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Remove DropAndReplace terminator

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PR 107844 made DropAndReplace unused, let's remove it completely
from the codebase.
This commit is contained in:
Giacomo Pasini 2023-03-05 21:02:14 +01:00
parent 14c54b637b
commit c5d4e4d907
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GPG key ID: A03851B78A6C9A46
48 changed files with 33 additions and 341 deletions
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119
compiler/rustc_mir_transform/src/elaborate_drops.rs
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@ -18,15 +18,14 @@ use rustc_span::{DesugaringKind, Span};
use rustc_target::abi::VariantIdx;
use std::fmt;
/// During MIR building, Drop and DropAndReplace terminators are inserted in every place where a drop may occur.
/// During MIR building, Drop terminators are inserted in every place where a drop may occur.
/// However, in this phase, the presence of these terminators does not guarantee that a destructor will run,
/// as the target of the drop may be uninitialized.
/// In general, the compiler cannot determine at compile time whether a destructor will run or not.
///
/// At a high level, this pass refines Drop and DropAndReplace to only run the destructor if the
/// At a high level, this pass refines Drop to only run the destructor if the
/// target is initialized. The way this is achievied is by inserting drop flags for every variable
/// that may be dropped, and then using those flags to determine whether a destructor should run.
/// This pass also removes DropAndReplace, replacing it with a Drop paired with an assign statement.
/// Once this is complete, Drop terminators in the MIR correspond to a call to the "drop glue" or
/// "drop shim" for the type of the dropped place.
///
@ -121,8 +120,7 @@ fn remove_dead_unwinds<'tcx>(
.into_results_cursor(body);
for (bb, bb_data) in body.basic_blocks.iter_enumerated() {
let place = match bb_data.terminator().kind {
TerminatorKind::Drop { ref place, unwind: Some(_), .. }
| TerminatorKind::DropAndReplace { ref place, unwind: Some(_), .. } => {
TerminatorKind::Drop { ref place, unwind: Some(_), .. } => {
und.derefer(place.as_ref(), body).unwrap_or(*place)
}
_ => continue,
@ -343,8 +341,7 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
}
let terminator = data.terminator();
let place = match terminator.kind {
TerminatorKind::Drop { ref place, .. }
| TerminatorKind::DropAndReplace { ref place, .. } => {
TerminatorKind::Drop { ref place, .. } => {
self.un_derefer.derefer(place.as_ref(), self.body).unwrap_or(*place)
}
_ => continue,
@ -441,103 +438,11 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
}
}
}
TerminatorKind::DropAndReplace { mut place, ref value, target, unwind } => {
assert!(!data.is_cleanup);
if let Some(new_place) = self.un_derefer.derefer(place.as_ref(), self.body) {
place = new_place;
}
self.elaborate_replace(loc, place, value, target, unwind);
}
_ => continue,
}
}
}
/// Elaborate a MIR `replace` terminator. This instruction
/// is not directly handled by codegen, and therefore
/// must be desugared.
///
/// The desugaring drops the location if needed, and then writes
/// the value (including setting the drop flag) over it in *both* arms.
///
/// The `replace` terminator can also be called on places that
/// are not tracked by elaboration (for example,
/// `replace x[i] <- tmp0`). The borrow checker requires that
/// these locations are initialized before the assignment,
/// so we just generate an unconditional drop.
fn elaborate_replace(
&mut self,
loc: Location,
place: Place<'tcx>,
value: &Operand<'tcx>,
target: BasicBlock,
unwind: Option<BasicBlock>,
) {
let bb = loc.block;
let data = &self.body[bb];
let terminator = data.terminator();
assert!(!data.is_cleanup, "DropAndReplace in unwind path not supported");
let assign = Statement {
kind: StatementKind::Assign(Box::new((place, Rvalue::Use(value.clone())))),
source_info: terminator.source_info,
};
let unwind = unwind.unwrap_or_else(|| self.patch.resume_block());
let unwind = self.patch.new_block(BasicBlockData {
statements: vec![assign.clone()],
terminator: Some(Terminator {
kind: TerminatorKind::Goto { target: unwind },
..*terminator
}),
is_cleanup: true,
});
let target = self.patch.new_block(BasicBlockData {
statements: vec![assign],
terminator: Some(Terminator { kind: TerminatorKind::Goto { target }, ..*terminator }),
is_cleanup: false,
});
match self.move_data().rev_lookup.find(place.as_ref()) {
LookupResult::Exact(path) => {
debug!("elaborate_drop_and_replace({:?}) - tracked {:?}", terminator, path);
self.init_data.seek_before(loc);
elaborate_drop(
&mut Elaborator { ctxt: self },
terminator.source_info,
place,
path,
target,
Unwind::To(unwind),
bb,
);
on_all_children_bits(self.tcx, self.body, self.move_data(), path, |child| {
self.set_drop_flag(
Location { block: target, statement_index: 0 },
child,
DropFlagState::Present,
);
self.set_drop_flag(
Location { block: unwind, statement_index: 0 },
child,
DropFlagState::Present,
);
});
}
LookupResult::Parent(parent) => {
// drop and replace behind a pointer/array/whatever. The location
// must be initialized.
debug!("elaborate_drop_and_replace({:?}) - untracked {:?}", terminator, parent);
self.patch.patch_terminator(
bb,
TerminatorKind::Drop { place, target, unwind: Some(unwind) },
);
}
}
}
fn constant_bool(&self, span: Span, val: bool) -> Rvalue<'tcx> {
Rvalue::Use(Operand::Constant(Box::new(Constant {
span,
@ -609,22 +514,12 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
debug!("drop_flags_for_locs({:?})", data);
for i in 0..(data.statements.len() + 1) {
debug!("drop_flag_for_locs: stmt {}", i);
let mut allow_initializations = true;
if i == data.statements.len() {
match data.terminator().kind {
TerminatorKind::Drop { .. } => {
// drop elaboration should handle that by itself
continue;
}
TerminatorKind::DropAndReplace { .. } => {
// this contains the move of the source and
// the initialization of the destination. We
// only want the former - the latter is handled
// by the elaboration code and must be done
// *after* the destination is dropped.
assert!(self.patch.is_patched(bb));
allow_initializations = false;
}
TerminatorKind::Resume => {
// It is possible for `Resume` to be patched
// (in particular it can be patched to be replaced with
@ -641,11 +536,7 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
self.body,
self.env,
loc,
|path, ds| {
if ds == DropFlagState::Absent || allow_initializations {
self.set_drop_flag(loc, path, ds)
}
},
|path, ds| self.set_drop_flag(loc, path, ds),
)
}