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Mark inactive enum variants along "otherwise" edge as uninitialized #77377

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Mark inactive enum variants along "otherwise" edge as uninitialized #77377

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7e2fc9f
Mark inactive enum variants along "otherwise" edge as unintitialized
ecstatic-morse Sep 30, 2020
d5ee723
Remove unused helper function
ecstatic-morse Sep 30, 2020
fac0ad6
Bless MIR test
ecstatic-morse Sep 30, 2020
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40 changes: 0 additions & 40 deletions compiler/rustc_mir/src/dataflow/drop_flag_effects.rs
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Original file line number Diff line number Diff line change
@@ -1,7 +1,6 @@
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};
Expand Down Expand Up @@ -229,42 +228,3 @@ 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)
});
}
}
}
171 changes: 102 additions & 69 deletions compiler/rustc_mir/src/dataflow/impls/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@ use super::MoveDataParamEnv;

use crate::util::elaborate_drops::DropFlagState;

use super::move_paths::{HasMoveData, InitIndex, InitKind, MoveData, MovePathIndex};
use super::move_paths::{HasMoveData, InitIndex, InitKind, LookupResult, MoveData, MovePathIndex};
use super::{lattice, AnalysisDomain, GenKill, GenKillAnalysis};

use super::drop_flag_effects_for_function_entry;
Expand Down Expand Up @@ -362,40 +362,17 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
return;
}

let enum_ = discr.place().and_then(|discr| {
switch_on_enum_discriminant(self.tcx, &self.body, &self.body[block], discr)
});

let (enum_place, enum_def) = match enum_ {
Some(x) => x,
None => return,
};

let mut discriminants = enum_def.discriminants(self.tcx);
edge_effects.apply(|trans, edge| {
let value = match edge.value {
Some(x) => x,
None => return,
};

// MIR building adds discriminants to the `values` array in the same order as they
// are yielded by `AdtDef::discriminants`. We rely on this to match each
// discriminant in `values` to its corresponding variant in linear time.
let (variant, _) = discriminants
.find(|&(_, discr)| discr.val == value)
.expect("Order of `AdtDef::discriminants` differed from `SwitchInt::values`");

// 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),
);
});
// Kill all move paths that correspond to variants we know to be inactive along a
// particular outgoing edge of a `SwitchInt`.
enum_discriminant_switch_inactive_variant_effects(
self.tcx,
self.body,
self.move_data(),
block,
discr,
edge_effects,
|trans, mpi| trans.kill(mpi),
);
}
}

Expand Down Expand Up @@ -481,40 +458,17 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeUninitializedPlaces<'_, 'tcx> {
return;
}

let enum_ = discr.place().and_then(|discr| {
switch_on_enum_discriminant(self.tcx, &self.body, &self.body[block], discr)
});

let (enum_place, enum_def) = match enum_ {
Some(x) => x,
None => return,
};

let mut discriminants = enum_def.discriminants(self.tcx);
edge_effects.apply(|trans, edge| {
let value = match edge.value {
Some(x) => x,
None => return,
};

// MIR building adds discriminants to the `values` array in the same order as they
// are yielded by `AdtDef::discriminants`. We rely on this to match each
// discriminant in `values` to its corresponding variant in linear time.
let (variant, _) = discriminants
.find(|&(_, discr)| discr.val == value)
.expect("Order of `AdtDef::discriminants` differed from `SwitchInt::values`");

// 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),
);
});
// Mark all move paths that correspond to variants that are inactive along a given edge as
// maybe uninitialized (in reality, they are *definitely* uninitialized).
enum_discriminant_switch_inactive_variant_effects(
self.tcx,
self.body,
self.move_data(),
block,
discr,
edge_effects,
|trans, mpi| trans.gen(mpi),
);
}
}

Expand Down Expand Up @@ -715,3 +669,82 @@ fn switch_on_enum_discriminant(
_ => None,
}
}

fn enum_discriminant_switch_inactive_variant_effects<D>(
tcx: TyCtxt<'tcx>,
body: &mir::Body<'tcx>,
move_data: &MoveData<'tcx>,
block: mir::BasicBlock,
discr: &mir::Operand<'tcx>,
edge_effects: &mut impl SwitchIntEdgeEffects<D>,
mut on_uninitialized_variant: impl FnMut(&mut D, MovePathIndex),
) {
let enum_ =
discr.place().and_then(|discr| switch_on_enum_discriminant(tcx, body, &body[block], discr));

let (enum_place, enum_def) = match enum_ {
Some(x) => x,
None => return,
};

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];
let mut discriminants = enum_def.discriminants(tcx);

// `MovePathIndex`s for those variants with their own outgoing edge. These
// get marked as uninitialized along the "otherwise" edge.
let mut variant_mpis_with_edge = vec![];

edge_effects.apply(|trans, edge| {
if let Some(value) = edge.value {
// MIR building adds discriminants to the `values` array in the same order as they
// are yielded by `AdtDef::discriminants`. We rely on this to match each
// discriminant in `values` to its corresponding variant in linear time.
let (active_variant, _) = discriminants
.find(|&(_, discr)| discr.val == value)
.expect("Order of `AdtDef::discriminants` differed from `SwitchInt::values`");

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 {
// If this is the move path for the variant that is active along this edge
// of the `SwitchInt` terminator, remember that it is not active as part of
// the "otherwise" edge.
variant_mpis_with_edge.push(variant_mpi);
} else {
// Otherwise, it is the move path for an *inactive* variant. Mark it and
// all of its descendants as uninitialzied.
drop_flag_effects::on_all_children_bits(
tcx,
body,
move_data,
variant_mpi,
|mpi| on_uninitialized_variant(trans, mpi),
);
}
}
} else {
// We're on the otherwise branch. All variants that we visited above are known to be
// uninitialized.
for &variant_mpi in &variant_mpis_with_edge {
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Is there a guarantee that the otherwise branch is always invoked last during the iteration performed by edge_effects.apply?

Because, as best as I can tell, it seems like you are relying here on such a guarantee (since you are accumulating all the values in variant_mpis_with_edge during the callback invocations on the non-otherwise edges.

But skimming over the doc comments added in PR #77242, I do not see any such guarantee in the documentation for apply_switch_int_edge_effects.

(And overall, I think the logic here would be easier to understand if the block structure in the code reflected that ordering constraint, even if it requires changes to the dataflow API. I don't know the best way to express it; perhaps by two distinct callbacks instead of just one, where the first callback is invoked for all the edge.value.is_some() cases here, and the second callback for the otherwise case?)

drop_flag_effects::on_all_children_bits(tcx, body, move_data, variant_mpi, |mpi| {
on_uninitialized_variant(trans, mpi)
});
}
}
});
}
16 changes: 5 additions & 11 deletions src/test/mir-opt/issue_41888.main.ElaborateDrops.after.mir
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Original file line number Diff line number Diff line change
Expand Up @@ -9,9 +9,8 @@ fn main() -> () {
let mut _5: isize; // in scope 0 at $DIR/issue-41888.rs:10:16: 10:24
let mut _7: bool; // in scope 0 at $DIR/issue-41888.rs:15:1: 15:2
let mut _8: bool; // in scope 0 at $DIR/issue-41888.rs:15:1: 15:2
let mut _9: bool; // in scope 0 at $DIR/issue-41888.rs:15:1: 15:2
let mut _9: isize; // in scope 0 at $DIR/issue-41888.rs:15:1: 15:2
let mut _10: isize; // in scope 0 at $DIR/issue-41888.rs:15:1: 15:2
let mut _11: isize; // in scope 0 at $DIR/issue-41888.rs:15:1: 15:2
scope 1 {
debug e => _1; // in scope 1 at $DIR/issue-41888.rs:7:9: 7:10
let _6: K; // in scope 1 at $DIR/issue-41888.rs:10:21: 10:23
Expand All @@ -21,7 +20,6 @@ fn main() -> () {
}

bb0: {
_9 = const false; // scope 0 at $DIR/issue-41888.rs:7:9: 7:10
_7 = const false; // scope 0 at $DIR/issue-41888.rs:7:9: 7:10
_8 = const false; // scope 0 at $DIR/issue-41888.rs:7:9: 7:10
StorageLive(_1); // scope 0 at $DIR/issue-41888.rs:7:9: 7:10
Expand Down Expand Up @@ -79,7 +77,6 @@ fn main() -> () {

bb10: {
StorageLive(_6); // scope 1 at $DIR/issue-41888.rs:10:21: 10:23
_9 = const false; // scope 1 at $DIR/issue-41888.rs:10:21: 10:23
_6 = move ((_1 as F).0: K); // scope 1 at $DIR/issue-41888.rs:10:21: 10:23
_0 = const (); // scope 2 at $DIR/issue-41888.rs:10:29: 13:10
StorageDead(_6); // scope 1 at $DIR/issue-41888.rs:13:9: 13:10
Expand All @@ -93,7 +90,6 @@ fn main() -> () {
bb12: {
_7 = const false; // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
_8 = const false; // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
_9 = const false; // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
StorageDead(_1); // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
StorageDead(_2); // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
return; // scope 0 at $DIR/issue-41888.rs:15:2: 15:2
Expand All @@ -102,15 +98,13 @@ fn main() -> () {
bb13 (cleanup): {
_7 = const true; // scope 1 at $DIR/issue-41888.rs:9:9: 9:10
_8 = const true; // scope 1 at $DIR/issue-41888.rs:9:9: 9:10
_9 = const true; // scope 1 at $DIR/issue-41888.rs:9:9: 9:10
_1 = move _3; // scope 1 at $DIR/issue-41888.rs:9:9: 9:10
goto -> bb7; // scope 1 at $DIR/issue-41888.rs:9:9: 9:10
}

bb14: {
_7 = const true; // scope 1 at $DIR/issue-41888.rs:9:9: 9:10
_8 = const true; // scope 1 at $DIR/issue-41888.rs:9:9: 9:10
_9 = const true; // scope 1 at $DIR/issue-41888.rs:9:9: 9:10
_1 = move _3; // scope 1 at $DIR/issue-41888.rs:9:9: 9:10
goto -> bb6; // scope 1 at $DIR/issue-41888.rs:9:9: 9:10
}
Expand Down Expand Up @@ -138,17 +132,17 @@ fn main() -> () {
}

bb20: {
_10 = discriminant(_1); // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
switchInt(move _10) -> [0_isize: bb15, otherwise: bb18]; // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
_9 = discriminant(_1); // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
switchInt(move _9) -> [0_isize: bb15, otherwise: bb18]; // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
}

bb21: {
switchInt(_7) -> [false: bb15, otherwise: bb20]; // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
}

bb22 (cleanup): {
_11 = discriminant(_1); // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
switchInt(move _11) -> [0_isize: bb17, otherwise: bb19]; // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
_10 = discriminant(_1); // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
switchInt(move _10) -> [0_isize: bb17, otherwise: bb19]; // scope 0 at $DIR/issue-41888.rs:15:1: 15:2
}

bb23 (cleanup): {
Expand Down