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Verify copies of mutable pointers in 2 stages in ReferencePropagation #111441

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May 12, 2023
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90 changes: 60 additions & 30 deletions compiler/rustc_mir_transform/src/ref_prop.rs
Original file line number Diff line number Diff line change
Expand Up @@ -85,7 +85,9 @@ fn propagate_ssa<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let ssa = SsaLocals::new(body);

let mut replacer = compute_replacement(tcx, body, &ssa);
debug!(?replacer.targets, ?replacer.allowed_replacements, ?replacer.storage_to_remove);
debug!(?replacer.targets);
debug!(?replacer.allowed_replacements);
debug!(?replacer.storage_to_remove);

replacer.visit_body_preserves_cfg(body);

Expand Down Expand Up @@ -190,8 +192,11 @@ fn compute_replacement<'tcx>(
continue;
}

// Whether the current local is subject to the uniqueness rule.
let needs_unique = ty.is_mutable_ptr();

// If this a mutable reference that we cannot fully replace, mark it as unknown.
if ty.is_mutable_ptr() && !fully_replacable_locals.contains(local) {
if needs_unique && !fully_replacable_locals.contains(local) {
debug!("not fully replaceable");
continue;
}
Expand All @@ -203,32 +208,33 @@ fn compute_replacement<'tcx>(
// have been visited before.
Rvalue::Use(Operand::Copy(place) | Operand::Move(place))
| Rvalue::CopyForDeref(place) => {
if let Some(rhs) = place.as_local() {
if let Some(rhs) = place.as_local() && ssa.is_ssa(rhs) {
let target = targets[rhs];
if matches!(target, Value::Pointer(..)) {
// Only see through immutable reference and pointers, as we do not know yet if
// mutable references are fully replaced.
if !needs_unique && matches!(target, Value::Pointer(..)) {
targets[local] = target;
} else if ssa.is_ssa(rhs) {
let refmut = body.local_decls[rhs].ty.is_mutable_ptr();
targets[local] = Value::Pointer(tcx.mk_place_deref(rhs.into()), refmut);
} else {
targets[local] = Value::Pointer(tcx.mk_place_deref(rhs.into()), needs_unique);
}
}
}
Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
let mut place = *place;
// Try to see through `place` in order to collapse reborrow chains.
if place.projection.first() == Some(&PlaceElem::Deref)
&& let Value::Pointer(target, refmut) = targets[place.local]
&& let Value::Pointer(target, inner_needs_unique) = targets[place.local]
// Only see through immutable reference and pointers, as we do not know yet if
// mutable references are fully replaced.
&& !refmut
&& !inner_needs_unique
// Only collapse chain if the pointee is definitely live.
&& can_perform_opt(target, location)
{
place = target.project_deeper(&place.projection[1..], tcx);
}
assert_ne!(place.local, local);
if is_constant_place(place) {
targets[local] = Value::Pointer(place, ty.is_mutable_ptr());
targets[local] = Value::Pointer(place, needs_unique);
}
}
// We do not know what to do, so keep as not-a-pointer.
Expand Down Expand Up @@ -276,16 +282,35 @@ fn compute_replacement<'tcx>(
return;
}

if let Value::Pointer(target, refmut) = self.targets[place.local]
&& place.projection.first() == Some(&PlaceElem::Deref)
{
let perform_opt = (self.can_perform_opt)(target, loc);
if perform_opt {
self.allowed_replacements.insert((target.local, loc));
} else if refmut {
// This mutable reference is not fully replacable, so drop it.
self.targets[place.local] = Value::Unknown;
if place.projection.first() != Some(&PlaceElem::Deref) {
// This is not a dereference, nothing to do.
return;
}

let mut place = place.as_ref();
loop {
if let Value::Pointer(target, needs_unique) = self.targets[place.local] {
let perform_opt = (self.can_perform_opt)(target, loc);
debug!(?place, ?target, ?needs_unique, ?perform_opt);

// This a reborrow chain, recursively allow the replacement.
//
// This also allows to detect cases where `target.local` is not replacable,
// and mark it as such.
if let &[PlaceElem::Deref] = &target.projection[..] {
assert!(perform_opt);
self.allowed_replacements.insert((target.local, loc));
place.local = target.local;
continue;
} else if perform_opt {
self.allowed_replacements.insert((target.local, loc));
} else if needs_unique {
// This mutable reference is not fully replacable, so drop it.
self.targets[place.local] = Value::Unknown;
}
}

break;
}
}
}
Expand Down Expand Up @@ -326,18 +351,23 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'tcx> {
}

fn visit_place(&mut self, place: &mut Place<'tcx>, ctxt: PlaceContext, loc: Location) {
if let Value::Pointer(target, _) = self.targets[place.local]
&& place.projection.first() == Some(&PlaceElem::Deref)
{
let perform_opt = matches!(ctxt, PlaceContext::NonUse(_))
|| self.allowed_replacements.contains(&(target.local, loc));

if perform_opt {
*place = target.project_deeper(&place.projection[1..], self.tcx);
self.any_replacement = true;
if place.projection.first() != Some(&PlaceElem::Deref) {
return;
}

loop {
if let Value::Pointer(target, _) = self.targets[place.local] {
let perform_opt = matches!(ctxt, PlaceContext::NonUse(_))
|| self.allowed_replacements.contains(&(target.local, loc));

if perform_opt {
*place = target.project_deeper(&place.projection[1..], self.tcx);
self.any_replacement = true;
continue;
}
}
} else {
self.super_place(place, ctxt, loc);

break;
}
}

Expand Down
Original file line number Diff line number Diff line change
@@ -0,0 +1,74 @@
- // MIR for `mut_raw_then_mut_shr` before ReferencePropagation
+ // MIR for `mut_raw_then_mut_shr` after ReferencePropagation

fn mut_raw_then_mut_shr() -> (i32, i32) {
let mut _0: (i32, i32); // return place in scope 0 at $DIR/reference_prop.rs:+0:30: +0:40
let mut _1: i32; // in scope 0 at $DIR/reference_prop.rs:+1:9: +1:14
let mut _4: *mut i32; // in scope 0 at $DIR/reference_prop.rs:+3:16: +3:36
let mut _5: &mut i32; // in scope 0 at $DIR/reference_prop.rs:+3:16: +3:26
let _8: (); // in scope 0 at $DIR/reference_prop.rs:+7:5: +7:26
let mut _9: i32; // in scope 0 at $DIR/reference_prop.rs:+8:6: +8:7
let mut _10: i32; // in scope 0 at $DIR/reference_prop.rs:+8:9: +8:10
scope 1 {
debug x => _1; // in scope 1 at $DIR/reference_prop.rs:+1:9: +1:14
let _2: &mut i32; // in scope 1 at $DIR/reference_prop.rs:+2:9: +2:13
scope 2 {
debug xref => _2; // in scope 2 at $DIR/reference_prop.rs:+2:9: +2:13
let _3: *mut i32; // in scope 2 at $DIR/reference_prop.rs:+3:9: +3:13
scope 3 {
debug xraw => _3; // in scope 3 at $DIR/reference_prop.rs:+3:9: +3:13
let _6: &i32; // in scope 3 at $DIR/reference_prop.rs:+4:9: +4:13
scope 4 {
debug xshr => _6; // in scope 4 at $DIR/reference_prop.rs:+4:9: +4:13
let _7: i32; // in scope 4 at $DIR/reference_prop.rs:+6:9: +6:10
scope 5 {
debug a => _7; // in scope 5 at $DIR/reference_prop.rs:+6:9: +6:10
scope 6 {
}
}
}
}
}
}

bb0: {
StorageLive(_1); // scope 0 at $DIR/reference_prop.rs:+1:9: +1:14
_1 = const 2_i32; // scope 0 at $DIR/reference_prop.rs:+1:17: +1:18
- StorageLive(_2); // scope 1 at $DIR/reference_prop.rs:+2:9: +2:13
_2 = &mut _1; // scope 1 at $DIR/reference_prop.rs:+2:16: +2:22
StorageLive(_3); // scope 2 at $DIR/reference_prop.rs:+3:9: +3:13
- StorageLive(_4); // scope 2 at $DIR/reference_prop.rs:+3:16: +3:36
- StorageLive(_5); // scope 2 at $DIR/reference_prop.rs:+3:16: +3:26
- _5 = &mut (*_2); // scope 2 at $DIR/reference_prop.rs:+3:16: +3:26
- _4 = &raw mut (*_5); // scope 2 at $DIR/reference_prop.rs:+3:16: +3:26
+ _4 = &raw mut _1; // scope 2 at $DIR/reference_prop.rs:+3:16: +3:26
_3 = _4; // scope 2 at $DIR/reference_prop.rs:+3:16: +3:36
- StorageDead(_5); // scope 2 at $DIR/reference_prop.rs:+3:36: +3:37
- StorageDead(_4); // scope 2 at $DIR/reference_prop.rs:+3:36: +3:37
StorageLive(_6); // scope 3 at $DIR/reference_prop.rs:+4:9: +4:13
- _6 = &(*_2); // scope 3 at $DIR/reference_prop.rs:+4:16: +4:22
+ _6 = &_1; // scope 3 at $DIR/reference_prop.rs:+4:16: +4:22
StorageLive(_7); // scope 4 at $DIR/reference_prop.rs:+6:9: +6:10
- _7 = (*_6); // scope 4 at $DIR/reference_prop.rs:+6:13: +6:18
- StorageLive(_8); // scope 5 at $DIR/reference_prop.rs:+7:5: +7:26
- (*_3) = const 4_i32; // scope 6 at $DIR/reference_prop.rs:+7:14: +7:23
- _8 = const (); // scope 6 at $DIR/reference_prop.rs:+7:5: +7:26
- StorageDead(_8); // scope 5 at $DIR/reference_prop.rs:+7:25: +7:26
+ _7 = _1; // scope 4 at $DIR/reference_prop.rs:+6:13: +6:18
+ _1 = const 4_i32; // scope 6 at $DIR/reference_prop.rs:+7:14: +7:23
StorageLive(_9); // scope 5 at $DIR/reference_prop.rs:+8:6: +8:7
_9 = _7; // scope 5 at $DIR/reference_prop.rs:+8:6: +8:7
StorageLive(_10); // scope 5 at $DIR/reference_prop.rs:+8:9: +8:10
_10 = _1; // scope 5 at $DIR/reference_prop.rs:+8:9: +8:10
_0 = (move _9, move _10); // scope 5 at $DIR/reference_prop.rs:+8:5: +8:11
StorageDead(_10); // scope 5 at $DIR/reference_prop.rs:+8:10: +8:11
StorageDead(_9); // scope 5 at $DIR/reference_prop.rs:+8:10: +8:11
StorageDead(_7); // scope 4 at $DIR/reference_prop.rs:+9:1: +9:2
StorageDead(_6); // scope 3 at $DIR/reference_prop.rs:+9:1: +9:2
StorageDead(_3); // scope 2 at $DIR/reference_prop.rs:+9:1: +9:2
- StorageDead(_2); // scope 1 at $DIR/reference_prop.rs:+9:1: +9:2
StorageDead(_1); // scope 0 at $DIR/reference_prop.rs:+9:1: +9:2
return; // scope 0 at $DIR/reference_prop.rs:+9:2: +9:2
}
}

Original file line number Diff line number Diff line change
Expand Up @@ -9,15 +9,14 @@
let mut _5: *mut usize; // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL

bb0: {
_2 = &mut (*_1); // scope 0 at $DIR/reference_prop.rs:+10:13: +10:25
- _2 = &mut (*_1); // scope 0 at $DIR/reference_prop.rs:+10:13: +10:25
- _3 = &mut (*_2); // scope 0 at $DIR/reference_prop.rs:+11:13: +11:26
- _4 = &raw mut (*_2); // scope 0 at $DIR/reference_prop.rs:+12:13: +12:30
- _5 = &raw mut (*_3); // scope 0 at $DIR/reference_prop.rs:+13:13: +13:30
- _0 = (*_4); // scope 0 at $DIR/reference_prop.rs:+15:13: +15:22
- _0 = (*_5); // scope 0 at $DIR/reference_prop.rs:+16:13: +16:22
+ _3 = &mut (*_1); // scope 0 at $DIR/reference_prop.rs:+11:13: +11:26
+ _0 = (*_2); // scope 0 at $DIR/reference_prop.rs:+15:13: +15:22
+ _0 = (*_3); // scope 0 at $DIR/reference_prop.rs:+16:13: +16:22
+ _0 = (*_1); // scope 0 at $DIR/reference_prop.rs:+15:13: +15:22
+ _0 = (*_1); // scope 0 at $DIR/reference_prop.rs:+16:13: +16:22
return; // scope 0 at $DIR/reference_prop.rs:+17:13: +17:21
}
}
Expand Down
27 changes: 27 additions & 0 deletions tests/mir-opt/reference_prop.rs
Original file line number Diff line number Diff line change
Expand Up @@ -433,6 +433,29 @@ fn maybe_dead(m: bool) {
)
}

fn mut_raw_then_mut_shr() -> (i32, i32) {
let mut x = 2;
let xref = &mut x;
let xraw = &mut *xref as *mut _;
let xshr = &*xref;
// Verify that we completely replace with `x` in both cases.
let a = *xshr;
unsafe { *xraw = 4; }
(a, x)
}

fn unique_with_copies() {
let y = {
let mut a = 0;
let x = &raw mut a;
// `*y` is not replacable below, so we must not replace `*x`.
unsafe { opaque(*x) };
x
};
// But rewriting as `*x` is ok.
unsafe { opaque(*y) };
}

fn main() {
let mut x = 5_usize;
let mut y = 7_usize;
Expand All @@ -444,6 +467,8 @@ fn main() {
multiple_storage();
dominate_storage();
maybe_dead(true);
mut_raw_then_mut_shr();
unique_with_copies();
}

// EMIT_MIR reference_prop.reference_propagation.ReferencePropagation.diff
Expand All @@ -454,3 +479,5 @@ fn main() {
// EMIT_MIR reference_prop.multiple_storage.ReferencePropagation.diff
// EMIT_MIR reference_prop.dominate_storage.ReferencePropagation.diff
// EMIT_MIR reference_prop.maybe_dead.ReferencePropagation.diff
// EMIT_MIR reference_prop.mut_raw_then_mut_shr.ReferencePropagation.diff
// EMIT_MIR reference_prop.unique_with_copies.ReferencePropagation.diff
Original file line number Diff line number Diff line change
@@ -0,0 +1,66 @@
- // MIR for `unique_with_copies` before ReferencePropagation
+ // MIR for `unique_with_copies` after ReferencePropagation

fn unique_with_copies() -> () {
let mut _0: (); // return place in scope 0 at $DIR/reference_prop.rs:+0:25: +0:25
let _1: *mut i32; // in scope 0 at $DIR/reference_prop.rs:+1:9: +1:10
let mut _2: i32; // in scope 0 at $DIR/reference_prop.rs:+2:13: +2:18
let _4: (); // in scope 0 at $DIR/reference_prop.rs:+5:18: +5:28
let mut _5: i32; // in scope 0 at $DIR/reference_prop.rs:+5:25: +5:27
let _6: (); // in scope 0 at $DIR/reference_prop.rs:+9:14: +9:24
let mut _7: i32; // in scope 0 at $DIR/reference_prop.rs:+9:21: +9:23
scope 1 {
debug y => _1; // in scope 1 at $DIR/reference_prop.rs:+1:9: +1:10
scope 5 {
}
}
scope 2 {
debug a => _2; // in scope 2 at $DIR/reference_prop.rs:+2:13: +2:18
let _3: *mut i32; // in scope 2 at $DIR/reference_prop.rs:+3:13: +3:14
scope 3 {
debug x => _3; // in scope 3 at $DIR/reference_prop.rs:+3:13: +3:14
scope 4 {
}
}
}

bb0: {
StorageLive(_1); // scope 0 at $DIR/reference_prop.rs:+1:9: +1:10
StorageLive(_2); // scope 0 at $DIR/reference_prop.rs:+2:13: +2:18
_2 = const 0_i32; // scope 0 at $DIR/reference_prop.rs:+2:21: +2:22
- StorageLive(_3); // scope 2 at $DIR/reference_prop.rs:+3:13: +3:14
_3 = &raw mut _2; // scope 2 at $DIR/reference_prop.rs:+3:17: +3:27
StorageLive(_4); // scope 3 at $DIR/reference_prop.rs:+5:9: +5:30
StorageLive(_5); // scope 4 at $DIR/reference_prop.rs:+5:25: +5:27
_5 = (*_3); // scope 4 at $DIR/reference_prop.rs:+5:25: +5:27
_4 = opaque::<i32>(move _5) -> bb1; // scope 4 at $DIR/reference_prop.rs:+5:18: +5:28
// mir::Constant
// + span: $DIR/reference_prop.rs:452:18: 452:24
// + literal: Const { ty: fn(i32) {opaque::<i32>}, val: Value(<ZST>) }
}

bb1: {
StorageDead(_5); // scope 4 at $DIR/reference_prop.rs:+5:27: +5:28
StorageDead(_4); // scope 3 at $DIR/reference_prop.rs:+5:30: +5:31
_1 = _3; // scope 3 at $DIR/reference_prop.rs:+6:9: +6:10
- StorageDead(_3); // scope 2 at $DIR/reference_prop.rs:+7:5: +7:6
StorageDead(_2); // scope 0 at $DIR/reference_prop.rs:+7:5: +7:6
StorageLive(_6); // scope 1 at $DIR/reference_prop.rs:+9:5: +9:26
StorageLive(_7); // scope 5 at $DIR/reference_prop.rs:+9:21: +9:23
- _7 = (*_1); // scope 5 at $DIR/reference_prop.rs:+9:21: +9:23
+ _7 = (*_3); // scope 5 at $DIR/reference_prop.rs:+9:21: +9:23
_6 = opaque::<i32>(move _7) -> bb2; // scope 5 at $DIR/reference_prop.rs:+9:14: +9:24
// mir::Constant
// + span: $DIR/reference_prop.rs:456:14: 456:20
// + literal: Const { ty: fn(i32) {opaque::<i32>}, val: Value(<ZST>) }
}

bb2: {
StorageDead(_7); // scope 5 at $DIR/reference_prop.rs:+9:23: +9:24
StorageDead(_6); // scope 1 at $DIR/reference_prop.rs:+9:26: +9:27
_0 = const (); // scope 0 at $DIR/reference_prop.rs:+0:25: +10:2
StorageDead(_1); // scope 0 at $DIR/reference_prop.rs:+10:1: +10:2
return; // scope 0 at $DIR/reference_prop.rs:+10:2: +10:2
}
}