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         let param_env = tcx.param_env(body.source.def_id());
  42         let bbs = body.basic_blocks_mut();
  43         'outer: for bb_idx in bbs.indices() {
  44             let (discr, val, switch_ty, first, second) = match bbs[bb_idx].terminator().kind {
  45                 TerminatorKind::SwitchInt {
  46                     discr: Operand::Copy(ref place) | Operand::Move(ref place),
  47                     switch_ty,
  48                     ref targets,
  49                     ..
  50                 } if targets.iter().len() == 1 => {
  51                     let (value, target) = targets.iter().next().unwrap();
  52                     if target == targets.otherwise() {
  53                         continue;
  54                     }
  55                     (place, value, switch_ty, target, targets.otherwise())
  56                 }
  57                 // Only optimize switch int statements
  58                 _ => continue,
  59             };
  60
  61             // Check that destinations are identical, and if not, then don't optimize this block
  62             if &bbs[first].terminator().kind != &bbs[second].terminator().kind {
  63                 continue;
  64             }
  65
  66             // Check that blocks are assignments of consts to the same place or same statement,
  67             // and match up 1-1, if not don't optimize this block.
  68             let first_stmts = &bbs[first].statements;
  69             let scnd_stmts = &bbs[second].statements;
  70             if first_stmts.len() != scnd_stmts.len() {
  71                 continue;
  72             }
  73             for (f, s) in first_stmts.iter().zip(scnd_stmts.iter()) {
  74                 match (&f.kind, &s.kind) {
  75                     // If two statements are exactly the same, we can optimize.
  76                     (f_s, s_s) if f_s == s_s => {}
  77
  78                     // If two statements are const bool assignments to the same place, we can optimize.
  79                     (
  80                         StatementKind::Assign(box (lhs_f, Rvalue::Use(Operand::Constant(f_c)))),
  81                         StatementKind::Assign(box (lhs_s, Rvalue::Use(Operand::Constant(s_c)))),
  82                     ) if lhs_f == lhs_s
  83                         && f_c.literal.ty.is_bool()
  84                         && s_c.literal.ty.is_bool()
  85                         && f_c.literal.try_eval_bool(tcx, param_env).is_some()
  86                         && s_c.literal.try_eval_bool(tcx, param_env).is_some() => {}
  87
  88                     // Otherwise we cannot optimize. Try another block.
  89                     _ => continue 'outer,
  90                 }
  91             }
  92             // Take ownership of items now that we know we can optimize.
  93             let discr = discr.clone();
  94
  95             // We already checked that first and second are different blocks,
  96             // and bb_idx has a different terminator from both of them.
  97             let (from, first, second) = bbs.pick3_mut(bb_idx, first, second);
  98
  99             let new_stmts = first.statements.iter().zip(second.statements.iter()).map(|(f, s)| {
 100                 match (&f.kind, &s.kind) {
 101                     (f_s, s_s) if f_s == s_s => (*f).clone(),
 102
 103                     (
 104                         StatementKind::Assign(box (lhs, Rvalue::Use(Operand::Constant(f_c)))),
 105                         StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(s_c)))),
 106                     ) => {
 107                         // From earlier loop we know that we are dealing with bool constants only:
 108                         let f_b = f_c.literal.try_eval_bool(tcx, param_env).unwrap();
 109                         let s_b = s_c.literal.try_eval_bool(tcx, param_env).unwrap();
 110                         if f_b == s_b {
 111                             // Same value in both blocks. Use statement as is.
 112                             (*f).clone()
 113                         } else {
 114                             // Different value between blocks. Make value conditional on switch condition.
 115                             let size = tcx.layout_of(param_env.and(switch_ty)).unwrap().size;
 116                             let const_cmp = Operand::const_from_scalar(
 117                                 tcx,
 118                                 switch_ty,
 119                                 crate::interpret::Scalar::from_uint(val, size),
 120                                 rustc_span::DUMMY_SP,
 121                             );
 122                             let op = if f_b { BinOp::Eq } else { BinOp::Ne };
 123                             let rhs = Rvalue::BinaryOp(op, Operand::Copy(discr.clone()), const_cmp);
 124                             Statement {
 125                                 source_info: f.source_info,
 126                                 kind: StatementKind::Assign(box (*lhs, rhs)),
 127                             }
 128                         }
 129                     }
 130
 131                     _ => unreachable!(),
 132                 }
 133             });
 134             from.statements.extend(new_stmts);
 135             from.terminator_mut().kind = first.terminator().kind.clone();
 136         }
 137     }
 138 }