compiler/rustc_mir_transform/src/deref_separator.rs

   1 use crate::MirPass;
   2 use rustc_index::vec::IndexVec;
   3 use rustc_middle::mir::patch::MirPatch;
   4 use rustc_middle::mir::visit::{MutVisitor, PlaceContext};
   5 use rustc_middle::mir::*;
   6 use rustc_middle::ty::TyCtxt;
   7 pub struct Derefer;
   8
   9 pub struct DerefChecker<'tcx> {
  10     tcx: TyCtxt<'tcx>,
  11     patcher: MirPatch<'tcx>,
  12     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
  13 }
  14
  15 impl<'tcx> MutVisitor<'tcx> for DerefChecker<'tcx> {
  16     fn tcx(&self) -> TyCtxt<'tcx> {
  17         self.tcx
  18     }
  19
  20     fn visit_place(&mut self, place: &mut Place<'tcx>, _: PlaceContext, loc: Location) {
  21         let mut place_local = place.local;
  22         let mut last_len = 0;
  23         let mut last_deref_idx = 0;
  24
  25         let mut prev_temp: Option<Local> = None;
  26
  27         for (idx, (p_ref, p_elem)) in place.iter_projections().enumerate() {
  28             if p_elem == ProjectionElem::Deref && !p_ref.projection.is_empty() {
  29                 last_deref_idx = idx;
  30             }
  31         }
  32
  33         for (idx, (p_ref, p_elem)) in place.iter_projections().enumerate() {
  34             if p_elem == ProjectionElem::Deref && !p_ref.projection.is_empty() {
  35                 let ty = p_ref.ty(&self.local_decls, self.tcx).ty;
  36                 let temp =
  37                     self.patcher.new_temp(ty, self.local_decls[p_ref.local].source_info.span);
  38
  39                 self.patcher.add_statement(loc, StatementKind::StorageLive(temp));
  40
  41                 // We are adding current p_ref's projections to our
  42                 // temp value, excluding projections we already covered.
  43                 let deref_place = Place::from(place_local)
  44                     .project_deeper(&p_ref.projection[last_len..], self.tcx);
  45                 self.patcher.add_assign(
  46                     loc,
  47                     Place::from(temp),
  48                     Rvalue::Use(Operand::Move(deref_place)),
  49                 );
  50
  51                 place_local = temp;
  52                 last_len = p_ref.projection.len();
  53
  54                 // Change `Place` only if we are actually at the Place's last deref
  55                 if idx == last_deref_idx {
  56                     let temp_place =
  57                         Place::from(temp).project_deeper(&place.projection[idx..], self.tcx);
  58                     *place = temp_place;
  59                 }
  60
  61                 // We are destroying last temp since it's no longer used.
  62                 if let Some(prev_temp) = prev_temp {
  63                     self.patcher.add_statement(loc, StatementKind::StorageDead(prev_temp));
  64                 }
  65
  66                 prev_temp = Some(temp);
  67             }
  68         }
  69
  70         // Since we won't be able to reach final temp, we destroy it outside the loop.
  71         if let Some(prev_temp) = prev_temp {
  72             let last_loc = Location { block: loc.block, statement_index: loc.statement_index + 1 };
  73             self.patcher.add_statement(last_loc, StatementKind::StorageDead(prev_temp));
  74         }
  75     }
  76 }
  77
  78 pub fn deref_finder<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  79     let patch = MirPatch::new(body);
  80     let mut checker = DerefChecker { tcx, patcher: patch, local_decls: body.local_decls.clone() };
  81
  82     for (bb, data) in body.basic_blocks_mut().iter_enumerated_mut() {
  83         checker.visit_basic_block_data(bb, data);
  84     }
  85
  86     checker.patcher.apply(body);
  87 }
  88
  89 impl<'tcx> MirPass<'tcx> for Derefer {
  90     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  91         deref_finder(tcx, body);
  92     }
  93 }