src/librustc_mir/transform/util.rs

// Copyright 2015 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::mir::repr::*;

/// Update basic block ids in all terminators using the given replacements,
/// useful e.g. after removal of several basic blocks to update all terminators
/// in a single pass
pub fn update_basic_block_ids(mir: &mut Mir, replacements: &[BasicBlock]) {
    for bb in mir.all_basic_blocks() {
        for target in mir.basic_block_data_mut(bb).terminator_mut().successors_mut() {
            *target = replacements[target.index()];
        }
    }
}

/// Mass removal of basic blocks to keep the ID-remapping cheap.
pub fn retain_basic_blocks(mir: &mut Mir, keep: &[bool]) {
    let num_blocks = mir.basic_blocks.len();

    // Check that we have a usage flag for every block
    assert_eq!(num_blocks, keep.len());

    let first_dead = match keep.iter().position(|&k| !k) {
        None => return,
        Some(first_dead) => first_dead,
    };

    // `replacements` maps the old block ids to the new ones
    let mut replacements: Vec<_> = (0..num_blocks).map(BasicBlock::new).collect();

    let mut dead = 0;
    for i in first_dead..num_blocks {
        if keep[i] {
            replacements[i] = BasicBlock::new(i - dead);
            mir.basic_blocks.swap(i, i - dead);
        } else {
            dead += 1;
        }
    }
    mir.basic_blocks.truncate(num_blocks - dead);

    update_basic_block_ids(mir, &replacements);
}