From e403654c8b49f7bf62e551c2dc5aaaca9ba71d76 Mon Sep 17 00:00:00 2001 From: Scott McMurray Date: Sun, 2 Mar 2025 23:47:24 -0800 Subject: [PATCH 1/2] Simplify `::size_hint` The current version is wrong (cc 137919); let's see if we can get away with a loose but trivially-correct one. --- compiler/rustc_middle/src/mir/traversal.rs | 17 ++++++----------- 1 file changed, 6 insertions(+), 11 deletions(-) diff --git a/compiler/rustc_middle/src/mir/traversal.rs b/compiler/rustc_middle/src/mir/traversal.rs index 5950ac295af19..5fb3c648d5a26 100644 --- a/compiler/rustc_middle/src/mir/traversal.rs +++ b/compiler/rustc_middle/src/mir/traversal.rs @@ -108,7 +108,6 @@ pub struct Postorder<'a, 'tcx> { basic_blocks: &'a IndexSlice>, visited: DenseBitSet, visit_stack: Vec<(BasicBlock, Successors<'a>)>, - root_is_start_block: bool, /// A non-empty `extra` allows for a precise calculation of the successors. extra: Option<(TyCtxt<'tcx>, Instance<'tcx>)>, } @@ -123,7 +122,6 @@ impl<'a, 'tcx> Postorder<'a, 'tcx> { basic_blocks, visited: DenseBitSet::new_empty(basic_blocks.len()), visit_stack: Vec::new(), - root_is_start_block: root == START_BLOCK, extra, }; @@ -211,16 +209,13 @@ impl<'tcx> Iterator for Postorder<'_, 'tcx> { } fn size_hint(&self) -> (usize, Option) { - // All the blocks, minus the number of blocks we've visited. - let upper = self.basic_blocks.len() - self.visited.count(); - - let lower = if self.root_is_start_block { - // We will visit all remaining blocks exactly once. - upper - } else { - self.visit_stack.len() - }; + // These bounds are not at all tight, but that's fine. + // It's not worth a popcnt loop in `DenseBitSet` to improve the upper, + // and in mono-reachable we can't be precise anyway. + // Leaning on amortized growth is fine. + let lower = self.visit_stack.len(); + let upper = self.basic_blocks.len(); (lower, Some(upper)) } } From fe6cf341479c91ef90d8c809aea70193ed42036a Mon Sep 17 00:00:00 2001 From: Scott McMurray Date: Mon, 3 Mar 2025 21:07:55 -0800 Subject: [PATCH 2/2] Also simplify `Preorder`'s `size_hint` --- compiler/rustc_middle/src/mir/traversal.rs | 20 +++++--------------- 1 file changed, 5 insertions(+), 15 deletions(-) diff --git a/compiler/rustc_middle/src/mir/traversal.rs b/compiler/rustc_middle/src/mir/traversal.rs index 5fb3c648d5a26..9308570d89d18 100644 --- a/compiler/rustc_middle/src/mir/traversal.rs +++ b/compiler/rustc_middle/src/mir/traversal.rs @@ -23,19 +23,13 @@ pub struct Preorder<'a, 'tcx> { body: &'a Body<'tcx>, visited: DenseBitSet, worklist: Vec, - root_is_start_block: bool, } impl<'a, 'tcx> Preorder<'a, 'tcx> { pub fn new(body: &'a Body<'tcx>, root: BasicBlock) -> Preorder<'a, 'tcx> { let worklist = vec![root]; - Preorder { - body, - visited: DenseBitSet::new_empty(body.basic_blocks.len()), - worklist, - root_is_start_block: root == START_BLOCK, - } + Preorder { body, visited: DenseBitSet::new_empty(body.basic_blocks.len()), worklist } } } @@ -71,15 +65,11 @@ impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> { } fn size_hint(&self) -> (usize, Option) { - // All the blocks, minus the number of blocks we've visited. - let upper = self.body.basic_blocks.len() - self.visited.count(); + // The worklist might be only things already visited. + let lower = 0; - let lower = if self.root_is_start_block { - // We will visit all remaining blocks exactly once. - upper - } else { - self.worklist.len() - }; + // This is extremely loose, but it's not worth a popcnt loop to do better. + let upper = self.body.basic_blocks.len(); (lower, Some(upper)) }