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The const propagator cannot trace references. #73613

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Jun 24, 2020
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The const propagator cannot trace references. #73613

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54 changes: 38 additions & 16 deletions src/librustc_mir/transform/const_prop.rs
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Expand Up @@ -575,8 +575,16 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
}

// Do not try creating references (#67862)
Rvalue::Ref(_, _, place_ref) => {
trace!("skipping Ref({:?})", place_ref);
Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
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Maybe this match should also be made exhaustive? It currently has a fallback case, basically assuming "every rvalue I don't know will definitely be harmless". That seems like a rather strong assumption.

trace!("skipping AddressOf | Ref for {:?}", place);

// This may be creating mutable references or immutable references to cells.
// If that happens, the pointed to value could be mutated via that reference.
// Since we aren't tracking references, the const propagator loses track of what
// value the local has right now.
// Thus, all locals that have their reference taken
// must not take part in propagation.
Self::remove_const(&mut self.ecx, place.local);
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I tried simply asserting here, but that won't work for

  • zsts or
  • locals that got a reference assigned from a constant instead of an Rvalue::Ref and are now hitting this Rvalue::Ref because the reference is getting reborrowed. This is basically Projection::Deref biting us again
  • arguments, because they don't get marked as NoPropagation


return None;
}
Expand Down Expand Up @@ -716,6 +724,9 @@ enum ConstPropMode {
OnlyInsideOwnBlock,
/// The `Local` can be propagated into but reads cannot be propagated.
OnlyPropagateInto,
/// The `Local` cannot be part of propagation at all. Any statement
/// referencing it either for reading or writing will not get propagated.
NoPropagation,
}

struct CanConstProp {
Expand Down Expand Up @@ -781,7 +792,9 @@ impl<'tcx> Visitor<'tcx> for CanConstProp {
// end of the block anyway, and inside the block we overwrite previous
// states as applicable.
ConstPropMode::OnlyInsideOwnBlock => {}
other => {
ConstPropMode::NoPropagation => {}
ConstPropMode::OnlyPropagateInto => {}
other @ ConstPropMode::FullConstProp => {
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Part of exhaustively matching on things, but also part of the bugfix, because we don't go from NoPropagation to OnlyPropagateInto anymore.

trace!(
"local {:?} can't be propagated because of multiple assignments",
local,
Expand Down Expand Up @@ -813,7 +826,7 @@ impl<'tcx> Visitor<'tcx> for CanConstProp {
| MutatingUse(MutatingUseContext::Borrow)
| MutatingUse(MutatingUseContext::AddressOf) => {
trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
self.can_const_prop[local] = ConstPropMode::OnlyPropagateInto;
self.can_const_prop[local] = ConstPropMode::NoPropagation;
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Main part of the bugfix, while we can do things for borrows, this is super hard to reason about and I don't think we should do it without dataflow

}
}
}
Expand Down Expand Up @@ -858,19 +871,22 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
}
}
}
if can_const_prop == ConstPropMode::OnlyInsideOwnBlock {
trace!(
"found local restricted to its block. Will remove it from const-prop after block is finished. Local: {:?}",
place.local
);
self.locals_of_current_block.insert(place.local);
}

if can_const_prop == ConstPropMode::OnlyPropagateInto {
trace!("can't propagate into {:?}", place);
if place.local != RETURN_PLACE {
Self::remove_const(&mut self.ecx, place.local);
match can_const_prop {
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more exhaustively matching.

ConstPropMode::OnlyInsideOwnBlock => {
trace!(
"found local restricted to its block. \
Will remove it from const-prop after block is finished. Local: {:?}",
place.local
);
self.locals_of_current_block.insert(place.local);
}
ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
trace!("can't propagate into {:?}", place);
if place.local != RETURN_PLACE {
Self::remove_const(&mut self.ecx, place.local);
}
}
ConstPropMode::FullConstProp => {}
}
} else {
// Const prop failed, so erase the destination, ensuring that whatever happens
Expand All @@ -890,6 +906,12 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
);
Self::remove_const(&mut self.ecx, place.local);
}
} else {
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This arm didn't exist before, it's the else for the layout_of check. If we can't compute the layout, erase anything that's in that local, even if I don't think that it can actually happen that there's something in the local.

trace!(
"cannot propagate into {:?}, because the type of the local is generic.",
place,
);
Self::remove_const(&mut self.ecx, place.local);
}
} else {
match statement.kind {
Expand Down
18 changes: 2 additions & 16 deletions src/test/mir-opt/const_prop/bad_op_unsafe_oob_for_slices/32bit/rustc.main.ConstProp.diff
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Expand Up @@ -46,22 +46,8 @@
// mir::Constant
// + span: $DIR/bad_op_unsafe_oob_for_slices.rs:7:23: 7:24
// + literal: Const { ty: usize, val: Value(Scalar(0x00000003)) }
- _7 = Len((*_1)); // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
- _8 = Lt(_6, _7); // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
+ _7 = const 3usize; // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
+ // ty::Const
+ // + ty: usize
+ // + val: Value(Scalar(0x00000003))
+ // mir::Constant
+ // + span: $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
+ // + literal: Const { ty: usize, val: Value(Scalar(0x00000003)) }
+ _8 = const false; // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
+ // ty::Const
+ // + ty: bool
+ // + val: Value(Scalar(0x00))
+ // mir::Constant
+ // + span: $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
+ // + literal: Const { ty: bool, val: Value(Scalar(0x00)) }
_7 = Len((*_1)); // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
_8 = Lt(_6, _7); // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
assert(move _8, "index out of bounds: the len is {} but the index is {}", move _7, _6) -> bb1; // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
}

Expand Down
18 changes: 2 additions & 16 deletions src/test/mir-opt/const_prop/bad_op_unsafe_oob_for_slices/64bit/rustc.main.ConstProp.diff
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Expand Up @@ -46,22 +46,8 @@
// mir::Constant
// + span: $DIR/bad_op_unsafe_oob_for_slices.rs:7:23: 7:24
// + literal: Const { ty: usize, val: Value(Scalar(0x0000000000000003)) }
- _7 = Len((*_1)); // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
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this is "fallout" of checking for Rvalue::AddressOf. I think we should exhaustively match every enum in const prop to prevent such things from falling through.

- _8 = Lt(_6, _7); // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
+ _7 = const 3usize; // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
+ // ty::Const
+ // + ty: usize
+ // + val: Value(Scalar(0x0000000000000003))
+ // mir::Constant
+ // + span: $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
+ // + literal: Const { ty: usize, val: Value(Scalar(0x0000000000000003)) }
+ _8 = const false; // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
+ // ty::Const
+ // + ty: bool
+ // + val: Value(Scalar(0x00))
+ // mir::Constant
+ // + span: $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
+ // + literal: Const { ty: bool, val: Value(Scalar(0x00)) }
_7 = Len((*_1)); // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
_8 = Lt(_6, _7); // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
assert(move _8, "index out of bounds: the len is {} but the index is {}", move _7, _6) -> bb1; // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
}

Expand Down
22 changes: 22 additions & 0 deletions src/test/mir-opt/const_prop_miscompile.rs
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@@ -0,0 +1,22 @@
#![feature(raw_ref_op)]

// EMIT_MIR rustc.foo.ConstProp.diff
fn foo() {
let mut u = (1,);
*&mut u.0 = 5;
let y = { u.0 } == 5;
}

// EMIT_MIR rustc.bar.ConstProp.diff
fn bar() {
let mut v = (1,);
unsafe {
*&raw mut v.0 = 5;
}
let y = { v.0 } == 5;
}

fn main() {
foo();
bar();
}
75 changes: 75 additions & 0 deletions src/test/mir-opt/const_prop_miscompile/rustc.bar.ConstProp.diff
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@@ -0,0 +1,75 @@
- // MIR for `bar` before ConstProp
+ // MIR for `bar` after ConstProp

fn bar() -> () {
let mut _0: (); // return place in scope 0 at $DIR/const_prop_miscompile.rs:11:10: 11:10
let mut _1: (i32,); // in scope 0 at $DIR/const_prop_miscompile.rs:12:9: 12:14
let _2: (); // in scope 0 at $DIR/const_prop_miscompile.rs:13:5: 15:6
let mut _3: *mut i32; // in scope 0 at $DIR/const_prop_miscompile.rs:14:10: 14:22
let mut _5: i32; // in scope 0 at $DIR/const_prop_miscompile.rs:16:13: 16:20
scope 1 {
debug v => _1; // in scope 1 at $DIR/const_prop_miscompile.rs:12:9: 12:14
let _4: bool; // in scope 1 at $DIR/const_prop_miscompile.rs:16:9: 16:10
scope 2 {
}
scope 3 {
debug y => _4; // in scope 3 at $DIR/const_prop_miscompile.rs:16:9: 16:10
}
}

bb0: {
StorageLive(_1); // scope 0 at $DIR/const_prop_miscompile.rs:12:9: 12:14
- _1 = (const 1i32,); // scope 0 at $DIR/const_prop_miscompile.rs:12:17: 12:21
+ _1 = const (1i32,); // scope 0 at $DIR/const_prop_miscompile.rs:12:17: 12:21
// ty::Const
- // + ty: i32
+ // + ty: (i32,)
// + val: Value(Scalar(0x00000001))
// mir::Constant
- // + span: $DIR/const_prop_miscompile.rs:12:18: 12:19
- // + literal: Const { ty: i32, val: Value(Scalar(0x00000001)) }
+ // + span: $DIR/const_prop_miscompile.rs:12:17: 12:21
+ // + literal: Const { ty: (i32,), val: Value(Scalar(0x00000001)) }
StorageLive(_2); // scope 1 at $DIR/const_prop_miscompile.rs:13:5: 15:6
StorageLive(_3); // scope 2 at $DIR/const_prop_miscompile.rs:14:10: 14:22
_3 = &raw mut (_1.0: i32); // scope 2 at $DIR/const_prop_miscompile.rs:14:10: 14:22
(*_3) = const 5i32; // scope 2 at $DIR/const_prop_miscompile.rs:14:9: 14:26
// ty::Const
// + ty: i32
// + val: Value(Scalar(0x00000005))
// mir::Constant
// + span: $DIR/const_prop_miscompile.rs:14:25: 14:26
// + literal: Const { ty: i32, val: Value(Scalar(0x00000005)) }
StorageDead(_3); // scope 2 at $DIR/const_prop_miscompile.rs:14:26: 14:27
_2 = const (); // scope 2 at $DIR/const_prop_miscompile.rs:13:5: 15:6
// ty::Const
// + ty: ()
// + val: Value(Scalar(<ZST>))
// mir::Constant
// + span: $DIR/const_prop_miscompile.rs:13:5: 15:6
// + literal: Const { ty: (), val: Value(Scalar(<ZST>)) }
StorageDead(_2); // scope 1 at $DIR/const_prop_miscompile.rs:15:5: 15:6
StorageLive(_4); // scope 1 at $DIR/const_prop_miscompile.rs:16:9: 16:10
StorageLive(_5); // scope 1 at $DIR/const_prop_miscompile.rs:16:13: 16:20
_5 = (_1.0: i32); // scope 1 at $DIR/const_prop_miscompile.rs:16:15: 16:18
_4 = Eq(move _5, const 5i32); // scope 1 at $DIR/const_prop_miscompile.rs:16:13: 16:25
// ty::Const
// + ty: i32
// + val: Value(Scalar(0x00000005))
// mir::Constant
// + span: $DIR/const_prop_miscompile.rs:16:24: 16:25
// + literal: Const { ty: i32, val: Value(Scalar(0x00000005)) }
StorageDead(_5); // scope 1 at $DIR/const_prop_miscompile.rs:16:24: 16:25
_0 = const (); // scope 0 at $DIR/const_prop_miscompile.rs:11:10: 17:2
// ty::Const
// + ty: ()
// + val: Value(Scalar(<ZST>))
// mir::Constant
// + span: $DIR/const_prop_miscompile.rs:11:10: 17:2
// + literal: Const { ty: (), val: Value(Scalar(<ZST>)) }
StorageDead(_4); // scope 1 at $DIR/const_prop_miscompile.rs:17:1: 17:2
StorageDead(_1); // scope 0 at $DIR/const_prop_miscompile.rs:17:1: 17:2
return; // scope 0 at $DIR/const_prop_miscompile.rs:17:2: 17:2
}
}

63 changes: 63 additions & 0 deletions src/test/mir-opt/const_prop_miscompile/rustc.foo.ConstProp.diff
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@@ -0,0 +1,63 @@
- // MIR for `foo` before ConstProp
+ // MIR for `foo` after ConstProp

fn foo() -> () {
let mut _0: (); // return place in scope 0 at $DIR/const_prop_miscompile.rs:4:10: 4:10
let mut _1: (i32,); // in scope 0 at $DIR/const_prop_miscompile.rs:5:9: 5:14
let mut _2: &mut i32; // in scope 0 at $DIR/const_prop_miscompile.rs:6:6: 6:14
let mut _4: i32; // in scope 0 at $DIR/const_prop_miscompile.rs:7:13: 7:20
scope 1 {
debug u => _1; // in scope 1 at $DIR/const_prop_miscompile.rs:5:9: 5:14
let _3: bool; // in scope 1 at $DIR/const_prop_miscompile.rs:7:9: 7:10
scope 2 {
debug y => _3; // in scope 2 at $DIR/const_prop_miscompile.rs:7:9: 7:10
}
}

bb0: {
StorageLive(_1); // scope 0 at $DIR/const_prop_miscompile.rs:5:9: 5:14
- _1 = (const 1i32,); // scope 0 at $DIR/const_prop_miscompile.rs:5:17: 5:21
+ _1 = const (1i32,); // scope 0 at $DIR/const_prop_miscompile.rs:5:17: 5:21
// ty::Const
- // + ty: i32
+ // + ty: (i32,)
// + val: Value(Scalar(0x00000001))
// mir::Constant
- // + span: $DIR/const_prop_miscompile.rs:5:18: 5:19
- // + literal: Const { ty: i32, val: Value(Scalar(0x00000001)) }
+ // + span: $DIR/const_prop_miscompile.rs:5:17: 5:21
+ // + literal: Const { ty: (i32,), val: Value(Scalar(0x00000001)) }
StorageLive(_2); // scope 1 at $DIR/const_prop_miscompile.rs:6:6: 6:14
_2 = &mut (_1.0: i32); // scope 1 at $DIR/const_prop_miscompile.rs:6:6: 6:14
(*_2) = const 5i32; // scope 1 at $DIR/const_prop_miscompile.rs:6:5: 6:18
// ty::Const
// + ty: i32
// + val: Value(Scalar(0x00000005))
// mir::Constant
// + span: $DIR/const_prop_miscompile.rs:6:17: 6:18
// + literal: Const { ty: i32, val: Value(Scalar(0x00000005)) }
StorageDead(_2); // scope 1 at $DIR/const_prop_miscompile.rs:6:18: 6:19
StorageLive(_3); // scope 1 at $DIR/const_prop_miscompile.rs:7:9: 7:10
StorageLive(_4); // scope 1 at $DIR/const_prop_miscompile.rs:7:13: 7:20
_4 = (_1.0: i32); // scope 1 at $DIR/const_prop_miscompile.rs:7:15: 7:18
_3 = Eq(move _4, const 5i32); // scope 1 at $DIR/const_prop_miscompile.rs:7:13: 7:25
// ty::Const
// + ty: i32
// + val: Value(Scalar(0x00000005))
// mir::Constant
// + span: $DIR/const_prop_miscompile.rs:7:24: 7:25
// + literal: Const { ty: i32, val: Value(Scalar(0x00000005)) }
StorageDead(_4); // scope 1 at $DIR/const_prop_miscompile.rs:7:24: 7:25
_0 = const (); // scope 0 at $DIR/const_prop_miscompile.rs:4:10: 8:2
// ty::Const
// + ty: ()
// + val: Value(Scalar(<ZST>))
// mir::Constant
// + span: $DIR/const_prop_miscompile.rs:4:10: 8:2
// + literal: Const { ty: (), val: Value(Scalar(<ZST>)) }
StorageDead(_3); // scope 1 at $DIR/const_prop_miscompile.rs:8:1: 8:2
StorageDead(_1); // scope 0 at $DIR/const_prop_miscompile.rs:8:1: 8:2
return; // scope 0 at $DIR/const_prop_miscompile.rs:8:2: 8:2
}
}

2 changes: 1 addition & 1 deletion src/test/ui/mir/mir_detects_invalid_ops.rs
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Expand Up @@ -19,6 +19,6 @@ fn mod_by_zero() {
fn oob_error_for_slices() {
let a: *const [_] = &[1, 2, 3];
unsafe {
let _b = (*a)[3]; //~ ERROR this operation will panic at runtime [unconditional_panic]
let _b = (*a)[3];
}
}
8 changes: 1 addition & 7 deletions src/test/ui/mir/mir_detects_invalid_ops.stderr
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Expand Up @@ -12,11 +12,5 @@ error: this operation will panic at runtime
LL | let _z = 1 % y;
| ^^^^^ attempt to calculate the remainder with a divisor of zero

error: this operation will panic at runtime
--> $DIR/mir_detects_invalid_ops.rs:22:18
|
LL | let _b = (*a)[3];
| ^^^^^^^ index out of bounds: the len is 3 but the index is 3

error: aborting due to 3 previous errors
error: aborting due to 2 previous errors