compiler/rustc_mir_transform/src/simplify_branches.rs

   1 use crate::MirPass;
   2 use rustc_middle::mir::*;
   3 use rustc_middle::ty::TyCtxt;
   4
   5 /// A pass that replaces a branch with a goto when its condition is known.
   6 pub struct SimplifyConstCondition {
   7     label: String,
   8 }
   9
  10 impl SimplifyConstCondition {
  11     pub fn new(label: &str) -> Self {
  12         SimplifyConstCondition { label: format!("SimplifyConstCondition-{}", label) }
  13     }
  14 }
  15
  16 impl<'tcx> MirPass<'tcx> for SimplifyConstCondition {
  17     fn name(&self) -> &str {
  18         &self.label
  19     }
  20
  21     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  22         let param_env = tcx.param_env(body.source.def_id());
  23         for block in body.basic_blocks_mut() {
  24             let terminator = block.terminator_mut();
  25             terminator.kind = match terminator.kind {
  26                 TerminatorKind::SwitchInt {
  27                     discr: Operand::Constant(ref c),
  28                     switch_ty,
  29                     ref targets,
  30                     ..
  31                 } => {
  32                     let constant = c.literal.try_eval_bits(tcx, param_env, switch_ty);
  33                     if let Some(constant) = constant {
  34                         let target = targets.target_for_value(constant);
  35                         TerminatorKind::Goto { target }
  36                     } else {
  37                         continue;
  38                     }
  39                 }
  40                 TerminatorKind::Assert {
  41                     target, cond: Operand::Constant(ref c), expected, ..
  42                 } => match c.literal.try_eval_bool(tcx, param_env) {
  43                     Some(v) if v == expected => TerminatorKind::Goto { target },
  44                     _ => continue,
  45                 },
  46                 _ => continue,
  47             };
  48         }
  49     }
  50 }