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// 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_data_structures::bitvec::BitVector;
use rustc::middle::const_eval::ConstVal;
use rustc::middle::infer;
use rustc::mir::repr::*;
use rustc::mir::transform::MirPass;
pub struct SimplifyCfg;
impl SimplifyCfg {
pub fn new() -> SimplifyCfg {
SimplifyCfg
}
fn remove_dead_blocks(&self, mir: &mut Mir) {
let mut seen = BitVector::new(mir.basic_blocks.len());
// These blocks are always required.
seen.insert(START_BLOCK.index());
seen.insert(END_BLOCK.index());
let mut worklist = Vec::with_capacity(4);
worklist.push(START_BLOCK);
while let Some(bb) = worklist.pop() {
for succ in mir.basic_block_data(bb).terminator().successors().iter() {
if seen.insert(succ.index()) {
worklist.push(*succ);
}
}
}
retain_basic_blocks(mir, &seen);
}
fn remove_goto_chains(&self, mir: &mut Mir) -> bool {
// Find the target at the end of the jump chain, return None if there is a loop
fn final_target(mir: &Mir, mut target: BasicBlock) -> Option<BasicBlock> {
// Keep track of already seen blocks to detect loops
let mut seen: Vec<BasicBlock> = Vec::with_capacity(8);
while mir.basic_block_data(target).statements.is_empty() {
match mir.basic_block_data(target).terminator {
Some(Terminator::Goto { target: next }) => {
if seen.contains(&next) {
return None;
}
seen.push(next);
target = next;
}
_ => break
}
}
Some(target)
}
let mut changed = false;
for bb in mir.all_basic_blocks() {
// Temporarily take ownership of the terminator we're modifying to keep borrowck happy
let mut terminator = mir.basic_block_data_mut(bb).terminator.take()
.expect("invalid terminator state");
for target in terminator.successors_mut() {
let new_target = match final_target(mir, *target) {
Some(new_target) => new_target,
None if mir.basic_block_data(bb).statements.is_empty() => bb,
None => continue
};
changed |= *target != new_target;
*target = new_target;
}
mir.basic_block_data_mut(bb).terminator = Some(terminator);
}
changed
}
fn simplify_branches(&self, mir: &mut Mir) -> bool {
let mut changed = false;
for bb in mir.all_basic_blocks() {
let basic_block = mir.basic_block_data_mut(bb);
let mut terminator = basic_block.terminator_mut();
*terminator = match *terminator {
Terminator::If { ref targets, .. } if targets.0 == targets.1 => {
changed = true;
Terminator::Goto { target: targets.0 }
}
Terminator::If { ref targets, cond: Operand::Constant(Constant {
literal: Literal::Value {
value: ConstVal::Bool(cond)
}, ..
}) } => {
changed = true;
if cond {
Terminator::Goto { target: targets.0 }
} else {
Terminator::Goto { target: targets.1 }
}
}
Terminator::SwitchInt { ref targets, .. } if targets.len() == 1 => {
Terminator::Goto { target: targets[0] }
}
_ => continue
}
}
changed
}
}
impl MirPass for SimplifyCfg {
fn run_on_mir<'a, 'tcx>(&mut self, mir: &mut Mir<'tcx>, _: &infer::InferCtxt<'a, 'tcx>) {
let mut changed = true;
while changed {
changed = self.simplify_branches(mir);
changed |= self.remove_goto_chains(mir);
self.remove_dead_blocks(mir);
}
// FIXME: Should probably be moved into some kind of pass manager
mir.basic_blocks.shrink_to_fit();
}
}
/// Mass removal of basic blocks to keep the ID-remapping cheap.
fn retain_basic_blocks(mir: &mut Mir, keep: &BitVector) {
let num_blocks = mir.basic_blocks.len();
let mut replacements: Vec<_> = (0..num_blocks).map(BasicBlock::new).collect();
let mut used_blocks = 0;
for alive_index in keep.iter() {
replacements[alive_index] = BasicBlock::new(used_blocks);
if alive_index != used_blocks {
// Swap the next alive block data with the current available slot. Since alive_index is
// non-decreasing this is a valid operation.
mir.basic_blocks.swap(alive_index, used_blocks);
}
used_blocks += 1;
}
mir.basic_blocks.truncate(used_blocks);
for bb in mir.all_basic_blocks() {
for target in mir.basic_block_data_mut(bb).terminator_mut().successors_mut() {
*target = replacements[target.index()];
}
}
}
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