src/librustc_mir/transform/simplify_branches.rs

   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 //! A pass that simplifies branches when their condition is known.
  12
  13 use rustc::ty::TyCtxt;
  14 use rustc::middle::const_val::ConstVal;
  15 use rustc::mir::transform::{MirPass, MirSource, Pass};
  16 use rustc::mir::*;
  17
  18 use std::fmt;
  19
  20 pub struct SimplifyBranches<'a> { label: &'a str }
  21
  22 impl<'a> SimplifyBranches<'a> {
  23     pub fn new(label: &'a str) -> Self {
  24         SimplifyBranches { label: label }
  25     }
  26 }
  27
  28 impl<'l, 'tcx> MirPass<'tcx> for SimplifyBranches<'l> {
  29     fn run_pass<'a>(&mut self, _tcx: TyCtxt<'a, 'tcx, 'tcx>, _src: MirSource, mir: &mut Mir<'tcx>) {
  30         for block in mir.basic_blocks_mut() {
  31             let terminator = block.terminator_mut();
  32             terminator.kind = match terminator.kind {
  33                 TerminatorKind::SwitchInt { discr: Operand::Constant(Constant {
  34                     literal: Literal::Value { ref value }, ..
  35                 }), ref values, ref targets, .. } => {
  36                     if let Some(ref constint) = value.to_const_int() {
  37                         let (otherwise, targets) = targets.split_last().unwrap();
  38                         let mut ret = TerminatorKind::Goto { target: *otherwise };
  39                         for (v, t) in values.iter().zip(targets.iter()) {
  40                             if v == constint {
  41                                 ret = TerminatorKind::Goto { target: *t };
  42                                 break;
  43                             }
  44                         }
  45                         ret
  46                     } else {
  47                         continue
  48                     }
  49                 },
  50                 TerminatorKind::Assert { target, cond: Operand::Constant(Constant {
  51                     literal: Literal::Value {
  52                         value: ConstVal::Bool(cond)
  53                     }, ..
  54                 }), expected, .. } if cond == expected => {
  55                     TerminatorKind::Goto { target: target }
  56                 },
  57                 _ => continue
  58             };
  59         }
  60     }
  61 }
  62
  63 impl<'l> Pass for SimplifyBranches<'l> {
  64     fn disambiguator<'a>(&'a self) -> Option<Box<fmt::Display+'a>> {
  65         Some(Box::new(self.label))
  66     }
  67
  68     // avoid calling `type_name` - it contains `<'static>`
  69     fn name(&self) -> ::std::borrow::Cow<'static, str> { "SimplifyBranches".into() }
  70 }