src/librustc_mir/transform/add_call_guards.rs

   1 // Copyright 2016 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use rustc::ty::TyCtxt;
  12 use rustc::mir::*;
  13 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
  14 use transform::{MirPass, MirSource};
  15
  16 #[derive(PartialEq)]
  17 pub enum AddCallGuards {
  18     AllCallEdges,
  19     CriticalCallEdges,
  20 }
  21 pub use self::AddCallGuards::*;
  22
  23 /**
  24  * Breaks outgoing critical edges for call terminators in the MIR.
  25  *
  26  * Critical edges are edges that are neither the only edge leaving a
  27  * block, nor the only edge entering one.
  28  *
  29  * When you want something to happen "along" an edge, you can either
  30  * do at the end of the predecessor block, or at the start of the
  31  * successor block. Critical edges have to be broken in order to prevent
  32  * "edge actions" from affecting other edges. We need this for calls that are
  33  * codegened to LLVM invoke instructions, because invoke is a block terminator
  34  * in LLVM so we can't insert any code to handle the call's result into the
  35  * block that performs the call.
  36  *
  37  * This function will break those edges by inserting new blocks along them.
  38  *
  39  * NOTE: Simplify CFG will happily undo most of the work this pass does.
  40  *
  41  */
  42
  43 impl MirPass for AddCallGuards {
  44     fn run_pass<'a, 'tcx>(&self,
  45                           _tcx: TyCtxt<'a, 'tcx, 'tcx>,
  46                           _src: MirSource,
  47                           mir: &mut Mir<'tcx>) {
  48         self.add_call_guards(mir);
  49     }
  50 }
  51
  52 impl AddCallGuards {
  53     pub fn add_call_guards(&self, mir: &mut Mir) {
  54         let pred_count: IndexVec<_, _> =
  55             mir.predecessors().iter().map(|ps| ps.len()).collect();
  56
  57         // We need a place to store the new blocks generated
  58         let mut new_blocks = Vec::new();
  59
  60         let cur_len = mir.basic_blocks().len();
  61
  62         for block in mir.basic_blocks_mut() {
  63             match block.terminator {
  64                 Some(Terminator {
  65                     kind: TerminatorKind::Call {
  66                         destination: Some((_, ref mut destination)),
  67                         cleanup,
  68                         ..
  69                     }, source_info
  70                 }) if pred_count[*destination] > 1 &&
  71                       (cleanup.is_some() || self == &AllCallEdges) =>
  72                 {
  73                     // It's a critical edge, break it
  74                     let call_guard = BasicBlockData {
  75                         statements: vec![],
  76                         is_cleanup: block.is_cleanup,
  77                         terminator: Some(Terminator {
  78                             source_info,
  79                             kind: TerminatorKind::Goto { target: *destination }
  80                         })
  81                     };
  82
  83                     // Get the index it will be when inserted into the MIR
  84                     let idx = cur_len + new_blocks.len();
  85                     new_blocks.push(call_guard);
  86                     *destination = BasicBlock::new(idx);
  87                 }
  88                 _ => {}
  89             }
  90         }
  91
  92         debug!("Broke {} N edges", new_blocks.len());
  93
  94         mir.basic_blocks_mut().extend(new_blocks);
  95     }
  96 }