bjoernager/rust
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Remove dead unwinds before drop elaboration

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As a part of drop elaboration, we identify dead unwinds, i.e., unwind
edges on a drop terminators which are known to be unreachable, because
there is no need to drop anything.

Previously, the data flow framework was informed about the dead unwinds,
and it assumed those edges are absent from MIR. Unfortunately, the data
flow framework wasn't consistent in maintaining this assumption.

In particular, if a block was reachable only through a dead unwind edge,
its state was propagated to other blocks still. This became an issue in
the context of change removes DropAndReplace terminator, since it
introduces initialization into cleanup blocks.

To avoid this issue, remove unreachable unwind edges before the drop
elaboration, and elaborate only blocks that remain reachable.
This commit is contained in:
Tomasz Miąsko 2023-01-05 10:15:33 +01:00
parent 00eec854f1
commit 23e3840014
3 changed files with 44 additions and 76 deletions
Download patch file Download diff file Expand all files Collapse all files

31
compiler/rustc_mir_dataflow/src/framework/direction.rs
View file

@ -1,4 +1,3 @@
use rustc_index::bit_set::BitSet;
use rustc_middle::mir::{self, BasicBlock, Location, SwitchTargets};
use rustc_middle::ty::TyCtxt;
use std::ops::RangeInclusive;
@ -54,7 +53,6 @@ pub trait Direction {
analysis: &A,
tcx: TyCtxt<'tcx>,
body: &mir::Body<'tcx>,
dead_unwinds: Option<&BitSet<BasicBlock>>,
exit_state: &mut A::Domain,
block: (BasicBlock, &'_ mir::BasicBlockData<'tcx>),
propagate: impl FnMut(BasicBlock, &A::Domain),
@ -221,7 +219,6 @@ impl Direction for Backward {
analysis: &A,
_tcx: TyCtxt<'tcx>,
body: &mir::Body<'tcx>,
dead_unwinds: Option<&BitSet<BasicBlock>>,
exit_state: &mut A::Domain,
(bb, _bb_data): (BasicBlock, &'_ mir::BasicBlockData<'tcx>),
mut propagate: impl FnMut(BasicBlock, &A::Domain),
@ -278,20 +275,6 @@ impl Direction for Backward {
}
}
// Ignore dead unwinds.
mir::TerminatorKind::Call { cleanup: Some(unwind), .. }
| mir::TerminatorKind::Assert { cleanup: Some(unwind), .. }
| mir::TerminatorKind::Drop { unwind: Some(unwind), .. }
| mir::TerminatorKind::DropAndReplace { unwind: Some(unwind), .. }
| mir::TerminatorKind::FalseUnwind { unwind: Some(unwind), .. }
| mir::TerminatorKind::InlineAsm { cleanup: Some(unwind), .. }
if unwind == bb =>
{
if dead_unwinds.map_or(true, |dead| !dead.contains(pred)) {
propagate(pred, exit_state);
}
}
_ => propagate(pred, exit_state),
}
}
@ -304,7 +287,6 @@ struct BackwardSwitchIntEdgeEffectsApplier<'a, 'tcx, D, F> {
exit_state: &'a mut D,
bb: BasicBlock,
propagate: &'a mut F,
effects_applied: bool,
}
@ -484,7 +466,6 @@ impl Direction for Forward {
analysis: &A,
_tcx: TyCtxt<'tcx>,
_body: &mir::Body<'tcx>,
dead_unwinds: Option<&BitSet<BasicBlock>>,
exit_state: &mut A::Domain,
(bb, bb_data): (BasicBlock, &'_ mir::BasicBlockData<'tcx>),
mut propagate: impl FnMut(BasicBlock, &A::Domain),
@ -502,9 +483,7 @@ impl Direction for Forward {
| DropAndReplace { target, unwind, value: _, place: _ }
| FalseUnwind { real_target: target, unwind } => {
if let Some(unwind) = unwind {
if dead_unwinds.map_or(true, |dead| !dead.contains(bb)) {
propagate(unwind, exit_state);
}
propagate(unwind, exit_state);
}
propagate(target, exit_state);
@ -534,9 +513,7 @@ impl Direction for Forward {
fn_span: _,
} => {
if let Some(unwind) = cleanup {
if dead_unwinds.map_or(true, |dead| !dead.contains(bb)) {
propagate(unwind, exit_state);
}
propagate(unwind, exit_state);
}
if let Some(target) = target {
@ -560,9 +537,7 @@ impl Direction for Forward {
cleanup,
} => {
if let Some(unwind) = cleanup {
if dead_unwinds.map_or(true, |dead| !dead.contains(bb)) {
propagate(unwind, exit_state);
}
propagate(unwind, exit_state);
}
if let Some(target) = destination {

32
compiler/rustc_mir_dataflow/src/framework/engine.rs
View file

@ -12,7 +12,6 @@ use rustc_ast as ast;
use rustc_data_structures::work_queue::WorkQueue;
use rustc_graphviz as dot;
use rustc_hir::def_id::DefId;
use rustc_index::bit_set::BitSet;
use rustc_index::vec::{Idx, IndexVec};
use rustc_middle::mir::{self, traversal, BasicBlock};
use rustc_middle::mir::{create_dump_file, dump_enabled};
@ -78,7 +77,6 @@ where
{
tcx: TyCtxt<'tcx>,
body: &'a mir::Body<'tcx>,
dead_unwinds: Option<&'a BitSet<BasicBlock>>,
entry_sets: IndexVec<BasicBlock, A::Domain>,
pass_name: Option<&'static str>,
analysis: A,
@ -154,25 +152,7 @@ where
bug!("`initialize_start_block` is not yet supported for backward dataflow analyses");
}
Engine {
analysis,
tcx,
body,
dead_unwinds: None,
pass_name: None,
entry_sets,
apply_trans_for_block,
}
}
/// Signals that we do not want dataflow state to propagate across unwind edges for these
/// `BasicBlock`s.
///
/// You must take care that `dead_unwinds` does not contain a `BasicBlock` that *can* actually
/// unwind during execution. Otherwise, your dataflow results will not be correct.
pub fn dead_unwinds(mut self, dead_unwinds: &'a BitSet<BasicBlock>) -> Self {
self.dead_unwinds = Some(dead_unwinds);
self
Engine { analysis, tcx, body, pass_name: None, entry_sets, apply_trans_for_block }
}
/// Adds an identifier to the graphviz output for this particular run of a dataflow analysis.
@ -190,14 +170,7 @@ where
A::Domain: DebugWithContext<A>,
{
let Engine {
analysis,
body,
dead_unwinds,
mut entry_sets,
tcx,
apply_trans_for_block,
pass_name,
..
analysis, body, mut entry_sets, tcx, apply_trans_for_block, pass_name, ..
} = self;
let mut dirty_queue: WorkQueue<BasicBlock> = WorkQueue::with_none(body.basic_blocks.len());
@ -236,7 +209,6 @@ where
&analysis,
tcx,
body,
dead_unwinds,
&mut state,
(bb, bb_data),
|target: BasicBlock, state: &A::Domain| {