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Avoid memory allocation when removing dead blocks #81470

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Jan 29, 2021
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Avoid memory allocation when removing dead blocks #81470

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13 changes: 6 additions & 7 deletions compiler/rustc_mir/src/transform/simplify.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,7 +28,6 @@
//! return.

use crate::transform::MirPass;
use rustc_index::bit_set::BitSet;
use rustc_index::vec::{Idx, IndexVec};
use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
use rustc_middle::mir::*;
Expand Down Expand Up @@ -288,17 +287,17 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
}

pub fn remove_dead_blocks(body: &mut Body<'_>) {
let mut seen = BitSet::new_empty(body.basic_blocks().len());
for (bb, _) in traversal::preorder(body) {
seen.insert(bb.index());
let reachable = traversal::reachable_as_bitset(body);
let num_blocks = body.basic_blocks().len();
if num_blocks == reachable.count() {
return;
}

let basic_blocks = body.basic_blocks_mut();

let num_blocks = basic_blocks.len();
let mut replacements: Vec<_> = (0..num_blocks).map(BasicBlock::new).collect();
let mut used_blocks = 0;
for alive_index in seen.iter() {
for alive_index in reachable.iter() {
let alive_index = alive_index.index();
replacements[alive_index] = BasicBlock::new(used_blocks);
if alive_index != used_blocks {
// Swap the next alive block data with the current available slot. Since
Expand Down