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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_val::ConstVal;
use rustc::ty::TyCtxt;
use rustc::mir::repr::*;
use rustc::mir::transform::{MirPass, MirSource, Pass};
use pretty;
use std::mem;
use super::remove_dead_blocks::RemoveDeadBlocks;
use traversal;
pub struct SimplifyCfg;
impl SimplifyCfg {
pub fn new() -> SimplifyCfg {
SimplifyCfg
}
}
impl<'tcx> MirPass<'tcx> for SimplifyCfg {
fn run_pass<'a>(&mut self, tcx: TyCtxt<'a, 'tcx, 'tcx>, src: MirSource, mir: &mut Mir<'tcx>) {
simplify_branches(mir);
RemoveDeadBlocks.run_pass(tcx, src, mir);
merge_consecutive_blocks(mir);
RemoveDeadBlocks.run_pass(tcx, src, mir);
pretty::dump_mir(tcx, "simplify_cfg", &0, src, mir, None);
// FIXME: Should probably be moved into some kind of pass manager
mir.basic_blocks.shrink_to_fit();
}
}
impl Pass for SimplifyCfg {}
fn merge_consecutive_blocks(mir: &mut Mir) {
// Build the precedecessor map for the MIR
let mut pred_count = vec![0u32; mir.basic_blocks.len()];
for (_, data) in traversal::preorder(mir) {
if let Some(ref term) = data.terminator {
for &tgt in term.successors().iter() {
pred_count[tgt.index()] += 1;
}
}
}
loop {
let mut changed = false;
let mut seen = BitVector::new(mir.basic_blocks.len());
let mut worklist = vec![START_BLOCK];
while let Some(bb) = worklist.pop() {
// 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");
// See if we can merge the target block into this one
loop {
let mut inner_change = false;
if let TerminatorKind::Goto { target } = terminator.kind {
// Don't bother trying to merge a block into itself
if target == bb {
break;
}
let num_insts = mir.basic_block_data(target).statements.len();
match mir.basic_block_data(target).terminator().kind {
TerminatorKind::Goto { target: new_target } if num_insts == 0 => {
inner_change = true;
terminator.kind = TerminatorKind::Goto { target: new_target };
pred_count[target.index()] -= 1;
pred_count[new_target.index()] += 1;
}
_ if pred_count[target.index()] == 1 => {
inner_change = true;
let mut stmts = Vec::new();
{
let target_data = mir.basic_block_data_mut(target);
mem::swap(&mut stmts, &mut target_data.statements);
mem::swap(&mut terminator, target_data.terminator_mut());
}
mir.basic_block_data_mut(bb).statements.append(&mut stmts);
}
_ => {}
};
}
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
};
if *target != new_target {
inner_change = true;
pred_count[target.index()] -= 1;
pred_count[new_target.index()] += 1;
*target = new_target;
}
}
changed |= inner_change;
if !inner_change {
break;
}
}
mir.basic_block_data_mut(bb).terminator = Some(terminator);
for succ in mir.basic_block_data(bb).terminator().successors().iter() {
if seen.insert(succ.index()) {
worklist.push(*succ);
}
}
}
if !changed {
break;
}
}
}
// 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() {
// NB -- terminator may have been swapped with `None` in
// merge_consecutive_blocks, in which case we have a cycle and just want
// to stop
match mir.basic_block_data(target).terminator {
Some(Terminator { kind: TerminatorKind::Goto { target: next }, .. }) => {
if seen.contains(&next) {
return None;
}
seen.push(next);
target = next;
}
_ => break
}
}
Some(target)
}
fn simplify_branches(mir: &mut Mir) {
loop {
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.kind = match terminator.kind {
TerminatorKind::If { ref targets, .. } if targets.0 == targets.1 => {
changed = true;
TerminatorKind::Goto { target: targets.0 }
}
TerminatorKind::If { ref targets, cond: Operand::Constant(Constant {
literal: Literal::Value {
value: ConstVal::Bool(cond)
}, ..
}) } => {
changed = true;
if cond {
TerminatorKind::Goto { target: targets.0 }
} else {
TerminatorKind::Goto { target: targets.1 }
}
}
TerminatorKind::SwitchInt { ref targets, .. } if targets.len() == 1 => {
TerminatorKind::Goto { target: targets[0] }
}
_ => continue
}
}
if !changed {
break;
}
}
}
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