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Handle discriminants in dataflow-const-prop.

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This commit is contained in:
Camille GILLOT 2023-01-21 22:28:54 +00:00
parent cd3649b2a5
commit 9a6c04f5d0
6 changed files with 311 additions and 65 deletions
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185
compiler/rustc_mir_dataflow/src/value_analysis.rs
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@ -65,10 +65,8 @@ pub trait ValueAnalysis<'tcx> {
StatementKind::Assign(box (place, rvalue)) => {
self.handle_assign(*place, rvalue, state);
}
StatementKind::SetDiscriminant { .. } => {
// Could treat this as writing a constant to a pseudo-place.
// But discriminants are currently not tracked, so we do nothing.
// Related: https://github.com/rust-lang/unsafe-code-guidelines/issues/84
StatementKind::SetDiscriminant { box ref place, .. } => {
state.flood_discr(place.as_ref(), self.map());
}
StatementKind::Intrinsic(box intrinsic) => {
self.handle_intrinsic(intrinsic, state);
@ -447,26 +445,29 @@ impl<V: Clone + HasTop + HasBottom> State<V> {
}
pub fn flood_with(&mut self, place: PlaceRef<'_>, map: &Map, value: V) {
if let Some(root) = map.find(place) {
self.flood_idx_with(root, map, value);
}
}
pub fn flood(&mut self, place: PlaceRef<'_>, map: &Map) {
self.flood_with(place, map, V::top())
}
pub fn flood_idx_with(&mut self, place: PlaceIndex, map: &Map, value: V) {
let StateData::Reachable(values) = &mut self.0 else { return };
map.preorder_invoke(place, &mut |place| {
map.for_each_aliasing_place(place, None, &mut |place| {
if let Some(vi) = map.places[place].value_index {
values[vi] = value.clone();
}
});
}
pub fn flood_idx(&mut self, place: PlaceIndex, map: &Map) {
self.flood_idx_with(place, map, V::top())
pub fn flood(&mut self, place: PlaceRef<'_>, map: &Map) {
self.flood_with(place, map, V::top())
}
pub fn flood_discr_with(&mut self, place: PlaceRef<'_>, map: &Map, value: V) {
let StateData::Reachable(values) = &mut self.0 else { return };
map.for_each_aliasing_place(place, Some(TrackElem::Discriminant), &mut |place| {
if let Some(vi) = map.places[place].value_index {
values[vi] = value.clone();
}
});
}
pub fn flood_discr(&mut self, place: PlaceRef<'_>, map: &Map) {
self.flood_discr_with(place, map, V::top())
}
/// Copies `source` to `target`, including all tracked places beneath.
@ -474,7 +475,9 @@ impl<V: Clone + HasTop + HasBottom> State<V> {
/// If `target` contains a place that is not contained in `source`, it will be overwritten with
/// Top. Also, because this will copy all entries one after another, it may only be used for
/// places that are non-overlapping or identical.
pub fn assign_place_idx(&mut self, target: PlaceIndex, source: PlaceIndex, map: &Map) {
///
/// The target place must have been flooded before calling this method.
fn assign_place_idx(&mut self, target: PlaceIndex, source: PlaceIndex, map: &Map) {
let StateData::Reachable(values) = &mut self.0 else { return };
// If both places are tracked, we copy the value to the target. If the target is tracked,
@ -492,26 +495,28 @@ impl<V: Clone + HasTop + HasBottom> State<V> {
let projection = map.places[target_child].proj_elem.unwrap();
if let Some(source_child) = map.projections.get(&(source, projection)) {
self.assign_place_idx(target_child, *source_child, map);
} else {
self.flood_idx(target_child, map);
}
}
}
pub fn assign(&mut self, target: PlaceRef<'_>, result: ValueOrPlace<V>, map: &Map) {
self.flood(target, map);
if let Some(target) = map.find(target) {
self.assign_idx(target, result, map);
} else {
// We don't track this place nor any projections, assignment can be ignored.
}
}
pub fn assign_discr(&mut self, target: PlaceRef<'_>, result: ValueOrPlace<V>, map: &Map) {
self.flood_discr(target, map);
if let Some(target) = map.find_discr(target) {
self.assign_idx(target, result, map);
}
}
/// The target place must have been flooded before calling this method.
pub fn assign_idx(&mut self, target: PlaceIndex, result: ValueOrPlace<V>, map: &Map) {
match result {
ValueOrPlace::Value(value) => {
// First flood the target place in case we also track any projections (although
// this scenario is currently not well-supported by the API).
self.flood_idx(target, map);
let StateData::Reachable(values) = &mut self.0 else { return };
if let Some(value_index) = map.places[target].value_index {
values[value_index] = value;
@ -526,6 +531,14 @@ impl<V: Clone + HasTop + HasBottom> State<V> {
map.find(place).map(|place| self.get_idx(place, map)).unwrap_or(V::top())
}
/// Retrieve the value stored for a place, or ⊤ if it is not tracked.
pub fn get_discr(&self, place: PlaceRef<'_>, map: &Map) -> V {
match map.find_discr(place) {
Some(place) => self.get_idx(place, map),
None => V::top(),
}
}
/// Retrieve the value stored for a place index, or ⊤ if it is not tracked.
pub fn get_idx(&self, place: PlaceIndex, map: &Map) -> V {
match &self.0 {
@ -582,7 +595,6 @@ impl Map {
/// This is currently the only way to create a [`Map`]. The way in which the tracked places are
/// chosen is an implementation detail and may not be relied upon (other than that their type
/// passes the filter).
#[instrument(skip_all, level = "debug")]
pub fn from_filter<'tcx>(
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,
@ -614,7 +626,7 @@ impl Map {
/// Potentially register the (local, projection) place and its fields, recursively.
///
/// Invariant: The projection must only contain fields.
/// Invariant: The projection must only contain trackable elements.
fn register_with_filter_rec<'tcx>(
&mut self,
tcx: TyCtxt<'tcx>,
@ -623,21 +635,46 @@ impl Map {
ty: Ty<'tcx>,
filter: &mut impl FnMut(Ty<'tcx>) -> bool,
) {
if filter(ty) {
// We know that the projection only contains trackable elements.
let place = self.make_place(local, projection).unwrap();
// We know that the projection only contains trackable elements.
let place = self.make_place(local, projection).unwrap();
// Allocate a value slot if it doesn't have one.
if self.places[place].value_index.is_none() {
self.places[place].value_index = Some(self.value_count.into());
self.value_count += 1;
// Allocate a value slot if it doesn't have one, and the user requested one.
if self.places[place].value_index.is_none() && filter(ty) {
self.places[place].value_index = Some(self.value_count.into());
self.value_count += 1;
}
if ty.is_enum() {
let discr_ty = ty.discriminant_ty(tcx);
if filter(discr_ty) {
let discr = *self
.projections
.entry((place, TrackElem::Discriminant))
.or_insert_with(|| {
// Prepend new child to the linked list.
let next = self.places.push(PlaceInfo::new(Some(TrackElem::Discriminant)));
self.places[next].next_sibling = self.places[place].first_child;
self.places[place].first_child = Some(next);
next
});
// Allocate a value slot if it doesn't have one.
if self.places[discr].value_index.is_none() {
self.places[discr].value_index = Some(self.value_count.into());
self.value_count += 1;
}
}
}
// Recurse with all fields of this place.
iter_fields(ty, tcx, |variant, field, ty| {
if variant.is_some() {
// Downcasts are currently not supported.
if let Some(variant) = variant {
projection.push(PlaceElem::Downcast(None, variant));
let _ = self.make_place(local, projection);
projection.push(PlaceElem::Field(field, ty));
self.register_with_filter_rec(tcx, local, projection, ty, filter);
projection.pop();
projection.pop();
return;
}
projection.push(PlaceElem::Field(field, ty));
@ -694,13 +731,77 @@ impl Map {
Some(index)
}
/// Locates the given place, if it exists in the tree.
pub fn find_discr(&self, place: PlaceRef<'_>) -> Option<PlaceIndex> {
let index = self.find(place)?;
self.apply(index, TrackElem::Discriminant)
}
/// Iterate over all direct children.
pub fn children(&self, parent: PlaceIndex) -> impl Iterator<Item = PlaceIndex> + '_ {
Children::new(self, parent)
}
/// Invoke a function on the given place and all places that may alias it.
///
/// In particular, when the given place has a variant downcast, we invoke the function on all
/// the other variants.
///
/// `tail_elem` allows to support discriminants that are not a place in MIR, but that we track
/// as such.
fn for_each_aliasing_place(
&self,
place: PlaceRef<'_>,
tail_elem: Option<TrackElem>,
f: &mut impl FnMut(PlaceIndex),
) {
let Some(&Some(mut index)) = self.locals.get(place.local) else {
// The local is not tracked at all, nothing to invalidate.
return;
};
let elems = place
.projection
.iter()
.map(|&elem| elem.try_into())
.chain(tail_elem.map(Ok).into_iter());
for elem in elems {
let Ok(elem) = elem else { return };
let sub = self.apply(index, elem);
if let TrackElem::Variant(..) | TrackElem::Discriminant = elem {
// Writing to an enum variant field invalidates the other variants and the discriminant.
self.for_each_variant_sibling(index, sub, f);
}
if let Some(sub) = sub {
index = sub
} else {
return;
}
}
self.preorder_invoke(index, f);
}
/// Invoke the given function on all the descendants of the given place, except one branch.
pub fn for_each_variant_sibling(
&self,
parent: PlaceIndex,
preserved_child: Option<PlaceIndex>,
f: &mut impl FnMut(PlaceIndex),
) {
for sibling in self.children(parent) {
let elem = self.places[sibling].proj_elem;
// Only invalidate variants and discriminant. Fields (for generators) are not
// invalidated by assignment to a variant.
if let Some(TrackElem::Variant(..) | TrackElem::Discriminant) = elem
// Only invalidate the other variants, the current one is fine.
&& Some(sibling) != preserved_child
{
self.preorder_invoke(sibling, f);
}
}
}
/// Invoke a function on the given place and all descendants.
pub fn preorder_invoke(&self, root: PlaceIndex, f: &mut impl FnMut(PlaceIndex)) {
fn preorder_invoke(&self, root: PlaceIndex, f: &mut impl FnMut(PlaceIndex)) {
f(root);
for child in self.children(root) {
self.preorder_invoke(child, f);
@ -759,6 +860,7 @@ impl<'a> Iterator for Children<'a> {
}
/// Used as the result of an operand or r-value.
#[derive(Debug)]
pub enum ValueOrPlace<V> {
Value(V),
Place(PlaceIndex),
@ -776,6 +878,8 @@ impl<V: HasTop> ValueOrPlace<V> {
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
pub enum TrackElem {
Field(Field),
Variant(VariantIdx),
Discriminant,
}
impl<V, T> TryFrom<ProjectionElem<V, T>> for TrackElem {
@ -784,6 +888,7 @@ impl<V, T> TryFrom<ProjectionElem<V, T>> for TrackElem {
fn try_from(value: ProjectionElem<V, T>) -> Result<Self, Self::Error> {
match value {
ProjectionElem::Field(field, _) => Ok(TrackElem::Field(field)),
ProjectionElem::Downcast(_, idx) => Ok(TrackElem::Variant(idx)),
_ => Err(()),
}
}
@ -900,6 +1005,12 @@ fn debug_with_context_rec<V: Debug + Eq>(
for child in map.children(place) {
let info_elem = map.places[child].proj_elem.unwrap();
let child_place_str = match info_elem {
TrackElem::Discriminant => {
format!("discriminant({})", place_str)
}
TrackElem::Variant(idx) => {
format!("({} as {:?})", place_str, idx)
}
TrackElem::Field(field) => {
if place_str.starts_with('*') {
format!("({}).{}", place_str, field.index())