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Sink unused const assignments #106613
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Sink unused const assignments #106613
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222 changes: 222 additions & 0 deletions
222
compiler/rustc_mir_transform/src/sink_const_assignments.rs
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,222 @@ | ||
| use crate::MirPass; | ||
| use rustc_data_structures::fx::{FxHashMap, FxHashSet}; | ||
| use rustc_index::bit_set::BitSet; | ||
| use rustc_middle::mir::visit::{NonUseContext, PlaceContext, Visitor}; | ||
| use rustc_middle::mir::*; | ||
| use rustc_middle::ty::TyCtxt; | ||
|
|
||
| const SUCCESSOR_LIMIT: usize = 100; | ||
|
|
||
| pub struct SinkConstAssignments; | ||
|
|
||
| impl<'tcx> MirPass<'tcx> for SinkConstAssignments { | ||
| fn is_enabled(&self, sess: &rustc_session::Session) -> bool { | ||
| sess.mir_opt_level() >= 1 | ||
| } | ||
|
|
||
| fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { | ||
| // The primary benefit of this pass is sinking assignments to drop flags and enabling | ||
| // ConstGoto and SimplifyCfg to merge the drop flag check into existing control flow. | ||
| // If we permit sinking assignments to any local, we will sometimes sink an assignment into | ||
| // but not completely through a goto chain, preventing SimplifyCfg from removing the | ||
| // blocks. | ||
| let mut optimizable_locals = branched_locals(body); | ||
| if optimizable_locals.is_empty() { | ||
| return; | ||
| } | ||
|
|
||
| let borrowed_locals = rustc_mir_dataflow::impls::borrowed_locals(body); | ||
| optimizable_locals.subtract(&borrowed_locals); | ||
| if optimizable_locals.is_empty() { | ||
| return; | ||
| } | ||
|
|
||
| 'outer: for block in 0..body.basic_blocks.len() { | ||
| let block = block.into(); | ||
| let block_data = &body.basic_blocks[block]; | ||
| let Some(terminator) = &block_data.terminator else { continue; }; | ||
|
|
||
| let mut successors = Vec::new(); | ||
| for succ in terminator.successors() { | ||
| // Successors which are just a Resume are okay | ||
| if is_empty_resume(&body.basic_blocks[succ]) { | ||
| continue; | ||
| } | ||
| if body.basic_blocks.predecessors()[succ].len() != 1 { | ||
| debug!("Bailing from {block:?} because {succ:?} has multiple predecessors"); | ||
| continue 'outer; | ||
| } | ||
| successors.push(succ); | ||
| } | ||
|
|
||
| if successors.len() > SUCCESSOR_LIMIT { | ||
| debug!("Will not sink assignment, its basic block has too many successors"); | ||
| } | ||
|
|
||
| let mut local_uses = None; | ||
| for statement_idx in 0..body.basic_blocks[block].statements.len() { | ||
| let statement = &body.basic_blocks[block].statements[statement_idx]; | ||
| if let StatementKind::Assign(box (place, Rvalue::Use(Operand::Constant(_)))) = | ||
| &statement.kind | ||
| { | ||
| let local = place.local; | ||
| if !place.projection.is_empty() { | ||
| debug!("Nonempty place projection: {statement:?}"); | ||
| continue; | ||
| } | ||
| if !optimizable_locals.contains(local) { | ||
| continue; | ||
| } | ||
|
|
||
| let uses = match &local_uses { | ||
| Some(uses) => uses, | ||
| None => { | ||
| let mut visitor = CountUsesVisitor::new(); | ||
| visitor.visit_basic_block_data(block, &body.basic_blocks[block]); | ||
| local_uses = Some(visitor); | ||
| local_uses.as_ref().unwrap() | ||
| } | ||
| }; | ||
|
|
||
| // If the local dies in this block, don't propagate it | ||
| if uses.dead.contains(&local) { | ||
| continue; | ||
| } | ||
| if !uses.is_used_once(local) { | ||
| debug!("Local used elsewhere in this block: {statement:?}"); | ||
| continue; | ||
| } | ||
| if !tcx.consider_optimizing(|| format!("Sinking const assignment to {local:?}")) | ||
| { | ||
| debug!("optimization fuel exhausted"); | ||
| break 'outer; | ||
| } | ||
| debug!("Sinking const assignment to {local:?}"); | ||
| let blocks = body.basic_blocks.as_mut_preserves_cfg(); | ||
| let statement = blocks[block].statements[statement_idx].replace_nop(); | ||
|
|
||
| for succ in &successors { | ||
| let mut successor_uses = SuccessorUsesVisitor::new(local); | ||
| successor_uses.visit_basic_block_data(*succ, &blocks[*succ]); | ||
|
|
||
| if let Some(used) = successor_uses.first_use() { | ||
| if used == blocks[*succ].statements.len() { | ||
| blocks[*succ].statements.push(statement.clone()); | ||
| continue; | ||
| } | ||
| // If the first use of our local in this block is another const | ||
| // assignment to it, do not paste a new assignment right before it | ||
| // because that would just create dead code. | ||
| if let StatementKind::Assign(box ( | ||
| place, | ||
| Rvalue::Use(Operand::Constant(_)), | ||
| )) = &blocks[*succ].statements[used].kind | ||
| { | ||
| if place.local == local && place.projection.is_empty() { | ||
| continue; | ||
| } | ||
| } | ||
| blocks[*succ].statements.insert(used, statement.clone()); | ||
| } else { | ||
| blocks[*succ].statements.push(statement.clone()); | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
|
||
| fn branched_locals(body: &Body<'_>) -> BitSet<Local> { | ||
| let mut visitor = BranchedLocals { | ||
| branched: BitSet::new_empty(body.local_decls.len()), | ||
| gets_const_assign: BitSet::new_empty(body.local_decls.len()), | ||
| }; | ||
| visitor.visit_body(body); | ||
| visitor.branched.intersect(&visitor.gets_const_assign); | ||
| visitor.branched | ||
| } | ||
|
|
||
| struct BranchedLocals { | ||
| branched: BitSet<Local>, | ||
| gets_const_assign: BitSet<Local>, | ||
| } | ||
|
|
||
| impl Visitor<'_> for BranchedLocals { | ||
| fn visit_terminator(&mut self, terminator: &Terminator<'_>, _location: Location) { | ||
| let TerminatorKind::SwitchInt { discr, .. } = &terminator.kind else { return; }; | ||
| if let Some(place) = discr.place() { | ||
| self.branched.insert(place.local); | ||
| } | ||
| } | ||
|
|
||
| fn visit_statement(&mut self, statement: &Statement<'_>, _location: Location) { | ||
| if let StatementKind::Assign(box (place, Rvalue::Use(Operand::Constant(_)))) = | ||
| &statement.kind | ||
| { | ||
| if place.projection.is_empty() { | ||
| self.gets_const_assign.insert(place.local); | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
|
||
| fn is_empty_resume<'tcx>(block: &BasicBlockData<'tcx>) -> bool { | ||
| block.statements.iter().all(|s| matches!(s.kind, StatementKind::Nop)) | ||
| && block.terminator.as_ref().map(|t| &t.kind) == Some(&TerminatorKind::Resume) | ||
| } | ||
|
|
||
| struct CountUsesVisitor { | ||
| counts: FxHashMap<Local, usize>, | ||
| dead: FxHashSet<Local>, | ||
| } | ||
|
|
||
| impl CountUsesVisitor { | ||
| fn new() -> Self { | ||
| Self { counts: FxHashMap::default(), dead: FxHashSet::default() } | ||
| } | ||
|
|
||
| fn is_used_once(&self, local: Local) -> bool { | ||
| self.counts.get(&local) == Some(&1) | ||
| } | ||
| } | ||
|
|
||
| impl Visitor<'_> for CountUsesVisitor { | ||
| fn visit_local(&mut self, local: Local, context: PlaceContext, _location: Location) { | ||
| match context { | ||
| PlaceContext::NonUse(NonUseContext::StorageDead) => { | ||
| self.dead.insert(local); | ||
| } | ||
| PlaceContext::NonUse(_) => {} | ||
| PlaceContext::MutatingUse(_) | PlaceContext::NonMutatingUse(_) => { | ||
| *self.counts.entry(local).or_default() += 1; | ||
| } | ||
| }; | ||
| } | ||
| } | ||
|
|
||
| struct SuccessorUsesVisitor { | ||
| local: Local, | ||
| first_use: Option<usize>, | ||
| } | ||
|
|
||
| impl SuccessorUsesVisitor { | ||
| fn new(local: Local) -> Self { | ||
| Self { local, first_use: None } | ||
| } | ||
|
|
||
| fn first_use(&self) -> Option<usize> { | ||
| self.first_use | ||
| } | ||
| } | ||
|
|
||
| impl Visitor<'_> for SuccessorUsesVisitor { | ||
| fn visit_local(&mut self, local: Local, _context: PlaceContext, location: Location) { | ||
| if local == self.local { | ||
| match self.first_use { | ||
| None => self.first_use = Some(location.statement_index), | ||
| Some(first) => self.first_use = Some(first.min(location.statement_index)), | ||
| } | ||
| } | ||
| } | ||
| } | ||
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Perhaps this should bail out when there are too many successors. It's probably not worth creating 100 new assign statements for a 100-arm match