Auto merge of #115138 - cjgillot:dse-move-packed, r=compiler-errors
Do not convert copies of packed projections to moves.
This code path was introduced in https://github.com/rust-lang/rust/pull/113758
After seeing https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/Packed.20fields.20and.20in-place.20function.20argument.2Freturn.20passing, this may be UB, so should be disallowed.
This should not appear in normally-built MIR, which introduces temporary copies for packed projections.
diff --git a/compiler/rustc_const_eval/src/util/alignment.rs b/compiler/rustc_const_eval/src/util/alignment.rs
index 4f39dad..c1f0ff2 100644
--- a/compiler/rustc_const_eval/src/util/alignment.rs
+++ b/compiler/rustc_const_eval/src/util/alignment.rs
@@ -40,7 +40,7 @@
}
}
-fn is_within_packed<'tcx, L>(
+pub fn is_within_packed<'tcx, L>(
tcx: TyCtxt<'tcx>,
local_decls: &L,
place: Place<'tcx>,
diff --git a/compiler/rustc_const_eval/src/util/mod.rs b/compiler/rustc_const_eval/src/util/mod.rs
index 289e342..0aef7fa 100644
--- a/compiler/rustc_const_eval/src/util/mod.rs
+++ b/compiler/rustc_const_eval/src/util/mod.rs
@@ -5,7 +5,7 @@
mod compare_types;
mod type_name;
-pub use self::alignment::is_disaligned;
+pub use self::alignment::{is_disaligned, is_within_packed};
pub use self::check_validity_requirement::check_validity_requirement;
pub use self::compare_types::{is_equal_up_to_subtyping, is_subtype};
pub use self::type_name::type_name;
diff --git a/compiler/rustc_mir_transform/src/dead_store_elimination.rs b/compiler/rustc_mir_transform/src/dead_store_elimination.rs
index 3f98893..ef14105 100644
--- a/compiler/rustc_mir_transform/src/dead_store_elimination.rs
+++ b/compiler/rustc_mir_transform/src/dead_store_elimination.rs
@@ -12,6 +12,7 @@
//! will still not cause any further changes.
//!
+use crate::util::is_within_packed;
use rustc_index::bit_set::BitSet;
use rustc_middle::mir::visit::Visitor;
use rustc_middle::mir::*;
@@ -49,6 +50,11 @@
&& !place.is_indirect()
&& !borrowed.contains(place.local)
&& !state.contains(place.local)
+ // If `place` is a projection of a disaligned field in a packed ADT,
+ // the move may be codegened as a pointer to that field.
+ // Using that disaligned pointer may trigger UB in the callee,
+ // so do nothing.
+ && is_within_packed(tcx, body, place).is_none()
{
call_operands_to_move.push((bb, index));
}
diff --git a/tests/mir-opt/dead-store-elimination/call_arg_copy.move_packed.DeadStoreElimination.panic-abort.diff b/tests/mir-opt/dead-store-elimination/call_arg_copy.move_packed.DeadStoreElimination.panic-abort.diff
new file mode 100644
index 0000000..dc0f907
--- /dev/null
+++ b/tests/mir-opt/dead-store-elimination/call_arg_copy.move_packed.DeadStoreElimination.panic-abort.diff
@@ -0,0 +1,15 @@
+- // MIR for `move_packed` before DeadStoreElimination
++ // MIR for `move_packed` after DeadStoreElimination
+
+ fn move_packed(_1: Packed) -> () {
+ let mut _0: ();
+
+ bb0: {
+ _0 = use_both(const 0_i32, (_1.1: i32)) -> [return: bb1, unwind unreachable];
+ }
+
+ bb1: {
+ return;
+ }
+ }
+
diff --git a/tests/mir-opt/dead-store-elimination/call_arg_copy.move_packed.DeadStoreElimination.panic-unwind.diff b/tests/mir-opt/dead-store-elimination/call_arg_copy.move_packed.DeadStoreElimination.panic-unwind.diff
new file mode 100644
index 0000000..86ef026
--- /dev/null
+++ b/tests/mir-opt/dead-store-elimination/call_arg_copy.move_packed.DeadStoreElimination.panic-unwind.diff
@@ -0,0 +1,15 @@
+- // MIR for `move_packed` before DeadStoreElimination
++ // MIR for `move_packed` after DeadStoreElimination
+
+ fn move_packed(_1: Packed) -> () {
+ let mut _0: ();
+
+ bb0: {
+ _0 = use_both(const 0_i32, (_1.1: i32)) -> [return: bb1, unwind continue];
+ }
+
+ bb1: {
+ return;
+ }
+ }
+
diff --git a/tests/mir-opt/dead-store-elimination/call_arg_copy.rs b/tests/mir-opt/dead-store-elimination/call_arg_copy.rs
index 41f91fc..f09cdee 100644
--- a/tests/mir-opt/dead-store-elimination/call_arg_copy.rs
+++ b/tests/mir-opt/dead-store-elimination/call_arg_copy.rs
@@ -2,6 +2,12 @@
// unit-test: DeadStoreElimination
// compile-flags: -Zmir-enable-passes=+CopyProp
+#![feature(core_intrinsics)]
+#![feature(custom_mir)]
+#![allow(internal_features)]
+
+use std::intrinsics::mir::*;
+
#[inline(never)]
fn use_both(_: i32, _: i32) {}
@@ -10,6 +16,26 @@
use_both(x, x);
}
+#[repr(packed)]
+struct Packed {
+ x: u8,
+ y: i32,
+}
+
+// EMIT_MIR call_arg_copy.move_packed.DeadStoreElimination.diff
+#[custom_mir(dialect = "analysis")]
+fn move_packed(packed: Packed) {
+ mir!(
+ {
+ Call(RET = use_both(0, packed.y), ret)
+ }
+ ret = {
+ Return()
+ }
+ )
+}
+
fn main() {
move_simple(1);
+ move_packed(Packed { x: 0, y: 1 });
}
