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, ParamEnv};
  14 use rustc::mir::*;
  15 use transform::{MirPass, MirSource};
  16
  17 use std::borrow::Cow;
  18
  19 pub struct SimplifyBranches { label: String }
  20
  21 impl SimplifyBranches {
  22     pub fn new(label: &str) -> Self {
  23         SimplifyBranches { label: format!("SimplifyBranches-{}", label) }
  24     }
  25 }
  26
  27 impl MirPass for SimplifyBranches {
  28     fn name<'a>(&'a self) -> Cow<'a, str> {
  29         Cow::Borrowed(&self.label)
  30     }
  31
  32     fn run_pass<'a, 'tcx>(&self,
  33                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  34                           _src: MirSource,
  35                           mir: &mut Mir<'tcx>) {
  36         for block in mir.basic_blocks_mut() {
  37             let terminator = block.terminator_mut();
  38             terminator.kind = match terminator.kind {
  39                 TerminatorKind::SwitchInt {
  40                     discr: Operand::Constant(ref c), switch_ty, ref values, ref targets, ..
  41                 } => {
  42                     let switch_ty = ParamEnv::empty().and(switch_ty);
  43                     if let Some(constint) = c.literal.assert_bits(tcx, switch_ty) {
  44                         let (otherwise, targets) = targets.split_last().unwrap();
  45                         let mut ret = TerminatorKind::Goto { target: *otherwise };
  46                         for (&v, t) in values.iter().zip(targets.iter()) {
  47                             if v == constint {
  48                                 ret = TerminatorKind::Goto { target: *t };
  49                                 break;
  50                             }
  51                         }
  52                         ret
  53                     } else {
  54                         continue
  55                     }
  56                 },
  57                 TerminatorKind::Assert {
  58                     target, cond: Operand::Constant(ref c), expected, ..
  59                 } if (c.literal.assert_bool(tcx) == Some(true)) == expected => {
  60                     TerminatorKind::Goto { target: target }
  61                 },
  62                 TerminatorKind::FalseEdges { real_target, .. } => {
  63                     TerminatorKind::Goto { target: real_target }
  64                 },
  65                 TerminatorKind::FalseUnwind { real_target, .. } => {
  66                     TerminatorKind::Goto { target: real_target }
  67                 },
  68                 _ => continue
  69             };
  70         }
  71     }
  72 }