src/librustc_mir/transform/add_call_guards.rs

   1 use rustc::ty::TyCtxt;
   2 use rustc::mir::*;
   3 use rustc_index::vec::{Idx, IndexVec};
   4 use crate::transform::{MirPass, MirSource};
   5
   6 #[derive(PartialEq)]
   7 pub enum AddCallGuards {
   8     AllCallEdges,
   9     CriticalCallEdges,
  10 }
  11 pub use self::AddCallGuards::*;
  12
  13 /**
  14  * Breaks outgoing critical edges for call terminators in the MIR.
  15  *
  16  * Critical edges are edges that are neither the only edge leaving a
  17  * block, nor the only edge entering one.
  18  *
  19  * When you want something to happen "along" an edge, you can either
  20  * do at the end of the predecessor block, or at the start of the
  21  * successor block. Critical edges have to be broken in order to prevent
  22  * "edge actions" from affecting other edges. We need this for calls that are
  23  * codegened to LLVM invoke instructions, because invoke is a block terminator
  24  * in LLVM so we can't insert any code to handle the call's result into the
  25  * block that performs the call.
  26  *
  27  * This function will break those edges by inserting new blocks along them.
  28  *
  29  * NOTE: Simplify CFG will happily undo most of the work this pass does.
  30  *
  31  */
  32
  33 impl<'tcx> MirPass<'tcx> for AddCallGuards {
  34     fn run_pass(
  35         &self, _tcx: TyCtxt<'tcx>, _src: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>
  36     ) {
  37         self.add_call_guards(body);
  38     }
  39 }
  40
  41 impl AddCallGuards {
  42     pub fn add_call_guards(&self, body: &mut BodyAndCache<'_>) {
  43         let pred_count: IndexVec<_, _> = body.predecessors().iter().map(|ps| ps.len()).collect();
  44
  45         // We need a place to store the new blocks generated
  46         let mut new_blocks = Vec::new();
  47
  48         let cur_len = body.basic_blocks().len();
  49
  50         for block in body.basic_blocks_mut() {
  51             match block.terminator {
  52                 Some(Terminator {
  53                     kind: TerminatorKind::Call {
  54                         destination: Some((_, ref mut destination)),
  55                         cleanup,
  56                         ..
  57                     }, source_info
  58                 }) if pred_count[*destination] > 1 &&
  59                       (cleanup.is_some() || self == &AllCallEdges) =>
  60                 {
  61                     // It's a critical edge, break it
  62                     let call_guard = BasicBlockData {
  63                         statements: vec![],
  64                         is_cleanup: block.is_cleanup,
  65                         terminator: Some(Terminator {
  66                             source_info,
  67                             kind: TerminatorKind::Goto { target: *destination }
  68                         })
  69                     };
  70
  71                     // Get the index it will be when inserted into the MIR
  72                     let idx = cur_len + new_blocks.len();
  73                     new_blocks.push(call_guard);
  74                     *destination = BasicBlock::new(idx);
  75                 }
  76                 _ => {}
  77             }
  78         }
  79
  80         debug!("Broke {} N edges", new_blocks.len());
  81
  82         body.basic_blocks_mut().extend(new_blocks);
  83     }
  84 }