From 5fc1dd11a93b57d795e352e6c8718d5ffbb74dbb Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Tue, 28 Jun 2022 11:03:50 -0400
Subject: [PATCH] emit Retag for compound types with reference fields
---
compiler/rustc_mir_transform/src/add_retag.rs | 23 +++++++++++++++----
...asts.SimplifyCfg-elaborate-drops.after.mir | 2 ++
2 files changed, 20 insertions(+), 5 deletions(-)
diff --git a/compiler/rustc_mir_transform/src/add_retag.rs b/compiler/rustc_mir_transform/src/add_retag.rs
index 0495439385bee..0f87e638d2618 100644
--- a/compiler/rustc_mir_transform/src/add_retag.rs
+++ b/compiler/rustc_mir_transform/src/add_retag.rs
@@ -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
@@ -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.
+ 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,
}
@@ -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>| {
diff --git a/src/test/mir-opt/retag.array_casts.SimplifyCfg-elaborate-drops.after.mir b/src/test/mir-opt/retag.array_casts.SimplifyCfg-elaborate-drops.after.mir
index 69742d6bc3b6a..c8c45da19137f 100644
--- a/src/test/mir-opt/retag.array_casts.SimplifyCfg-elaborate-drops.after.mir
+++ b/src/test/mir-opt/retag.array_casts.SimplifyCfg-elaborate-drops.after.mir
@@ -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
@@ -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