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::mir::transform::{MirPass, MirSource};
  14 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
  15
  16 pub struct AddCallGuards;
  17
  18 /**
  19  * Breaks outgoing critical edges for call terminators in the MIR.
  20  *
  21  * Critical edges are edges that are neither the only edge leaving a
  22  * block, nor the only edge entering one.
  23  *
  24  * When you want something to happen "along" an edge, you can either
  25  * do at the end of the predecessor block, or at the start of the
  26  * successor block. Critical edges have to be broken in order to prevent
  27  * "edge actions" from affecting other edges. We need this for calls that are
  28  * translated to LLVM invoke instructions, because invoke is a block terminator
  29  * in LLVM so we can't insert any code to handle the call's result into the
  30  * block that performs the call.
  31  *
  32  * This function will break those edges by inserting new blocks along them.
  33  *
  34  * NOTE: Simplify CFG will happily undo most of the work this pass does.
  35  *
  36  */
  37
  38 impl MirPass for AddCallGuards {
  39     fn run_pass<'a, 'tcx>(&self,
  40                           _tcx: TyCtxt<'a, 'tcx, 'tcx>,
  41                           _src: MirSource,
  42                           mir: &mut Mir<'tcx>) {
  43         add_call_guards(mir);
  44     }
  45 }
  46
  47 pub fn add_call_guards(mir: &mut Mir) {
  48     let pred_count: IndexVec<_, _> =
  49         mir.predecessors().iter().map(|ps| ps.len()).collect();
  50
  51     // We need a place to store the new blocks generated
  52     let mut new_blocks = Vec::new();
  53
  54     let cur_len = mir.basic_blocks().len();
  55
  56     for block in mir.basic_blocks_mut() {
  57         match block.terminator {
  58             Some(Terminator {
  59                 kind: TerminatorKind::Call {
  60                     destination: Some((_, ref mut destination)),
  61                     cleanup: Some(_),
  62                     ..
  63                 }, source_info
  64             }) if pred_count[*destination] > 1 => {
  65                 // It's a critical edge, break it
  66                 let call_guard = BasicBlockData {
  67                     statements: vec![],
  68                     is_cleanup: block.is_cleanup,
  69                     terminator: Some(Terminator {
  70                         source_info: source_info,
  71                         kind: TerminatorKind::Goto { target: *destination }
  72                     })
  73                 };
  74
  75                 // Get the index it will be when inserted into the MIR
  76                 let idx = cur_len + new_blocks.len();
  77                 new_blocks.push(call_guard);
  78                 *destination = BasicBlock::new(idx);
  79             }
  80             _ => {}
  81         }
  82     }
  83
  84     debug!("Broke {} N edges", new_blocks.len());
  85
  86     mir.basic_blocks_mut().extend(new_blocks);
  87 }