src/librustc_mir/transform/simplify_branches.rs

   1 //! A pass that simplifies branches when their condition is known.
   2
   3 use crate::transform::{MirPass, MirSource};
   4 use rustc::mir::*;
   5 use rustc::ty::TyCtxt;
   6
   7 use std::borrow::Cow;
   8
   9 pub struct SimplifyBranches {
  10     label: String,
  11 }
  12
  13 impl SimplifyBranches {
  14     pub fn new(label: &str) -> Self {
  15         SimplifyBranches { label: format!("SimplifyBranches-{}", label) }
  16     }
  17 }
  18
  19 impl<'tcx> MirPass<'tcx> for SimplifyBranches {
  20     fn name(&self) -> Cow<'_, str> {
  21         Cow::Borrowed(&self.label)
  22     }
  23
  24     fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
  25         let param_env = tcx.param_env(src.def_id());
  26         for block in body.basic_blocks_mut() {
  27             let terminator = block.terminator_mut();
  28             terminator.kind = match terminator.kind {
  29                 TerminatorKind::SwitchInt {
  30                     discr: Operand::Constant(ref c),
  31                     switch_ty,
  32                     ref values,
  33                     ref targets,
  34                     ..
  35                 } => {
  36                     let constant = c.literal.try_eval_bits(tcx, param_env, switch_ty);
  37                     if let Some(constant) = constant {
  38                         let (otherwise, targets) = targets.split_last().unwrap();
  39                         let mut ret = TerminatorKind::Goto { target: *otherwise };
  40                         for (&v, t) in values.iter().zip(targets.iter()) {
  41                             if v == constant {
  42                                 ret = TerminatorKind::Goto { target: *t };
  43                                 break;
  44                             }
  45                         }
  46                         ret
  47                     } else {
  48                         continue;
  49                     }
  50                 }
  51                 TerminatorKind::Assert {
  52                     target, cond: Operand::Constant(ref c), expected, ..
  53                 } if (c.literal.try_eval_bool(tcx, param_env) == Some(true)) == expected => {
  54                     TerminatorKind::Goto { target }
  55                 }
  56                 TerminatorKind::FalseEdges { real_target, .. } => {
  57                     TerminatorKind::Goto { target: real_target }
  58                 }
  59                 TerminatorKind::FalseUnwind { real_target, .. } => {
  60                     TerminatorKind::Goto { target: real_target }
  61                 }
  62                 _ => continue,
  63             };
  64         }
  65     }
  66 }