compiler/rustc_mir_transform/src/add_moves_for_packed_drops.rs

   1 use rustc_middle::mir::*;
   2 use rustc_middle::ty::TyCtxt;
   3
   4 use crate::util;
   5 use crate::MirPass;
   6 use rustc_middle::mir::patch::MirPatch;
   7
   8 // This pass moves values being dropped that are within a packed
   9 // struct to a separate local before dropping them, to ensure that
  10 // they are dropped from an aligned address.
  11 //
  12 // For example, if we have something like
  13 // ```Rust
  14 //     #[repr(packed)]
  15 //     struct Foo {
  16 //         dealign: u8,
  17 //         data: Vec<u8>
  18 //     }
  19 //
  20 //     let foo = ...;
  21 // ```
  22 //
  23 // We want to call `drop_in_place::<Vec<u8>>` on `data` from an aligned
  24 // address. This means we can't simply drop `foo.data` directly, because
  25 // its address is not aligned.
  26 //
  27 // Instead, we move `foo.data` to a local and drop that:
  28 // ```
  29 //     storage.live(drop_temp)
  30 //     drop_temp = foo.data;
  31 //     drop(drop_temp) -> next
  32 // next:
  33 //     storage.dead(drop_temp)
  34 // ```
  35 //
  36 // The storage instructions are required to avoid stack space
  37 // blowup.
  38
  39 pub struct AddMovesForPackedDrops;
  40
  41 impl<'tcx> MirPass<'tcx> for AddMovesForPackedDrops {
  42     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  43         debug!("add_moves_for_packed_drops({:?} @ {:?})", body.source, body.span);
  44         add_moves_for_packed_drops(tcx, body);
  45     }
  46 }
  47
  48 pub fn add_moves_for_packed_drops<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  49     let patch = add_moves_for_packed_drops_patch(tcx, body);
  50     patch.apply(body);
  51 }
  52
  53 fn add_moves_for_packed_drops_patch<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> MirPatch<'tcx> {
  54     let def_id = body.source.def_id();
  55     let mut patch = MirPatch::new(body);
  56     let param_env = tcx.param_env(def_id);
  57
  58     for (bb, data) in body.basic_blocks.iter_enumerated() {
  59         let loc = Location { block: bb, statement_index: data.statements.len() };
  60         let terminator = data.terminator();
  61
  62         match terminator.kind {
  63             TerminatorKind::Drop { place, .. }
  64                 if util::is_disaligned(tcx, body, param_env, place) =>
  65             {
  66                 add_move_for_packed_drop(tcx, body, &mut patch, terminator, loc, data.is_cleanup);
  67             }
  68             TerminatorKind::DropAndReplace { .. } => {
  69                 span_bug!(terminator.source_info.span, "replace in AddMovesForPackedDrops");
  70             }
  71             _ => {}
  72         }
  73     }
  74
  75     patch
  76 }
  77
  78 fn add_move_for_packed_drop<'tcx>(
  79     tcx: TyCtxt<'tcx>,
  80     body: &Body<'tcx>,
  81     patch: &mut MirPatch<'tcx>,
  82     terminator: &Terminator<'tcx>,
  83     loc: Location,
  84     is_cleanup: bool,
  85 ) {
  86     debug!("add_move_for_packed_drop({:?} @ {:?})", terminator, loc);
  87     let TerminatorKind::Drop { ref place, target, unwind } = terminator.kind else {
  88         unreachable!();
  89     };
  90
  91     let source_info = terminator.source_info;
  92     let ty = place.ty(body, tcx).ty;
  93     let temp = patch.new_temp(ty, terminator.source_info.span);
  94
  95     let storage_dead_block = patch.new_block(BasicBlockData {
  96         statements: vec![Statement { source_info, kind: StatementKind::StorageDead(temp) }],
  97         terminator: Some(Terminator { source_info, kind: TerminatorKind::Goto { target } }),
  98         is_cleanup,
  99     });
 100
 101     patch.add_statement(loc, StatementKind::StorageLive(temp));
 102     patch.add_assign(loc, Place::from(temp), Rvalue::Use(Operand::Move(*place)));
 103     patch.patch_terminator(
 104         loc.block,
 105         TerminatorKind::Drop { place: Place::from(temp), target: storage_dead_block, unwind },
 106     );
 107 }