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| 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::repr::*; |
| 13 | +use rustc::mir::transform::{MirPass, Pass}; |
| 14 | +use syntax::ast::NodeId; |
| 15 | + |
| 16 | +use rustc_data_structures::bitvec::BitVector; |
| 17 | + |
| 18 | +use traversal; |
| 19 | + |
| 20 | +pub struct BreakCriticalEdges; |
| 21 | + |
| 22 | +/** |
| 23 | + * Breaks critical edges in the MIR. |
| 24 | + * |
| 25 | + * Critical edges are edges that are neither the only edge leaving a |
| 26 | + * block, nor the only edge entering one. |
| 27 | + * |
| 28 | + * When you want something to happen "along" an edge, you can either |
| 29 | + * do at the end of the predecessor block, or at the start of the |
| 30 | + * successor block. Critical edges have to be broken in order to prevent |
| 31 | + * "edge actions" from affecting other edges. |
| 32 | + * |
| 33 | + * This function will break those edges by inserting new blocks along them. |
| 34 | + * |
| 35 | + * A special case is Drop and Call terminators with unwind/cleanup successors, |
| 36 | + * They use `invoke` in LLVM, which terminates a block, meaning that code cannot |
| 37 | + * be inserted after them, so even if an edge is the only edge leaving a block |
| 38 | + * like that, we still insert blocks if the edge is one of many entering the |
| 39 | + * target. |
| 40 | + * |
| 41 | + * NOTE: Simplify CFG will happily undo most of the work this pass does. |
| 42 | + * |
| 43 | + */ |
| 44 | + |
| 45 | +impl<'tcx> MirPass<'tcx> for BreakCriticalEdges { |
| 46 | + fn run_pass(&mut self, _: &TyCtxt<'tcx>, _: NodeId, mir: &mut Mir<'tcx>) { |
| 47 | + break_critical_edges(mir); |
| 48 | + } |
| 49 | +} |
| 50 | + |
| 51 | +impl Pass for BreakCriticalEdges {} |
| 52 | + |
| 53 | +fn break_critical_edges(mir: &mut Mir) { |
| 54 | + let mut pred_count = vec![0u32; mir.basic_blocks.len()]; |
| 55 | + |
| 56 | + // Build the precedecessor map for the MIR |
| 57 | + for (_, data) in traversal::preorder(mir) { |
| 58 | + if let Some(ref term) = data.terminator { |
| 59 | + for &tgt in term.successors().iter() { |
| 60 | + pred_count[tgt.index()] += 1; |
| 61 | + } |
| 62 | + } |
| 63 | + } |
| 64 | + |
| 65 | + let cleanup_map : BitVector = mir.basic_blocks |
| 66 | + .iter().map(|bb| bb.is_cleanup).collect(); |
| 67 | + |
| 68 | + // We need a place to store the new blocks generated |
| 69 | + let mut new_blocks = Vec::new(); |
| 70 | + |
| 71 | + let bbs = mir.all_basic_blocks(); |
| 72 | + let cur_len = mir.basic_blocks.len(); |
| 73 | + |
| 74 | + for &bb in &bbs { |
| 75 | + let data = mir.basic_block_data_mut(bb); |
| 76 | + |
| 77 | + if let Some(ref mut term) = data.terminator { |
| 78 | + let is_invoke = term_is_invoke(term); |
| 79 | + let term_span = term.span; |
| 80 | + let term_scope = term.scope; |
| 81 | + let succs = term.successors_mut(); |
| 82 | + if succs.len() > 1 || (succs.len() > 0 && is_invoke) { |
| 83 | + for tgt in succs { |
| 84 | + let num_preds = pred_count[tgt.index()]; |
| 85 | + if num_preds > 1 { |
| 86 | + // It's a critical edge, break it |
| 87 | + let goto = Terminator { |
| 88 | + span: term_span, |
| 89 | + scope: term_scope, |
| 90 | + kind: TerminatorKind::Goto { target: *tgt } |
| 91 | + }; |
| 92 | + let mut data = BasicBlockData::new(Some(goto)); |
| 93 | + data.is_cleanup = cleanup_map.contains(tgt.index()); |
| 94 | + |
| 95 | + // Get the index it will be when inserted into the MIR |
| 96 | + let idx = cur_len + new_blocks.len(); |
| 97 | + new_blocks.push(data); |
| 98 | + *tgt = BasicBlock::new(idx); |
| 99 | + } |
| 100 | + } |
| 101 | + } |
| 102 | + } |
| 103 | + } |
| 104 | + |
| 105 | + debug!("Broke {} N edges", new_blocks.len()); |
| 106 | + |
| 107 | + mir.basic_blocks.extend_from_slice(&new_blocks); |
| 108 | +} |
| 109 | + |
| 110 | +// Returns true if the terminator would use an invoke in LLVM. |
| 111 | +fn term_is_invoke(term: &Terminator) -> bool { |
| 112 | + match term.kind { |
| 113 | + TerminatorKind::Call { cleanup: Some(_), .. } | |
| 114 | + TerminatorKind::Drop { unwind: Some(_), .. } => true, |
| 115 | + _ => false |
| 116 | + } |
| 117 | +} |
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