rustc_mir_dataflow/framework/

mod.rs

//! A framework that can express both [gen-kill] and generic dataflow problems.
//!
//! To use this framework, implement the [`Analysis`] trait. There used to be a `GenKillAnalysis`
//! alternative trait for gen-kill analyses that would pre-compute the transfer function for each
//! block. It was intended as an optimization, but it ended up not being any faster than
//! `Analysis`.
//!
//! The `impls` module contains several examples of dataflow analyses.
//!
//! Then call `iterate_to_fixpoint` on your type that impls `Analysis` to get a `Results`. From
//! there, you can use a `ResultsCursor` to inspect the fixpoint solution to your dataflow problem
//! (good for inspecting a small number of locations), or implement the `ResultsVisitor` interface
//! and use `visit_results` (good for inspecting many or all locations). The following example uses
//! the `ResultsCursor` approach.
//!
//! ```ignore (cross-crate-imports)
//! use rustc_const_eval::dataflow::Analysis; // Makes `iterate_to_fixpoint` available.
//!
//! fn do_my_analysis(tcx: TyCtxt<'tcx>, body: &mir::Body<'tcx>) {
//!     let analysis = MyAnalysis::new()
//!         .iterate_to_fixpoint(tcx, body, None)
//!         .into_results_cursor(body);
//!
//!     // Print the dataflow state *after* each statement in the start block.
//!     for (_, statement_index) in body.block_data[START_BLOCK].statements.iter_enumerated() {
//!         cursor.seek_after(Location { block: START_BLOCK, statement_index });
//!         let state = cursor.get();
//!         println!("{:?}", state);
//!     }
//! }
//! ```
//!
//! [gen-kill]: https://en.wikipedia.org/wiki/Data-flow_analysis#Bit_vector_problems

use rustc_index::bit_set::{DenseBitSet, MixedBitSet};
use rustc_index::{Idx, IndexVec};
use rustc_middle::bug;
use rustc_middle::mir::{
    self, BasicBlock, BasicBlockData, CallReturnPlaces, Location, TerminatorEdges,
};
use rustc_middle::ty::TyCtxt;
use tracing::error;

use self::graphviz::write_graphviz_results;
use super::fmt::DebugWithContext;

mod cursor;
mod direction;
pub mod fmt;
pub mod graphviz;
pub mod lattice;
mod results;
mod visitor;

pub use self::cursor::ResultsCursor;
pub use self::direction::{Backward, Direction, Forward};
pub use self::lattice::{JoinSemiLattice, MaybeReachable};
pub use self::results::{EntryStates, Results};
pub use self::visitor::{ResultsVisitor, visit_reachable_results, visit_results};

/// Analysis domains are all bitsets of various kinds. This trait holds
/// operations needed by all of them.
pub trait BitSetExt<T> {
    fn contains(&self, elem: T) -> bool;
}

impl<T: Idx> BitSetExt<T> for DenseBitSet<T> {
    fn contains(&self, elem: T) -> bool {
        self.contains(elem)
    }
}

impl<T: Idx> BitSetExt<T> for MixedBitSet<T> {
    fn contains(&self, elem: T) -> bool {
        self.contains(elem)
    }
}

/// A dataflow problem with an arbitrarily complex transfer function.
///
/// This trait specifies the lattice on which this analysis operates (the domain), its
/// initial value at the entry point of each basic block, and various operations.
///
/// # Convergence
///
/// When implementing this trait it's possible to choose a transfer function such that the analysis
/// does not reach fixpoint. To guarantee convergence, your transfer functions must maintain the
/// following invariant:
///
/// > If the dataflow state **before** some point in the program changes to be greater
/// than the prior state **before** that point, the dataflow state **after** that point must
/// also change to be greater than the prior state **after** that point.
///
/// This invariant guarantees that the dataflow state at a given point in the program increases
/// monotonically until fixpoint is reached. Note that this monotonicity requirement only applies
/// to the same point in the program at different points in time. The dataflow state at a given
/// point in the program may or may not be greater than the state at any preceding point.
pub trait Analysis<'tcx> {
    /// The type that holds the dataflow state at any given point in the program.
    type Domain: Clone + JoinSemiLattice;

    /// The direction of this analysis. Either `Forward` or `Backward`.
    type Direction: Direction = Forward;

    /// Auxiliary data used for analyzing `SwitchInt` terminators, if necessary.
    type SwitchIntData = !;

    /// A descriptive name for this analysis. Used only for debugging.
    ///
    /// This name should be brief and contain no spaces, periods or other characters that are not
    /// suitable as part of a filename.
    const NAME: &'static str;

    /// Returns the initial value of the dataflow state upon entry to each basic block.
    fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain;

    /// Mutates the initial value of the dataflow state upon entry to the `START_BLOCK`.
    ///
    /// For backward analyses, initial state (besides the bottom value) is not yet supported. Trying
    /// to mutate the initial state will result in a panic.
    //
    // FIXME: For backward dataflow analyses, the initial state should be applied to every basic
    // block where control flow could exit the MIR body (e.g., those terminated with `return` or
    // `resume`). It's not obvious how to handle `yield` points in coroutines, however.
    fn initialize_start_block(&self, body: &mir::Body<'tcx>, state: &mut Self::Domain);

    /// Given an `EffectIndex`, calls the appropriate `apply_*` method in the
    /// {early,primary} x {statement,terminator} space.
    ///
    /// Do not override this; instead override one or more of the `apply_*` methods.
    #[inline]
    fn apply_effect<'mir>(
        &self,
        state: &mut Self::Domain,
        block: BasicBlock,
        block_data: &'mir BasicBlockData<'tcx>,
        idx: EffectIndex,
    ) {
        let statement_index = idx.statement_index;
        let terminator_index = block_data.statements.len();
        let loc = Location { block, statement_index };
        let is_terminator = statement_index == terminator_index;

        if !is_terminator {
            let statement = &block_data.statements[statement_index];
            match idx.effect {
                Effect::Early => self.apply_early_statement_effect(state, statement, loc),
                Effect::Primary => self.apply_primary_statement_effect(state, statement, loc),
            }
        } else {
            let terminator = block_data.terminator();
            match idx.effect {
                Effect::Early => self.apply_early_terminator_effect(state, terminator, loc),
                Effect::Primary => {
                    self.apply_primary_terminator_effect(state, terminator, loc);
                }
            }
        }
    }

    /// Updates the current dataflow state with an "early" effect, i.e. one
    /// that occurs immediately before the given statement.
    ///
    /// This method is useful if the consumer of the results of this analysis only needs to observe
    /// *part* of the effect of a statement (e.g. for two-phase borrows). As a general rule,
    /// analyses should not implement this without also implementing
    /// `apply_primary_statement_effect`.
    fn apply_early_statement_effect(
        &self,
        _state: &mut Self::Domain,
        _statement: &mir::Statement<'tcx>,
        _location: Location,
    ) {
    }

    /// Updates the current dataflow state with the effect of evaluating a statement.
    fn apply_primary_statement_effect(
        &self,
        state: &mut Self::Domain,
        statement: &mir::Statement<'tcx>,
        location: Location,
    );

    /// Updates the current dataflow state with an effect that occurs immediately *before* the
    /// given terminator.
    ///
    /// This method is useful if the consumer of the results of this analysis needs only to observe
    /// *part* of the effect of a terminator (e.g. for two-phase borrows). As a general rule,
    /// analyses should not implement this without also implementing
    /// `apply_primary_terminator_effect`.
    fn apply_early_terminator_effect(
        &self,
        _state: &mut Self::Domain,
        _terminator: &mir::Terminator<'tcx>,
        _location: Location,
    ) {
    }

    /// Updates the current dataflow state with the effect of evaluating a terminator.
    ///
    /// The effect of a successful return from a `Call` terminator should **not** be accounted for
    /// in this function. That should go in `apply_call_return_effect`. For example, in the
    /// `InitializedPlaces` analyses, the return place for a function call is not marked as
    /// initialized here.
    fn apply_primary_terminator_effect<'mir>(
        &self,
        _state: &mut Self::Domain,
        terminator: &'mir mir::Terminator<'tcx>,
        _location: Location,
    ) -> TerminatorEdges<'mir, 'tcx> {
        terminator.edges()
    }

    /* Edge-specific effects */

    /// Updates the current dataflow state with the effect of a successful return from a `Call`
    /// terminator.
    ///
    /// This is separate from `apply_primary_terminator_effect` to properly track state across
    /// unwind edges.
    fn apply_call_return_effect(
        &self,
        _state: &mut Self::Domain,
        _block: BasicBlock,
        _return_places: CallReturnPlaces<'_, 'tcx>,
    ) {
    }

    /// Used to update the current dataflow state with the effect of taking a particular branch in
    /// a `SwitchInt` terminator.
    ///
    /// Unlike the other edge-specific effects, which are allowed to mutate `Self::Domain`
    /// directly, overriders of this method must return a `Self::SwitchIntData` value (wrapped in
    /// `Some`). The `apply_switch_int_edge_effect` method will then be called once for each
    /// outgoing edge and will have access to the dataflow state that will be propagated along that
    /// edge, and also the `Self::SwitchIntData` value.
    ///
    /// This interface is somewhat more complex than the other visitor-like "effect" methods.
    /// However, it is both more ergonomic—callers don't need to recompute or cache information
    /// about a given `SwitchInt` terminator for each one of its edges—and more efficient—the
    /// engine doesn't need to clone the exit state for a block unless
    /// `get_switch_int_data` is actually called.
    fn get_switch_int_data(
        &self,
        _block: mir::BasicBlock,
        _targets: &mir::SwitchTargets,
        _discr: &mir::Operand<'tcx>,
    ) -> Option<Self::SwitchIntData> {
        None
    }

    /// See comments on `get_switch_int_data`.
    fn apply_switch_int_edge_effect(
        &self,
        _state: &mut Self::Domain,
        _data: &mut Self::SwitchIntData,
        _target_idx: SwitchTargetIndex,
    ) {
        unreachable!();
    }

    /* Extension methods */

    /// Finds the fixpoint for this dataflow problem.
    ///
    /// You shouldn't need to override this. Its purpose is to enable method chaining like so:
    ///
    /// ```ignore (cross-crate-imports)
    /// let results = MyAnalysis::new(tcx, body)
    ///     .iterate_to_fixpoint(tcx, body, None)
    ///     .into_results_cursor(body);
    /// ```
    /// You can optionally add a `pass_name` to the graphviz output for this particular run of a
    /// dataflow analysis. Some analyses are run multiple times in the compilation pipeline.
    /// Without a `pass_name` to differentiates them, only the results for the latest run will be
    /// saved.
    fn iterate_to_fixpoint<'mir>(
        self,
        tcx: TyCtxt<'tcx>,
        body: &'mir mir::Body<'tcx>,
        pass_name: Option<&'static str>,
    ) -> Results<'tcx, Self>
    where
        Self: Sized,
        Self::Domain: DebugWithContext<Self>,
    {
        let mut entry_states =
            IndexVec::from_fn_n(|_| self.bottom_value(body), body.basic_blocks.len());
        self.initialize_start_block(body, &mut entry_states[mir::START_BLOCK]);

        if Self::Direction::IS_BACKWARD && entry_states[mir::START_BLOCK] != self.bottom_value(body)
        {
            bug!("`initialize_start_block` is not yet supported for backward dataflow analyses");
        }

        // Forward analyses use a reverse postorder (`rpo`). Every reachable basic block has a
        // *rank*: its position within `rpo`. Rank order is dataflow order: for every edge A -> B
        // that is not a back edge, rank(A) < rank(B). This is independent of basic block numbering
        // (which depends on the vagaries of CFG construction).
        //
        // The CFG traversal uses a "min-rank" algorithm. First, all reachable basic blocks are
        // marked as dirty. The loop-head invariant is that `curr_rank` always points to the
        // minimum-rank dirty block in `rpo`. Before processing that block we mark it as clean. If
        // the processing dirties a block with a rank lower than or equal to `curr_rank` (via a
        // back edge, which could be an edge-to-self) then `curr_rank` is set to that
        // lower-or-equal rank. After the block is processed, if `curr_rank` doesn't point to a
        // dirty block it is moved to the next dirty block, and we iterate again.
        //
        // This algorithm ensures each basic block is processed only after all its dirty
        // predecessors (ignoring back edges). When a back edge dirties an earlier block we return
        // to that earlier block immediately, which avoids processing later blocks with possibly
        // soon-to-be-stale information. Loop-free code is processed in a single pass.
        //
        // Backward analyses: we want a postorder instead of a reverse postorder, but we also want
        // to avoid the cost of adding a `postorder` field to `mir::basic_blocks::Cache`. We can
        // fake a postorder traversal cheaply by using a reverse postorder and flipping the rank
        // mapping. There is also one wrinkle involving unreachable blocks; see below.

        rustc_index::newtype_index! {
            #[orderable]
            #[debug_format = "bbr{}"]
            struct BasicBlockRank {}
        }

        let rpo: &[BasicBlock] = body.basic_blocks.reverse_postorder();
        let last = rpo.len() - 1;

        let mut ranks: IndexVec<BasicBlock, Option<BasicBlockRank>> =
            IndexVec::from_elem_n(None, body.basic_blocks.len());
        for (i, &bb) in rpo.iter().enumerate() {
            let rank = if Self::Direction::IS_FORWARD { i } else { last - i };
            ranks[bb] = Some(BasicBlockRank::new(rank));
        }

        let mut dirty: DenseBitSet<BasicBlockRank> = DenseBitSet::new_filled(rpo.len());
        let mut curr_rank = BasicBlockRank::ZERO;

        // `state` is not actually used between iterations; this is just an optimization to avoid
        // reallocating every iteration.
        let mut state = self.bottom_value(body);

        loop {
            let i = curr_rank.as_usize();
            let bb = rpo[if Self::Direction::IS_FORWARD { i } else { last - i }];
            debug_assert!(dirty.contains(curr_rank)); // check invariant
            dirty.remove(curr_rank); // invariant temporarily broken

            state.clone_from(&entry_states[bb]);
            let prop = |target: BasicBlock, state: &Self::Domain| {
                // A backward analysis may encounter an unreachable block, because a predecessor
                // of a reachable block may be unreachable. Ignore any such block. (In contrast, in
                // a forward analysis any successor of a reachable block must be reachable.)
                let target_rank = ranks[target];
                if Self::Direction::IS_BACKWARD && target_rank.is_none() {
                    return;
                }
                let target_rank = target_rank.unwrap();

                let set_changed = entry_states[target].join(state);
                if set_changed {
                    dirty.insert(target_rank);
                    curr_rank = curr_rank.min(target_rank);
                }
            };
            Self::Direction::apply_effects_in_block(&self, body, &mut state, bb, &body[bb], prop);

            match dirty.first_set_at_or_after(curr_rank) {
                Some(rank) => curr_rank = rank, // broken invariant re-established
                None => break,                  // no more dirty blocks; finish
            }
        }

        let results = Results { analysis: self, entry_states };

        if tcx.sess.opts.unstable_opts.dump_mir_dataflow {
            let res = write_graphviz_results(tcx, body, &results, pass_name);
            if let Err(e) = res {
                error!("Failed to write graphviz dataflow results: {}", e);
            }
        }

        results
    }
}

#[derive(Debug, Clone, Copy)]
pub enum SwitchTargetIndex {
    // Index of a normal switch target.
    Normal(usize),
    // The final "otherwise" fallback target.
    Otherwise,
}

/// The legal operations for a transfer function in a gen/kill problem.
pub trait GenKill<T> {
    /// Inserts `elem` into the state vector.
    fn gen_(&mut self, elem: T);

    /// Removes `elem` from the state vector.
    fn kill(&mut self, elem: T);

    /// Calls `gen` for each element in `elems`.
    fn gen_all(&mut self, elems: impl IntoIterator<Item = T>) {
        for elem in elems {
            self.gen_(elem);
        }
    }

    /// Calls `kill` for each element in `elems`.
    fn kill_all(&mut self, elems: impl IntoIterator<Item = T>) {
        for elem in elems {
            self.kill(elem);
        }
    }
}

impl<T: Idx> GenKill<T> for DenseBitSet<T> {
    fn gen_(&mut self, elem: T) {
        self.insert(elem);
    }

    fn kill(&mut self, elem: T) {
        self.remove(elem);
    }
}

impl<T: Idx> GenKill<T> for MixedBitSet<T> {
    fn gen_(&mut self, elem: T) {
        self.insert(elem);
    }

    fn kill(&mut self, elem: T) {
        self.remove(elem);
    }
}

impl<T, S: GenKill<T>> GenKill<T> for MaybeReachable<S> {
    fn gen_(&mut self, elem: T) {
        match self {
            // If the state is not reachable, adding an element does nothing.
            MaybeReachable::Unreachable => {}
            MaybeReachable::Reachable(set) => set.gen_(elem),
        }
    }

    fn kill(&mut self, elem: T) {
        match self {
            // If the state is not reachable, killing an element does nothing.
            MaybeReachable::Unreachable => {}
            MaybeReachable::Reachable(set) => set.kill(elem),
        }
    }
}

// NOTE: DO NOT CHANGE VARIANT ORDER. The derived `Ord` impls rely on the current order.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
enum Effect {
    /// The "early" effect (e.g., `apply_early_statement_effect`) for a statement/terminator.
    Early,

    /// The "primary" effect (e.g., `apply_primary_statement_effect`) for a statement/terminator.
    Primary,
}

impl Effect {
    const fn at_index(self, statement_index: usize) -> EffectIndex {
        EffectIndex { effect: self, statement_index }
    }
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct EffectIndex {
    statement_index: usize,
    effect: Effect,
}

#[cfg(test)]
mod tests;