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Auto merge of #76044 - ecstatic-morse:dataflow-lattice, r=oli-obk

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Support dataflow problems on arbitrary lattices

This PR implements last of the proposed extensions I mentioned in the design meeting for the original dataflow refactor. It extends the current dataflow framework to work with arbitrary lattices, not just `BitSet`s. This is a prerequisite for dataflow-enabled MIR const-propagation. Personally, I am skeptical of the usefulness of doing const-propagation pre-monomorphization, since many useful constants only become known after monomorphization (e.g. `size_of::<T>()`) and users have a natural tendency to hand-optimize the rest. It's probably worth exprimenting with, however, and others have shown interest cc `@rust-lang/wg-mir-opt.`

The `Idx` associated type is moved from `AnalysisDomain` to `GenKillAnalysis` and replaced with an associated `Domain` type that must implement `JoinSemiLattice`. Like before, each `Analysis` defines the "bottom value" for its domain, but can no longer override the dataflow join operator. Analyses that want to use set intersection must now use the `lattice::Dual` newtype. `GenKillAnalysis` impls have an additional requirement that `Self::Domain: BorrowMut<BitSet<Self::Idx>>`, which effectively means that they must use `BitSet<Self::Idx>` or `lattice::Dual<BitSet<Self::Idx>>` as their domain.

Most of these changes were mechanical. However, because a `Domain` is no longer always a powerset of some index type, we can no longer use an `IndexVec<BasicBlock, GenKillSet<A::Idx>>>` to store cached block transfer functions. Instead, we use a boxed `dyn Fn` trait object. I discuss a few alternatives to the current approach in a commit message.

The majority of new lines of code are to preserve existing Graphviz diagrams for those unlucky enough to have to debug dataflow analyses. I find these diagrams incredibly useful when things are going wrong and considered regressing them unacceptable, especially the pretty-printing of `MovePathIndex`s, which are used in many dataflow analyses. This required a parallel `fmt` trait used only for printing dataflow domains, as well as a refactoring of the `graphviz` module now that we cannot expect the domain to be a `BitSet`. Some features did have to be removed, such as the gen/kill display mode (which I didn't use but existed to mirror the output of the old dataflow framework) and line wrapping. Since I had to rewrite much of it anyway, I took the opportunity to switch to a `Visitor` for printing dataflow state diffs instead of using cursors, which are error prone for code that must be generic over both forward and backward analyses. As a side-effect of this change, we no longer have quadratic behavior when writing graphviz diagrams for backward dataflow analyses.

r? `@pnkfelix`
This commit is contained in:
bors 2020-09-07 21:29:43 +00:00
parent 4286d9c87a 1c5b0fbe53
commit 3ffe9f843c
1 changed files with 7 additions and 10 deletions
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17
clippy_lints/src/redundant_clone.rs
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@ -14,7 +14,6 @@ use rustc_middle::mir::{
visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor as _},
};
use rustc_middle::ty::{self, fold::TypeVisitor, Ty};
use rustc_mir::dataflow::BottomValue;
use rustc_mir::dataflow::{Analysis, AnalysisDomain, GenKill, GenKillAnalysis, ResultsCursor};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::{BytePos, Span};
@ -411,14 +410,15 @@ impl<'tcx> mir::visit::Visitor<'tcx> for LocalUseVisitor {
struct MaybeStorageLive;
impl<'tcx> AnalysisDomain<'tcx> for MaybeStorageLive {
type Idx = mir::Local;
type Domain = BitSet<mir::Local>;
const NAME: &'static str = "maybe_storage_live";
fn bits_per_block(&self, body: &mir::Body<'tcx>) -> usize {
body.local_decls.len()
fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
// bottom = dead
BitSet::new_empty(body.local_decls.len())
}
fn initialize_start_block(&self, body: &mir::Body<'tcx>, state: &mut BitSet<Self::Idx>) {
fn initialize_start_block(&self, body: &mir::Body<'tcx>, state: &mut Self::Domain) {
for arg in body.args_iter() {
state.insert(arg);
}
@ -426,6 +426,8 @@ impl<'tcx> AnalysisDomain<'tcx> for MaybeStorageLive {
}
impl<'tcx> GenKillAnalysis<'tcx> for MaybeStorageLive {
type Idx = mir::Local;
fn statement_effect(&self, trans: &mut impl GenKill<Self::Idx>, stmt: &mir::Statement<'tcx>, _: mir::Location) {
match stmt.kind {
mir::StatementKind::StorageLive(l) => trans.gen(l),
@ -454,11 +456,6 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeStorageLive {
}
}
impl BottomValue for MaybeStorageLive {
/// bottom = dead
const BOTTOM_VALUE: bool = false;
}
/// Collects the possible borrowers of each local.
/// For example, `b = &a; c = &a;` will make `b` and (transitively) `c`
/// possible borrowers of `a`.