| //! A pass that promotes borrows of constant rvalues. |
| //! |
| //! The rvalues considered constant are trees of temps, |
| //! each with exactly one initialization, and holding |
| //! a constant value with no interior mutability. |
| //! They are placed into a new MIR constant body in |
| //! `promoted` and the borrow rvalue is replaced with |
| //! a `Literal::Promoted` using the index into `promoted` |
| //! of that constant MIR. |
| //! |
| //! This pass assumes that every use is dominated by an |
| //! initialization and can otherwise silence errors, if |
| //! move analysis runs after promotion on broken MIR. |
| |
| use rustc::hir::def_id::DefId; |
| use rustc::mir::*; |
| use rustc::mir::visit::{PlaceContext, MutatingUseContext, MutVisitor, Visitor}; |
| use rustc::mir::traversal::ReversePostorder; |
| use rustc::ty::subst::InternalSubsts; |
| use rustc::ty::TyCtxt; |
| use syntax_pos::Span; |
| |
| use rustc_data_structures::indexed_vec::{IndexVec, Idx}; |
| |
| use std::{iter, mem, usize}; |
| |
| /// State of a temporary during collection and promotion. |
| #[derive(Copy, Clone, PartialEq, Eq, Debug)] |
| pub enum TempState { |
| /// No references to this temp. |
| Undefined, |
| /// One direct assignment and any number of direct uses. |
| /// A borrow of this temp is promotable if the assigned |
| /// value is qualified as constant. |
| Defined { |
| location: Location, |
| uses: usize |
| }, |
| /// Any other combination of assignments/uses. |
| Unpromotable, |
| /// This temp was part of an rvalue which got extracted |
| /// during promotion and needs cleanup. |
| PromotedOut |
| } |
| |
| impl TempState { |
| pub fn is_promotable(&self) -> bool { |
| debug!("is_promotable: self={:?}", self); |
| if let TempState::Defined { .. } = *self { |
| true |
| } else { |
| false |
| } |
| } |
| } |
| |
| /// A "root candidate" for promotion, which will become the |
| /// returned value in a promoted MIR, unless it's a subset |
| /// of a larger candidate. |
| #[derive(Debug)] |
| pub enum Candidate { |
| /// Borrow of a constant temporary. |
| Ref(Location), |
| |
| /// Promotion of the `x` in `[x; 32]`. |
| Repeat(Location), |
| |
| /// Currently applied to function calls where the callee has the unstable |
| /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle |
| /// intrinsic. The intrinsic requires the arguments are indeed constant and |
| /// the attribute currently provides the semantic requirement that arguments |
| /// must be constant. |
| Argument { bb: BasicBlock, index: usize }, |
| } |
| |
| struct TempCollector<'tcx> { |
| temps: IndexVec<Local, TempState>, |
| span: Span, |
| body: &'tcx Body<'tcx>, |
| } |
| |
| impl<'tcx> Visitor<'tcx> for TempCollector<'tcx> { |
| fn visit_local(&mut self, |
| &index: &Local, |
| context: PlaceContext, |
| location: Location) { |
| debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location); |
| // We're only interested in temporaries and the return place |
| match self.body.local_kind(index) { |
| | LocalKind::Temp |
| | LocalKind::ReturnPointer |
| => {}, |
| | LocalKind::Arg |
| | LocalKind::Var |
| => return, |
| } |
| |
| // Ignore drops, if the temp gets promoted, |
| // then it's constant and thus drop is noop. |
| // Non-uses are also irrelevent. |
| if context.is_drop() || !context.is_use() { |
| debug!( |
| "visit_local: context.is_drop={:?} context.is_use={:?}", |
| context.is_drop(), context.is_use(), |
| ); |
| return; |
| } |
| |
| let temp = &mut self.temps[index]; |
| debug!("visit_local: temp={:?}", temp); |
| if *temp == TempState::Undefined { |
| match context { |
| PlaceContext::MutatingUse(MutatingUseContext::Store) | |
| PlaceContext::MutatingUse(MutatingUseContext::Call) => { |
| *temp = TempState::Defined { |
| location, |
| uses: 0 |
| }; |
| return; |
| } |
| _ => { /* mark as unpromotable below */ } |
| } |
| } else if let TempState::Defined { ref mut uses, .. } = *temp { |
| // We always allow borrows, even mutable ones, as we need |
| // to promote mutable borrows of some ZSTs e.g., `&mut []`. |
| let allowed_use = context.is_borrow() || context.is_nonmutating_use(); |
| debug!("visit_local: allowed_use={:?}", allowed_use); |
| if allowed_use { |
| *uses += 1; |
| return; |
| } |
| /* mark as unpromotable below */ |
| } |
| *temp = TempState::Unpromotable; |
| } |
| |
| fn visit_source_info(&mut self, source_info: &SourceInfo) { |
| self.span = source_info.span; |
| } |
| } |
| |
| pub fn collect_temps(body: &Body<'_>, |
| rpo: &mut ReversePostorder<'_, '_>) -> IndexVec<Local, TempState> { |
| let mut collector = TempCollector { |
| temps: IndexVec::from_elem(TempState::Undefined, &body.local_decls), |
| span: body.span, |
| body, |
| }; |
| for (bb, data) in rpo { |
| collector.visit_basic_block_data(bb, data); |
| } |
| collector.temps |
| } |
| |
| struct Promoter<'a, 'tcx> { |
| tcx: TyCtxt<'tcx>, |
| source: &'a mut Body<'tcx>, |
| promoted: Body<'tcx>, |
| temps: &'a mut IndexVec<Local, TempState>, |
| |
| /// If true, all nested temps are also kept in the |
| /// source MIR, not moved to the promoted MIR. |
| keep_original: bool, |
| } |
| |
| impl<'a, 'tcx> Promoter<'a, 'tcx> { |
| fn new_block(&mut self) -> BasicBlock { |
| let span = self.promoted.span; |
| self.promoted.basic_blocks_mut().push(BasicBlockData { |
| statements: vec![], |
| terminator: Some(Terminator { |
| source_info: SourceInfo { |
| span, |
| scope: OUTERMOST_SOURCE_SCOPE |
| }, |
| kind: TerminatorKind::Return |
| }), |
| is_cleanup: false |
| }) |
| } |
| |
| fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) { |
| let last = self.promoted.basic_blocks().last().unwrap(); |
| let data = &mut self.promoted[last]; |
| data.statements.push(Statement { |
| source_info: SourceInfo { |
| span, |
| scope: OUTERMOST_SOURCE_SCOPE |
| }, |
| kind: StatementKind::Assign(box(Place::from(dest), rvalue)) |
| }); |
| } |
| |
| /// Copies the initialization of this temp to the |
| /// promoted MIR, recursing through temps. |
| fn promote_temp(&mut self, temp: Local) -> Local { |
| let old_keep_original = self.keep_original; |
| let loc = match self.temps[temp] { |
| TempState::Defined { location, uses } if uses > 0 => { |
| if uses > 1 { |
| self.keep_original = true; |
| } |
| location |
| } |
| state => { |
| span_bug!(self.promoted.span, "{:?} not promotable: {:?}", |
| temp, state); |
| } |
| }; |
| if !self.keep_original { |
| self.temps[temp] = TempState::PromotedOut; |
| } |
| |
| let no_stmts = self.source[loc.block].statements.len(); |
| let new_temp = self.promoted.local_decls.push( |
| LocalDecl::new_temp(self.source.local_decls[temp].ty, |
| self.source.local_decls[temp].source_info.span)); |
| |
| debug!("promote({:?} @ {:?}/{:?}, {:?})", |
| temp, loc, no_stmts, self.keep_original); |
| |
| // First, take the Rvalue or Call out of the source MIR, |
| // or duplicate it, depending on keep_original. |
| if loc.statement_index < no_stmts { |
| let (mut rvalue, source_info) = { |
| let statement = &mut self.source[loc.block].statements[loc.statement_index]; |
| let rhs = match statement.kind { |
| StatementKind::Assign(box(_, ref mut rhs)) => rhs, |
| _ => { |
| span_bug!(statement.source_info.span, "{:?} is not an assignment", |
| statement); |
| } |
| }; |
| |
| (if self.keep_original { |
| rhs.clone() |
| } else { |
| let unit = Rvalue::Aggregate(box AggregateKind::Tuple, vec![]); |
| mem::replace(rhs, unit) |
| }, statement.source_info) |
| }; |
| |
| self.visit_rvalue(&mut rvalue, loc); |
| self.assign(new_temp, rvalue, source_info.span); |
| } else { |
| let terminator = if self.keep_original { |
| self.source[loc.block].terminator().clone() |
| } else { |
| let terminator = self.source[loc.block].terminator_mut(); |
| let target = match terminator.kind { |
| TerminatorKind::Call { destination: Some((_, target)), .. } => target, |
| ref kind => { |
| span_bug!(terminator.source_info.span, "{:?} not promotable", kind); |
| } |
| }; |
| Terminator { |
| source_info: terminator.source_info, |
| kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto { |
| target, |
| }) |
| } |
| }; |
| |
| match terminator.kind { |
| TerminatorKind::Call { mut func, mut args, from_hir_call, .. } => { |
| self.visit_operand(&mut func, loc); |
| for arg in &mut args { |
| self.visit_operand(arg, loc); |
| } |
| |
| let last = self.promoted.basic_blocks().last().unwrap(); |
| let new_target = self.new_block(); |
| |
| *self.promoted[last].terminator_mut() = Terminator { |
| kind: TerminatorKind::Call { |
| func, |
| args, |
| cleanup: None, |
| destination: Some( |
| (Place::from(new_temp), new_target) |
| ), |
| from_hir_call, |
| }, |
| ..terminator |
| }; |
| } |
| ref kind => { |
| span_bug!(terminator.source_info.span, "{:?} not promotable", kind); |
| } |
| }; |
| }; |
| |
| self.keep_original = old_keep_original; |
| new_temp |
| } |
| |
| fn promote_candidate( |
| mut self, |
| def_id: DefId, |
| candidate: Candidate, |
| next_promoted_id: usize, |
| ) -> Option<Body<'tcx>> { |
| let mut operand = { |
| let promoted = &mut self.promoted; |
| let promoted_id = Promoted::new(next_promoted_id); |
| let tcx = self.tcx; |
| let mut promoted_place = |ty, span| { |
| promoted.span = span; |
| promoted.local_decls[RETURN_PLACE] = LocalDecl::new_return_place(ty, span); |
| Place { |
| base: PlaceBase::Static(box Static { |
| kind: |
| StaticKind::Promoted( |
| promoted_id, |
| InternalSubsts::identity_for_item(tcx, def_id), |
| ), |
| ty, |
| def_id, |
| }), |
| projection: box [], |
| } |
| }; |
| let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut(); |
| match candidate { |
| Candidate::Ref(loc) => { |
| let ref mut statement = blocks[loc.block].statements[loc.statement_index]; |
| match statement.kind { |
| StatementKind::Assign(box(_, Rvalue::Ref(_, _, ref mut place))) => { |
| // Use the underlying local for this (necessarily interior) borrow. |
| let ty = place.base.ty(local_decls).ty; |
| let span = statement.source_info.span; |
| |
| Operand::Move(Place { |
| base: mem::replace( |
| &mut place.base, |
| promoted_place(ty, span).base, |
| ), |
| projection: box [], |
| }) |
| } |
| _ => bug!() |
| } |
| } |
| Candidate::Repeat(loc) => { |
| let ref mut statement = blocks[loc.block].statements[loc.statement_index]; |
| match statement.kind { |
| StatementKind::Assign(box(_, Rvalue::Repeat(ref mut operand, _))) => { |
| let ty = operand.ty(local_decls, self.tcx); |
| let span = statement.source_info.span; |
| mem::replace( |
| operand, |
| Operand::Copy(promoted_place(ty, span)) |
| ) |
| } |
| _ => bug!() |
| } |
| }, |
| Candidate::Argument { bb, index } => { |
| let terminator = blocks[bb].terminator_mut(); |
| match terminator.kind { |
| TerminatorKind::Call { ref mut args, .. } => { |
| let ty = args[index].ty(local_decls, self.tcx); |
| let span = terminator.source_info.span; |
| let operand = Operand::Copy(promoted_place(ty, span)); |
| mem::replace(&mut args[index], operand) |
| } |
| // We expected a `TerminatorKind::Call` for which we'd like to promote an |
| // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but |
| // we are seeing a `Goto`. That means that the `promote_temps` method |
| // already promoted this call away entirely. This case occurs when calling |
| // a function requiring a constant argument and as that constant value |
| // providing a value whose computation contains another call to a function |
| // requiring a constant argument. |
| TerminatorKind::Goto { .. } => return None, |
| _ => bug!() |
| } |
| } |
| } |
| }; |
| |
| assert_eq!(self.new_block(), START_BLOCK); |
| self.visit_operand(&mut operand, Location { |
| block: BasicBlock::new(0), |
| statement_index: usize::MAX |
| }); |
| |
| let span = self.promoted.span; |
| self.assign(RETURN_PLACE, Rvalue::Use(operand), span); |
| Some(self.promoted) |
| } |
| } |
| |
| /// Replaces all temporaries with their promoted counterparts. |
| impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> { |
| fn visit_local(&mut self, |
| local: &mut Local, |
| _: PlaceContext, |
| _: Location) { |
| if self.source.local_kind(*local) == LocalKind::Temp { |
| *local = self.promote_temp(*local); |
| } |
| } |
| } |
| |
| pub fn promote_candidates<'tcx>( |
| def_id: DefId, |
| body: &mut Body<'tcx>, |
| tcx: TyCtxt<'tcx>, |
| mut temps: IndexVec<Local, TempState>, |
| candidates: Vec<Candidate>, |
| ) -> IndexVec<Promoted, Body<'tcx>> { |
| // Visit candidates in reverse, in case they're nested. |
| debug!("promote_candidates({:?})", candidates); |
| |
| let mut promotions = IndexVec::new(); |
| |
| for candidate in candidates.into_iter().rev() { |
| match candidate { |
| Candidate::Repeat(Location { block, statement_index }) | |
| Candidate::Ref(Location { block, statement_index }) => { |
| match body[block].statements[statement_index].kind { |
| StatementKind::Assign(box(Place { |
| base: PlaceBase::Local(local), |
| projection: box [], |
| }, _)) => { |
| if temps[local] == TempState::PromotedOut { |
| // Already promoted. |
| continue; |
| } |
| } |
| _ => {} |
| } |
| } |
| Candidate::Argument { .. } => {} |
| } |
| |
| |
| // Declare return place local so that `mir::Body::new` doesn't complain. |
| let initial_locals = iter::once( |
| LocalDecl::new_return_place(tcx.types.never, body.span) |
| ).collect(); |
| |
| let promoter = Promoter { |
| promoted: Body::new( |
| IndexVec::new(), |
| // FIXME: maybe try to filter this to avoid blowing up |
| // memory usage? |
| body.source_scopes.clone(), |
| body.source_scope_local_data.clone(), |
| None, |
| initial_locals, |
| IndexVec::new(), |
| 0, |
| vec![], |
| body.span, |
| vec![], |
| ), |
| tcx, |
| source: body, |
| temps: &mut temps, |
| keep_original: false |
| }; |
| |
| //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice |
| if let Some(promoted) = promoter.promote_candidate(def_id, candidate, promotions.len()) { |
| promotions.push(promoted); |
| } |
| } |
| |
| // Eliminate assignments to, and drops of promoted temps. |
| let promoted = |index: Local| temps[index] == TempState::PromotedOut; |
| for block in body.basic_blocks_mut() { |
| block.statements.retain(|statement| { |
| match statement.kind { |
| StatementKind::Assign(box(Place { |
| base: PlaceBase::Local(index), |
| projection: box [], |
| }, _)) | |
| StatementKind::StorageLive(index) | |
| StatementKind::StorageDead(index) => { |
| !promoted(index) |
| } |
| _ => true |
| } |
| }); |
| let terminator = block.terminator_mut(); |
| match terminator.kind { |
| TerminatorKind::Drop { location: Place { |
| base: PlaceBase::Local(index), |
| projection: box [], |
| }, target, .. } => { |
| if promoted(index) { |
| terminator.kind = TerminatorKind::Goto { |
| target, |
| }; |
| } |
| } |
| _ => {} |
| } |
| } |
| |
| promotions |
| } |