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Propagate from borrowed locals in CopyProp

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This commit is contained in:
Tomasz Miąsko
2025-07-07 21:49:20 +02:00
parent 32cd911471
commit 9681786409
7 changed files with 98 additions and 143 deletions
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26
compiler/rustc_mir_transform/src/ssa.rs
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@@ -293,10 +293,6 @@ impl<'tcx> Visitor<'tcx> for SsaVisitor<'_, 'tcx> {
fn compute_copy_classes(ssa: &mut SsaLocals, body: &Body<'_>) {
let mut direct_uses = std::mem::take(&mut ssa.direct_uses);
let mut copies = IndexVec::from_fn_n(|l| l, body.local_decls.len());
// We must not unify two locals that are borrowed. But this is fine if one is borrowed and
// the other is not. This bitset is keyed by *class head* and contains whether any member of
// the class is borrowed.
let mut borrowed_classes = ssa.borrowed_locals().clone();
for (local, rvalue, _) in ssa.assignments(body) {
let (Rvalue::Use(Operand::Copy(place) | Operand::Move(place))
@@ -322,8 +318,12 @@ fn compute_copy_classes(ssa: &mut SsaLocals, body: &Body<'_>) {
// visited before `local`, and we just have to copy the representing local.
let head = copies[rhs];
// Do not unify borrowed locals.
if borrowed_classes.contains(local) || borrowed_classes.contains(head) {
// When propagating from `head` to `local` we need to ensure that changes to the address
// are not observable, so at most one the locals involved can be borrowed. Additionally, we
// need to ensure that the definition of `head` dominates all uses of `local`. When `local`
// is borrowed, there might exist an indirect use of `local` that isn't dominated by the
// definition, so we have to reject copy propagation.
if ssa.borrowed_locals().contains(local) {
continue;
}
@@ -339,21 +339,14 @@ fn compute_copy_classes(ssa: &mut SsaLocals, body: &Body<'_>) {
*h = RETURN_PLACE;
}
}
if borrowed_classes.contains(head) {
borrowed_classes.insert(RETURN_PLACE);
}
} else {
copies[local] = head;
if borrowed_classes.contains(local) {
borrowed_classes.insert(head);
}
}
direct_uses[rhs] -= 1;
}
debug!(?copies);
debug!(?direct_uses);
debug!(?borrowed_classes);
// Invariant: `copies` must point to the head of an equivalence class.
#[cfg(debug_assertions)]
@@ -362,13 +355,6 @@ fn compute_copy_classes(ssa: &mut SsaLocals, body: &Body<'_>) {
}
debug_assert_eq!(copies[RETURN_PLACE], RETURN_PLACE);
// Invariant: `borrowed_classes` must be true if any member of the class is borrowed.
#[cfg(debug_assertions)]
for &head in copies.iter() {
let any_borrowed = ssa.borrowed_locals.iter().any(|l| copies[l] == head);
assert_eq!(borrowed_classes.contains(head), any_borrowed);
}
ssa.direct_uses = direct_uses;
ssa.copy_classes = copies;
}