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Auto merge of #116454 - tmiasko:small-dominators, r=cjgillot

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Generalize small dominators optimization

* Use small dominators optimization from 640ede7b0a1840415cb6ec881c2210302bfeba18 more generally.
* Merge `DefLocation` and `LocationExtended` since they serve the same purpose.
This commit is contained in:
bors 2023-10-08 12:38:14 +00:00
commit 4f4a413fe6
5 changed files with 134 additions and 115 deletions
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17
compiler/rustc_codegen_ssa/src/mir/analyze.rs
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@ -8,7 +8,7 @@ use rustc_index::bit_set::BitSet;
use rustc_index::{IndexSlice, IndexVec};
use rustc_middle::mir::traversal;
use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
use rustc_middle::mir::{self, Location, TerminatorKind};
use rustc_middle::mir::{self, DefLocation, Location, TerminatorKind};
use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf};
pub fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
@ -67,21 +67,6 @@ enum LocalKind {
SSA(DefLocation),
}
#[derive(Copy, Clone, PartialEq, Eq)]
enum DefLocation {
Argument,
Body(Location),
}
impl DefLocation {
fn dominates(self, location: Location, dominators: &Dominators<mir::BasicBlock>) -> bool {
match self {
DefLocation::Argument => true,
DefLocation::Body(def) => def.successor_within_block().dominates(location, dominators),
}
}
}
struct LocalAnalyzer<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
fx: &'mir FunctionCx<'a, 'tcx, Bx>,
dominators: &'mir Dominators<mir::BasicBlock>,

76
compiler/rustc_data_structures/src/graph/dominators/mod.rs
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@ -26,7 +26,42 @@ rustc_index::newtype_index! {
struct PreorderIndex {}
}
pub fn dominators<G: ControlFlowGraph>(graph: &G) -> Dominators<G::Node> {
#[derive(Clone, Debug)]
pub struct Dominators<Node: Idx> {
kind: Kind<Node>,
}
#[derive(Clone, Debug)]
enum Kind<Node: Idx> {
/// A representation optimized for a small path graphs.
Path,
General(Inner<Node>),
}
pub fn dominators<G: ControlFlowGraph>(g: &G) -> Dominators<G::Node> {
// We often encounter MIR bodies with 1 or 2 basic blocks. Special case the dominators
// computation and representation for those cases.
if is_small_path_graph(g) {
Dominators { kind: Kind::Path }
} else {
Dominators { kind: Kind::General(dominators_impl(g)) }
}
}
fn is_small_path_graph<G: ControlFlowGraph>(g: &G) -> bool {
if g.start_node().index() != 0 {
return false;
}
if g.num_nodes() == 1 {
return true;
}
if g.num_nodes() == 2 {
return g.successors(g.start_node()).any(|n| n.index() == 1);
}
false
}
fn dominators_impl<G: ControlFlowGraph>(graph: &G) -> Inner<G::Node> {
// compute the post order index (rank) for each node
let mut post_order_rank = IndexVec::from_elem_n(0, graph.num_nodes());
@ -245,7 +280,7 @@ pub fn dominators<G: ControlFlowGraph>(graph: &G) -> Dominators<G::Node> {
let time = compute_access_time(start_node, &immediate_dominators);
Dominators { start_node, post_order_rank, immediate_dominators, time }
Inner { post_order_rank, immediate_dominators, time }
}
/// Evaluate the link-eval virtual forest, providing the currently minimum semi
@ -310,8 +345,7 @@ fn compress(
/// Tracks the list of dominators for each node.
#[derive(Clone, Debug)]
pub struct Dominators<N: Idx> {
start_node: N,
struct Inner<N: Idx> {
post_order_rank: IndexVec<N, usize>,
// Even though we track only the immediate dominator of each node, it's
// possible to get its full list of dominators by looking up the dominator
@ -323,12 +357,24 @@ pub struct Dominators<N: Idx> {
impl<Node: Idx> Dominators<Node> {
/// Returns true if node is reachable from the start node.
pub fn is_reachable(&self, node: Node) -> bool {
node == self.start_node || self.immediate_dominators[node].is_some()
match &self.kind {
Kind::Path => true,
Kind::General(g) => g.time[node].start != 0,
}
}
/// Returns the immediate dominator of node, if any.
pub fn immediate_dominator(&self, node: Node) -> Option<Node> {
self.immediate_dominators[node]
match &self.kind {
Kind::Path => {
if 0 < node.index() {
Some(Node::new(node.index() - 1))
} else {
None
}
}
Kind::General(g) => g.immediate_dominators[node],
}
}
/// Provides an iterator over each dominator up the CFG, for the given Node.
@ -343,7 +389,10 @@ impl<Node: Idx> Dominators<Node> {
/// of two unrelated nodes will also be consistent, but otherwise the order has no
/// meaning.) This method cannot be used to determine if either Node dominates the other.
pub fn cmp_in_dominator_order(&self, lhs: Node, rhs: Node) -> Ordering {
self.post_order_rank[rhs].cmp(&self.post_order_rank[lhs])
match &self.kind {
Kind::Path => lhs.index().cmp(&rhs.index()),
Kind::General(g) => g.post_order_rank[rhs].cmp(&g.post_order_rank[lhs]),
}
}
/// Returns true if `a` dominates `b`.
@ -352,10 +401,15 @@ impl<Node: Idx> Dominators<Node> {
///
/// Panics if `b` is unreachable.
pub fn dominates(&self, a: Node, b: Node) -> bool {
let a = self.time[a];
let b = self.time[b];
assert!(b.start != 0, "node {b:?} is not reachable");
a.start <= b.start && b.finish <= a.finish
match &self.kind {
Kind::Path => a.index() <= b.index(),
Kind::General(g) => {
let a = g.time[a];
let b = g.time[b];
assert!(b.start != 0, "node {b:?} is not reachable");
a.start <= b.start && b.finish <= a.finish
}
}
}
}

45
compiler/rustc_data_structures/src/graph/dominators/tests.rs
View File

@ -6,12 +6,11 @@ use super::super::tests::TestGraph;
fn diamond() {
let graph = TestGraph::new(0, &[(0, 1), (0, 2), (1, 3), (2, 3)]);
let dominators = dominators(&graph);
let immediate_dominators = &dominators.immediate_dominators;
assert_eq!(immediate_dominators[0], None);
assert_eq!(immediate_dominators[1], Some(0));
assert_eq!(immediate_dominators[2], Some(0));
assert_eq!(immediate_dominators[3], Some(0));
let d = dominators(&graph);
assert_eq!(d.immediate_dominator(0), None);
assert_eq!(d.immediate_dominator(1), Some(0));
assert_eq!(d.immediate_dominator(2), Some(0));
assert_eq!(d.immediate_dominator(3), Some(0));
}
#[test]
@ -22,15 +21,14 @@ fn paper() {
&[(6, 5), (6, 4), (5, 1), (4, 2), (4, 3), (1, 2), (2, 3), (3, 2), (2, 1)],
);
let dominators = dominators(&graph);
let immediate_dominators = &dominators.immediate_dominators;
assert_eq!(immediate_dominators[0], None); // <-- note that 0 is not in graph
assert_eq!(immediate_dominators[1], Some(6));
assert_eq!(immediate_dominators[2], Some(6));
assert_eq!(immediate_dominators[3], Some(6));
assert_eq!(immediate_dominators[4], Some(6));
assert_eq!(immediate_dominators[5], Some(6));
assert_eq!(immediate_dominators[6], None);
let d = dominators(&graph);
assert_eq!(d.immediate_dominator(0), None); // <-- note that 0 is not in graph
assert_eq!(d.immediate_dominator(1), Some(6));
assert_eq!(d.immediate_dominator(2), Some(6));
assert_eq!(d.immediate_dominator(3), Some(6));
assert_eq!(d.immediate_dominator(4), Some(6));
assert_eq!(d.immediate_dominator(5), Some(6));
assert_eq!(d.immediate_dominator(6), None);
}
#[test]
@ -47,11 +45,11 @@ fn paper_slt() {
#[test]
fn immediate_dominator() {
let graph = TestGraph::new(1, &[(1, 2), (2, 3)]);
let dominators = dominators(&graph);
assert_eq!(dominators.immediate_dominator(0), None);
assert_eq!(dominators.immediate_dominator(1), None);
assert_eq!(dominators.immediate_dominator(2), Some(1));
assert_eq!(dominators.immediate_dominator(3), Some(2));
let d = dominators(&graph);
assert_eq!(d.immediate_dominator(0), None);
assert_eq!(d.immediate_dominator(1), None);
assert_eq!(d.immediate_dominator(2), Some(1));
assert_eq!(d.immediate_dominator(3), Some(2));
}
#[test]
@ -75,8 +73,7 @@ fn transitive_dominator() {
],
);
let dom_tree = dominators(&graph);
let immediate_dominators = &dom_tree.immediate_dominators;
assert_eq!(immediate_dominators[2], Some(0));
assert_eq!(immediate_dominators[3], Some(0)); // This used to return Some(1).
let d = dominators(&graph);
assert_eq!(d.immediate_dominator(2), Some(0));
assert_eq!(d.immediate_dominator(3), Some(0)); // This used to return Some(1).
}

17
compiler/rustc_middle/src/mir/mod.rs
View File

@ -1562,6 +1562,23 @@ impl Location {
}
}
/// `DefLocation` represents the location of a definition - either an argument or an assignment
/// within MIR body.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum DefLocation {
Argument,
Body(Location),
}
impl DefLocation {
pub fn dominates(self, location: Location, dominators: &Dominators<BasicBlock>) -> bool {
match self {
DefLocation::Argument => true,
DefLocation::Body(def) => def.successor_within_block().dominates(location, dominators),
}
}
}
// Some nodes are used a lot. Make sure they don't unintentionally get bigger.
#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
mod size_asserts {

94
compiler/rustc_mir_transform/src/ssa.rs
View File

@ -15,7 +15,7 @@ use rustc_middle::mir::*;
pub struct SsaLocals {
/// Assignments to each local. This defines whether the local is SSA.
assignments: IndexVec<Local, Set1<LocationExtended>>,
assignments: IndexVec<Local, Set1<DefLocation>>,
/// We visit the body in reverse postorder, to ensure each local is assigned before it is used.
/// We remember the order in which we saw the assignments to compute the SSA values in a single
/// pass.
@ -27,55 +27,18 @@ pub struct SsaLocals {
direct_uses: IndexVec<Local, u32>,
}
/// We often encounter MIR bodies with 1 or 2 basic blocks. In those cases, it's unnecessary to
/// actually compute dominators, we can just compare block indices because bb0 is always the first
/// block, and in any body all other blocks are always dominated by bb0.
struct SmallDominators<'a> {
inner: Option<&'a Dominators<BasicBlock>>,
}
impl SmallDominators<'_> {
fn dominates(&self, first: Location, second: Location) -> bool {
if first.block == second.block {
first.statement_index <= second.statement_index
} else if let Some(inner) = &self.inner {
inner.dominates(first.block, second.block)
} else {
first.block < second.block
}
}
fn check_dominates(&mut self, set: &mut Set1<LocationExtended>, loc: Location) {
let assign_dominates = match *set {
Set1::Empty | Set1::Many => false,
Set1::One(LocationExtended::Arg) => true,
Set1::One(LocationExtended::Plain(assign)) => {
self.dominates(assign.successor_within_block(), loc)
}
};
// 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 SsaLocals {
pub fn new<'tcx>(body: &Body<'tcx>) -> SsaLocals {
let assignment_order = Vec::with_capacity(body.local_decls.len());
let assignments = IndexVec::from_elem(Set1::Empty, &body.local_decls);
let dominators =
if body.basic_blocks.len() > 2 { Some(body.basic_blocks.dominators()) } else { None };
let dominators = SmallDominators { inner: dominators };
let dominators = body.basic_blocks.dominators();
let direct_uses = IndexVec::from_elem(0, &body.local_decls);
let mut visitor = SsaVisitor { assignments, assignment_order, dominators, direct_uses };
for local in body.args_iter() {
visitor.assignments[local] = Set1::One(LocationExtended::Arg);
visitor.assignments[local] = Set1::One(DefLocation::Argument);
}
// For SSA assignments, a RPO visit will see the assignment before it sees any use.
@ -131,14 +94,7 @@ impl SsaLocals {
location: Location,
) -> bool {
match self.assignments[local] {
Set1::One(LocationExtended::Arg) => true,
Set1::One(LocationExtended::Plain(ass)) => {
if ass.block == location.block {
ass.statement_index < location.statement_index
} else {
dominators.dominates(ass.block, location.block)
}
}
Set1::One(def) => def.dominates(location, dominators),
_ => false,
}
}
@ -148,7 +104,7 @@ impl SsaLocals {
body: &'a Body<'tcx>,
) -> impl Iterator<Item = (Local, &'a Rvalue<'tcx>, Location)> + 'a {
self.assignment_order.iter().filter_map(|&local| {
if let Set1::One(LocationExtended::Plain(loc)) = self.assignments[local] {
if let Set1::One(DefLocation::Body(loc)) = self.assignments[local] {
// `loc` must point to a direct assignment to `local`.
let Either::Left(stmt) = body.stmt_at(loc) else { bug!() };
let Some((target, rvalue)) = stmt.kind.as_assign() else { bug!() };
@ -166,7 +122,7 @@ impl SsaLocals {
mut f: impl FnMut(Local, &mut Rvalue<'tcx>, Location),
) {
for &local in &self.assignment_order {
if let Set1::One(LocationExtended::Plain(loc)) = self.assignments[local] {
if let Set1::One(DefLocation::Body(loc)) = self.assignments[local] {
// `loc` must point to a direct assignment to `local`.
let bbs = basic_blocks.as_mut_preserves_cfg();
let bb = &mut bbs[loc.block];
@ -224,19 +180,29 @@ impl SsaLocals {
}
}
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
enum LocationExtended {
Plain(Location),
Arg,
}
struct SsaVisitor<'a> {
dominators: SmallDominators<'a>,
assignments: IndexVec<Local, Set1<LocationExtended>>,
dominators: &'a Dominators<BasicBlock>,
assignments: IndexVec<Local, Set1<DefLocation>>,
assignment_order: Vec<Local>,
direct_uses: IndexVec<Local, u32>,
}
impl SsaVisitor<'_> {
fn check_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(def) => def.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 {
@ -254,7 +220,7 @@ impl<'tcx> Visitor<'tcx> for SsaVisitor<'_> {
self.assignments[local] = Set1::Many;
}
PlaceContext::NonMutatingUse(_) => {
self.dominators.check_dominates(&mut self.assignments[local], loc);
self.check_dominates(local, loc);
self.direct_uses[local] += 1;
}
PlaceContext::NonUse(_) => {}
@ -269,7 +235,7 @@ impl<'tcx> Visitor<'tcx> for SsaVisitor<'_> {
let new_ctxt = PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy);
self.visit_projection(place.as_ref(), new_ctxt, loc);
self.dominators.check_dominates(&mut self.assignments[place.local], loc);
self.check_dominates(place.local, loc);
}
return;
} else {
@ -280,7 +246,7 @@ impl<'tcx> Visitor<'tcx> for SsaVisitor<'_> {
fn visit_assign(&mut self, place: &Place<'tcx>, rvalue: &Rvalue<'tcx>, loc: Location) {
if let Some(local) = place.as_local() {
self.assignments[local].insert(LocationExtended::Plain(loc));
self.assignments[local].insert(DefLocation::Body(loc));
if let Set1::One(_) = self.assignments[local] {
// Only record if SSA-like, to avoid growing the vector needlessly.
self.assignment_order.push(local);
@ -356,7 +322,7 @@ fn compute_copy_classes(ssa: &mut SsaLocals, body: &Body<'_>) {
#[derive(Debug)]
pub(crate) struct StorageLiveLocals {
/// Set of "StorageLive" statements for each local.
storage_live: IndexVec<Local, Set1<LocationExtended>>,
storage_live: IndexVec<Local, Set1<DefLocation>>,
}
impl StorageLiveLocals {
@ -366,13 +332,13 @@ impl StorageLiveLocals {
) -> StorageLiveLocals {
let mut storage_live = IndexVec::from_elem(Set1::Empty, &body.local_decls);
for local in always_storage_live_locals.iter() {
storage_live[local] = Set1::One(LocationExtended::Arg);
storage_live[local] = Set1::One(DefLocation::Argument);
}
for (block, bbdata) in body.basic_blocks.iter_enumerated() {
for (statement_index, statement) in bbdata.statements.iter().enumerate() {
if let StatementKind::StorageLive(local) = statement.kind {
storage_live[local]
.insert(LocationExtended::Plain(Location { block, statement_index }));
.insert(DefLocation::Body(Location { block, statement_index }));
}
}
}