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Simplify <Postorder as Iterator>::size_hint

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The current version is wrong (cc 137919); let's see if we can get away with a loose but trivially-correct one.
This commit is contained in:
Scott McMurray 2025-03-02 23:47:24 -08:00
parent 1c3b035542
commit e403654c8b
1 changed files with 6 additions and 11 deletions
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17
compiler/rustc_middle/src/mir/traversal.rs
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@ -108,7 +108,6 @@ pub struct Postorder<'a, 'tcx> {
basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
visited: DenseBitSet<BasicBlock>,
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<usize>) {
// 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))
}
}