compiler/rustc_mir_transform/src/add_call_guards.rs

   1 use crate::MirPass;
   2 use rustc_index::{Idx, IndexVec};
   3 use rustc_middle::mir::*;
   4 use rustc_middle::ty::TyCtxt;
   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(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  35         self.add_call_guards(body);
  36     }
  37 }
  38
  39 impl AddCallGuards {
  40     pub fn add_call_guards(&self, body: &mut Body<'_>) {
  41         let mut pred_count: IndexVec<_, _> =
  42             body.basic_blocks.predecessors().iter().map(|ps| ps.len()).collect();
  43         pred_count[START_BLOCK] += 1;
  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 { target: Some(ref mut destination), unwind, .. },
  54                     source_info,
  55                 }) if pred_count[*destination] > 1
  56                     && (matches!(
  57                         unwind,
  58                         UnwindAction::Cleanup(_) | UnwindAction::Terminate(_)
  59                     ) || 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 }