MIR dominators: computed on-demand and cached

 Cached inside MIR body - motivated by potential reuse opportunity when
 code generating different instantiations of the same generic code.

Author: tmiasko

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -2193,7 +2193,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         let mut back_edge_stack = Vec::new();
 
         predecessor_locations(self.body, location).for_each(|predecessor| {
-            if location.dominates(predecessor, self.dominators()) {
+            if location.dominates(predecessor, self.body.basic_blocks.dominators()) {
                 back_edge_stack.push(predecessor)
             } else {
                 stack.push(predecessor);
@@ -2305,7 +2305,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
             let mut has_predecessor = false;
             predecessor_locations(self.body, location).for_each(|predecessor| {
-                if location.dominates(predecessor, self.dominators()) {
+                if location.dominates(predecessor, self.body.basic_blocks.dominators()) {
                     back_edge_stack.push(predecessor)
                 } else {
                     stack.push(predecessor);

compiler/rustc_borrowck/src/invalidation.rs

@@ -1,8 +1,7 @@
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
-use rustc_data_structures::graph::dominators::Dominators;
 use rustc_middle::mir::visit::Visitor;
-use rustc_middle::mir::{self, BasicBlock, Body, Location, NonDivergingIntrinsic, Place, Rvalue};
+use rustc_middle::mir::{self, Body, Location, NonDivergingIntrinsic, Place, Rvalue};
 use rustc_middle::mir::{BorrowKind, Mutability, Operand};
 use rustc_middle::mir::{InlineAsmOperand, Terminator, TerminatorKind};
 use rustc_middle::mir::{Statement, StatementKind};
@@ -28,15 +27,8 @@ pub(super) fn generate_invalidates<'tcx>(
 
     if let Some(all_facts) = all_facts {
         let _prof_timer = tcx.prof.generic_activity("polonius_fact_generation");
-        let dominators = body.basic_blocks.dominators();
-        let mut ig = InvalidationGenerator {
-            all_facts,
-            borrow_set,
-            tcx,
-            location_table,
-            body: &body,
-            dominators,
-        };
+        let mut ig =
+            InvalidationGenerator { all_facts, borrow_set, tcx, location_table, body: &body };
         ig.visit_body(body);
     }
 }
@@ -46,7 +38,6 @@ struct InvalidationGenerator<'cx, 'tcx> {
     all_facts: &'cx mut AllFacts,
     location_table: &'cx LocationTable,
     body: &'cx Body<'tcx>,
-    dominators: Dominators<BasicBlock>,
     borrow_set: &'cx BorrowSet<'tcx>,
 }
 
@@ -389,7 +380,7 @@ impl<'cx, 'tcx> InvalidationGenerator<'cx, 'tcx> {
 
                     (Read(_), BorrowKind::Unique | BorrowKind::Mut { .. }) => {
                         // Reading from mere reservations of mutable-borrows is OK.
-                        if !is_active(&this.dominators, borrow, location) {
+                        if !is_active(this.body.basic_blocks.dominators(), borrow, location) {
                             // If the borrow isn't active yet, reads don't invalidate it
                             assert!(allow_two_phase_borrow(borrow.kind));
                             return Control::Continue;

compiler/rustc_borrowck/src/lib.rs

@@ -5,7 +5,6 @@
 #![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(never_type)]
-#![feature(once_cell)]
 #![feature(rustc_attrs)]
 #![feature(stmt_expr_attributes)]
 #![feature(trusted_step)]
@@ -18,7 +17,6 @@ extern crate rustc_middle;
 extern crate tracing;
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-use rustc_data_structures::graph::dominators::Dominators;
 use rustc_data_structures::vec_map::VecMap;
 use rustc_errors::{Diagnostic, DiagnosticBuilder};
 use rustc_hir as hir;
@@ -30,7 +28,7 @@ use rustc_middle::mir::{
     traversal, Body, ClearCrossCrate, Local, Location, Mutability, NonDivergingIntrinsic, Operand,
     Place, PlaceElem, PlaceRef, VarDebugInfoContents,
 };
-use rustc_middle::mir::{AggregateKind, BasicBlock, BorrowCheckResult, BorrowKind};
+use rustc_middle::mir::{AggregateKind, BorrowCheckResult, BorrowKind};
 use rustc_middle::mir::{Field, ProjectionElem, Promoted, Rvalue, Statement, StatementKind};
 use rustc_middle::mir::{InlineAsmOperand, Terminator, TerminatorKind};
 use rustc_middle::ty::query::Providers;
@@ -40,7 +38,6 @@ use rustc_span::{Span, Symbol};
 
 use either::Either;
 use smallvec::SmallVec;
-use std::cell::OnceCell;
 use std::cell::RefCell;
 use std::collections::BTreeMap;
 use std::rc::Rc;
@@ -335,7 +332,6 @@ fn do_mir_borrowck<'tcx>(
                 used_mut: Default::default(),
                 used_mut_upvars: SmallVec::new(),
                 borrow_set: Rc::clone(&borrow_set),
-                dominators: Default::default(),
                 upvars: Vec::new(),
                 local_names: IndexVec::from_elem(None, &promoted_body.local_decls),
                 region_names: RefCell::default(),
@@ -364,7 +360,6 @@ fn do_mir_borrowck<'tcx>(
         used_mut: Default::default(),
         used_mut_upvars: SmallVec::new(),
         borrow_set: Rc::clone(&borrow_set),
-        dominators: Default::default(),
         upvars,
         local_names,
         region_names: RefCell::default(),
@@ -533,9 +528,6 @@ struct MirBorrowckCtxt<'cx, 'tcx> {
     /// The set of borrows extracted from the MIR
     borrow_set: Rc<BorrowSet<'tcx>>,
 
-    /// Dominators for MIR
-    dominators: OnceCell<Dominators<BasicBlock>>,
-
     /// Information about upvars not necessarily preserved in types or MIR
     upvars: Vec<Upvar<'tcx>>,
 
@@ -1051,7 +1043,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
                 (Read(kind), BorrowKind::Unique | BorrowKind::Mut { .. }) => {
                     // Reading from mere reservations of mutable-borrows is OK.
-                    if !is_active(this.dominators(), borrow, location) {
+                    if !is_active(this.body.basic_blocks.dominators(), borrow, location) {
                         assert!(allow_two_phase_borrow(borrow.kind));
                         return Control::Continue;
                     }
@@ -2219,10 +2211,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     fn is_upvar_field_projection(&self, place_ref: PlaceRef<'tcx>) -> Option<Field> {
         path_utils::is_upvar_field_projection(self.infcx.tcx, &self.upvars, place_ref, self.body())
     }
-
-    fn dominators(&self) -> &Dominators<BasicBlock> {
-        self.dominators.get_or_init(|| self.body.basic_blocks.dominators())
-    }
 }
 
 mod error {

compiler/rustc_codegen_ssa/src/mir/analyze.rs

@@ -69,7 +69,7 @@ enum LocalKind {
 
 struct LocalAnalyzer<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
     fx: &'mir FunctionCx<'a, 'tcx, Bx>,
-    dominators: Dominators<mir::BasicBlock>,
+    dominators: &'mir Dominators<mir::BasicBlock>,
     locals: IndexVec<mir::Local, LocalKind>,
 }
 

compiler/rustc_middle/src/mir/basic_blocks.rs

@@ -23,6 +23,7 @@ pub type SwitchSources = FxHashMap<(BasicBlock, BasicBlock), SmallVec<[Option<u1
 
 #[derive(Clone, Default, Debug)]
 struct Cache {
+    dominators: OnceCell<Dominators<BasicBlock>>,
     predecessors: OnceCell<Predecessors>,
     switch_sources: OnceCell<SwitchSources>,
     is_cyclic: OnceCell<bool>,
@@ -42,8 +43,8 @@ impl<'tcx> BasicBlocks<'tcx> {
     }
 
     #[inline]
-    pub fn dominators(&self) -> Dominators<BasicBlock> {
-        dominators(&self)
+    pub fn dominators(&self) -> &Dominators<BasicBlock> {
+        self.cache.dominators.get_or_init(|| dominators(self))
     }
 
     /// Returns predecessors for each basic block.