1 //! See the docs for [`RenameReturnPlace`].
3 use rustc_hir::Mutability;
4 use rustc_index::bit_set::HybridBitSet;
5 use rustc_middle::mir::visit::{MutVisitor, NonUseContext, PlaceContext, Visitor};
6 use rustc_middle::mir::{self, BasicBlock, Local, Location};
7 use rustc_middle::ty::TyCtxt;
11 /// This pass looks for MIR that always copies the same local into the return place and eliminates
12 /// the copy by renaming all uses of that local to `_0`.
14 /// This allows LLVM to perform an optimization similar to the named return value optimization
15 /// (NRVO) that is guaranteed in C++. This avoids a stack allocation and `memcpy` for the
16 /// relatively common pattern of allocating a buffer on the stack, mutating it, and returning it by
20 /// fn foo(init: fn(&mut [u8; 1024])) -> [u8; 1024] {
21 /// let mut buf = [0; 1024];
27 /// For now, this pass is very simple and only capable of eliminating a single copy. A more general
28 /// version of copy propagation, such as the one based on non-overlapping live ranges in [#47954] and
29 /// [#71003], could yield even more benefits.
31 /// [#47954]: https://github.com/rust-lang/rust/pull/47954
32 /// [#71003]: https://github.com/rust-lang/rust/pull/71003
33 pub struct RenameReturnPlace;
35 impl<'tcx> MirPass<'tcx> for RenameReturnPlace {
36 fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
37 sess.mir_opt_level() > 0
40 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut mir::Body<'tcx>) {
41 let def_id = body.source.def_id();
42 let Some(returned_local) = local_eligible_for_nrvo(body) else {
43 debug!("`{:?}` was ineligible for NRVO", def_id);
47 if !tcx.consider_optimizing(|| format!("RenameReturnPlace {:?}", def_id)) {
52 "`{:?}` was eligible for NRVO, making {:?} the return place",
53 def_id, returned_local
56 RenameToReturnPlace { tcx, to_rename: returned_local }.visit_body_preserves_cfg(body);
58 // Clean up the `NOP`s we inserted for statements made useless by our renaming.
59 for block_data in body.basic_blocks.as_mut_preserves_cfg() {
60 block_data.statements.retain(|stmt| stmt.kind != mir::StatementKind::Nop);
63 // Overwrite the debuginfo of `_0` with that of the renamed local.
64 let (renamed_decl, ret_decl) =
65 body.local_decls.pick2_mut(returned_local, mir::RETURN_PLACE);
67 // Sometimes, the return place is assigned a local of a different but coercible type, for
68 // example `&mut T` instead of `&T`. Overwriting the `LocalInfo` for the return place means
69 // its type may no longer match the return type of its function. This doesn't cause a
70 // problem in codegen because these two types are layout-compatible, but may be unexpected.
71 debug!("_0: {:?} = {:?}: {:?}", ret_decl.ty, returned_local, renamed_decl.ty);
72 ret_decl.clone_from(renamed_decl);
74 // The return place is always mutable.
75 ret_decl.mutability = Mutability::Mut;
79 /// MIR that is eligible for the NRVO must fulfill two conditions:
80 /// 1. The return place must not be read prior to the `Return` terminator.
81 /// 2. A simple assignment of a whole local to the return place (e.g., `_0 = _1`) must be the
82 /// only definition of the return place reaching the `Return` terminator.
84 /// If the MIR fulfills both these conditions, this function returns the `Local` that is assigned
85 /// to the return place along all possible paths through the control-flow graph.
86 fn local_eligible_for_nrvo(body: &mut mir::Body<'_>) -> Option<Local> {
87 if IsReturnPlaceRead::run(body) {
91 let mut copied_to_return_place = None;
92 for block in body.basic_blocks.indices() {
93 // Look for blocks with a `Return` terminator.
94 if !matches!(body[block].terminator().kind, mir::TerminatorKind::Return) {
98 // Look for an assignment of a single local to the return place prior to the `Return`.
99 let returned_local = find_local_assigned_to_return_place(block, body)?;
100 match body.local_kind(returned_local) {
101 // FIXME: Can we do this for arguments as well?
102 mir::LocalKind::Arg => return None,
104 mir::LocalKind::ReturnPointer => bug!("Return place was assigned to itself?"),
105 mir::LocalKind::Var | mir::LocalKind::Temp => {}
108 // If multiple different locals are copied to the return place. We can't pick a
109 // single one to rename.
110 if copied_to_return_place.map_or(false, |old| old != returned_local) {
114 copied_to_return_place = Some(returned_local);
117 copied_to_return_place
120 fn find_local_assigned_to_return_place(
122 body: &mut mir::Body<'_>,
124 let mut block = start;
125 let mut seen = HybridBitSet::new_empty(body.basic_blocks.len());
127 // Iterate as long as `block` has exactly one predecessor that we have not yet visited.
128 while seen.insert(block) {
129 trace!("Looking for assignments to `_0` in {:?}", block);
131 let local = body[block].statements.iter().rev().find_map(as_local_assigned_to_return_place);
136 match body.basic_blocks.predecessors()[block].as_slice() {
137 &[pred] => block = pred,
145 // If this statement is an assignment of an unprojected local to the return place,
146 // return that local.
147 fn as_local_assigned_to_return_place(stmt: &mir::Statement<'_>) -> Option<Local> {
148 if let mir::StatementKind::Assign(box (lhs, rhs)) = &stmt.kind {
149 if lhs.as_local() == Some(mir::RETURN_PLACE) {
150 if let mir::Rvalue::Use(mir::Operand::Copy(rhs) | mir::Operand::Move(rhs)) = rhs {
151 return rhs.as_local();
159 struct RenameToReturnPlace<'tcx> {
164 /// Replaces all uses of `self.to_rename` with `_0`.
165 impl<'tcx> MutVisitor<'tcx> for RenameToReturnPlace<'tcx> {
166 fn tcx(&self) -> TyCtxt<'tcx> {
170 fn visit_statement(&mut self, stmt: &mut mir::Statement<'tcx>, loc: Location) {
171 // Remove assignments of the local being replaced to the return place, since it is now the
174 if as_local_assigned_to_return_place(stmt) == Some(self.to_rename) {
175 stmt.kind = mir::StatementKind::Nop;
179 // Remove storage annotations for the local being replaced:
181 if let mir::StatementKind::StorageLive(local) | mir::StatementKind::StorageDead(local) =
184 if local == self.to_rename {
185 stmt.kind = mir::StatementKind::Nop;
190 self.super_statement(stmt, loc)
193 fn visit_terminator(&mut self, terminator: &mut mir::Terminator<'tcx>, loc: Location) {
194 // Ignore the implicit "use" of the return place in a `Return` statement.
195 if let mir::TerminatorKind::Return = terminator.kind {
199 self.super_terminator(terminator, loc);
202 fn visit_local(&mut self, l: &mut Local, ctxt: PlaceContext, _: Location) {
203 if *l == mir::RETURN_PLACE {
204 assert_eq!(ctxt, PlaceContext::NonUse(NonUseContext::VarDebugInfo));
205 } else if *l == self.to_rename {
206 *l = mir::RETURN_PLACE;
211 struct IsReturnPlaceRead(bool);
213 impl IsReturnPlaceRead {
214 fn run(body: &mir::Body<'_>) -> bool {
215 let mut vis = IsReturnPlaceRead(false);
216 vis.visit_body(body);
221 impl<'tcx> Visitor<'tcx> for IsReturnPlaceRead {
222 fn visit_local(&mut self, l: Local, ctxt: PlaceContext, _: Location) {
223 if l == mir::RETURN_PLACE && ctxt.is_use() && !ctxt.is_place_assignment() {
228 fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, loc: Location) {
229 // Ignore the implicit "use" of the return place in a `Return` statement.
230 if let mir::TerminatorKind::Return = terminator.kind {
234 self.super_terminator(terminator, loc);