Implement new dataflow framework and cursor

Author: ecstatic-morse

src/librustc_mir/dataflow/generic/cursor.rs

@@ -0,0 +1,265 @@
+//! Random access inspection of the results of a dataflow analysis.
+
+use std::borrow::Borrow;
+
+use rustc::mir::{self, BasicBlock, Location};
+use rustc_index::bit_set::BitSet;
+
+use super::{Analysis, Results};
+
+/// A `ResultsCursor` that borrows the underlying `Results`.
+pub type ResultsRefCursor<'a, 'mir, 'tcx, A> = ResultsCursor<'mir, 'tcx, A, &'a Results<'tcx, A>>;
+
+/// Allows random access inspection of the results of a dataflow analysis.
+///
+/// This cursor only has linear performance within a basic block when its statements are visited in
+/// order. In the worst case—when statements are visited in *reverse* order—performance will be
+/// quadratic in the number of statements in the block. The order in which basic blocks are
+/// inspected has no impact on performance.
+///
+/// A `ResultsCursor` can either own (the default) or borrow the dataflow results it inspects. The
+/// type of ownership is determined by `R` (see `ResultsRefCursor` above).
+pub struct ResultsCursor<'mir, 'tcx, A, R = Results<'tcx, A>>
+where
+    A: Analysis<'tcx>,
+{
+    body: &'mir mir::Body<'tcx>,
+    results: R,
+    state: BitSet<A::Idx>,
+
+    pos: CursorPosition,
+
+    /// When this flag is set, the cursor is pointing at a `Call` terminator whose call return
+    /// effect has been applied to `state`.
+    ///
+    /// This flag helps to ensure that multiple calls to `seek_after_assume_call_returns` with the
+    /// same target will result in exactly one invocation of `apply_call_return_effect`. It is
+    /// sufficient to clear this only in `seek_to_block_start`, since seeking away from a
+    /// terminator will always require a cursor reset.
+    call_return_effect_applied: bool,
+}
+
+impl<'mir, 'tcx, A, R> ResultsCursor<'mir, 'tcx, A, R>
+where
+    A: Analysis<'tcx>,
+    R: Borrow<Results<'tcx, A>>,
+{
+    /// Returns a new cursor for `results` that points to the start of the `START_BLOCK`.
+    pub fn new(body: &'mir mir::Body<'tcx>, results: R) -> Self {
+        ResultsCursor {
+            body,
+            pos: CursorPosition::BlockStart(mir::START_BLOCK),
+            state: results.borrow().entry_sets[mir::START_BLOCK].clone(),
+            call_return_effect_applied: false,
+            results,
+        }
+    }
+
+    /// Returns the `Analysis` used to generate the underlying results.
+    pub fn analysis(&self) -> &A {
+        &self.results.borrow().analysis
+    }
+
+    /// Returns the dataflow state at the current location.
+    pub fn get(&self) -> &BitSet<A::Idx> {
+        &self.state
+    }
+
+    /// Resets the cursor to the start of the given basic block.
+    pub fn seek_to_block_start(&mut self, block: BasicBlock) {
+        self.state.overwrite(&self.results.borrow().entry_sets[block]);
+        self.pos = CursorPosition::BlockStart(block);
+        self.call_return_effect_applied = false;
+    }
+
+    /// Advances the cursor to hold all effects up to and including to the "before" effect of the
+    /// statement (or terminator) at the given location.
+    ///
+    /// If you wish to observe the full effect of a statement or terminator, not just the "before"
+    /// effect, use `seek_after` or `seek_after_assume_call_returns`.
+    pub fn seek_before(&mut self, target: Location) {
+        assert!(target <= self.body.terminator_loc(target.block));
+        self._seek(target, false);
+    }
+
+    /// Advances the cursor to hold the full effect of all statements (and possibly closing
+    /// terminators) up to and including the `target`.
+    ///
+    /// If the `target` is a `Call` terminator, any call return effect for that terminator will
+    /// **not** be observed. Use `seek_after_assume_call_returns` if you wish to observe the call
+    /// return effect.
+    pub fn seek_after(&mut self, target: Location) {
+        assert!(target <= self.body.terminator_loc(target.block));
+
+        // If we have already applied the call return effect, we are currently pointing at a `Call`
+        // terminator. Unconditionally reset the dataflow cursor, since there is no way to "undo"
+        // the call return effect.
+        if self.call_return_effect_applied {
+            self.seek_to_block_start(target.block);
+        }
+
+        self._seek(target, true);
+    }
+
+    /// Advances the cursor to hold all effects up to and including of the statement (or
+    /// terminator) at the given location.
+    ///
+    /// If the `target` is a `Call` terminator, any call return effect for that terminator will
+    /// be observed. Use `seek_after` if you do **not** wish to observe the call return effect.
+    pub fn seek_after_assume_call_returns(&mut self, target: Location) {
+        let terminator_loc = self.body.terminator_loc(target.block);
+        assert!(target.statement_index <= terminator_loc.statement_index);
+
+        self._seek(target, true);
+
+        if target != terminator_loc {
+            return;
+        }
+
+        let terminator = self.body.basic_blocks()[target.block].terminator();
+        if let mir::TerminatorKind::Call {
+            destination: Some((return_place, _)), func, args, ..
+        } = &terminator.kind
+        {
+            if !self.call_return_effect_applied {
+                self.call_return_effect_applied = true;
+                self.results.borrow().analysis.apply_call_return_effect(
+                    &mut self.state,
+                    target.block,
+                    func,
+                    args,
+                    return_place,
+                );
+            }
+        }
+    }
+
+    fn _seek(&mut self, target: Location, apply_after_effect_at_target: bool) {
+        use CursorPosition::*;
+
+        match self.pos {
+            // Return early if we are already at the target location.
+            Before(curr) if curr == target && !apply_after_effect_at_target => return,
+            After(curr) if curr == target && apply_after_effect_at_target => return,
+
+            // Otherwise, we must reset to the start of the target block if...
+
+            // we are in a different block entirely.
+            BlockStart(block) | Before(Location { block, .. }) | After(Location { block, .. })
+                if block != target.block =>
+            {
+                self.seek_to_block_start(target.block)
+            }
+
+            // we are in the same block but have advanced past the target statement.
+            Before(curr) | After(curr) if curr.statement_index > target.statement_index => {
+                self.seek_to_block_start(target.block)
+            }
+
+            // we have already applied the entire effect of a statement but only wish to observe
+            // its "before" effect.
+            After(curr)
+                if curr.statement_index == target.statement_index
+                    && !apply_after_effect_at_target =>
+            {
+                self.seek_to_block_start(target.block)
+            }
+
+            // N.B., `call_return_effect_applied` is checked in `seek_after`, not here.
+            _ => (),
+        }
+
+        let analysis = &self.results.borrow().analysis;
+        let block_data = &self.body.basic_blocks()[target.block];
+
+        // At this point, the cursor is in the same block as the target location at an earlier
+        // statement.
+        debug_assert_eq!(target.block, self.pos.block());
+
+        // Find the first statement whose transfer function has not yet been applied.
+        let first_unapplied_statement = match self.pos {
+            BlockStart(_) => 0,
+            After(Location { statement_index, .. }) => statement_index + 1,
+
+            // If we have only applied the "before" effect for the current statement, apply the
+            // remainder before continuing.
+            Before(curr) => {
+                if curr.statement_index == block_data.statements.len() {
+                    let terminator = block_data.terminator();
+                    analysis.apply_terminator_effect(&mut self.state, terminator, curr);
+                } else {
+                    let statement = &block_data.statements[curr.statement_index];
+                    analysis.apply_statement_effect(&mut self.state, statement, curr);
+                }
+
+                // If all we needed to do was go from `Before` to `After` in the same statement,
+                // we are now done.
+                if curr.statement_index == target.statement_index {
+                    debug_assert!(apply_after_effect_at_target);
+                    self.pos = After(target);
+                    return;
+                }
+
+                curr.statement_index + 1
+            }
+        };
+
+        // We have now applied all effects prior to `first_unapplied_statement`.
+
+        // Apply the effects of all statements before `target`.
+        let mut location = Location { block: target.block, statement_index: 0 };
+        for statement_index in first_unapplied_statement..target.statement_index {
+            location.statement_index = statement_index;
+            let statement = &block_data.statements[statement_index];
+            analysis.apply_before_statement_effect(&mut self.state, statement, location);
+            analysis.apply_statement_effect(&mut self.state, statement, location);
+        }
+
+        // Apply the effect of the statement (or terminator) at `target`.
+        location.statement_index = target.statement_index;
+        if target.statement_index == block_data.statements.len() {
+            let terminator = &block_data.terminator();
+            analysis.apply_before_terminator_effect(&mut self.state, terminator, location);
+
+            if apply_after_effect_at_target {
+                analysis.apply_terminator_effect(&mut self.state, terminator, location);
+                self.pos = After(target);
+            } else {
+                self.pos = Before(target);
+            }
+        } else {
+            let statement = &block_data.statements[target.statement_index];
+            analysis.apply_before_statement_effect(&mut self.state, statement, location);
+
+            if apply_after_effect_at_target {
+                analysis.apply_statement_effect(&mut self.state, statement, location);
+                self.pos = After(target)
+            } else {
+                self.pos = Before(target);
+            }
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug)]
+enum CursorPosition {
+    /// No effects within this block have been applied.
+    BlockStart(BasicBlock),
+
+    /// Only the "before" effect of the statement (or terminator) at this location has been
+    /// applied (along with the effects of all previous statements).
+    Before(Location),
+
+    /// The effects of all statements up to and including the one at this location have been
+    /// applied.
+    After(Location),
+}
+
+impl CursorPosition {
+    fn block(&self) -> BasicBlock {
+        match *self {
+            Self::BlockStart(block) => block,
+            Self::Before(loc) | Self::After(loc) => loc.block,
+        }
+    }
+}