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Improve documentation, plus some small changes

This commit is contained in:
Jannis Christopher Köhl 2022-10-19 15:56:58 +02:00
parent 931d99f61f
commit 274a49132b
4 changed files with 160 additions and 106 deletions
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236
compiler/rustc_mir_dataflow/src/value_analysis.rs
View File

@ -192,13 +192,27 @@ pub trait ValueAnalysis<'tcx> {
.map(ValueOrPlaceOrRef::Ref)
.unwrap_or(ValueOrPlaceOrRef::top()),
Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
// This is not a `&x` reference and could be used for modification.
state.flood(place.as_ref(), self.map());
ValueOrPlaceOrRef::top()
}
Rvalue::CopyForDeref(place) => {
self.handle_operand(&Operand::Copy(*place), state).into()
}
_ => ValueOrPlaceOrRef::top(),
Rvalue::Repeat(..)
| Rvalue::ThreadLocalRef(..)
| Rvalue::Len(..)
| Rvalue::Cast(..)
| Rvalue::BinaryOp(..)
| Rvalue::CheckedBinaryOp(..)
| Rvalue::NullaryOp(..)
| Rvalue::UnaryOp(..)
| Rvalue::Discriminant(..)
| Rvalue::Aggregate(..)
| Rvalue::ShallowInitBox(..) => {
// No modification is possible through these r-values.
ValueOrPlaceOrRef::top()
}
}
}
@ -419,7 +433,7 @@ enum StateData<V> {
#[derive(PartialEq, Eq, Clone, Debug)]
pub struct State<V>(StateData<V>);
impl<V: Clone + HasTop> State<V> {
impl<V: Clone + HasTop + HasBottom> State<V> {
pub fn is_reachable(&self) -> bool {
matches!(&self.0, StateData::Reachable(_))
}
@ -460,6 +474,10 @@ impl<V: Clone + HasTop> State<V> {
self.flood_idx_with(place, map, V::top())
}
/// Copies `source` to `target`, including all tracked places beneath.
///
/// 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
pub fn assign_place_idx(&mut self, target: PlaceIndex, source: PlaceIndex, map: &Map) {
// We use (A3) and copy all entries one after another.
let StateData::Reachable(values) = &mut self.0 else { return };
@ -514,7 +532,7 @@ impl<V: Clone + HasTop> State<V> {
// track *what* it points to (as in, what do we know about the target). For an
// assignment `x = &y`, we thus copy the info we have for `y` to `*x`. This is
// sound because we only track places that are `Freeze`, and (A4).
if let Some(target_deref) = map.apply_elem(target, ProjElem::Deref) {
if let Some(target_deref) = map.apply(target, TrackElem::Deref) {
self.assign_place_idx(target_deref, source, map);
}
}
@ -530,7 +548,10 @@ impl<V: Clone + HasTop> State<V> {
StateData::Reachable(values) => {
map.places[place].value_index.map(|v| values[v].clone()).unwrap_or(V::top())
}
StateData::Unreachable => V::top(),
StateData::Unreachable => {
// Because this is unreachable, we can return any value we want.
V::bottom()
}
}
}
}
@ -548,10 +569,15 @@ impl<V: JoinSemiLattice + Clone> JoinSemiLattice for State<V> {
}
}
/// A partial mapping from `Place` to `PlaceIndex`.
///
/// Some additioanl bookkeeping is done to speed up traversal:
/// - For iteration, every [`PlaceInfo`] contains an intrusive linked list of its children.
/// - To directly get the child for a specific projection, there is `projections` map.
#[derive(Debug)]
pub struct Map {
locals: IndexVec<Local, Option<PlaceIndex>>,
projections: FxHashMap<(PlaceIndex, ProjElem), PlaceIndex>,
projections: FxHashMap<(PlaceIndex, TrackElem), PlaceIndex>,
places: IndexVec<PlaceIndex, PlaceInfo>,
value_count: usize,
}
@ -566,7 +592,10 @@ impl Map {
}
}
/// Register all suitable places with matching types (up to a certain depth).
/// Returns a map that only tracks places whose type passes the filter.
///
/// This is currently the only way to create a [`Map`]. The way in which the tracked places are
/// chosen is an implementation detail an may not be relied upon.
pub fn from_filter<'tcx>(
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,
@ -579,7 +608,9 @@ impl Map {
// the correctness relies on an aliasing model similar to Stacked Borrows (which is
// not yet guaranteed).
if tcx.sess.opts.unstable_opts.unsound_mir_opts {
map.register_with_filter(tcx, body, 3, filter, &[]);
// We might want to add additional limitations. If a struct has 10 boxed fields of
// itself, will currently be `10.pow(max_derefs)` tracked places.
map.register_with_filter(tcx, body, 2, filter, &[]);
} else {
map.register_with_filter(tcx, body, 0, filter, &escaped_places(body));
}
@ -587,6 +618,7 @@ impl Map {
map
}
/// Register all non-excluded places that pass the filter, up to a certain dereference depth.
fn register_with_filter<'tcx>(
&mut self,
tcx: TyCtxt<'tcx>,
@ -595,7 +627,9 @@ impl Map {
mut filter: impl FnMut(Ty<'tcx>) -> bool,
exclude: &[Place<'tcx>],
) {
// This is used to tell whether a type is `!Freeze`.
let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
let mut projection = Vec::new();
for (local, decl) in body.local_decls.iter_enumerated() {
self.register_with_filter_rec(
@ -622,7 +656,7 @@ impl Map {
param_env: ty::ParamEnv<'tcx>,
exclude: &[Place<'tcx>],
) {
// This check could be improved.
// This currently does a linear scan, could be improved.
if exclude.contains(&Place { local, projection: tcx.intern_place_elems(projection) }) {
return;
}
@ -664,6 +698,9 @@ impl Map {
});
}
/// Tries to add the place to the map, without allocating a value slot.
///
/// Can fail if the projection contains non-trackable elements.
fn make_place<'tcx>(
&mut self,
local: Local,
@ -675,10 +712,6 @@ impl Map {
// Apply the projection.
for &elem in projection {
match elem {
PlaceElem::Downcast(..) => return Err(()),
_ => (),
}
let elem = elem.try_into()?;
index = *self.projections.entry((index, elem)).or_insert_with(|| {
// Prepend new child to the linked list.
@ -713,6 +746,7 @@ impl Map {
self.register_with_ty(local, projection, place_ty.ty)
}
/// Tries to track the given place. Fails if type is non-scalar or projection is not trackable.
fn register_with_ty<'tcx>(
&mut self,
local: Local,
@ -736,7 +770,7 @@ impl Map {
Ok(())
}
pub fn apply_elem(&self, place: PlaceIndex, elem: ProjElem) -> Option<PlaceIndex> {
pub fn apply(&self, place: PlaceIndex, elem: TrackElem) -> Option<PlaceIndex> {
self.projections.get(&(place, elem)).copied()
}
@ -744,12 +778,13 @@ impl Map {
let mut index = *self.locals.get(place.local)?.as_ref()?;
for &elem in place.projection {
index = self.apply_elem(index, elem.try_into().ok()?)?;
index = self.apply(index, elem.try_into().ok()?)?;
}
Some(index)
}
/// Iterate over all direct children.
pub fn children(&self, parent: PlaceIndex) -> impl Iterator<Item = PlaceIndex> + '_ {
Children::new(self, parent)
}
@ -762,40 +797,31 @@ impl Map {
}
}
/// This is the information tracked for every [`PlaceIndex`] and is stored by [`Map`].
///
/// Together, `first_child` and `next_sibling` form an intrusive linked list, which is used to
/// model a tree structure (a replacement for a member like `children: Vec<PlaceIndex>`).
#[derive(Debug)]
struct PlaceInfo {
next_sibling: Option<PlaceIndex>,
first_child: Option<PlaceIndex>,
/// The projection used to go from parent to this node (only None for root).
proj_elem: Option<ProjElem>,
/// We store a [`ValueIndex`] if and only if the placed is tracked by the analysis.
value_index: Option<ValueIndex>,
/// The projection used to go from parent to this node (only None for root).
proj_elem: Option<TrackElem>,
/// The left-most child.
first_child: Option<PlaceIndex>,
/// Index of the sibling to the right of this node.
next_sibling: Option<PlaceIndex>,
}
impl PlaceInfo {
fn new(proj_elem: Option<ProjElem>) -> Self {
fn new(proj_elem: Option<TrackElem>) -> Self {
Self { next_sibling: None, first_child: None, proj_elem, value_index: None }
}
}
/// Returns all places, that have their reference or address taken.
fn escaped_places<'tcx>(body: &Body<'tcx>) -> Vec<Place<'tcx>> {
struct Collector<'tcx> {
result: Vec<Place<'tcx>>,
}
impl<'tcx> Visitor<'tcx> for Collector<'tcx> {
fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
if context.is_borrow() || context.is_address_of() {
self.result.push(*place);
}
}
}
let mut collector = Collector { result: Vec::new() };
collector.visit_body(body);
collector.result
}
struct Children<'a> {
map: &'a Map,
next: Option<PlaceIndex>,
@ -820,25 +846,28 @@ impl<'a> Iterator for Children<'a> {
}
}
}
/// Used as the result of an operand.
pub enum ValueOrPlace<V> {
Value(V),
Place(PlaceIndex),
}
impl<V: HasTop> ValueOrPlace<V> {
pub fn top() -> Self {
impl<V: HasTop> HasTop for ValueOrPlace<V> {
fn top() -> Self {
ValueOrPlace::Value(V::top())
}
}
/// Used as the result of an r-value.
pub enum ValueOrPlaceOrRef<V> {
Value(V),
Place(PlaceIndex),
Ref(PlaceIndex),
}
impl<V: HasTop> ValueOrPlaceOrRef<V> {
pub fn top() -> Self {
impl<V: HasTop> HasTop for ValueOrPlaceOrRef<V> {
fn top() -> Self {
ValueOrPlaceOrRef::Value(V::top())
}
}
@ -872,27 +901,28 @@ impl<V> HasTop for FlatSet<V> {
}
}
/// Currently, we only track places through deref and field projections.
/// The set of projection elements that can be used by a tracked place.
///
/// For now, downcast is not allowed due to aliasing between variants (see #101168).
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
pub enum ProjElem {
pub enum TrackElem {
Deref,
Field(Field),
}
impl<V, T> TryFrom<ProjectionElem<V, T>> for ProjElem {
impl<V, T> TryFrom<ProjectionElem<V, T>> for TrackElem {
type Error = ();
fn try_from(value: ProjectionElem<V, T>) -> Result<Self, Self::Error> {
match value {
ProjectionElem::Deref => Ok(ProjElem::Deref),
ProjectionElem::Field(field, _) => Ok(ProjElem::Field(field)),
ProjectionElem::Deref => Ok(TrackElem::Deref),
ProjectionElem::Field(field, _) => Ok(TrackElem::Field(field)),
_ => Err(()),
}
}
}
/// Invokes `f` on all direct fields of `ty`.
fn iter_fields<'tcx>(
ty: Ty<'tcx>,
tcx: TyCtxt<'tcx>,
@ -922,58 +952,26 @@ fn iter_fields<'tcx>(
}
}
fn debug_with_context_rec<V: Debug + Eq>(
place: PlaceIndex,
place_str: &str,
new: &IndexVec<ValueIndex, V>,
old: Option<&IndexVec<ValueIndex, V>>,
map: &Map,
f: &mut Formatter<'_>,
) -> std::fmt::Result {
if let Some(value) = map.places[place].value_index {
match old {
None => writeln!(f, "{}: {:?}", place_str, new[value])?,
Some(old) => {
if new[value] != old[value] {
writeln!(f, "\u{001f}-{}: {:?}", place_str, old[value])?;
writeln!(f, "\u{001f}+{}: {:?}", place_str, new[value])?;
}
/// Returns all places, that have their reference or address taken.
fn escaped_places<'tcx>(body: &Body<'tcx>) -> Vec<Place<'tcx>> {
struct Collector<'tcx> {
result: Vec<Place<'tcx>>,
}
impl<'tcx> Visitor<'tcx> for Collector<'tcx> {
fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
if context.is_borrow() || context.is_address_of() {
self.result.push(*place);
}
}
}
for child in map.children(place) {
let info_elem = map.places[child].proj_elem.unwrap();
let child_place_str = match info_elem {
ProjElem::Deref => format!("*{}", place_str),
ProjElem::Field(field) => {
if place_str.starts_with("*") {
format!("({}).{}", place_str, field.index())
} else {
format!("{}.{}", place_str, field.index())
}
}
};
debug_with_context_rec(child, &child_place_str, new, old, map, f)?;
}
Ok(())
}
fn debug_with_context<V: Debug + Eq>(
new: &IndexVec<ValueIndex, V>,
old: Option<&IndexVec<ValueIndex, V>>,
map: &Map,
f: &mut Formatter<'_>,
) -> std::fmt::Result {
for (local, place) in map.locals.iter_enumerated() {
if let Some(place) = place {
debug_with_context_rec(*place, &format!("{:?}", local), new, old, map, f)?;
}
}
Ok(())
let mut collector = Collector { result: Vec::new() };
collector.visit_body(body);
collector.result
}
/// This is used to visualize the dataflow analysis.
impl<'tcx, T> DebugWithContext<ValueAnalysisWrapper<T>> for State<T::Value>
where
T: ValueAnalysis<'tcx>,
@ -1000,3 +998,55 @@ where
}
}
}
fn debug_with_context_rec<V: Debug + Eq>(
place: PlaceIndex,
place_str: &str,
new: &IndexVec<ValueIndex, V>,
old: Option<&IndexVec<ValueIndex, V>>,
map: &Map,
f: &mut Formatter<'_>,
) -> std::fmt::Result {
if let Some(value) = map.places[place].value_index {
match old {
None => writeln!(f, "{}: {:?}", place_str, new[value])?,
Some(old) => {
if new[value] != old[value] {
writeln!(f, "\u{001f}-{}: {:?}", place_str, old[value])?;
writeln!(f, "\u{001f}+{}: {:?}", place_str, new[value])?;
}
}
}
}
for child in map.children(place) {
let info_elem = map.places[child].proj_elem.unwrap();
let child_place_str = match info_elem {
TrackElem::Deref => format!("*{}", place_str),
TrackElem::Field(field) => {
if place_str.starts_with("*") {
format!("({}).{}", place_str, field.index())
} else {
format!("{}.{}", place_str, field.index())
}
}
};
debug_with_context_rec(child, &child_place_str, new, old, map, f)?;
}
Ok(())
}
fn debug_with_context<V: Debug + Eq>(
new: &IndexVec<ValueIndex, V>,
old: Option<&IndexVec<ValueIndex, V>>,
map: &Map,
f: &mut Formatter<'_>,
) -> std::fmt::Result {
for (local, place) in map.locals.iter_enumerated() {
if let Some(place) = place {
debug_with_context_rec(*place, &format!("{:?}", local), new, old, map, f)?;
}
}
Ok(())
}

24
compiler/rustc_mir_transform/src/dataflow_const_prop.rs
View File

@ -1,10 +1,14 @@
//! A constant propagation optimization pass based on dataflow analysis.
//!
//! Tracks places that have a scalar type.
use rustc_const_eval::interpret::{ConstValue, ImmTy, Immediate, InterpCx, Scalar};
use rustc_data_structures::fx::FxHashMap;
use rustc_middle::mir::visit::{MutVisitor, Visitor};
use rustc_middle::mir::*;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_mir_dataflow::value_analysis::{
Map, ProjElem, State, ValueAnalysis, ValueOrPlace, ValueOrPlaceOrRef,
HasTop, Map, State, TrackElem, ValueAnalysis, ValueOrPlace, ValueOrPlaceOrRef,
};
use rustc_mir_dataflow::{lattice::FlatSet, Analysis, ResultsVisitor, SwitchIntEdgeEffects};
use rustc_span::{sym, DUMMY_SP};
@ -20,7 +24,7 @@ impl<'tcx> MirPass<'tcx> for DataflowConstProp {
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
// Decide which places to track during the analysis.
let map = Map::from_filter(tcx, body, |ty| ty.is_scalar() && !ty.is_unsafe_ptr());
let map = Map::from_filter(tcx, body, Ty::is_scalar);
// Perform the actual dataflow analysis.
let analysis = ConstAnalysis::new(tcx, body, map);
@ -65,12 +69,10 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'tcx> {
state.flood_idx(target, self.map());
}
let value_target = target.and_then(|target| {
self.map().apply_elem(target, ProjElem::Field(0_u32.into()))
});
let overflow_target = target.and_then(|target| {
self.map().apply_elem(target, ProjElem::Field(1_u32.into()))
});
let value_target = target
.and_then(|target| self.map().apply(target, TrackElem::Field(0_u32.into())));
let overflow_target = target
.and_then(|target| self.map().apply(target, TrackElem::Field(1_u32.into())));
if value_target.is_some() || overflow_target.is_some() {
let (val, overflow) = self.binary_op(state, *op, left, right);
@ -211,6 +213,7 @@ struct ScalarTy<'tcx>(Scalar, Ty<'tcx>);
impl<'tcx> std::fmt::Debug for ScalarTy<'tcx> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
// This is used for dataflow visualization, so we return something more concise.
std::fmt::Display::fmt(&ConstantKind::Val(ConstValue::Scalar(self.0), self.1), f)
}
}
@ -341,13 +344,16 @@ impl<'mir, 'tcx, 'map> ResultsVisitor<'mir, 'tcx> for CollectAndPatch<'tcx, 'map
location: Location,
) {
match statement.kind {
StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(_)))) => {
// Don't overwrite the assignment if it already uses a constant (to keep the span).
}
StatementKind::Assign(box (place, _)) => match state.get(place.as_ref(), self.map) {
FlatSet::Top => (),
FlatSet::Elem(value) => {
self.assignments.insert(location, value);
}
FlatSet::Bottom => {
// This statement is not reachable. Do nothing, it will (hopefully) be removed.
// This assignment is either unreachable, or an uninitialized value is assigned.
}
},
_ => (),

3
src/test/mir-opt/dataflow-const-prop/checked.main.DataflowConstProp.diff
View File

@ -53,8 +53,7 @@
StorageDead(_5); // scope 2 at $DIR/checked.rs:+3:17: +3:18
StorageDead(_4); // scope 2 at $DIR/checked.rs:+3:17: +3:18
StorageLive(_7); // scope 3 at $DIR/checked.rs:+5:9: +5:10
- _7 = const _; // scope 3 at $DIR/checked.rs:+5:13: +5:21
+ _7 = const i32::MAX; // scope 3 at $DIR/checked.rs:+5:13: +5:21
_7 = const _; // scope 3 at $DIR/checked.rs:+5:13: +5:21
StorageLive(_8); // scope 4 at $DIR/checked.rs:+6:9: +6:10
StorageLive(_9); // scope 4 at $DIR/checked.rs:+6:13: +6:14
- _9 = _7; // scope 4 at $DIR/checked.rs:+6:13: +6:14

3
src/test/mir-opt/dataflow-const-prop/previous/control_flow_simplification.hello.DataflowConstProp.diff
View File

@ -8,8 +8,7 @@
bb0: {
StorageLive(_1); // scope 0 at $DIR/control-flow-simplification.rs:+1:8: +1:21
- _1 = const _; // scope 0 at $DIR/control-flow-simplification.rs:+1:8: +1:21
+ _1 = const false; // scope 0 at $DIR/control-flow-simplification.rs:+1:8: +1:21
_1 = const _; // scope 0 at $DIR/control-flow-simplification.rs:+1:8: +1:21
switchInt(const false) -> [false: bb2, otherwise: bb1]; // scope 0 at $DIR/control-flow-simplification.rs:+1:8: +1:21
}