compiler/rustc_mir/src/transform/multiple_return_terminators.rs

   1 //! This pass removes jumps to basic blocks containing only a return, and replaces them with a
   2 //! return instead.
   3
   4 use crate::transform::{simplify, MirPass};
   5 use rustc_index::bit_set::BitSet;
   6 use rustc_middle::mir::*;
   7 use rustc_middle::ty::TyCtxt;
   8
   9 pub struct MultipleReturnTerminators;
  10
  11 impl<'tcx> MirPass<'tcx> for MultipleReturnTerminators {
  12     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  13         if tcx.sess.mir_opt_level() < 4 {
  14             return;
  15         }
  16
  17         // find basic blocks with no statement and a return terminator
  18         let mut bbs_simple_returns = BitSet::new_empty(body.basic_blocks().len());
  19         let def_id = body.source.def_id();
  20         let bbs = body.basic_blocks_mut();
  21         for idx in bbs.indices() {
  22             if bbs[idx].statements.is_empty()
  23                 && bbs[idx].terminator().kind == TerminatorKind::Return
  24             {
  25                 bbs_simple_returns.insert(idx);
  26             }
  27         }
  28
  29         for bb in bbs {
  30             if !tcx.consider_optimizing(|| format!("MultipleReturnTerminators {:?} ", def_id)) {
  31                 break;
  32             }
  33
  34             if let TerminatorKind::Goto { target } = bb.terminator().kind {
  35                 if bbs_simple_returns.contains(target) {
  36                     bb.terminator_mut().kind = TerminatorKind::Return;
  37                 }
  38             }
  39         }
  40
  41         simplify::remove_dead_blocks(body)
  42     }
  43 }