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Auto merge of #108178 - cjgillot:ssa-deref, r=oli-obk

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Do not consider `&mut *x` as mutating `x` in `CopyProp`

This PR removes an unfortunate overly cautious case from the current implementation.

Found by https://github.com/rust-lang/rust/pull/105274 cc `@saethlin`
This commit is contained in:
bors 2023-03-09 04:50:38 +00:00
commit 66a2d62210
6 changed files with 294 additions and 16 deletions
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56
compiler/rustc_mir_transform/src/ssa.rs
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@ -53,7 +53,7 @@ impl SsaLocals {
body: &Body<'tcx>,
borrowed_locals: &BitSet<Local>,
) -> SsaLocals {
let assignment_order = Vec::new();
let assignment_order = Vec::with_capacity(body.local_decls.len());
let assignments = IndexVec::from_elem(Set1::Empty, &body.local_decls);
let dominators =
@ -179,12 +179,34 @@ struct SsaVisitor {
assignment_order: Vec<Local>,
}
impl SsaVisitor {
fn check_assignment_dominates(&mut self, local: Local, loc: Location) {
let set = &mut self.assignments[local];
let assign_dominates = match *set {
Set1::Empty | Set1::Many => false,
Set1::One(LocationExtended::Arg) => true,
Set1::One(LocationExtended::Plain(assign)) => {
assign.successor_within_block().dominates(loc, &self.dominators)
}
};
// We are visiting a use that is not dominated by an assignment.
// Either there is a cycle involved, or we are reading for uninitialized local.
// Bail out.
if !assign_dominates {
*set = Set1::Many;
}
}
}
impl<'tcx> Visitor<'tcx> for SsaVisitor {
fn visit_local(&mut self, local: Local, ctxt: PlaceContext, loc: Location) {
match ctxt {
PlaceContext::MutatingUse(MutatingUseContext::Store) => {
self.assignments[local].insert(LocationExtended::Plain(loc));
self.assignment_order.push(local);
if let Set1::One(_) = self.assignments[local] {
// Only record if SSA-like, to avoid growing the vector needlessly.
self.assignment_order.push(local);
}
}
// Anything can happen with raw pointers, so remove them.
PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
@ -192,24 +214,26 @@ impl<'tcx> Visitor<'tcx> for SsaVisitor {
// Immutable borrows are taken into account in `SsaLocals::new` by
// removing non-freeze locals.
PlaceContext::NonMutatingUse(_) => {
let set = &mut self.assignments[local];
let assign_dominates = match *set {
Set1::Empty | Set1::Many => false,
Set1::One(LocationExtended::Arg) => true,
Set1::One(LocationExtended::Plain(assign)) => {
assign.successor_within_block().dominates(loc, &self.dominators)
}
};
// We are visiting a use that is not dominated by an assignment.
// Either there is a cycle involved, or we are reading for uninitialized local.
// Bail out.
if !assign_dominates {
*set = Set1::Many;
}
self.check_assignment_dominates(local, loc);
}
PlaceContext::NonUse(_) => {}
}
}
fn visit_place(&mut self, place: &Place<'tcx>, ctxt: PlaceContext, loc: Location) {
if place.projection.first() == Some(&PlaceElem::Deref) {
// Do not do anything for storage statements and debuginfo.
if ctxt.is_use() {
// A use through a `deref` only reads from the local, and cannot write to it.
let new_ctxt = PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection);
self.visit_projection(place.as_ref(), new_ctxt, loc);
self.check_assignment_dominates(place.local, loc);
}
return;
}
self.super_place(place, ctxt, loc);
}
}
#[instrument(level = "trace", skip(ssa, body))]