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 use std::iter;
   5
   6 use super::simplify::simplify_cfg;
   7
   8 pub struct MatchBranchSimplification;
   9
  10 /// If a source block is found that switches between two blocks that are exactly
  11 /// the same modulo const bool assignments (e.g., one assigns true another false
  12 /// to the same place), merge a target block statements into the source block,
  13 /// using Eq / Ne comparison with switch value where const bools value differ.
  14 ///
  15 /// For example:
  16 ///
  17 /// ```rust
  18 /// bb0: {
  19 ///     switchInt(move _3) -> [42_isize: bb1, otherwise: bb2];
  20 /// }
  21 ///
  22 /// bb1: {
  23 ///     _2 = const true;
  24 ///     goto -> bb3;
  25 /// }
  26 ///
  27 /// bb2: {
  28 ///     _2 = const false;
  29 ///     goto -> bb3;
  30 /// }
  31 /// ```
  32 ///
  33 /// into:
  34 ///
  35 /// ```rust
  36 /// bb0: {
  37 ///    _2 = Eq(move _3, const 42_isize);
  38 ///    goto -> bb3;
  39 /// }
  40 /// ```
  41
  42 impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
  43     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  44         if tcx.sess.mir_opt_level() < 3 {
  45             return;
  46         }
  47
  48         let def_id = body.source.def_id();
  49         let param_env = tcx.param_env(def_id);
  50
  51         let (bbs, local_decls) = body.basic_blocks_and_local_decls_mut();
  52         let mut should_cleanup = false;
  53         'outer: for bb_idx in bbs.indices() {
  54             if !tcx.consider_optimizing(|| format!("MatchBranchSimplification {:?} ", def_id)) {
  55                 continue;
  56             }
  57
  58             let (discr, val, switch_ty, first, second) = match bbs[bb_idx].terminator().kind {
  59                 TerminatorKind::SwitchInt {
  60                     discr: ref discr @ (Operand::Copy(_) | Operand::Move(_)),
  61                     switch_ty,
  62                     ref targets,
  63                     ..
  64                 } if targets.iter().len() == 1 => {
  65                     let (value, target) = targets.iter().next().unwrap();
  66                     if target == targets.otherwise() {
  67                         continue;
  68                     }
  69                     (discr, value, switch_ty, target, targets.otherwise())
  70                 }
  71                 // Only optimize switch int statements
  72                 _ => continue,
  73             };
  74
  75             // Check that destinations are identical, and if not, then don't optimize this block
  76             if bbs[first].terminator().kind != bbs[second].terminator().kind {
  77                 continue;
  78             }
  79
  80             // Check that blocks are assignments of consts to the same place or same statement,
  81             // and match up 1-1, if not don't optimize this block.
  82             let first_stmts = &bbs[first].statements;
  83             let scnd_stmts = &bbs[second].statements;
  84             if first_stmts.len() != scnd_stmts.len() {
  85                 continue;
  86             }
  87             for (f, s) in iter::zip(first_stmts, scnd_stmts) {
  88                 match (&f.kind, &s.kind) {
  89                     // If two statements are exactly the same, we can optimize.
  90                     (f_s, s_s) if f_s == s_s => {}
  91
  92                     // If two statements are const bool assignments to the same place, we can optimize.
  93                     (
  94                         StatementKind::Assign(box (lhs_f, Rvalue::Use(Operand::Constant(f_c)))),
  95                         StatementKind::Assign(box (lhs_s, Rvalue::Use(Operand::Constant(s_c)))),
  96                     ) if lhs_f == lhs_s
  97                         && f_c.literal.ty().is_bool()
  98                         && s_c.literal.ty().is_bool()
  99                         && f_c.literal.try_eval_bool(tcx, param_env).is_some()
 100                         && s_c.literal.try_eval_bool(tcx, param_env).is_some() => {}
 101
 102                     // Otherwise we cannot optimize. Try another block.
 103                     _ => continue 'outer,
 104                 }
 105             }
 106             // Take ownership of items now that we know we can optimize.
 107             let discr = discr.clone();
 108
 109             // Introduce a temporary for the discriminant value.
 110             let source_info = bbs[bb_idx].terminator().source_info;
 111             let discr_local = local_decls.push(LocalDecl::new(switch_ty, source_info.span));
 112
 113             // We already checked that first and second are different blocks,
 114             // and bb_idx has a different terminator from both of them.
 115             let (from, first, second) = bbs.pick3_mut(bb_idx, first, second);
 116
 117             let new_stmts = iter::zip(&first.statements, &second.statements).map(|(f, s)| {
 118                 match (&f.kind, &s.kind) {
 119                     (f_s, s_s) if f_s == s_s => (*f).clone(),
 120
 121                     (
 122                         StatementKind::Assign(box (lhs, Rvalue::Use(Operand::Constant(f_c)))),
 123                         StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(s_c)))),
 124                     ) => {
 125                         // From earlier loop we know that we are dealing with bool constants only:
 126                         let f_b = f_c.literal.try_eval_bool(tcx, param_env).unwrap();
 127                         let s_b = s_c.literal.try_eval_bool(tcx, param_env).unwrap();
 128                         if f_b == s_b {
 129                             // Same value in both blocks. Use statement as is.
 130                             (*f).clone()
 131                         } else {
 132                             // Different value between blocks. Make value conditional on switch condition.
 133                             let size = tcx.layout_of(param_env.and(switch_ty)).unwrap().size;
 134                             let const_cmp = Operand::const_from_scalar(
 135                                 tcx,
 136                                 switch_ty,
 137                                 crate::interpret::Scalar::from_uint(val, size),
 138                                 rustc_span::DUMMY_SP,
 139                             );
 140                             let op = if f_b { BinOp::Eq } else { BinOp::Ne };
 141                             let rhs = Rvalue::BinaryOp(
 142                                 op,
 143                                 box (Operand::Copy(Place::from(discr_local)), const_cmp),
 144                             );
 145                             Statement {
 146                                 source_info: f.source_info,
 147                                 kind: StatementKind::Assign(box (*lhs, rhs)),
 148                             }
 149                         }
 150                     }
 151
 152                     _ => unreachable!(),
 153                 }
 154             });
 155
 156             from.statements
 157                 .push(Statement { source_info, kind: StatementKind::StorageLive(discr_local) });
 158             from.statements.push(Statement {
 159                 source_info,
 160                 kind: StatementKind::Assign(box (Place::from(discr_local), Rvalue::Use(discr))),
 161             });
 162             from.statements.extend(new_stmts);
 163             from.statements
 164                 .push(Statement { source_info, kind: StatementKind::StorageDead(discr_local) });
 165             from.terminator_mut().kind = first.terminator().kind.clone();
 166             should_cleanup = true;
 167         }
 168
 169         if should_cleanup {
 170             simplify_cfg(body);
 171         }
 172     }
 173 }