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Move BottomValue into framework/mod.rs

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This commit is contained in:
Dylan MacKenzie 2020-03-02 09:32:12 -08:00
parent a8cc89f975
commit 7108cea14e
2 changed files with 43 additions and 47 deletions
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43
src/librustc_mir/dataflow/framework/mod.rs
View File

@ -41,8 +41,6 @@ use rustc_hir::def_id::DefId;
use rustc_index::bit_set::{BitSet, HybridBitSet};
use rustc_index::vec::{Idx, IndexVec};
use crate::dataflow::BottomValue;
mod cursor;
mod engine;
mod graphviz;
@ -95,6 +93,47 @@ where
}
}
/// Parameterization for the precise form of data flow that is used.
///
/// `BottomValue` determines whether the initial entry set for each basic block is empty or full.
/// This also determines the semantics of the lattice `join` operator used to merge dataflow
/// results, since dataflow works by starting at the bottom and moving monotonically to a fixed
/// point.
///
/// This means, for propagation across the graph, that you either want to start at all-zeroes and
/// then use Union as your merge when propagating, or you start at all-ones and then use Intersect
/// as your merge when propagating.
pub trait BottomValue {
/// Specifies the initial value for each bit in the entry set for each basic block.
const BOTTOM_VALUE: bool;
/// Merges `in_set` into `inout_set`, returning `true` if `inout_set` changed.
///
/// It is almost certainly wrong to override this, since it automatically applies
/// * `inout_set & in_set` if `BOTTOM_VALUE == true`
/// * `inout_set | in_set` if `BOTTOM_VALUE == false`
///
/// This means that if a bit is not `BOTTOM_VALUE`, it is propagated into all target blocks.
/// For clarity, the above statement again from a different perspective:
/// A bit in the block's entry set is `!BOTTOM_VALUE` if *any* predecessor block's bit value is
/// `!BOTTOM_VALUE`.
///
/// There are situations where you want the opposite behaviour: propagate only if *all*
/// predecessor blocks's value is `!BOTTOM_VALUE`.
/// E.g. if you want to know whether a bit is *definitely* set at a specific location. This
/// means that all code paths leading to the location must have set the bit, instead of any
/// code path leading there.
///
/// If you want this kind of "definitely set" analysis, you need to
/// 1. Invert `BOTTOM_VALUE`
/// 2. Reset the `entry_set` in `start_block_effect` to `!BOTTOM_VALUE`
/// 3. Override `join` to do the opposite from what it's doing now.
#[inline]
fn join<T: Idx>(&self, inout_set: &mut BitSet<T>, in_set: &BitSet<T>) -> bool {
if !Self::BOTTOM_VALUE { inout_set.union(in_set) } else { inout_set.intersect(in_set) }
}
}
/// Define the domain of a dataflow problem.
///
/// This trait specifies the lattice on which this analysis operates. For now, this must be a

47
src/librustc_mir/dataflow/mod.rs
View File

@ -1,13 +1,11 @@
use rustc::ty;
use rustc_ast::ast::{self, MetaItem};
use rustc_index::bit_set::BitSet;
use rustc_index::vec::Idx;
use rustc_span::symbol::{sym, Symbol};
pub(crate) use self::drop_flag_effects::*;
pub use self::framework::{
visit_results, Analysis, AnalysisDomain, BorrowckFlowState, BorrowckResults, Engine, GenKill,
GenKillAnalysis, Results, ResultsCursor, ResultsRefCursor, ResultsVisitor,
visit_results, Analysis, AnalysisDomain, BorrowckFlowState, BorrowckResults, BottomValue,
Engine, GenKill, GenKillAnalysis, Results, ResultsCursor, ResultsRefCursor, ResultsVisitor,
};
pub use self::impls::{
borrows::Borrows, DefinitelyInitializedPlaces, EverInitializedPlaces, MaybeBorrowedLocals,
@ -48,44 +46,3 @@ pub(crate) fn has_rustc_mir_with(attrs: &[ast::Attribute], name: Symbol) -> Opti
}
None
}
/// Parameterization for the precise form of data flow that is used.
///
/// `BottomValue` determines whether the initial entry set for each basic block is empty or full.
/// This also determines the semantics of the lattice `join` operator used to merge dataflow
/// results, since dataflow works by starting at the bottom and moving monotonically to a fixed
/// point.
///
/// This means, for propagation across the graph, that you either want to start at all-zeroes and
/// then use Union as your merge when propagating, or you start at all-ones and then use Intersect
/// as your merge when propagating.
pub trait BottomValue {
/// Specifies the initial value for each bit in the entry set for each basic block.
const BOTTOM_VALUE: bool;
/// Merges `in_set` into `inout_set`, returning `true` if `inout_set` changed.
///
/// It is almost certainly wrong to override this, since it automatically applies
/// * `inout_set & in_set` if `BOTTOM_VALUE == true`
/// * `inout_set | in_set` if `BOTTOM_VALUE == false`
///
/// This means that if a bit is not `BOTTOM_VALUE`, it is propagated into all target blocks.
/// For clarity, the above statement again from a different perspective:
/// A bit in the block's entry set is `!BOTTOM_VALUE` if *any* predecessor block's bit value is
/// `!BOTTOM_VALUE`.
///
/// There are situations where you want the opposite behaviour: propagate only if *all*
/// predecessor blocks's value is `!BOTTOM_VALUE`.
/// E.g. if you want to know whether a bit is *definitely* set at a specific location. This
/// means that all code paths leading to the location must have set the bit, instead of any
/// code path leading there.
///
/// If you want this kind of "definitely set" analysis, you need to
/// 1. Invert `BOTTOM_VALUE`
/// 2. Reset the `entry_set` in `start_block_effect` to `!BOTTOM_VALUE`
/// 3. Override `join` to do the opposite from what it's doing now.
#[inline]
fn join<T: Idx>(&self, inout_set: &mut BitSet<T>, in_set: &BitSet<T>) -> bool {
if !Self::BOTTOM_VALUE { inout_set.union(in_set) } else { inout_set.intersect(in_set) }
}
}