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Auto merge of #81470 - tmiasko:remove-allocations, r=matthewjasper

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Avoid memory allocation when removing dead blocks

Use `reachable_as_bitset` to reuse a bitset from the traversal rather
than allocating it seprately. Additionally check if there are any
unreachable blocks before proceeding.
This commit is contained in:
bors 2021-01-29 13:10:09 +00:00
commit b122908617
1 changed files with 6 additions and 7 deletions
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13
compiler/rustc_mir/src/transform/simplify.rs
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@ -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::*;
@ -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