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//! See the docs for [`RenameReturnPlace`].
use rustc_hir::Mutability;
use rustc_index::bit_set::BitSet;
use rustc_middle::bug;
use rustc_middle::mir::visit::{MutVisitor, NonUseContext, PlaceContext, Visitor};
use rustc_middle::mir::{self, BasicBlock, Local, Location};
use rustc_middle::ty::TyCtxt;
use tracing::{debug, trace};
use crate::MirPass;
/// This pass looks for MIR that always copies the same local into the return place and eliminates
/// the copy by renaming all uses of that local to `_0`.
///
/// This allows LLVM to perform an optimization similar to the named return value optimization
/// (NRVO) that is guaranteed in C++. This avoids a stack allocation and `memcpy` for the
/// relatively common pattern of allocating a buffer on the stack, mutating it, and returning it by
/// value like so:
///
/// ```rust
/// fn foo(init: fn(&mut [u8; 1024])) -> [u8; 1024] {
/// let mut buf = [0; 1024];
/// init(&mut buf);
/// buf
/// }
/// ```
///
/// For now, this pass is very simple and only capable of eliminating a single copy. A more general
/// version of copy propagation, such as the one based on non-overlapping live ranges in [#47954] and
/// [#71003], could yield even more benefits.
///
/// [#47954]: https://github.com/rust-lang/rust/pull/47954
/// [#71003]: https://github.com/rust-lang/rust/pull/71003
pub struct RenameReturnPlace;
impl<'tcx> MirPass<'tcx> for RenameReturnPlace {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
// unsound: #111005
sess.mir_opt_level() > 0 && sess.opts.unstable_opts.unsound_mir_opts
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut mir::Body<'tcx>) {
let def_id = body.source.def_id();
let Some(returned_local) = local_eligible_for_nrvo(body) else {
debug!("`{:?}` was ineligible for NRVO", def_id);
return;
};
if !tcx.consider_optimizing(|| format!("RenameReturnPlace {def_id:?}")) {
return;
}
debug!(
"`{:?}` was eligible for NRVO, making {:?} the return place",
def_id, returned_local
);
RenameToReturnPlace { tcx, to_rename: returned_local }.visit_body_preserves_cfg(body);
// Clean up the `NOP`s we inserted for statements made useless by our renaming.
for block_data in body.basic_blocks.as_mut_preserves_cfg() {
block_data.statements.retain(|stmt| stmt.kind != mir::StatementKind::Nop);
}
// Overwrite the debuginfo of `_0` with that of the renamed local.
let (renamed_decl, ret_decl) =
body.local_decls.pick2_mut(returned_local, mir::RETURN_PLACE);
// Sometimes, the return place is assigned a local of a different but coercible type, for
// example `&mut T` instead of `&T`. Overwriting the `LocalInfo` for the return place means
// its type may no longer match the return type of its function. This doesn't cause a
// problem in codegen because these two types are layout-compatible, but may be unexpected.
debug!("_0: {:?} = {:?}: {:?}", ret_decl.ty, returned_local, renamed_decl.ty);
ret_decl.clone_from(renamed_decl);
// The return place is always mutable.
ret_decl.mutability = Mutability::Mut;
}
}
/// MIR that is eligible for the NRVO must fulfill two conditions:
/// 1. The return place must not be read prior to the `Return` terminator.
/// 2. A simple assignment of a whole local to the return place (e.g., `_0 = _1`) must be the
/// only definition of the return place reaching the `Return` terminator.
///
/// If the MIR fulfills both these conditions, this function returns the `Local` that is assigned
/// to the return place along all possible paths through the control-flow graph.
fn local_eligible_for_nrvo(body: &mir::Body<'_>) -> Option<Local> {
if IsReturnPlaceRead::run(body) {
return None;
}
let mut copied_to_return_place = None;
for block in body.basic_blocks.indices() {
// Look for blocks with a `Return` terminator.
if !matches!(body[block].terminator().kind, mir::TerminatorKind::Return) {
continue;
}
// Look for an assignment of a single local to the return place prior to the `Return`.
let returned_local = find_local_assigned_to_return_place(block, body)?;
match body.local_kind(returned_local) {
// FIXME: Can we do this for arguments as well?
mir::LocalKind::Arg => return None,
mir::LocalKind::ReturnPointer => bug!("Return place was assigned to itself?"),
mir::LocalKind::Temp => {}
}
// If multiple different locals are copied to the return place. We can't pick a
// single one to rename.
if copied_to_return_place.is_some_and(|old| old != returned_local) {
return None;
}
copied_to_return_place = Some(returned_local);
}
copied_to_return_place
}
fn find_local_assigned_to_return_place(start: BasicBlock, body: &mir::Body<'_>) -> Option<Local> {
let mut block = start;
let mut seen = BitSet::new_empty(body.basic_blocks.len());
// Iterate as long as `block` has exactly one predecessor that we have not yet visited.
while seen.insert(block) {
trace!("Looking for assignments to `_0` in {:?}", block);
let local = body[block].statements.iter().rev().find_map(as_local_assigned_to_return_place);
if local.is_some() {
return local;
}
match body.basic_blocks.predecessors()[block].as_slice() {
&[pred] => block = pred,
_ => return None,
}
}
None
}
// If this statement is an assignment of an unprojected local to the return place,
// return that local.
fn as_local_assigned_to_return_place(stmt: &mir::Statement<'_>) -> Option<Local> {
if let mir::StatementKind::Assign(box (lhs, rhs)) = &stmt.kind {
if lhs.as_local() == Some(mir::RETURN_PLACE) {
if let mir::Rvalue::Use(mir::Operand::Copy(rhs) | mir::Operand::Move(rhs)) = rhs {
return rhs.as_local();
}
}
}
None
}
struct RenameToReturnPlace<'tcx> {
to_rename: Local,
tcx: TyCtxt<'tcx>,
}
/// Replaces all uses of `self.to_rename` with `_0`.
impl<'tcx> MutVisitor<'tcx> for RenameToReturnPlace<'tcx> {
fn tcx(&self) -> TyCtxt<'tcx> {
self.tcx
}
fn visit_statement(&mut self, stmt: &mut mir::Statement<'tcx>, loc: Location) {
// Remove assignments of the local being replaced to the return place, since it is now the
// return place:
// _0 = _1
if as_local_assigned_to_return_place(stmt) == Some(self.to_rename) {
stmt.kind = mir::StatementKind::Nop;
return;
}
// Remove storage annotations for the local being replaced:
// StorageLive(_1)
if let mir::StatementKind::StorageLive(local) | mir::StatementKind::StorageDead(local) =
stmt.kind
{
if local == self.to_rename {
stmt.kind = mir::StatementKind::Nop;
return;
}
}
self.super_statement(stmt, loc)
}
fn visit_terminator(&mut self, terminator: &mut mir::Terminator<'tcx>, loc: Location) {
// Ignore the implicit "use" of the return place in a `Return` statement.
if let mir::TerminatorKind::Return = terminator.kind {
return;
}
self.super_terminator(terminator, loc);
}
fn visit_local(&mut self, l: &mut Local, ctxt: PlaceContext, _: Location) {
if *l == mir::RETURN_PLACE {
assert_eq!(ctxt, PlaceContext::NonUse(NonUseContext::VarDebugInfo));
} else if *l == self.to_rename {
*l = mir::RETURN_PLACE;
}
}
}
struct IsReturnPlaceRead(bool);
impl IsReturnPlaceRead {
fn run(body: &mir::Body<'_>) -> bool {
let mut vis = IsReturnPlaceRead(false);
vis.visit_body(body);
vis.0
}
}
impl<'tcx> Visitor<'tcx> for IsReturnPlaceRead {
fn visit_local(&mut self, l: Local, ctxt: PlaceContext, _: Location) {
if l == mir::RETURN_PLACE && ctxt.is_use() && !ctxt.is_place_assignment() {
self.0 = true;
}
}
fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, loc: Location) {
// Ignore the implicit "use" of the return place in a `Return` statement.
if let mir::TerminatorKind::Return = terminator.kind {
return;
}
self.super_terminator(terminator, loc);
}
}