 11, entries, index_in_bucket: idx as usize - entries } } // SAFETY: Buckets must be managed solely by functions here (i.e., get/put on SlotIndex) and -- cgit 1.4.1-3-g733a5