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::NonUseContext::VarDebugInfo;
   5 use rustc_middle::mir::visit::{MutVisitor, PlaceContext};
   6 use rustc_middle::mir::*;
   7 use rustc_middle::ty::TyCtxt;
   8
   9 pub struct Derefer;
  10
  11 pub struct DerefChecker<'tcx> {
  12     tcx: TyCtxt<'tcx>,
  13     patcher: MirPatch<'tcx>,
  14     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
  15 }
  16
  17 impl<'tcx> MutVisitor<'tcx> for DerefChecker<'tcx> {
  18     fn tcx(&self) -> TyCtxt<'tcx> {
  19         self.tcx
  20     }
  21
  22     fn visit_place(&mut self, place: &mut Place<'tcx>, cntxt: PlaceContext, loc: Location) {
  23         if !place.projection.is_empty()
  24             && cntxt != PlaceContext::NonUse(VarDebugInfo)
  25             && place.projection[1..].contains(&ProjectionElem::Deref)
  26         {
  27             let mut place_local = place.local;
  28             let mut last_len = 0;
  29             let mut last_deref_idx = 0;
  30
  31             let mut prev_temp: Option<Local> = None;
  32
  33             for (idx, elem) in place.projection[0..].iter().enumerate() {
  34                 if *elem == ProjectionElem::Deref {
  35                     last_deref_idx = idx;
  36                 }
  37             }
  38             for (idx, (p_ref, p_elem)) in place.iter_projections().enumerate() {
  39                 if !p_ref.projection.is_empty() && p_elem == ProjectionElem::Deref {
  40                     let ty = p_ref.ty(&self.local_decls, self.tcx).ty;
  41                     let temp = self.patcher.new_local_with_info(
  42                         ty,
  43                         self.local_decls[p_ref.local].source_info.span,
  44                         Some(Box::new(LocalInfo::DerefTemp)),
  45                     );
  46
  47                     self.patcher.add_statement(loc, StatementKind::StorageLive(temp));
  48
  49                     // We are adding current p_ref's projections to our
  50                     // temp value, excluding projections we already covered.
  51                     let deref_place = Place::from(place_local)
  52                         .project_deeper(&p_ref.projection[last_len..], self.tcx);
  53
  54                     self.patcher.add_assign(
  55                         loc,
  56                         Place::from(temp),
  57                         Rvalue::Use(Operand::Move(deref_place)),
  58                     );
  59                     place_local = temp;
  60                     last_len = p_ref.projection.len();
  61
  62                     // Change `Place` only if we are actually at the Place's last deref
  63                     if idx == last_deref_idx {
  64                         let temp_place =
  65                             Place::from(temp).project_deeper(&place.projection[idx..], self.tcx);
  66                         *place = temp_place;
  67                     }
  68
  69                     // We are destroying the previous temp since it's no longer used.
  70                     if let Some(prev_temp) = prev_temp {
  71                         self.patcher.add_statement(loc, StatementKind::StorageDead(prev_temp));
  72                     }
  73
  74                     prev_temp = Some(temp);
  75                 }
  76             }
  77
  78             // Since we won't be able to reach final temp, we destroy it outside the loop.
  79             if let Some(prev_temp) = prev_temp {
  80                 let last_loc =
  81                     Location { block: loc.block, statement_index: loc.statement_index + 1 };
  82                 self.patcher.add_statement(last_loc, StatementKind::StorageDead(prev_temp));
  83             }
  84         }
  85     }
  86 }
  87
  88 pub fn deref_finder<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  89     let patch = MirPatch::new(body);
  90     let mut checker = DerefChecker { tcx, patcher: patch, local_decls: body.local_decls.clone() };
  91
  92     for (bb, data) in body.basic_blocks_mut().iter_enumerated_mut() {
  93         checker.visit_basic_block_data(bb, data);
  94     }
  95
  96     checker.patcher.apply(body);
  97 }
  98
  99 impl<'tcx> MirPass<'tcx> for Derefer {
 100     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 101         deref_finder(tcx, body);
 102         body.phase = MirPhase::Derefered;
 103     }
 104 }