Collect conflict information in GeneratorLayout

Author: tmandry

src/librustc/mir/mod.rs

@@ -9,6 +9,7 @@ use crate::hir::def_id::DefId;
 use crate::hir::{self, InlineAsm as HirInlineAsm};
 use crate::mir::interpret::{ConstValue, InterpError, Scalar};
 use crate::mir::visit::MirVisitable;
+use rustc_data_structures::bit_set::BitSet;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::graph::dominators::{dominators, Dominators};
 use rustc_data_structures::graph::{self, GraphPredecessors, GraphSuccessors};
@@ -1500,6 +1501,13 @@ impl<'tcx> BasicBlockData<'tcx> {
         self.terminator.as_mut().expect("invalid terminator state")
     }
 
+    pub fn is_unreachable(&self) -> bool {
+        match self.terminator().kind {
+            TerminatorKind::Unreachable => true,
+            _ => false,
+        }
+    }
+
     pub fn retain_statements<F>(&mut self, mut f: F)
     where
         F: FnMut(&mut Statement<'_>) -> bool,
@@ -2997,6 +3005,11 @@ pub struct GeneratorLayout<'tcx> {
     /// be stored in multiple variants.
     pub variant_fields: IndexVec<VariantIdx, IndexVec<Field, GeneratorSavedLocal>>,
 
+    /// Which saved locals are storage-live at the same time. Locals that do not
+    /// have conflicts with each other are allowed to overlap in the computed
+    /// layout.
+    pub storage_conflicts: IndexVec<GeneratorSavedLocal, BitSet<GeneratorSavedLocal>>,
+
     /// Names and scopes of all the stored generator locals.
     /// NOTE(tmandry) This is *strictly* a temporary hack for codegen
     /// debuginfo generation, and will be removed at some point.
@@ -3193,6 +3206,7 @@ BraceStructTypeFoldableImpl! {
     impl<'tcx> TypeFoldable<'tcx> for GeneratorLayout<'tcx> {
         field_tys,
         variant_fields,
+        storage_conflicts,
         __local_debuginfo_codegen_only_do_not_use,
     }
 }
@@ -3572,6 +3586,15 @@ impl<'tcx> TypeFoldable<'tcx> for GeneratorSavedLocal {
     }
 }
 
+impl<'tcx, T: Idx> TypeFoldable<'tcx> for BitSet<T> {
+    fn super_fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, _: &mut F) -> Self {
+        self.clone()
+    }
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _: &mut V) -> bool {
+        false
+    }
+}
+
 impl<'tcx> TypeFoldable<'tcx> for Constant<'tcx> {
     fn super_fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {
         Constant {

src/librustc_mir/dataflow/mod.rs

@@ -380,6 +380,62 @@ pub fn state_for_location<'tcx, T: BitDenotation<'tcx>>(loc: Location,
     gen_set.to_dense()
 }
 
+/// Calls `f` with the dataflow state at every location in `mir`.
+/// Ignores blocks that terminate in `unreachable`.
+pub fn for_each_location<'tcx, T: BitDenotation<'tcx>>(
+    mir: &Body<'tcx>,
+    analysis: &T,
+    result: &DataflowResults<'tcx, T>,
+    mut f: impl FnMut(&HybridBitSet<T::Idx>, Location)
+) {
+    for (block, bb_data) in mir.basic_blocks().iter_enumerated() {
+        if bb_data.is_unreachable() {
+            continue;
+        }
+        for_each_block_location(mir, block, bb_data, analysis, result, &mut f);
+    }
+}
+
+fn for_each_block_location<'tcx, T: BitDenotation<'tcx>>(
+    mir: &Body<'tcx>,
+    block: BasicBlock,
+    bb_data: &BasicBlockData<'tcx>,
+    analysis: &T,
+    result: &DataflowResults<'tcx, T>,
+    f: &mut impl FnMut(&HybridBitSet<T::Idx>, Location)
+) {
+    let statements = &bb_data.statements;
+
+    let mut on_entry = result.sets().on_entry_set_for(block.index()).to_owned();
+    let mut kill_set = on_entry.to_hybrid();
+    let mut gen_set = kill_set.clone();
+
+    {
+        let mut sets = BlockSets {
+            on_entry: &mut on_entry,
+            kill_set: &mut kill_set,
+            gen_set: &mut gen_set,
+        };
+        // FIXME: This location is technically wrong, but there isn't a way to
+        // denote the start of a block.
+        f(sets.gen_set, Location { block, statement_index: 0 });
+
+        for statement_index in 0..statements.len() {
+            let loc = Location { block, statement_index };
+            analysis.before_statement_effect(&mut sets, loc);
+            f(sets.gen_set, loc);
+            analysis.statement_effect(&mut sets, loc);
+            f(sets.gen_set, loc);
+        }
+
+        let term_loc = Location { block, statement_index: mir[block].statements.len() };
+        analysis.before_terminator_effect(&mut sets, term_loc);
+        f(sets.gen_set, term_loc);
+        analysis.before_statement_effect(&mut sets, term_loc);
+        f(sets.gen_set, term_loc);
+    }
+}
+
 pub struct DataflowAnalysis<'a, 'tcx: 'a, O> where O: BitDenotation<'tcx>
 {
     flow_state: DataflowState<'tcx, O>,

src/librustc_mir/transform/generator.rs

@@ -66,7 +66,8 @@ use std::mem;
 use crate::transform::{MirPass, MirSource};
 use crate::transform::simplify;
 use crate::transform::no_landing_pads::no_landing_pads;
-use crate::dataflow::{do_dataflow, DebugFormatted, state_for_location};
+use crate::dataflow::{DataflowResults};
+use crate::dataflow::{do_dataflow, DebugFormatted, state_for_location, for_each_location};
 use crate::dataflow::{MaybeStorageLive, HaveBeenBorrowedLocals};
 use crate::util::dump_mir;
 use crate::util::liveness;
@@ -400,6 +401,7 @@ fn locals_live_across_suspend_points(
     movable: bool,
 ) -> (
     liveness::LiveVarSet,
+    IndexVec<GeneratorSavedLocal, BitSet<GeneratorSavedLocal>>,
     FxHashMap<BasicBlock, liveness::LiveVarSet>,
     BitSet<BasicBlock>,
 ) {
@@ -503,7 +505,99 @@ fn locals_live_across_suspend_points(
     // The generator argument is ignored
     set.remove(self_arg());
 
-    (set, storage_liveness_map, suspending_blocks)
+    let storage_conflicts = compute_storage_conflicts(
+        body,
+        &set,
+        &ignored,
+        storage_live,
+        storage_live_analysis);
+
+    (set, storage_conflicts, storage_liveness_map, suspending_blocks)
+}
+
+/// For every saved local, looks for which locals are StorageLive at the same
+/// time. Generates a bitset for every local of all the other locals that may be
+/// StorageLive simultaneously with that local. This is used in the layout
+/// computation; see `GeneratorLayout` for more.
+fn compute_storage_conflicts(
+    body: &'mir Body<'tcx>,
+    stored_locals: &liveness::LiveVarSet,
+    ignored: &StorageIgnored,
+    storage_live: DataflowResults<'tcx, MaybeStorageLive<'mir, 'tcx>>,
+    storage_live_analysis: MaybeStorageLive<'mir, 'tcx>,
+) -> IndexVec<GeneratorSavedLocal, BitSet<GeneratorSavedLocal>> {
+    debug!("compute_storage_conflicts({:?})", body.span);
+    debug!("ignored = {:?}", ignored.0);
+
+    // Storage ignored locals are not eligible for overlap, since their storage
+    // is always live.
+    let mut ineligible_locals = ignored.0.clone();
+    ineligible_locals.intersect(&stored_locals);
+
+    // Of our remaining candidates, find out if any have overlapping storage
+    // liveness. Those that do must be in the same variant to remain candidates.
+    // FIXME(tmandry): Consider using sparse bitsets here once we have good
+    // benchmarks for generators.
+    let mut local_conflicts: IndexVec<Local, _> =
+        // Add conflicts for every ineligible local.
+        iter::repeat(ineligible_locals.clone())
+        .take(body.local_decls.len())
+        .collect();
+
+    for_each_location(body, &storage_live_analysis, &storage_live, |state, loc| {
+        let mut eligible_storage_live = state.clone().to_dense();
+        eligible_storage_live.intersect(&stored_locals);
+
+        for local in eligible_storage_live.iter() {
+            let mut overlaps = eligible_storage_live.clone();
+            overlaps.remove(local);
+            local_conflicts[local].union(&overlaps);
+
+            if !overlaps.is_empty() {
+                trace!("at {:?}, local {:?} conflicts with {:?}",
+                       loc, local, overlaps);
+            }
+        }
+    });
+
+    // NOTE: Today we store a full conflict bitset for every local. Technically
+    // this is twice as many bits as we need, since the relation is symmetric.
+    // However, in practice these bitsets are not usually large. The layout code
+    // also needs to keep track of how many conflicts each local has, so it's
+    // simpler to keep it this way for now.
+    let storage_conflicts: IndexVec<GeneratorSavedLocal, _> = stored_locals
+        .iter()
+        .map(|local_a| {
+            if ineligible_locals.contains(local_a) {
+                // Conflicts with everything.
+                BitSet::new_filled(stored_locals.count())
+            } else {
+                // Keep overlap information only for stored locals.
+                renumber_bitset(&local_conflicts[local_a], stored_locals)
+            }
+        })
+        .collect();
+
+    storage_conflicts
+}
+
+/// Renumbers the items present in `stored_locals` and applies the renumbering
+/// to 'input`.
+///
+/// For example, if `stored_locals = [1, 3, 5]`, this would be renumbered to
+/// `[0, 1, 2]`. Thus, if `input = [3, 5]` we would return `[1, 2]`.
+fn renumber_bitset(input: &BitSet<Local>, stored_locals: &liveness::LiveVarSet)
+-> BitSet<GeneratorSavedLocal> {
+    assert!(stored_locals.superset(&input), "{:?} not a superset of {:?}", stored_locals, input);
+    let mut out = BitSet::new_empty(stored_locals.count());
+    for (idx, local) in stored_locals.iter().enumerate() {
+        let saved_local = GeneratorSavedLocal::from(idx);
+        if input.contains(local) {
+            out.insert(saved_local);
+        }
+    }
+    debug!("renumber_bitset({:?}, {:?}) => {:?}", input, stored_locals, out);
+    out
 }
 
 fn compute_layout<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
@@ -517,7 +611,7 @@ fn compute_layout<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         FxHashMap<BasicBlock, liveness::LiveVarSet>)
 {
     // Use a liveness analysis to compute locals which are live across a suspension point
-    let (live_locals, storage_liveness, suspending_blocks) =
+    let (live_locals, storage_conflicts, storage_liveness, suspending_blocks) =
         locals_live_across_suspend_points(tcx, body, source, movable);
 
     // Erase regions from the types passed in from typeck so we can compare them with
@@ -547,7 +641,7 @@ fn compute_layout<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
     let dummy_local = LocalDecl::new_internal(tcx.mk_unit(), body.span);
 
-    // Gather live locals and their indices replacing values in mir.local_decls with a dummy
+    // Gather live locals and their indices replacing values in body.local_decls with a dummy
     // to avoid changing local indices
     let live_decls = live_locals.iter().map(|local| {
         let var = mem::replace(&mut body.local_decls[local], dummy_local.clone());
@@ -568,6 +662,7 @@ fn compute_layout<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         remap.insert(local, (var.ty, variant_index, idx));
         decls.push(var);
     }
+    debug!("generator saved local mappings: {:?}", decls);
     let field_tys = decls.iter().map(|field| field.ty).collect::<IndexVec<_, _>>();
 
     // Put every var in each variant, for now.
@@ -578,6 +673,7 @@ fn compute_layout<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     let layout = GeneratorLayout {
         field_tys,
         variant_fields: empty_variants.chain(state_variants).collect(),
+        storage_conflicts,
         __local_debuginfo_codegen_only_do_not_use: decls,
     };