Refactoring: pull bitvector initialization out from other parts of dataflow.

This is meant to ease development of multi-stage dataflow analyses
where the output from one analysis is used to initialize the state
for the next; in such a context, you cannot start with `bottom_value`
for all the bits.

Author: pnkfelix

src/librustc_mir/dataflow/impls/borrows.rs

@@ -22,7 +22,7 @@ use rustc_data_structures::bitslice::{BitwiseOperator};
 use rustc_data_structures::indexed_set::{IdxSet};
 use rustc_data_structures::indexed_vec::{IndexVec};
 
-use dataflow::{BitDenotation, BlockSets, DataflowOperator};
+use dataflow::{BitDenotation, BlockSets, InitialFlow};
 pub use dataflow::indexes::BorrowIndex;
 use borrow_check::nll::region_infer::RegionInferenceContext;
 use borrow_check::nll::ToRegionVid;
@@ -339,7 +339,7 @@ impl<'a, 'gcx, 'tcx> BitwiseOperator for Borrows<'a, 'gcx, 'tcx> {
     }
 }
 
-impl<'a, 'gcx, 'tcx> DataflowOperator for Borrows<'a, 'gcx, 'tcx> {
+impl<'a, 'gcx, 'tcx> InitialFlow for Borrows<'a, 'gcx, 'tcx> {
     #[inline]
     fn bottom_value() -> bool {
         false // bottom = no Rvalue::Refs are active by default

src/librustc_mir/dataflow/impls/mod.rs

@@ -23,7 +23,7 @@ use util::elaborate_drops::DropFlagState;
 
 use super::move_paths::{HasMoveData, MoveData, MoveOutIndex, MovePathIndex, InitIndex};
 use super::move_paths::{LookupResult, InitKind};
-use super::{BitDenotation, BlockSets, DataflowOperator};
+use super::{BitDenotation, BlockSets, InitialFlow};
 
 use super::drop_flag_effects_for_function_entry;
 use super::drop_flag_effects_for_location;
@@ -702,35 +702,35 @@ impl<'a, 'gcx, 'tcx> BitwiseOperator for EverInitializedLvals<'a, 'gcx, 'tcx> {
 // propagating, or you start at all-ones and then use Intersect as
 // your merge when propagating.
 
-impl<'a, 'gcx, 'tcx> DataflowOperator for MaybeInitializedLvals<'a, 'gcx, 'tcx> {
+impl<'a, 'gcx, 'tcx> InitialFlow for MaybeInitializedLvals<'a, 'gcx, 'tcx> {
     #[inline]
     fn bottom_value() -> bool {
         false // bottom = uninitialized
     }
 }
 
-impl<'a, 'gcx, 'tcx> DataflowOperator for MaybeUninitializedLvals<'a, 'gcx, 'tcx> {
+impl<'a, 'gcx, 'tcx> InitialFlow for MaybeUninitializedLvals<'a, 'gcx, 'tcx> {
     #[inline]
     fn bottom_value() -> bool {
         false // bottom = initialized (start_block_effect counters this at outset)
     }
 }
 
-impl<'a, 'gcx, 'tcx> DataflowOperator for DefinitelyInitializedLvals<'a, 'gcx, 'tcx> {
+impl<'a, 'gcx, 'tcx> InitialFlow for DefinitelyInitializedLvals<'a, 'gcx, 'tcx> {
     #[inline]
     fn bottom_value() -> bool {
         true // bottom = initialized (start_block_effect counters this at outset)
     }
 }
 
-impl<'a, 'gcx, 'tcx> DataflowOperator for MovingOutStatements<'a, 'gcx, 'tcx> {
+impl<'a, 'gcx, 'tcx> InitialFlow for MovingOutStatements<'a, 'gcx, 'tcx> {
     #[inline]
     fn bottom_value() -> bool {
         false // bottom = no loans in scope by default
     }
 }
 
-impl<'a, 'gcx, 'tcx> DataflowOperator for EverInitializedLvals<'a, 'gcx, 'tcx> {
+impl<'a, 'gcx, 'tcx> InitialFlow for EverInitializedLvals<'a, 'gcx, 'tcx> {
     #[inline]
     fn bottom_value() -> bool {
         false // bottom = no initialized variables by default

src/librustc_mir/dataflow/impls/storage_liveness.rs

@@ -74,7 +74,7 @@ impl<'a, 'tcx> BitwiseOperator for MaybeStorageLive<'a, 'tcx> {
     }
 }
 
-impl<'a, 'tcx> DataflowOperator for MaybeStorageLive<'a, 'tcx> {
+impl<'a, 'tcx> InitialFlow for MaybeStorageLive<'a, 'tcx> {
     #[inline]
     fn bottom_value() -> bool {
         false // bottom = dead

src/librustc_mir/dataflow/mod.rs

@@ -127,7 +127,7 @@ pub(crate) fn do_dataflow<'a, 'gcx, 'tcx, BD, P>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
                                                  bd: BD,
                                                  p: P)
                                                  -> DataflowResults<BD>
-    where BD: BitDenotation,
+    where BD: BitDenotation + InitialFlow,
           P: Fn(&BD, BD::Idx) -> DebugFormatted
 {
     let name_found = |sess: &Session, attrs: &[ast::Attribute], name| -> Option<String> {
@@ -176,7 +176,6 @@ impl<'a, 'tcx: 'a, BD> DataflowAnalysis<'a, 'tcx, BD> where BD: BitDenotation
         };
         while propcx.changed {
             propcx.changed = false;
-            propcx.reset(&mut temp);
             propcx.walk_cfg(&mut temp);
         }
     }
@@ -212,13 +211,6 @@ impl<'a, 'tcx: 'a, BD> DataflowAnalysis<'a, 'tcx, BD> where BD: BitDenotation
 
 impl<'b, 'a: 'b, 'tcx: 'a, BD> PropagationContext<'b, 'a, 'tcx, BD> where BD: BitDenotation
 {
-    fn reset(&mut self, bits: &mut IdxSet<BD::Idx>) {
-        let e = if BD::bottom_value() {!0} else {0};
-        for b in bits.words_mut() {
-            *b = e;
-        }
-    }
-
     fn walk_cfg(&mut self, in_out: &mut IdxSet<BD::Idx>) {
         let mir = self.builder.mir;
         for (bb_idx, bb_data) in mir.basic_blocks().iter().enumerate() {
@@ -554,12 +546,17 @@ impl<E:Idx> AllSets<E> {
 }
 
 /// Parameterization for the precise form of data flow that is used.
-pub trait DataflowOperator: BitwiseOperator {
+/// `InitialFlow` handles initializing the bitvectors before any
+/// code is inspected by the analysis. Analyses that need more nuanced
+/// initialization (e.g. they need to consult the results of some other
+/// dataflow analysis to set up the initial bitvectors) should not
+/// implement this.
+pub trait InitialFlow {
     /// Specifies the initial value for each bit in the `on_entry` set
     fn bottom_value() -> bool;
 }
 
-pub trait BitDenotation: DataflowOperator {
+pub trait BitDenotation: BitwiseOperator {
     /// Specifies what index type is used to access the bitvector.
     type Idx: Idx;
 
@@ -642,7 +639,7 @@ impl<'a, 'gcx, 'tcx: 'a, D> DataflowAnalysis<'a, 'tcx, D> where D: BitDenotation
     pub fn new(_tcx: TyCtxt<'a, 'gcx, 'tcx>,
                mir: &'a Mir<'tcx>,
                dead_unwinds: &'a IdxSet<mir::BasicBlock>,
-               denotation: D) -> Self {
+               denotation: D) -> Self where D: InitialFlow {
         let bits_per_block = denotation.bits_per_block();
         let usize_bits = mem::size_of::<usize>() * 8;
         let words_per_block = (bits_per_block + usize_bits - 1) / usize_bits;