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[mir-opt] Allow debuginfo to be generated for a constant or a Place
Prior to this commit, debuginfo was always generated by mapping a name to a Place. This has the side-effect that `SimplifyLocals` cannot remove locals that are only used for debuginfo because their other uses have been const-propagated. To allow these locals to be removed, we now allow debuginfo to point to a constant value. The `ConstProp` pass detects when debuginfo points to a local with a known constant value and replaces it with the value. This allows the later `SimplifyLocals` pass to remove the local.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,98 @@ | ||
| //! Finds locals which are assigned once to a const and unused except for debuginfo and converts | ||
| //! their debuginfo to use the const directly, allowing the local to be removed. | ||
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| use rustc_middle::{ | ||
| mir::{ | ||
| visit::{PlaceContext, Visitor}, | ||
| Body, Constant, Local, Location, Operand, Rvalue, StatementKind, VarDebugInfoContents, | ||
| }, | ||
| ty::TyCtxt, | ||
| }; | ||
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| use crate::transform::MirPass; | ||
| use rustc_index::{bit_set::BitSet, vec::IndexVec}; | ||
|
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| pub struct ConstDebugInfo; | ||
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| impl<'tcx> MirPass<'tcx> for ConstDebugInfo { | ||
| fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { | ||
| trace!("running ConstDebugInfo on {:?}", body.source); | ||
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| for (local, constant) in find_optimization_oportunities(body) { | ||
| for debuginfo in &mut body.var_debug_info { | ||
| if let VarDebugInfoContents::Place(p) = debuginfo.value { | ||
| if p.local == local && p.projection.is_empty() { | ||
| trace!( | ||
| "changing debug info for {:?} from place {:?} to constant {:?}", | ||
| debuginfo.name, | ||
| p, | ||
| constant | ||
| ); | ||
| debuginfo.value = VarDebugInfoContents::Const(constant); | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
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| struct LocalUseVisitor { | ||
| local_mutating_uses: IndexVec<Local, u8>, | ||
| local_assignment_locations: IndexVec<Local, Option<Location>>, | ||
| } | ||
|
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| fn find_optimization_oportunities<'tcx>(body: &Body<'tcx>) -> Vec<(Local, Constant<'tcx>)> { | ||
| let mut visitor = LocalUseVisitor { | ||
| local_mutating_uses: IndexVec::from_elem(0, &body.local_decls), | ||
| local_assignment_locations: IndexVec::from_elem(None, &body.local_decls), | ||
| }; | ||
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| visitor.visit_body(body); | ||
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| let mut locals_to_debuginfo = BitSet::new_empty(body.local_decls.len()); | ||
| for debuginfo in &body.var_debug_info { | ||
| if let VarDebugInfoContents::Place(p) = debuginfo.value { | ||
| if let Some(l) = p.as_local() { | ||
| locals_to_debuginfo.insert(l); | ||
| } | ||
| } | ||
| } | ||
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| let mut eligable_locals = Vec::new(); | ||
| for (local, mutating_uses) in visitor.local_mutating_uses.drain_enumerated(..) { | ||
| if mutating_uses != 1 || !locals_to_debuginfo.contains(local) { | ||
| continue; | ||
| } | ||
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| if let Some(location) = visitor.local_assignment_locations[local] { | ||
| let bb = &body[location.block]; | ||
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| // The value is assigned as the result of a call, not a constant | ||
| if bb.statements.len() == location.statement_index { | ||
| continue; | ||
| } | ||
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| if let StatementKind::Assign(box (p, Rvalue::Use(Operand::Constant(box c)))) = | ||
| &bb.statements[location.statement_index].kind | ||
| { | ||
| if let Some(local) = p.as_local() { | ||
| eligable_locals.push((local, *c)); | ||
| } | ||
| } | ||
| } | ||
| } | ||
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| eligable_locals | ||
| } | ||
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| impl<'tcx> Visitor<'tcx> for LocalUseVisitor { | ||
| fn visit_local(&mut self, local: &Local, context: PlaceContext, location: Location) { | ||
| if context.is_mutating_use() { | ||
| self.local_mutating_uses[*local] += 1; | ||
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| if context.is_place_assignment() { | ||
| self.local_assignment_locations[*local] = Some(location); | ||
| } | ||
| } | ||
| } | ||
| } |
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