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// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
use rustc::ty::TyCtxt;
use rustc::mir::*;
use rustc::mir::transform::{MirPass, MirSource};
use rustc_data_structures::indexed_vec::{Idx, IndexVec};
pub struct AddCallGuards;
/**
* Breaks outgoing critical edges for call terminators in the MIR.
*
* Critical edges are edges that are neither the only edge leaving a
* block, nor the only edge entering one.
*
* When you want something to happen "along" an edge, you can either
* do at the end of the predecessor block, or at the start of the
* successor block. Critical edges have to be broken in order to prevent
* "edge actions" from affecting other edges. We need this for calls that are
* translated to LLVM invoke instructions, because invoke is a block terminator
* in LLVM so we can't insert any code to handle the call's result into the
* block that performs the call.
*
* This function will break those edges by inserting new blocks along them.
*
* NOTE: Simplify CFG will happily undo most of the work this pass does.
*
*/
impl MirPass for AddCallGuards {
fn run_pass<'a, 'tcx>(&self,
_tcx: TyCtxt<'a, 'tcx, 'tcx>,
_src: MirSource,
mir: &mut Mir<'tcx>) {
add_call_guards(mir);
}
}
pub fn add_call_guards(mir: &mut Mir) {
let pred_count: IndexVec<_, _> =
mir.predecessors().iter().map(|ps| ps.len()).collect();
// We need a place to store the new blocks generated
let mut new_blocks = Vec::new();
let cur_len = mir.basic_blocks().len();
for block in mir.basic_blocks_mut() {
match block.terminator {
Some(Terminator {
kind: TerminatorKind::Call {
destination: Some((_, ref mut destination)),
cleanup: Some(_),
..
}, source_info
}) if pred_count[*destination] > 1 => {
// It's a critical edge, break it
let call_guard = BasicBlockData {
statements: vec![],
is_cleanup: block.is_cleanup,
terminator: Some(Terminator {
source_info: source_info,
kind: TerminatorKind::Goto { target: *destination }
})
};
// Get the index it will be when inserted into the MIR
let idx = cur_len + new_blocks.len();
new_blocks.push(call_guard);
*destination = BasicBlock::new(idx);
}
_ => {}
}
}
debug!("Broke {} N edges", new_blocks.len());
mir.basic_blocks_mut().extend(new_blocks);
}
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