1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107

use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;

use crate::util;
use crate::MirPass;
use rustc_middle::mir::patch::MirPatch;

// This pass moves values being dropped that are within a packed
// struct to a separate local before dropping them, to ensure that
// they are dropped from an aligned address.
//
// For example, if we have something like
// ```Rust
//     #[repr(packed)]
//     struct Foo {
//         dealign: u8,
//         data: Vec<u8>
//     }
//
//     let foo = ...;
// ```
//
// We want to call `drop_in_place::<Vec<u8>>` on `data` from an aligned
// address. This means we can't simply drop `foo.data` directly, because
// its address is not aligned.
//
// Instead, we move `foo.data` to a local and drop that:
// ```
//     storage.live(drop_temp)
//     drop_temp = foo.data;
//     drop(drop_temp) -> next
// next:
//     storage.dead(drop_temp)
// ```
//
// The storage instructions are required to avoid stack space
// blowup.

pub struct AddMovesForPackedDrops;

impl<'tcx> MirPass<'tcx> for AddMovesForPackedDrops {
    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
        debug!("add_moves_for_packed_drops({:?} @ {:?})", body.source, body.span);
        add_moves_for_packed_drops(tcx, body);
    }
}

pub fn add_moves_for_packed_drops<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
    let patch = add_moves_for_packed_drops_patch(tcx, body);
    patch.apply(body);
}

fn add_moves_for_packed_drops_patch<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> MirPatch<'tcx> {
    let def_id = body.source.def_id();
    let mut patch = MirPatch::new(body);
    let param_env = tcx.param_env(def_id);

    for (bb, data) in body.basic_blocks.iter_enumerated() {
        let loc = Location { block: bb, statement_index: data.statements.len() };
        let terminator = data.terminator();

        match terminator.kind {
            TerminatorKind::Drop { place, .. }
                if util::is_disaligned(tcx, body, param_env, place) =>
            {
                add_move_for_packed_drop(tcx, body, &mut patch, terminator, loc, data.is_cleanup);
            }
            TerminatorKind::DropAndReplace { .. } => {
                span_bug!(terminator.source_info.span, "replace in AddMovesForPackedDrops");
            }
            _ => {}
        }
    }

    patch
}

fn add_move_for_packed_drop<'tcx>(
    tcx: TyCtxt<'tcx>,
    body: &Body<'tcx>,
    patch: &mut MirPatch<'tcx>,
    terminator: &Terminator<'tcx>,
    loc: Location,
    is_cleanup: bool,
) {
    debug!("add_move_for_packed_drop({:?} @ {:?})", terminator, loc);
    let TerminatorKind::Drop { ref place, target, unwind } = terminator.kind else {
        unreachable!();
    };

    let source_info = terminator.source_info;
    let ty = place.ty(body, tcx).ty;
    let temp = patch.new_temp(ty, terminator.source_info.span);

    let storage_dead_block = patch.new_block(BasicBlockData {
        statements: vec![Statement { source_info, kind: StatementKind::StorageDead(temp) }],
        terminator: Some(Terminator { source_info, kind: TerminatorKind::Goto { target } }),
        is_cleanup,
    });

    patch.add_statement(loc, StatementKind::StorageLive(temp));
    patch.add_assign(loc, Place::from(temp), Rvalue::Use(Operand::Move(*place)));
    patch.patch_terminator(
        loc.block,
        TerminatorKind::Drop { place: Place::from(temp), target: storage_dead_block, unwind },
    );
}