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emit Retag for compound types with reference fields #98625

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Jun 29, 2022
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emit Retag for compound types with reference fields #98625

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23 changes: 18 additions & 5 deletions compiler/rustc_mir_transform/src/add_retag.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -33,8 +33,9 @@ fn is_stable(place: PlaceRef<'_>) -> bool {
})
}

/// Determine whether this type may be a reference (or box), and thus needs retagging.
fn may_be_reference(ty: Ty<'_>) -> bool {
/// Determine whether this type may contain a reference (or box), and thus needs retagging.
/// We will only recurse `depth` times into Tuples/ADTs to bound the cost of this.
fn may_contain_reference<'tcx>(ty: Ty<'tcx>, depth: u32, tcx: TyCtxt<'tcx>) -> bool {
match ty.kind() {
// Primitive types that are not references
ty::Bool
Expand All @@ -50,8 +51,20 @@ fn may_be_reference(ty: Ty<'_>) -> bool {
// References
ty::Ref(..) => true,
ty::Adt(..) if ty.is_box() => true,
// Compound types are not references
ty::Array(..) | ty::Slice(..) | ty::Tuple(..) | ty::Adt(..) => false,
// Compound types: recurse
ty::Array(ty, _) | ty::Slice(ty) => {
// This does not branch so we keep the depth the same.
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I don't understand the reasoning behind this. I can now ICE miri by syntactically nesting 10000 arrays, why is branching out relevant?

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This is meant to guard against time complexity, not stack overflows. Branching is a lot more expensive, if every level has N>1 fields then it is exponential in the depth, whereas array/slice checking is linear in the depth.

may_contain_reference(*ty, depth, tcx)
}
ty::Tuple(tys) => {
depth == 0 || tys.iter().any(|ty| may_contain_reference(ty, depth - 1, tcx))
}
ty::Adt(adt, subst) => {
depth == 0
|| adt.variants().iter().any(|v| {
v.fields.iter().any(|f| may_contain_reference(f.ty(tcx, subst), depth - 1, tcx))
})
}
// Conservative fallback
_ => true,
}
Expand Down Expand Up @@ -83,7 +96,7 @@ impl<'tcx> MirPass<'tcx> for AddRetag {
// FIXME: Instead of giving up for unstable places, we should introduce
// a temporary and retag on that.
is_stable(place.as_ref())
&& may_be_reference(place.ty(&*local_decls, tcx).ty)
&& may_contain_reference(place.ty(&*local_decls, tcx).ty, /*depth*/ 3, tcx)
&& is_not_temp(&local_decls[place.local])
};
let place_base_raw = |place: &Place<'tcx>| {
Expand Down
2 changes: 2 additions & 0 deletions src/test/mir-opt/retag.array_casts.SimplifyCfg-elaborate-drops.after.mir
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Original file line number Diff line number Diff line change
Expand Up @@ -129,6 +129,7 @@ fn array_casts() -> () {
_18 = &(*_35); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Retag(_18); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
_13 = (move _14, move _18); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Retag(_13); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
StorageDead(_18); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
StorageDead(_14); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
StorageLive(_20); // scope 5 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Expand Down Expand Up @@ -171,6 +172,7 @@ fn array_casts() -> () {
Retag(_32); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
StorageLive(_34); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
_34 = Option::<Arguments>::None; // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
Retag(_34); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
_28 = core::panicking::assert_failed::<usize, usize>(move _29, move _30, move _32, move _34); // scope 8 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
// mir::Constant
// + span: $SRC_DIR/core/src/macros/mod.rs:LL:COL
Expand Down