compiler/rustc_mir/src/transform/match_branches.rs

   1 use crate::transform::MirPass;
   2 use rustc_middle::mir::*;
   3 use rustc_middle::ty::TyCtxt;
   4
   5 pub struct MatchBranchSimplification;
   6
   7 /// If a source block is found that switches between two blocks that are exactly
   8 /// the same modulo const bool assignments (e.g., one assigns true another false
   9 /// to the same place), merge a target block statements into the source block,
  10 /// using Eq / Ne comparison with switch value where const bools value differ.
  11 ///
  12 /// For example:
  13 ///
  14 /// ```rust
  15 /// bb0: {
  16 ///     switchInt(move _3) -> [42_isize: bb1, otherwise: bb2];
  17 /// }
  18 ///
  19 /// bb1: {
  20 ///     _2 = const true;
  21 ///     goto -> bb3;
  22 /// }
  23 ///
  24 /// bb2: {
  25 ///     _2 = const false;
  26 ///     goto -> bb3;
  27 /// }
  28 /// ```
  29 ///
  30 /// into:
  31 ///
  32 /// ```rust
  33 /// bb0: {
  34 ///    _2 = Eq(move _3, const 42_isize);
  35 ///    goto -> bb3;
  36 /// }
  37 /// ```
  38
  39 impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
  40     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  41         if tcx.sess.opts.debugging_opts.mir_opt_level <= 1 {
  42             return;
  43         }
  44
  45         let param_env = tcx.param_env(body.source.def_id());
  46         let (bbs, local_decls) = body.basic_blocks_and_local_decls_mut();
  47         'outer: for bb_idx in bbs.indices() {
  48             let (discr, val, switch_ty, first, second) = match bbs[bb_idx].terminator().kind {
  49                 TerminatorKind::SwitchInt {
  50                     discr: ref discr @ (Operand::Copy(_) | Operand::Move(_)),
  51                     switch_ty,
  52                     ref targets,
  53                     ..
  54                 } if targets.iter().len() == 1 => {
  55                     let (value, target) = targets.iter().next().unwrap();
  56                     if target == targets.otherwise() {
  57                         continue;
  58                     }
  59                     (discr, value, switch_ty, target, targets.otherwise())
  60                 }
  61                 // Only optimize switch int statements
  62                 _ => continue,
  63             };
  64
  65             // Check that destinations are identical, and if not, then don't optimize this block
  66             if bbs[first].terminator().kind != bbs[second].terminator().kind {
  67                 continue;
  68             }
  69
  70             // Check that blocks are assignments of consts to the same place or same statement,
  71             // and match up 1-1, if not don't optimize this block.
  72             let first_stmts = &bbs[first].statements;
  73             let scnd_stmts = &bbs[second].statements;
  74             if first_stmts.len() != scnd_stmts.len() {
  75                 continue;
  76             }
  77             for (f, s) in first_stmts.iter().zip(scnd_stmts.iter()) {
  78                 match (&f.kind, &s.kind) {
  79                     // If two statements are exactly the same, we can optimize.
  80                     (f_s, s_s) if f_s == s_s => {}
  81
  82                     // If two statements are const bool assignments to the same place, we can optimize.
  83                     (
  84                         StatementKind::Assign(box (lhs_f, Rvalue::Use(Operand::Constant(f_c)))),
  85                         StatementKind::Assign(box (lhs_s, Rvalue::Use(Operand::Constant(s_c)))),
  86                     ) if lhs_f == lhs_s
  87                         && f_c.literal.ty.is_bool()
  88                         && s_c.literal.ty.is_bool()
  89                         && f_c.literal.try_eval_bool(tcx, param_env).is_some()
  90                         && s_c.literal.try_eval_bool(tcx, param_env).is_some() => {}
  91
  92                     // Otherwise we cannot optimize. Try another block.
  93                     _ => continue 'outer,
  94                 }
  95             }
  96             // Take ownership of items now that we know we can optimize.
  97             let discr = discr.clone();
  98
  99             // Introduce a temporary for the discriminant value.
 100             let source_info = bbs[bb_idx].terminator().source_info;
 101             let discr_local = local_decls.push(LocalDecl::new(switch_ty, source_info.span));
 102
 103             // We already checked that first and second are different blocks,
 104             // and bb_idx has a different terminator from both of them.
 105             let (from, first, second) = bbs.pick3_mut(bb_idx, first, second);
 106
 107             let new_stmts = first.statements.iter().zip(second.statements.iter()).map(|(f, s)| {
 108                 match (&f.kind, &s.kind) {
 109                     (f_s, s_s) if f_s == s_s => (*f).clone(),
 110
 111                     (
 112                         StatementKind::Assign(box (lhs, Rvalue::Use(Operand::Constant(f_c)))),
 113                         StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(s_c)))),
 114                     ) => {
 115                         // From earlier loop we know that we are dealing with bool constants only:
 116                         let f_b = f_c.literal.try_eval_bool(tcx, param_env).unwrap();
 117                         let s_b = s_c.literal.try_eval_bool(tcx, param_env).unwrap();
 118                         if f_b == s_b {
 119                             // Same value in both blocks. Use statement as is.
 120                             (*f).clone()
 121                         } else {
 122                             // Different value between blocks. Make value conditional on switch condition.
 123                             let size = tcx.layout_of(param_env.and(switch_ty)).unwrap().size;
 124                             let const_cmp = Operand::const_from_scalar(
 125                                 tcx,
 126                                 switch_ty,
 127                                 crate::interpret::Scalar::from_uint(val, size),
 128                                 rustc_span::DUMMY_SP,
 129                             );
 130                             let op = if f_b { BinOp::Eq } else { BinOp::Ne };
 131                             let rhs = Rvalue::BinaryOp(
 132                                 op,
 133                                 Operand::Copy(Place::from(discr_local)),
 134                                 const_cmp,
 135                             );
 136                             Statement {
 137                                 source_info: f.source_info,
 138                                 kind: StatementKind::Assign(box (*lhs, rhs)),
 139                             }
 140                         }
 141                     }
 142
 143                     _ => unreachable!(),
 144                 }
 145             });
 146
 147             from.statements
 148                 .push(Statement { source_info, kind: StatementKind::StorageLive(discr_local) });
 149             from.statements.push(Statement {
 150                 source_info,
 151                 kind: StatementKind::Assign(box (Place::from(discr_local), Rvalue::Use(discr))),
 152             });
 153             from.statements.extend(new_stmts);
 154             from.statements
 155                 .push(Statement { source_info, kind: StatementKind::StorageDead(discr_local) });
 156             from.terminator_mut().kind = first.terminator().kind.clone();
 157         }
 158     }
 159 }