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Rollup merge of #111441 - cjgillot:issue-111422, r=JakobDegen

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Verify copies of mutable pointers in 2 stages in ReferencePropagation

Fixes #111422

In the first stage, we mark the copies as reborrows, to be checked later.
In the second stage, we walk the reborrow chains to verify that all stages are fully replacable.

The replacement itself mirrors the check, and iterates through the reborrow chain.

r? ``````@RalfJung``````
cc ``````@JakobDegen``````
This commit is contained in:
Matthias Krüger 2023-05-12 07:11:13 +02:00 • committed by GitHub
commit ab18da61f4
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5 changed files with 229 additions and 33 deletions
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88
compiler/rustc_mir_transform/src/ref_prop.rs
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@ -85,7 +85,9 @@ fn propagate_ssa<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let ssa = SsaLocals::new(body);
let mut replacer = compute_replacement(tcx, body, &ssa);
debug!(?replacer.targets, ?replacer.allowed_replacements, ?replacer.storage_to_remove);
debug!(?replacer.targets);
debug!(?replacer.allowed_replacements);
debug!(?replacer.storage_to_remove);
replacer.visit_body_preserves_cfg(body);
@ -190,8 +192,11 @@ fn compute_replacement<'tcx>(
continue;
}
// Whether the current local is subject to the uniqueness rule.
let needs_unique = ty.is_mutable_ptr();
// If this a mutable reference that we cannot fully replace, mark it as unknown.
if ty.is_mutable_ptr() && !fully_replacable_locals.contains(local) {
if needs_unique && !fully_replacable_locals.contains(local) {
debug!("not fully replaceable");
continue;
}
@ -203,13 +208,14 @@ fn compute_replacement<'tcx>(
// have been visited before.
Rvalue::Use(Operand::Copy(place) | Operand::Move(place))
| Rvalue::CopyForDeref(place) => {
if let Some(rhs) = place.as_local() {
if let Some(rhs) = place.as_local() && ssa.is_ssa(rhs) {
let target = targets[rhs];
if matches!(target, Value::Pointer(..)) {
// Only see through immutable reference and pointers, as we do not know yet if
// mutable references are fully replaced.
if !needs_unique && matches!(target, Value::Pointer(..)) {
targets[local] = target;
} else if ssa.is_ssa(rhs) {
let refmut = body.local_decls[rhs].ty.is_mutable_ptr();
targets[local] = Value::Pointer(tcx.mk_place_deref(rhs.into()), refmut);
} else {
targets[local] = Value::Pointer(tcx.mk_place_deref(rhs.into()), needs_unique);
}
}
}
@ -217,10 +223,10 @@ fn compute_replacement<'tcx>(
let mut place = *place;
// Try to see through `place` in order to collapse reborrow chains.
if place.projection.first() == Some(&PlaceElem::Deref)
&& let Value::Pointer(target, refmut) = targets[place.local]
&& let Value::Pointer(target, inner_needs_unique) = targets[place.local]
// Only see through immutable reference and pointers, as we do not know yet if
// mutable references are fully replaced.
&& !refmut
&& !inner_needs_unique
// Only collapse chain if the pointee is definitely live.
&& can_perform_opt(target, location)
{
@ -228,7 +234,7 @@ fn compute_replacement<'tcx>(
}
assert_ne!(place.local, local);
if is_constant_place(place) {
targets[local] = Value::Pointer(place, ty.is_mutable_ptr());
targets[local] = Value::Pointer(place, needs_unique);
}
}
// We do not know what to do, so keep as not-a-pointer.
@ -276,16 +282,35 @@ fn compute_replacement<'tcx>(
return;
}
if let Value::Pointer(target, refmut) = self.targets[place.local]
&& place.projection.first() == Some(&PlaceElem::Deref)
{
let perform_opt = (self.can_perform_opt)(target, loc);
if perform_opt {
self.allowed_replacements.insert((target.local, loc));
} else if refmut {
// This mutable reference is not fully replacable, so drop it.
self.targets[place.local] = Value::Unknown;
if place.projection.first() != Some(&PlaceElem::Deref) {
// This is not a dereference, nothing to do.
return;
}
let mut place = place.as_ref();
loop {
if let Value::Pointer(target, needs_unique) = self.targets[place.local] {
let perform_opt = (self.can_perform_opt)(target, loc);
debug!(?place, ?target, ?needs_unique, ?perform_opt);
// This a reborrow chain, recursively allow the replacement.
//
// This also allows to detect cases where `target.local` is not replacable,
// and mark it as such.
if let &[PlaceElem::Deref] = &target.projection[..] {
assert!(perform_opt);
self.allowed_replacements.insert((target.local, loc));
place.local = target.local;
continue;
} else if perform_opt {
self.allowed_replacements.insert((target.local, loc));
} else if needs_unique {
// This mutable reference is not fully replacable, so drop it.
self.targets[place.local] = Value::Unknown;
}
}
break;
}
}
}
@ -326,18 +351,23 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'tcx> {
}
fn visit_place(&mut self, place: &mut Place<'tcx>, ctxt: PlaceContext, loc: Location) {
if let Value::Pointer(target, _) = self.targets[place.local]
&& place.projection.first() == Some(&PlaceElem::Deref)
{
let perform_opt = matches!(ctxt, PlaceContext::NonUse(_))
|| self.allowed_replacements.contains(&(target.local, loc));
if place.projection.first() != Some(&PlaceElem::Deref) {
return;
}
if perform_opt {
*place = target.project_deeper(&place.projection[1..], self.tcx);
self.any_replacement = true;
loop {
if let Value::Pointer(target, _) = self.targets[place.local] {
let perform_opt = matches!(ctxt, PlaceContext::NonUse(_))
|| self.allowed_replacements.contains(&(target.local, loc));
if perform_opt {
*place = target.project_deeper(&place.projection[1..], self.tcx);
self.any_replacement = true;
continue;
}
}
} else {
self.super_place(place, ctxt, loc);
break;
}
}