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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
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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 {