src/librustc_mir/transform/simplify_branches.rs

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