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Auto merge of #73879 - ecstatic-morse:discr-switch-uninit, r=oli-obk

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Handle inactive enum variants in `MaybeUninitializedPlaces`

Resolves the first part of #69715.

This is the equivalent of #68528 but for `MaybeUninitializedPlaces`. Because we now notify drop elaboration that inactive enum variants might be uninitialized, some drops get marked as ["open" that were previously "static"](e0e5d82e16/src/librustc_mir/transform/elaborate_drops.rs (L191)). Unlike in #69715, this isn't strictly better: An "open" drop expands to more MIR than a simple call to the drop shim. However, because drop elaboration considers each field of an "open" drop separately, it can sometimes eliminate unnecessary drops of moved-from or unit-like enum variants. This is the case for `Option::unwrap`, which is reflected in the `mir-opt` test.

cc @eddyb
r? @oli-obk
This commit is contained in:
bors 2020-07-05 18:01:47 +00:00
commit 2753fab7ce
6 changed files with 133 additions and 39 deletions
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40
src/librustc_mir/dataflow/drop_flag_effects.rs
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@ -1,6 +1,7 @@
use crate::util::elaborate_drops::DropFlagState;
use rustc_middle::mir::{self, Body, Location};
use rustc_middle::ty::{self, TyCtxt};
use rustc_target::abi::VariantIdx;
use super::indexes::MovePathIndex;
use super::move_paths::{InitKind, LookupResult, MoveData};
@ -228,3 +229,42 @@ pub(crate) fn for_location_inits<'tcx, F>(
}
}
}
/// Calls `handle_inactive_variant` for each descendant move path of `enum_place` that contains a
/// `Downcast` to a variant besides the `active_variant`.
///
/// NOTE: If there are no move paths corresponding to an inactive variant,
/// `handle_inactive_variant` will not be called for that variant.
pub(crate) fn on_all_inactive_variants<'tcx>(
tcx: TyCtxt<'tcx>,
body: &mir::Body<'tcx>,
move_data: &MoveData<'tcx>,
enum_place: mir::Place<'tcx>,
active_variant: VariantIdx,
mut handle_inactive_variant: impl FnMut(MovePathIndex),
) {
let enum_mpi = match move_data.rev_lookup.find(enum_place.as_ref()) {
LookupResult::Exact(mpi) => mpi,
LookupResult::Parent(_) => return,
};
let enum_path = &move_data.move_paths[enum_mpi];
for (variant_mpi, variant_path) in enum_path.children(&move_data.move_paths) {
// Because of the way we build the `MoveData` tree, each child should have exactly one more
// projection than `enum_place`. This additional projection must be a downcast since the
// base is an enum.
let (downcast, base_proj) = variant_path.place.projection.split_last().unwrap();
assert_eq!(enum_place.projection.len(), base_proj.len());
let variant_idx = match *downcast {
mir::ProjectionElem::Downcast(_, idx) => idx,
_ => unreachable!(),
};
if variant_idx != active_variant {
on_all_children_bits(tcx, body, move_data, variant_mpi, |mpi| {
handle_inactive_variant(mpi)
});
}
}
}

71
src/librustc_mir/dataflow/impls/mod.rs
View File

@ -12,7 +12,7 @@ use super::MoveDataParamEnv;
use crate::util::elaborate_drops::DropFlagState;
use super::move_paths::{HasMoveData, InitIndex, InitKind, LookupResult, MoveData, MovePathIndex};
use super::move_paths::{HasMoveData, InitIndex, InitKind, MoveData, MovePathIndex};
use super::{AnalysisDomain, BottomValue, GenKill, GenKillAnalysis};
use super::drop_flag_effects_for_function_entry;
@ -124,11 +124,23 @@ pub struct MaybeUninitializedPlaces<'a, 'tcx> {
tcx: TyCtxt<'tcx>,
body: &'a Body<'tcx>,
mdpe: &'a MoveDataParamEnv<'tcx>,
mark_inactive_variants_as_uninit: bool,
}
impl<'a, 'tcx> MaybeUninitializedPlaces<'a, 'tcx> {
pub fn new(tcx: TyCtxt<'tcx>, body: &'a Body<'tcx>, mdpe: &'a MoveDataParamEnv<'tcx>) -> Self {
MaybeUninitializedPlaces { tcx, body, mdpe }
MaybeUninitializedPlaces { tcx, body, mdpe, mark_inactive_variants_as_uninit: false }
}
/// Causes inactive enum variants to be marked as "maybe uninitialized" after a switch on an
/// enum discriminant.
///
/// This is correct in a vacuum but is not the default because it causes problems in the borrow
/// checker, where this information gets propagated along `FakeEdge`s.
pub fn mark_inactive_variants_as_uninit(mut self) -> Self {
self.mark_inactive_variants_as_uninit = true;
self
}
}
@ -350,27 +362,16 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
_adt: &ty::AdtDef,
variant: VariantIdx,
) {
let enum_mpi = match self.move_data().rev_lookup.find(enum_place.as_ref()) {
LookupResult::Exact(mpi) => mpi,
LookupResult::Parent(_) => return,
};
// Kill all move paths that correspond to variants other than this one
let move_paths = &self.move_data().move_paths;
let enum_path = &move_paths[enum_mpi];
for (mpi, variant_path) in enum_path.children(move_paths) {
trans.kill(mpi);
match variant_path.place.projection.last().unwrap() {
mir::ProjectionElem::Downcast(_, idx) if *idx == variant => continue,
_ => drop_flag_effects::on_all_children_bits(
self.tcx,
self.body,
self.move_data(),
mpi,
|mpi| trans.kill(mpi),
),
}
}
// Kill all move paths that correspond to variants we know to be inactive along this
// particular outgoing edge of a `SwitchInt`.
drop_flag_effects::on_all_inactive_variants(
self.tcx,
self.body,
self.move_data(),
enum_place,
variant,
|mpi| trans.kill(mpi),
);
}
}
@ -443,6 +444,30 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeUninitializedPlaces<'_, 'tcx> {
},
);
}
fn discriminant_switch_effect(
&self,
trans: &mut impl GenKill<Self::Idx>,
_block: mir::BasicBlock,
enum_place: mir::Place<'tcx>,
_adt: &ty::AdtDef,
variant: VariantIdx,
) {
if !self.mark_inactive_variants_as_uninit {
return;
}
// Mark all move paths that correspond to variants other than this one as maybe
// uninitialized (in reality, they are *definitely* uninitialized).
drop_flag_effects::on_all_inactive_variants(
self.tcx,
self.body,
self.move_data(),
enum_place,
variant,
|mpi| trans.gen(mpi),
);
}
}
impl<'a, 'tcx> AnalysisDomain<'tcx> for DefinitelyInitializedPlaces<'a, 'tcx> {

1
src/librustc_mir/transform/elaborate_drops.rs
View File

@ -48,6 +48,7 @@ impl<'tcx> MirPass<'tcx> for ElaborateDrops {
.into_results_cursor(body);
let uninits = MaybeUninitializedPlaces::new(tcx, body, &env)
.mark_inactive_variants_as_uninit()
.into_engine(tcx, body, def_id)
.dead_unwinds(&dead_unwinds)
.iterate_to_fixpoint()

22
src/test/mir-opt/no-drop-for-inactive-variant/rustc.unwrap.SimplifyCfg-elaborate-drops.after.mir
View File

@ -8,22 +8,19 @@ fn unwrap(_1: std::option::Option<T>) -> T {
let mut _4: !; // in scope 0 at $SRC_DIR/libstd/macros.rs:LL:COL
let mut _5: isize; // in scope 0 at $DIR/no-drop-for-inactive-variant.rs:12:1: 12:2
let mut _6: isize; // in scope 0 at $DIR/no-drop-for-inactive-variant.rs:12:1: 12:2
let mut _7: isize; // in scope 0 at $DIR/no-drop-for-inactive-variant.rs:12:1: 12:2
scope 1 {
debug x => _3; // in scope 1 at $DIR/no-drop-for-inactive-variant.rs:9:14: 9:15
}
bb0: {
_2 = discriminant(_1); // scope 0 at $DIR/no-drop-for-inactive-variant.rs:9:9: 9:16
switchInt(move _2) -> [0_isize: bb2, 1_isize: bb4, otherwise: bb3]; // scope 0 at $DIR/no-drop-for-inactive-variant.rs:9:9: 9:16
switchInt(move _2) -> [0_isize: bb1, 1_isize: bb3, otherwise: bb2]; // scope 0 at $DIR/no-drop-for-inactive-variant.rs:9:9: 9:16
}
bb1 (cleanup): {
resume; // scope 0 at $DIR/no-drop-for-inactive-variant.rs:7:1: 12:2
}
bb2: {
bb1: {
StorageLive(_4); // scope 0 at $SRC_DIR/libstd/macros.rs:LL:COL
const std::rt::begin_panic::<&str>(const "explicit panic") -> bb5; // scope 0 at $SRC_DIR/libstd/macros.rs:LL:COL
const std::rt::begin_panic::<&str>(const "explicit panic") -> bb4; // scope 0 at $SRC_DIR/libstd/macros.rs:LL:COL
// ty::Const
// + ty: fn(&str) -> ! {std::rt::begin_panic::<&str>}
// + val: Value(Scalar(<ZST>))
@ -38,20 +35,21 @@ fn unwrap(_1: std::option::Option<T>) -> T {
// + literal: Const { ty: &str, val: Value(Slice { data: Allocation { bytes: [101, 120, 112, 108, 105, 99, 105, 116, 32, 112, 97, 110, 105, 99], relocations: Relocations(SortedMap { data: [] }), init_mask: InitMask { blocks: [16383], len: Size { raw: 14 } }, size: Size { raw: 14 }, align: Align { pow2: 0 }, mutability: Not, extra: () }, start: 0, end: 14 }) }
}
bb3: {
bb2: {
unreachable; // scope 0 at $DIR/no-drop-for-inactive-variant.rs:8:11: 8:14
}
bb4: {
bb3: {
StorageLive(_3); // scope 0 at $DIR/no-drop-for-inactive-variant.rs:9:14: 9:15
_3 = move ((_1 as Some).0: T); // scope 0 at $DIR/no-drop-for-inactive-variant.rs:9:14: 9:15
_0 = move _3; // scope 1 at $DIR/no-drop-for-inactive-variant.rs:9:20: 9:21
StorageDead(_3); // scope 0 at $DIR/no-drop-for-inactive-variant.rs:9:21: 9:22
_5 = discriminant(_1); // scope 0 at $DIR/no-drop-for-inactive-variant.rs:12:1: 12:2
_6 = discriminant(_1); // scope 0 at $DIR/no-drop-for-inactive-variant.rs:12:1: 12:2
return; // scope 0 at $DIR/no-drop-for-inactive-variant.rs:12:2: 12:2
}
bb5 (cleanup): {
drop(_1) -> bb1; // scope 0 at $DIR/no-drop-for-inactive-variant.rs:12:1: 12:2
bb4 (cleanup): {
_5 = discriminant(_1); // scope 0 at $DIR/no-drop-for-inactive-variant.rs:12:1: 12:2
resume; // scope 0 at $DIR/no-drop-for-inactive-variant.rs:7:1: 12:2
}
}

19
src/test/mir-opt/simplify-locals-removes-unused-discriminant-reads/32bit/rustc.map.SimplifyLocals.diff
View File

@ -7,13 +7,28 @@
let mut _2: isize; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
- let _3: std::boxed::Box<()>; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:4:14: 4:15
- let mut _4: std::boxed::Box<()>; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:4:25: 4:26
- let mut _5: isize; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
- let mut _5: bool; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
- let mut _6: isize; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
- let mut _7: isize; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
scope 1 {
debug x => ((_0 as Some).0: std::boxed::Box<()>); // in scope 1 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:4:14: 4:15
}
bb0: {
- _5 = const false; // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
- // ty::Const
- // + ty: bool
- // + val: Value(Scalar(0x00))
- // mir::Constant
- // + span: $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
- // + literal: Const { ty: bool, val: Value(Scalar(0x00)) }
- _5 = const true; // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
- // ty::Const
- // + ty: bool
- // + val: Value(Scalar(0x01))
- // mir::Constant
- // + span: $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
- // + literal: Const { ty: bool, val: Value(Scalar(0x01)) }
_2 = discriminant(_1); // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
switchInt(move _2) -> [0_isize: bb2, otherwise: bb1]; // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
}
@ -35,7 +50,7 @@
}
bb3: {
- _5 = discriminant(_1); // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
- _6 = discriminant(_1); // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
return; // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:2: 6:2
}
}

19
src/test/mir-opt/simplify-locals-removes-unused-discriminant-reads/64bit/rustc.map.SimplifyLocals.diff
View File

@ -7,13 +7,28 @@
let mut _2: isize; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
- let _3: std::boxed::Box<()>; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:4:14: 4:15
- let mut _4: std::boxed::Box<()>; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:4:25: 4:26
- let mut _5: isize; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
- let mut _5: bool; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
- let mut _6: isize; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
- let mut _7: isize; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
scope 1 {
debug x => ((_0 as Some).0: std::boxed::Box<()>); // in scope 1 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:4:14: 4:15
}
bb0: {
- _5 = const false; // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
- // ty::Const
- // + ty: bool
- // + val: Value(Scalar(0x00))
- // mir::Constant
- // + span: $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
- // + literal: Const { ty: bool, val: Value(Scalar(0x00)) }
- _5 = const true; // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
- // ty::Const
- // + ty: bool
- // + val: Value(Scalar(0x01))
- // mir::Constant
- // + span: $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
- // + literal: Const { ty: bool, val: Value(Scalar(0x01)) }
_2 = discriminant(_1); // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
switchInt(move _2) -> [0_isize: bb2, otherwise: bb1]; // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
}
@ -35,7 +50,7 @@
}
bb3: {
- _5 = discriminant(_1); // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
- _6 = discriminant(_1); // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
return; // scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:2: 6:2
}
}