Rollup merge of #33552 - dotdash:scfg, r=luqmana
[MIR] Enhance the SimplifyCfg pass to merge consecutive blocks Updated from #30238, including the changes suggested by @Aatch.
This commit is contained in:
Manish Goregaokar
commit
ea68dd8def
@ -8,73 +8,155 @@
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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use rustc_data_structures::bitvec::BitVector;
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use rustc::middle::const_val::ConstVal;
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use rustc::ty::TyCtxt;
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use rustc::mir::repr::*;
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use rustc::mir::transform::{MirPass, MirSource, Pass};
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use pretty;
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use std::mem;
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use super::remove_dead_blocks::RemoveDeadBlocks;
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use traversal;
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pub struct SimplifyCfg;
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impl SimplifyCfg {
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pub fn new() -> SimplifyCfg {
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SimplifyCfg
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}
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}
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fn remove_goto_chains(&self, mir: &mut Mir) -> bool {
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// Find the target at the end of the jump chain, return None if there is a loop
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fn final_target(mir: &Mir, mut target: BasicBlock) -> Option<BasicBlock> {
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// Keep track of already seen blocks to detect loops
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let mut seen: Vec<BasicBlock> = Vec::with_capacity(8);
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impl<'tcx> MirPass<'tcx> for SimplifyCfg {
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fn run_pass<'a>(&mut self, tcx: TyCtxt<'a, 'tcx, 'tcx>, src: MirSource, mir: &mut Mir<'tcx>) {
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simplify_branches(mir);
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RemoveDeadBlocks.run_pass(tcx, src, mir);
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merge_consecutive_blocks(mir);
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RemoveDeadBlocks.run_pass(tcx, src, mir);
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pretty::dump_mir(tcx, "simplify_cfg", &0, src, mir, None);
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while mir.basic_block_data(target).statements.is_empty() {
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// NB -- terminator may have been swapped with `None`
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// below, in which case we have a cycle and just want
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// to stop
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if let Some(ref terminator) = mir.basic_block_data(target).terminator {
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match terminator.kind {
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TerminatorKind::Goto { target: next } => {
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if seen.contains(&next) {
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return None;
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}
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seen.push(next);
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target = next;
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}
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_ => break
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// FIXME: Should probably be moved into some kind of pass manager
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mir.basic_blocks.shrink_to_fit();
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}
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}
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impl Pass for SimplifyCfg {}
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fn merge_consecutive_blocks(mir: &mut Mir) {
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// Build the precedecessor map for the MIR
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let mut pred_count = vec![0u32; mir.basic_blocks.len()];
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for (_, data) in traversal::preorder(mir) {
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if let Some(ref term) = data.terminator {
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for &tgt in term.successors().iter() {
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pred_count[tgt.index()] += 1;
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}
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}
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}
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loop {
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let mut changed = false;
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let mut seen = BitVector::new(mir.basic_blocks.len());
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let mut worklist = vec![START_BLOCK];
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while let Some(bb) = worklist.pop() {
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// Temporarily take ownership of the terminator we're modifying to keep borrowck happy
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let mut terminator = mir.basic_block_data_mut(bb).terminator.take()
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.expect("invalid terminator state");
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// See if we can merge the target block into this one
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loop {
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let mut inner_change = false;
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if let TerminatorKind::Goto { target } = terminator.kind {
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// Don't bother trying to merge a block into itself
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if target == bb {
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break;
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}
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} else {
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break
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let num_insts = mir.basic_block_data(target).statements.len();
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match mir.basic_block_data(target).terminator().kind {
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TerminatorKind::Goto { target: new_target } if num_insts == 0 => {
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inner_change = true;
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terminator.kind = TerminatorKind::Goto { target: new_target };
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pred_count[target.index()] -= 1;
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pred_count[new_target.index()] += 1;
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}
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_ if pred_count[target.index()] == 1 => {
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inner_change = true;
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let mut stmts = Vec::new();
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{
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let target_data = mir.basic_block_data_mut(target);
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mem::swap(&mut stmts, &mut target_data.statements);
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mem::swap(&mut terminator, target_data.terminator_mut());
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}
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mir.basic_block_data_mut(bb).statements.append(&mut stmts);
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}
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_ => {}
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};
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}
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for target in terminator.successors_mut() {
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let new_target = match final_target(mir, *target) {
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Some(new_target) => new_target,
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None if mir.basic_block_data(bb).statements.is_empty() => bb,
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None => continue
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};
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if *target != new_target {
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inner_change = true;
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pred_count[target.index()] -= 1;
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pred_count[new_target.index()] += 1;
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*target = new_target;
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}
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}
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changed |= inner_change;
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if !inner_change {
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break;
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}
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}
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Some(target)
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}
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let mut changed = false;
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for bb in mir.all_basic_blocks() {
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// Temporarily take ownership of the terminator we're modifying to keep borrowck happy
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let mut terminator = mir.basic_block_data_mut(bb).terminator.take()
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.expect("invalid terminator state");
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debug!("remove_goto_chains: bb={:?} terminator={:?}", bb, terminator);
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for target in terminator.successors_mut() {
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let new_target = match final_target(mir, *target) {
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Some(new_target) => new_target,
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None if mir.basic_block_data(bb).statements.is_empty() => bb,
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None => continue
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};
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changed |= *target != new_target;
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*target = new_target;
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}
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mir.basic_block_data_mut(bb).terminator = Some(terminator);
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for succ in mir.basic_block_data(bb).terminator().successors().iter() {
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if seen.insert(succ.index()) {
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worklist.push(*succ);
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}
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}
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}
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if !changed {
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break;
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}
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}
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}
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// Find the target at the end of the jump chain, return None if there is a loop
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fn final_target(mir: &Mir, mut target: BasicBlock) -> Option<BasicBlock> {
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// Keep track of already seen blocks to detect loops
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let mut seen: Vec<BasicBlock> = Vec::with_capacity(8);
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while mir.basic_block_data(target).statements.is_empty() {
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// NB -- terminator may have been swapped with `None` in
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// merge_consecutive_blocks, in which case we have a cycle and just want
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// to stop
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match mir.basic_block_data(target).terminator {
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Some(Terminator { kind: TerminatorKind::Goto { target: next }, .. }) => {
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if seen.contains(&next) {
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return None;
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}
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seen.push(next);
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target = next;
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}
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_ => break
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}
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changed
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}
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fn simplify_branches(&self, mir: &mut Mir) -> bool {
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Some(target)
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}
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fn simplify_branches(mir: &mut Mir) {
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loop {
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let mut changed = false;
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for bb in mir.all_basic_blocks() {
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@ -106,25 +188,8 @@ impl SimplifyCfg {
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}
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}
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changed
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}
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}
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impl<'tcx> MirPass<'tcx> for SimplifyCfg {
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fn run_pass<'a>(&mut self, tcx: TyCtxt<'a, 'tcx, 'tcx>,
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src: MirSource, mir: &mut Mir<'tcx>) {
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let mut counter = 0;
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let mut changed = true;
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while changed {
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pretty::dump_mir(tcx, "simplify_cfg", &counter, src, mir, None);
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counter += 1;
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changed = self.simplify_branches(mir);
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changed |= self.remove_goto_chains(mir);
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RemoveDeadBlocks.run_pass(tcx, src, mir);
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if !changed {
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break;
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}
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// FIXME: Should probably be moved into some kind of pass manager
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mir.basic_blocks.shrink_to_fit();
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}
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}
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impl Pass for SimplifyCfg {}
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