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Do not use for_each_assignment_mut to iterate over assignment statements

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`for_each_assignment_mut` can skip assignment statements with side effects,
which can result in some assignment statements retrieving outdated value.
For example, it may skip a dereference assignment statement.
This commit is contained in:
dianqk 2025-03-24 16:55:24 +08:00
parent 79de6c0bbe
commit 9d999bb035
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4 changed files with 41 additions and 80 deletions
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77
compiler/rustc_mir_transform/src/gvn.rs
View file

@ -3,14 +3,16 @@
//! MIR may contain repeated and/or redundant computations. The objective of this pass is to detect
//! such redundancies and re-use the already-computed result when possible.
//!
//! In a first pass, we compute a symbolic representation of values that are assigned to SSA
//! locals. This symbolic representation is defined by the `Value` enum. Each produced instance of
//! `Value` is interned as a `VnIndex`, which allows us to cheaply compute identical values.
//!
//! From those assignments, we construct a mapping `VnIndex -> Vec<(Local, Location)>` of available
//! values, the locals in which they are stored, and the assignment location.
//!
//! In a second pass, we traverse all (non SSA) assignments `x = rvalue` and operands. For each
//! We traverse all assignments `x = rvalue` and operands.
//!
//! For each SSA one, we compute a symbolic representation of values that are assigned to SSA
//! locals. This symbolic representation is defined by the `Value` enum. Each produced instance of
//! `Value` is interned as a `VnIndex`, which allows us to cheaply compute identical values.
//!
//! For each non-SSA
//! one, we compute the `VnIndex` of the rvalue. If this `VnIndex` is associated to a constant, we
//! replace the rvalue/operand by that constant. Otherwise, if there is an SSA local `y`
//! associated to this `VnIndex`, and if its definition location strictly dominates the assignment
@ -107,7 +109,7 @@ use rustc_span::def_id::DefId;
use smallvec::SmallVec;
use tracing::{debug, instrument, trace};
use crate::ssa::{AssignedValue, SsaLocals};
use crate::ssa::SsaLocals;
pub(super) struct GVN;
@ -126,31 +128,11 @@ impl<'tcx> crate::MirPass<'tcx> for GVN {
let dominators = body.basic_blocks.dominators().clone();
let mut state = VnState::new(tcx, body, typing_env, &ssa, dominators, &body.local_decls);
ssa.for_each_assignment_mut(
body.basic_blocks.as_mut_preserves_cfg(),
|local, value, location| {
let value = match value {
// We do not know anything of this assigned value.
AssignedValue::Arg | AssignedValue::Terminator => None,
// Try to get some insight.
AssignedValue::Rvalue(rvalue) => {
let value = state.simplify_rvalue(rvalue, location);
// FIXME(#112651) `rvalue` may have a subtype to `local`. We can only mark
// `local` as reusable if we have an exact type match.
if state.local_decls[local].ty != rvalue.ty(state.local_decls, tcx) {
return;
}
value
}
};
// `next_opaque` is `Some`, so `new_opaque` must return `Some`.
let value = value.or_else(|| state.new_opaque()).unwrap();
state.assign(local, value);
},
);
// Stop creating opaques during replacement as it is useless.
state.next_opaque = None;
for local in body.args_iter().filter(|&local| ssa.is_ssa(local)) {
let opaque = state.new_opaque().unwrap();
state.assign(local, opaque);
}
let reverse_postorder = body.basic_blocks.reverse_postorder().to_vec();
for bb in reverse_postorder {
@ -250,7 +232,6 @@ struct VnState<'body, 'tcx> {
locals: IndexVec<Local, Option<VnIndex>>,
/// Locals that are assigned that value.
// This vector does not hold all the values of `VnIndex` that we create.
// It stops at the largest value created in the first phase of collecting assignments.
rev_locals: IndexVec<VnIndex, SmallVec<[Local; 1]>>,
values: FxIndexSet<Value<'tcx>>,
/// Values evaluated as constants if possible.
@ -345,6 +326,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
/// Record that `local` is assigned `value`. `local` must be SSA.
#[instrument(level = "trace", skip(self))]
fn assign(&mut self, local: Local, value: VnIndex) {
debug_assert!(self.ssa.is_ssa(local));
self.locals[local] = Some(value);
// Only register the value if its type is `Sized`, as we will emit copies of it.
@ -1751,15 +1733,19 @@ impl<'tcx> MutVisitor<'tcx> for VnState<'_, 'tcx> {
if let StatementKind::Assign(box (ref mut lhs, ref mut rvalue)) = stmt.kind {
self.simplify_place_projection(lhs, location);
// Do not try to simplify a constant, it's already in canonical shape.
if matches!(rvalue, Rvalue::Use(Operand::Constant(_))) {
return;
}
let value = lhs
.as_local()
.and_then(|local| self.locals[local])
.or_else(|| self.simplify_rvalue(rvalue, location));
let value = self.simplify_rvalue(rvalue, location);
let value = if let Some(local) = lhs.as_local()
&& self.ssa.is_ssa(local)
// FIXME(#112651) `rvalue` may have a subtype to `local`. We can only mark
// `local` as reusable if we have an exact type match.
&& self.local_decls[local].ty == rvalue.ty(self.local_decls, self.tcx)
{
let value = value.or_else(|| self.new_opaque()).unwrap();
self.assign(local, value);
Some(value)
} else {
value
};
let Some(value) = value else { return };
if let Some(const_) = self.try_as_constant(value) {
@ -1775,6 +1761,17 @@ impl<'tcx> MutVisitor<'tcx> for VnState<'_, 'tcx> {
}
self.super_statement(stmt, location);
}
fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
if let Terminator { kind: TerminatorKind::Call { destination, .. }, .. } = terminator
&& let Some(local) = destination.as_local()
&& self.ssa.is_ssa(local)
{
let opaque = self.new_opaque().unwrap();
self.assign(local, opaque);
}
self.super_terminator(terminator, location);
}
}
struct StorageRemover<'tcx> {

38
compiler/rustc_mir_transform/src/ssa.rs
View file

@ -32,12 +32,6 @@ pub(super) struct SsaLocals {
borrowed_locals: DenseBitSet<Local>,
}
pub(super) enum AssignedValue<'a, 'tcx> {
Arg,
Rvalue(&'a mut Rvalue<'tcx>),
Terminator,
}
impl SsaLocals {
pub(super) fn new<'tcx>(
tcx: TyCtxt<'tcx>,
@ -152,38 +146,6 @@ impl SsaLocals {
})
}
pub(super) fn for_each_assignment_mut<'tcx>(
&self,
basic_blocks: &mut IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
mut f: impl FnMut(Local, AssignedValue<'_, 'tcx>, Location),
) {
for &local in &self.assignment_order {
match self.assignments[local] {
Set1::One(DefLocation::Argument) => f(
local,
AssignedValue::Arg,
Location { block: START_BLOCK, statement_index: 0 },
),
Set1::One(DefLocation::Assignment(loc)) => {
let bb = &mut basic_blocks[loc.block];
// `loc` must point to a direct assignment to `local`.
let stmt = &mut bb.statements[loc.statement_index];
let StatementKind::Assign(box (target, ref mut rvalue)) = stmt.kind else {
bug!()
};
assert_eq!(target.as_local(), Some(local));
f(local, AssignedValue::Rvalue(rvalue), loc)
}
Set1::One(DefLocation::CallReturn { call, .. }) => {
let bb = &mut basic_blocks[call];
let loc = Location { block: call, statement_index: bb.statements.len() };
f(local, AssignedValue::Terminator, loc)
}
_ => {}
}
}
}
/// Compute the equivalence classes for locals, based on copy statements.
///
/// The returned vector maps each local to the one it copies. In the following case:

3
tests/mir-opt/const_prop/control_flow_simplification.hello.GVN.panic-abort.diff
View file

@ -8,8 +8,9 @@
bb0: {
StorageLive(_1);
_1 = const <bool as NeedsDrop>::NEEDS;
- _1 = const <bool as NeedsDrop>::NEEDS;
- switchInt(move _1) -> [0: bb2, otherwise: bb1];
+ _1 = const false;
+ switchInt(const false) -> [0: bb2, otherwise: bb1];
}

3
tests/mir-opt/const_prop/control_flow_simplification.hello.GVN.panic-unwind.diff
View file

@ -8,8 +8,9 @@
bb0: {
StorageLive(_1);
_1 = const <bool as NeedsDrop>::NEEDS;
- _1 = const <bool as NeedsDrop>::NEEDS;
- switchInt(move _1) -> [0: bb2, otherwise: bb1];
+ _1 = const false;
+ switchInt(const false) -> [0: bb2, otherwise: bb1];
}