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GVN: Only introduce new derefs if the immutable borrow is always valid. #147886

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GVN: Only introduce new derefs if the immutable borrow is always valid. #147886

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8df9dbc
Add miscompiled test cases for GVN
dianqk Oct 18, 2025
c0cb50a
GVN: Only introduce new derefs if the immutable borrow is always valid.
dianqk Oct 18, 2025
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102 changes: 65 additions & 37 deletions compiler/rustc_mir_transform/src/gvn.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -136,8 +136,8 @@ impl<'tcx> crate::MirPass<'tcx> for GVN {
VnState::new(tcx, body, typing_env, &ssa, dominators, &body.local_decls, &arena);

for local in body.args_iter().filter(|&local| ssa.is_ssa(local)) {
let opaque = state.new_opaque(body.local_decls[local].ty);
state.assign(local, opaque);
let argument = state.insert_parameter(body.local_decls[local].ty);
state.assign(local, argument);
}

let reverse_postorder = body.basic_blocks.reverse_postorder().to_vec();
Expand Down Expand Up @@ -204,8 +204,9 @@ enum AddressBase {
enum Value<'a, 'tcx> {
// Root values.
/// Used to represent values we know nothing about.
/// The `usize` is a counter incremented by `new_opaque`.
Opaque(VnOpaque),
/// Used to represent values we know nothing except that it is a function parameter.
Parameter(VnOpaque),
/// Evaluated or unevaluated constant value.
Constant {
value: Const<'tcx>,
Expand Down Expand Up @@ -290,7 +291,7 @@ impl<'a, 'tcx> ValueSet<'a, 'tcx> {
let value = value(VnOpaque);

debug_assert!(match value {
Value::Opaque(_) | Value::Address { .. } => true,
Value::Opaque(_) | Value::Parameter(_) | Value::Address { .. } => true,
Value::Constant { disambiguator, .. } => disambiguator.is_some(),
_ => false,
});
Expand All @@ -308,7 +309,7 @@ impl<'a, 'tcx> ValueSet<'a, 'tcx> {
#[allow(rustc::pass_by_value)] // closures take `&VnIndex`
fn insert(&mut self, ty: Ty<'tcx>, value: Value<'a, 'tcx>) -> (VnIndex, bool) {
debug_assert!(match value {
Value::Opaque(_) | Value::Address { .. } => false,
Value::Opaque(_) | Value::Parameter(_) | Value::Address { .. } => false,
Value::Constant { disambiguator, .. } => disambiguator.is_none(),
_ => true,
});
Expand Down Expand Up @@ -420,6 +421,20 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
self.ecx.typing_env()
}

fn insert_unique(
&mut self,
ty: Ty<'tcx>,
evaluated: bool,
value: impl FnOnce(VnOpaque) -> Value<'a, 'tcx>,
) -> VnIndex {
let index = self.values.insert_unique(ty, value);
let _index = self.evaluated.push(if evaluated { Some(None) } else { None });
debug_assert_eq!(index, _index);
let _index = self.rev_locals.push(SmallVec::new());
debug_assert_eq!(index, _index);
index
}

#[instrument(level = "trace", skip(self), ret)]
fn insert(&mut self, ty: Ty<'tcx>, value: Value<'a, 'tcx>) -> VnIndex {
let (index, new) = self.values.insert(ty, value);
Expand All @@ -437,12 +452,7 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
/// from all the others.
#[instrument(level = "trace", skip(self), ret)]
fn new_opaque(&mut self, ty: Ty<'tcx>) -> VnIndex {
let index = self.values.insert_unique(ty, Value::Opaque);
let _index = self.evaluated.push(Some(None));
debug_assert_eq!(index, _index);
let _index = self.rev_locals.push(SmallVec::new());
debug_assert_eq!(index, _index);
index
self.insert_unique(ty, true, Value::Opaque)
}

/// Create a new `Value::Address` distinct from all the others.
Expand Down Expand Up @@ -470,42 +480,30 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
projection.map(|proj| proj.try_map(|index| self.locals[index], |ty| ty).ok_or(()));
let projection = self.arena.try_alloc_from_iter(projection).ok()?;

let index = self.values.insert_unique(ty, |provenance| Value::Address {
let index = self.insert_unique(ty, false, |provenance| Value::Address {
base,
projection,
kind,
provenance,
});
let _index = self.evaluated.push(None);
debug_assert_eq!(index, _index);
let _index = self.rev_locals.push(SmallVec::new());
debug_assert_eq!(index, _index);

Some(index)
}

#[instrument(level = "trace", skip(self), ret)]
fn insert_constant(&mut self, value: Const<'tcx>) -> VnIndex {
let (index, new) = if value.is_deterministic() {
if value.is_deterministic() {
// The constant is deterministic, no need to disambiguate.
let constant = Value::Constant { value, disambiguator: None };
self.values.insert(value.ty(), constant)
self.insert(value.ty(), constant)
} else {
// Multiple mentions of this constant will yield different values,
// so assign a different `disambiguator` to ensure they do not get the same `VnIndex`.
let index = self.values.insert_unique(value.ty(), |disambiguator| Value::Constant {
self.insert_unique(value.ty(), false, |disambiguator| Value::Constant {
value,
disambiguator: Some(disambiguator),
});
(index, true)
};
if new {
let _index = self.evaluated.push(None);
debug_assert_eq!(index, _index);
let _index = self.rev_locals.push(SmallVec::new());
debug_assert_eq!(index, _index);
})
}
index
}

#[inline]
Expand Down Expand Up @@ -540,13 +538,20 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
self.insert(ty, Value::Constant { value, disambiguator: None })
}

fn insert_parameter(&mut self, ty: Ty<'tcx>) -> VnIndex {
self.insert_unique(ty, true, Value::Parameter)
}

fn insert_tuple(&mut self, ty: Ty<'tcx>, values: &[VnIndex]) -> VnIndex {
self.insert(ty, Value::Aggregate(VariantIdx::ZERO, self.arena.alloc_slice(values)))
}

fn insert_deref(&mut self, ty: Ty<'tcx>, value: VnIndex) -> VnIndex {
fn insert_deref(&mut self, ty: Ty<'tcx>, value: VnIndex, always_valid: bool) -> VnIndex {
let value = self.insert(ty, Value::Projection(value, ProjectionElem::Deref));
self.derefs.push(value);
// If the borrow lifetime is the whole body, we don't need to invalidate it.
if !always_valid {
self.derefs.push(value);
}
value
}

Expand All @@ -569,7 +574,7 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
let op = match self.get(value) {
_ if ty.is_zst() => ImmTy::uninit(ty).into(),

Opaque(_) => return None,
Opaque(_) | Parameter(_) => return None,
// Do not bother evaluating repeat expressions. This would uselessly consume memory.
Repeat(..) => return None,

Expand Down Expand Up @@ -817,15 +822,38 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
if let Some(Mutability::Not) = place_ty.ty.ref_mutability()
&& projection_ty.ty.is_freeze(self.tcx, self.typing_env())
{
if let Value::Address { base, projection, .. } = self.get(value)
&& let Some(value) = self.dereference_address(base, projection)
{
if let Value::Address { base, projection, .. } = self.get(value) {
// Bail out if the address cannot be dereferenced.
let value = self.dereference_address(base, projection)?;
return Some((projection_ty, value));
}

// An immutable borrow `_x` always points to the same value for the
// lifetime of the borrow, so we can merge all instances of `*_x`.
return Some((projection_ty, self.insert_deref(projection_ty.ty, value)));
if let Value::Parameter(_) | Value::Constant { .. } = self.get(value) {
// An immutable borrow `_x` that is an parameter or a constant always points to the same value for the
// lifetime of the borrow, so we can merge all instances of `*_x`.
// The lifetime here is unrelated to storage markers, but rather does not violate the Stacked Borrows rules.
return Some((
projection_ty,
self.insert_deref(projection_ty.ty, value, true),
));
}

// FIXME: We could introduce new deref that the immutable borrow is not valid in the whole body,
// but we must invalidate it when out of the immutable borrow lifetime.
// Known related issues:
// - https://github.com/rust-lang/rust/issues/130853 (Fixed by invalidating)
// - https://github.com/rust-lang/rust/issues/132353 (Fixed by invalidating)
// - https://github.com/rust-lang/rust/issues/141038 (Not fixed well)
// - https://github.com/rust-lang/rust/issues/141251 (Fixed by #144477)
// - https://github.com/rust-lang/rust/issues/141313
// - https://github.com/rust-lang/rust/pull/147607 (Fixed by invalidating)
if self.tcx.sess.opts.unstable_opts.unsound_mir_opts {
return Some((
projection_ty,
self.insert_deref(projection_ty.ty, value, false),
));
}
return None;
} else {
return None;
}
Expand Down
2 changes: 1 addition & 1 deletion tests/mir-opt/const_prop/ref_deref_project.rs
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@@ -1,6 +1,6 @@
// This does not currently propagate (#67862)
//@ test-mir-pass: GVN

// Checks that GVN knows a is 5.
// EMIT_MIR ref_deref_project.main.GVN.diff
fn main() {
// CHECK-LABEL: fn main(
Expand Down
2 changes: 1 addition & 1 deletion tests/mir-opt/const_prop/slice_len.main.GVN.32bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -40,7 +40,7 @@

bb1: {
- _1 = copy (*_2)[_6];
+ _1 = const 2_u32;
+ _1 = copy (*_2)[1 of 2];
StorageDead(_6);
StorageDead(_4);
StorageDead(_2);
Expand Down
2 changes: 1 addition & 1 deletion tests/mir-opt/const_prop/slice_len.main.GVN.32bit.panic-unwind.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -40,7 +40,7 @@

bb1: {
- _1 = copy (*_2)[_6];
+ _1 = const 2_u32;
+ _1 = copy (*_2)[1 of 2];
StorageDead(_6);
StorageDead(_4);
StorageDead(_2);
Expand Down
2 changes: 1 addition & 1 deletion tests/mir-opt/const_prop/slice_len.main.GVN.64bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -40,7 +40,7 @@

bb1: {
- _1 = copy (*_2)[_6];
+ _1 = const 2_u32;
+ _1 = copy (*_2)[1 of 2];
StorageDead(_6);
StorageDead(_4);
StorageDead(_2);
Expand Down
2 changes: 1 addition & 1 deletion tests/mir-opt/const_prop/slice_len.main.GVN.64bit.panic-unwind.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -40,7 +40,7 @@

bb1: {
- _1 = copy (*_2)[_6];
+ _1 = const 2_u32;
+ _1 = copy (*_2)[1 of 2];
StorageDead(_6);
StorageDead(_4);
StorageDead(_2);
Expand Down
3 changes: 2 additions & 1 deletion tests/mir-opt/const_prop/slice_len.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,12 +3,13 @@
// EMIT_MIR_FOR_EACH_PANIC_STRATEGY
// EMIT_MIR_FOR_EACH_BIT_WIDTH

// FIXME: GVN should be able to know `a` is 2;
// EMIT_MIR slice_len.main.GVN.diff
fn main() {
// CHECK-LABEL: fn main(
// CHECK: debug a => [[a:_.*]];
// CHECK: [[slice:_.*]] = copy {{.*}} as &[u32] (PointerCoercion(Unsize, AsCast));
// CHECK: assert(const true,
// CHECK: [[a]] = const 2_u32;
// CHECK: [[a]] = copy (*[[slice]])[1 of 2];
let a = (&[1u32, 2, 3] as &[u32])[1];
}
9 changes: 3 additions & 6 deletions tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.32bit.panic-abort.diff
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Expand Up @@ -72,8 +72,7 @@

bb0: {
StorageLive(_1);
- StorageLive(_2);
+ nop;
StorageLive(_2);
StorageLive(_3);
StorageLive(_4);
- _4 = ();
Expand Down Expand Up @@ -134,12 +133,10 @@
StorageDead(_4);
_2 = &_3;
_1 = &(*_2);
- StorageDead(_2);
StorageDead(_2);
- StorageLive(_5);
- _10 = copy (*_1);
+ nop;
+ nop;
+ _10 = copy (*_2);
_10 = copy (*_1);
_11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
_5 = &raw const (*_11);
- StorageLive(_6);
Expand Down
9 changes: 3 additions & 6 deletions tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.32bit.panic-unwind.diff
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Expand Up @@ -37,8 +37,7 @@

bb0: {
StorageLive(_1);
- StorageLive(_2);
+ nop;
StorageLive(_2);
StorageLive(_3);
StorageLive(_4);
- _4 = ();
Expand All @@ -51,12 +50,10 @@
StorageDead(_4);
_2 = &_3;
_1 = &(*_2);
- StorageDead(_2);
StorageDead(_2);
- StorageLive(_5);
- _10 = copy (*_1);
+ nop;
+ nop;
+ _10 = copy (*_2);
_10 = copy (*_1);
_11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
_5 = &raw const (*_11);
- StorageLive(_6);
Expand Down
9 changes: 3 additions & 6 deletions tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.64bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -72,8 +72,7 @@

bb0: {
StorageLive(_1);
- StorageLive(_2);
+ nop;
StorageLive(_2);
StorageLive(_3);
StorageLive(_4);
- _4 = ();
Expand Down Expand Up @@ -134,12 +133,10 @@
StorageDead(_4);
_2 = &_3;
_1 = &(*_2);
- StorageDead(_2);
StorageDead(_2);
- StorageLive(_5);
- _10 = copy (*_1);
+ nop;
+ nop;
+ _10 = copy (*_2);
_10 = copy (*_1);
_11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
_5 = &raw const (*_11);
- StorageLive(_6);
Expand Down
9 changes: 3 additions & 6 deletions tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.64bit.panic-unwind.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -37,8 +37,7 @@

bb0: {
StorageLive(_1);
- StorageLive(_2);
+ nop;
StorageLive(_2);
StorageLive(_3);
StorageLive(_4);
- _4 = ();
Expand All @@ -51,12 +50,10 @@
StorageDead(_4);
_2 = &_3;
_1 = &(*_2);
- StorageDead(_2);
StorageDead(_2);
- StorageLive(_5);
- _10 = copy (*_1);
+ nop;
+ nop;
+ _10 = copy (*_2);
_10 = copy (*_1);
_11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
_5 = &raw const (*_11);
- StorageLive(_6);
Expand Down
15 changes: 10 additions & 5 deletions tests/mir-opt/gvn.dereferences.GVN.panic-abort.diff
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Expand Up @@ -152,18 +152,23 @@
StorageDead(_28);
StorageDead(_27);
StorageLive(_29);
StorageLive(_30);
- StorageLive(_30);
+ nop;
_30 = copy ((*_3).0: u32);
_29 = opaque::<u32>(move _30) -> [return: bb12, unwind unreachable];
- _29 = opaque::<u32>(move _30) -> [return: bb12, unwind unreachable];
+ _29 = opaque::<u32>(copy _30) -> [return: bb12, unwind unreachable];
}

bb12: {
StorageDead(_30);
- StorageDead(_30);
+ nop;
StorageDead(_29);
StorageLive(_31);
StorageLive(_32);
_32 = copy ((*_3).0: u32);
_31 = opaque::<u32>(move _32) -> [return: bb13, unwind unreachable];
- _32 = copy ((*_3).0: u32);
- _31 = opaque::<u32>(move _32) -> [return: bb13, unwind unreachable];
+ _32 = copy _30;
+ _31 = opaque::<u32>(copy _30) -> [return: bb13, unwind unreachable];
}

bb13: {
Expand Down
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