| //! An analysis to determine which locals require allocas and |
| //! which do not. |
| |
| use super::FunctionCx; |
| use crate::traits::*; |
| use rustc_data_structures::graph::dominators::Dominators; |
| use rustc_index::bit_set::BitSet; |
| use rustc_index::{IndexSlice, IndexVec}; |
| use rustc_middle::mir::traversal; |
| use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor}; |
| use rustc_middle::mir::{self, DefLocation, Location, TerminatorKind}; |
| use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf}; |
| |
| pub fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>( |
| fx: &FunctionCx<'a, 'tcx, Bx>, |
| ) -> BitSet<mir::Local> { |
| let mir = fx.mir; |
| let dominators = mir.basic_blocks.dominators(); |
| let locals = mir |
| .local_decls |
| .iter() |
| .map(|decl| { |
| let ty = fx.monomorphize(decl.ty); |
| let layout = fx.cx.spanned_layout_of(ty, decl.source_info.span); |
| if layout.is_zst() { |
| LocalKind::ZST |
| } else if fx.cx.is_backend_immediate(layout) || fx.cx.is_backend_scalar_pair(layout) { |
| LocalKind::Unused |
| } else { |
| LocalKind::Memory |
| } |
| }) |
| .collect(); |
| |
| let mut analyzer = LocalAnalyzer { fx, dominators, locals }; |
| |
| // Arguments get assigned to by means of the function being called |
| for arg in mir.args_iter() { |
| analyzer.assign(arg, DefLocation::Argument); |
| } |
| |
| // If there exists a local definition that dominates all uses of that local, |
| // the definition should be visited first. Traverse blocks in an order that |
| // is a topological sort of dominance partial order. |
| for (bb, data) in traversal::reverse_postorder(&mir) { |
| analyzer.visit_basic_block_data(bb, data); |
| } |
| |
| let mut non_ssa_locals = BitSet::new_empty(analyzer.locals.len()); |
| for (local, kind) in analyzer.locals.iter_enumerated() { |
| if matches!(kind, LocalKind::Memory) { |
| non_ssa_locals.insert(local); |
| } |
| } |
| |
| non_ssa_locals |
| } |
| |
| #[derive(Copy, Clone, PartialEq, Eq)] |
| enum LocalKind { |
| ZST, |
| /// A local that requires an alloca. |
| Memory, |
| /// A scalar or a scalar pair local that is neither defined nor used. |
| Unused, |
| /// A scalar or a scalar pair local with a single definition that dominates all uses. |
| SSA(DefLocation), |
| } |
| |
| struct LocalAnalyzer<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> { |
| fx: &'mir FunctionCx<'a, 'tcx, Bx>, |
| dominators: &'mir Dominators<mir::BasicBlock>, |
| locals: IndexVec<mir::Local, LocalKind>, |
| } |
| |
| impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> LocalAnalyzer<'mir, 'a, 'tcx, Bx> { |
| fn assign(&mut self, local: mir::Local, location: DefLocation) { |
| let kind = &mut self.locals[local]; |
| match *kind { |
| LocalKind::ZST => {} |
| LocalKind::Memory => {} |
| LocalKind::Unused => *kind = LocalKind::SSA(location), |
| LocalKind::SSA(_) => *kind = LocalKind::Memory, |
| } |
| } |
| |
| fn process_place( |
| &mut self, |
| place_ref: &mir::PlaceRef<'tcx>, |
| context: PlaceContext, |
| location: Location, |
| ) { |
| let cx = self.fx.cx; |
| |
| if let Some((place_base, elem)) = place_ref.last_projection() { |
| let mut base_context = if context.is_mutating_use() { |
| PlaceContext::MutatingUse(MutatingUseContext::Projection) |
| } else { |
| PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) |
| }; |
| |
| // Allow uses of projections that are ZSTs or from scalar fields. |
| let is_consume = matches!( |
| context, |
| PlaceContext::NonMutatingUse( |
| NonMutatingUseContext::Copy | NonMutatingUseContext::Move, |
| ) |
| ); |
| if is_consume { |
| let base_ty = place_base.ty(self.fx.mir, cx.tcx()); |
| let base_ty = self.fx.monomorphize(base_ty); |
| |
| // ZSTs don't require any actual memory access. |
| let elem_ty = base_ty.projection_ty(cx.tcx(), self.fx.monomorphize(elem)).ty; |
| let span = self.fx.mir.local_decls[place_ref.local].source_info.span; |
| if cx.spanned_layout_of(elem_ty, span).is_zst() { |
| return; |
| } |
| |
| if let mir::ProjectionElem::Field(..) = elem { |
| let layout = cx.spanned_layout_of(base_ty.ty, span); |
| if cx.is_backend_immediate(layout) || cx.is_backend_scalar_pair(layout) { |
| // Recurse with the same context, instead of `Projection`, |
| // potentially stopping at non-operand projections, |
| // which would trigger `not_ssa` on locals. |
| base_context = context; |
| } |
| } |
| } |
| |
| if let mir::ProjectionElem::Deref = elem { |
| // Deref projections typically only read the pointer. |
| base_context = PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy); |
| } |
| |
| self.process_place(&place_base, base_context, location); |
| // HACK(eddyb) this emulates the old `visit_projection_elem`, this |
| // entire `visit_place`-like `process_place` method should be rewritten, |
| // now that we have moved to the "slice of projections" representation. |
| if let mir::ProjectionElem::Index(local) = elem { |
| self.visit_local( |
| local, |
| PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy), |
| location, |
| ); |
| } |
| } else { |
| self.visit_local(place_ref.local, context, location); |
| } |
| } |
| } |
| |
| impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx> |
| for LocalAnalyzer<'mir, 'a, 'tcx, Bx> |
| { |
| fn visit_assign( |
| &mut self, |
| place: &mir::Place<'tcx>, |
| rvalue: &mir::Rvalue<'tcx>, |
| location: Location, |
| ) { |
| debug!("visit_assign(place={:?}, rvalue={:?})", place, rvalue); |
| |
| if let Some(local) = place.as_local() { |
| self.assign(local, DefLocation::Body(location)); |
| if self.locals[local] != LocalKind::Memory { |
| let decl_span = self.fx.mir.local_decls[local].source_info.span; |
| if !self.fx.rvalue_creates_operand(rvalue, decl_span) { |
| self.locals[local] = LocalKind::Memory; |
| } |
| } |
| } else { |
| self.visit_place(place, PlaceContext::MutatingUse(MutatingUseContext::Store), location); |
| } |
| |
| self.visit_rvalue(rvalue, location); |
| } |
| |
| fn visit_place(&mut self, place: &mir::Place<'tcx>, context: PlaceContext, location: Location) { |
| debug!("visit_place(place={:?}, context={:?})", place, context); |
| self.process_place(&place.as_ref(), context, location); |
| } |
| |
| fn visit_local(&mut self, local: mir::Local, context: PlaceContext, location: Location) { |
| match context { |
| PlaceContext::MutatingUse(MutatingUseContext::Call) |
| | PlaceContext::MutatingUse(MutatingUseContext::Yield) => { |
| self.assign(local, DefLocation::Body(location)); |
| } |
| |
| PlaceContext::NonUse(_) |
| | PlaceContext::NonMutatingUse(NonMutatingUseContext::PlaceMention) |
| | PlaceContext::MutatingUse(MutatingUseContext::Retag) => {} |
| |
| PlaceContext::NonMutatingUse( |
| NonMutatingUseContext::Copy | NonMutatingUseContext::Move, |
| ) => match &mut self.locals[local] { |
| LocalKind::ZST => {} |
| LocalKind::Memory => {} |
| LocalKind::SSA(def) if def.dominates(location, &self.dominators) => {} |
| // Reads from uninitialized variables (e.g., in dead code, after |
| // optimizations) require locals to be in (uninitialized) memory. |
| // N.B., there can be uninitialized reads of a local visited after |
| // an assignment to that local, if they happen on disjoint paths. |
| kind @ (LocalKind::Unused | LocalKind::SSA(_)) => { |
| *kind = LocalKind::Memory; |
| } |
| }, |
| |
| PlaceContext::MutatingUse( |
| MutatingUseContext::Store |
| | MutatingUseContext::Deinit |
| | MutatingUseContext::SetDiscriminant |
| | MutatingUseContext::AsmOutput |
| | MutatingUseContext::Borrow |
| | MutatingUseContext::AddressOf |
| | MutatingUseContext::Projection, |
| ) |
| | PlaceContext::NonMutatingUse( |
| NonMutatingUseContext::Inspect |
| | NonMutatingUseContext::SharedBorrow |
| | NonMutatingUseContext::FakeBorrow |
| | NonMutatingUseContext::AddressOf |
| | NonMutatingUseContext::Projection, |
| ) => { |
| self.locals[local] = LocalKind::Memory; |
| } |
| |
| PlaceContext::MutatingUse(MutatingUseContext::Drop) => { |
| let kind = &mut self.locals[local]; |
| if *kind != LocalKind::Memory { |
| let ty = self.fx.mir.local_decls[local].ty; |
| let ty = self.fx.monomorphize(ty); |
| if self.fx.cx.type_needs_drop(ty) { |
| // Only need the place if we're actually dropping it. |
| *kind = LocalKind::Memory; |
| } |
| } |
| } |
| } |
| } |
| } |
| |
| #[derive(Copy, Clone, Debug, PartialEq, Eq)] |
| pub enum CleanupKind { |
| NotCleanup, |
| Funclet, |
| Internal { funclet: mir::BasicBlock }, |
| } |
| |
| impl CleanupKind { |
| pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> { |
| match self { |
| CleanupKind::NotCleanup => None, |
| CleanupKind::Funclet => Some(for_bb), |
| CleanupKind::Internal { funclet } => Some(funclet), |
| } |
| } |
| } |
| |
| /// MSVC requires unwinding code to be split to a tree of *funclets*, where each funclet can only |
| /// branch to itself or to its parent. Luckily, the code we generates matches this pattern. |
| /// Recover that structure in an analyze pass. |
| pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKind> { |
| fn discover_masters<'tcx>( |
| result: &mut IndexSlice<mir::BasicBlock, CleanupKind>, |
| mir: &mir::Body<'tcx>, |
| ) { |
| for (bb, data) in mir.basic_blocks.iter_enumerated() { |
| match data.terminator().kind { |
| TerminatorKind::Goto { .. } |
| | TerminatorKind::UnwindResume |
| | TerminatorKind::UnwindTerminate(_) |
| | TerminatorKind::Return |
| | TerminatorKind::CoroutineDrop |
| | TerminatorKind::Unreachable |
| | TerminatorKind::SwitchInt { .. } |
| | TerminatorKind::Yield { .. } |
| | TerminatorKind::FalseEdge { .. } |
| | TerminatorKind::FalseUnwind { .. } => { /* nothing to do */ } |
| TerminatorKind::Call { unwind, .. } |
| | TerminatorKind::InlineAsm { unwind, .. } |
| | TerminatorKind::Assert { unwind, .. } |
| | TerminatorKind::Drop { unwind, .. } => { |
| if let mir::UnwindAction::Cleanup(unwind) = unwind { |
| debug!( |
| "cleanup_kinds: {:?}/{:?} registering {:?} as funclet", |
| bb, data, unwind |
| ); |
| result[unwind] = CleanupKind::Funclet; |
| } |
| } |
| } |
| } |
| } |
| |
| fn propagate<'tcx>( |
| result: &mut IndexSlice<mir::BasicBlock, CleanupKind>, |
| mir: &mir::Body<'tcx>, |
| ) { |
| let mut funclet_succs = IndexVec::from_elem(None, &mir.basic_blocks); |
| |
| let mut set_successor = |funclet: mir::BasicBlock, succ| match funclet_succs[funclet] { |
| ref mut s @ None => { |
| debug!("set_successor: updating successor of {:?} to {:?}", funclet, succ); |
| *s = Some(succ); |
| } |
| Some(s) => { |
| if s != succ { |
| span_bug!( |
| mir.span, |
| "funclet {:?} has 2 parents - {:?} and {:?}", |
| funclet, |
| s, |
| succ |
| ); |
| } |
| } |
| }; |
| |
| for (bb, data) in traversal::reverse_postorder(mir) { |
| let funclet = match result[bb] { |
| CleanupKind::NotCleanup => continue, |
| CleanupKind::Funclet => bb, |
| CleanupKind::Internal { funclet } => funclet, |
| }; |
| |
| debug!( |
| "cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}", |
| bb, data, result[bb], funclet |
| ); |
| |
| for succ in data.terminator().successors() { |
| let kind = result[succ]; |
| debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}", funclet, succ, kind); |
| match kind { |
| CleanupKind::NotCleanup => { |
| result[succ] = CleanupKind::Internal { funclet }; |
| } |
| CleanupKind::Funclet => { |
| if funclet != succ { |
| set_successor(funclet, succ); |
| } |
| } |
| CleanupKind::Internal { funclet: succ_funclet } => { |
| if funclet != succ_funclet { |
| // `succ` has 2 different funclet going into it, so it must |
| // be a funclet by itself. |
| |
| debug!( |
| "promoting {:?} to a funclet and updating {:?}", |
| succ, succ_funclet |
| ); |
| result[succ] = CleanupKind::Funclet; |
| set_successor(succ_funclet, succ); |
| set_successor(funclet, succ); |
| } |
| } |
| } |
| } |
| } |
| } |
| |
| let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, &mir.basic_blocks); |
| |
| discover_masters(&mut result, mir); |
| propagate(&mut result, mir); |
| debug!("cleanup_kinds: result={:?}", result); |
| result |
| } |
