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| 1 | +//! This pass attempts to merge all branches to eliminate switch terminator. |
| 2 | +//! Ideally, we could combine it with `MatchBranchSimplification`, as these two passes |
| 3 | +//! match and merge statements with different patterns. Given the compile time and |
| 4 | +//! code complexity, we have not merged them into a more general pass for now. |
| 5 | +use rustc_const_eval::const_eval::mk_eval_cx_for_const_val; |
| 6 | +use rustc_index::bit_set::BitSet; |
| 7 | +use rustc_middle::mir::patch::MirPatch; |
| 8 | +use rustc_middle::mir::*; |
| 9 | +use rustc_middle::ty; |
| 10 | +use rustc_middle::ty::util::Discr; |
| 11 | +use rustc_middle::ty::{ParamEnv, TyCtxt}; |
| 12 | +use rustc_mir_dataflow::impls::borrowed_locals; |
| 13 | + |
| 14 | +use crate::dead_store_elimination::DeadStoreAnalysis; |
| 15 | + |
| 16 | +pub(super) struct MergeBranchSimplification; |
| 17 | + |
| 18 | +impl<'tcx> crate::MirPass<'tcx> for MergeBranchSimplification { |
| 19 | + fn is_enabled(&self, sess: &rustc_session::Session) -> bool { |
| 20 | + sess.mir_opt_level() >= 2 |
| 21 | + } |
| 22 | + |
| 23 | + fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { |
| 24 | + let def_id = body.source.def_id(); |
| 25 | + let param_env = tcx.param_env_reveal_all_normalized(def_id); |
| 26 | + |
| 27 | + let borrowed_locals = borrowed_locals(body); |
| 28 | + let mut dead_store_analysis = DeadStoreAnalysis::new(tcx, body, &borrowed_locals); |
| 29 | + |
| 30 | + for switch_bb_idx in body.basic_blocks.indices() { |
| 31 | + let bbs = &*body.basic_blocks; |
| 32 | + let Some((switch_discr, targets)) = bbs[switch_bb_idx].terminator().kind.as_switch() |
| 33 | + else { |
| 34 | + continue; |
| 35 | + }; |
| 36 | + // Check that destinations are identical, and if not, then don't optimize this block. |
| 37 | + let mut targets_iter = targets.iter(); |
| 38 | + let first_terminator_kind = &bbs[targets_iter.next().unwrap().1].terminator().kind; |
| 39 | + if targets_iter.any(|(_, other_target)| { |
| 40 | + first_terminator_kind != &bbs[other_target].terminator().kind |
| 41 | + }) { |
| 42 | + continue; |
| 43 | + } |
| 44 | + // We require that the possible target blocks all be distinct. |
| 45 | + if !targets.is_distinct() { |
| 46 | + continue; |
| 47 | + } |
| 48 | + if !bbs[targets.otherwise()].is_empty_unreachable() { |
| 49 | + continue; |
| 50 | + } |
| 51 | + // Check if the copy source matches the following pattern. |
| 52 | + // _2 = discriminant(*_1); // "*_1" is the expected the copy source. |
| 53 | + // switchInt(move _2) -> [0: bb3, 1: bb2, otherwise: bb1]; |
| 54 | + let Some(&Statement { |
| 55 | + kind: StatementKind::Assign(box (discr_place, Rvalue::Discriminant(src_place))), |
| 56 | + .. |
| 57 | + }) = bbs[switch_bb_idx].statements.last() |
| 58 | + else { |
| 59 | + continue; |
| 60 | + }; |
| 61 | + if switch_discr.place() != Some(discr_place) { |
| 62 | + continue; |
| 63 | + } |
| 64 | + let src_ty = src_place.ty(body.local_decls(), tcx); |
| 65 | + if let Some(dest_place) = can_simplify_to_copy( |
| 66 | + tcx, |
| 67 | + param_env, |
| 68 | + body, |
| 69 | + targets, |
| 70 | + src_place, |
| 71 | + src_ty, |
| 72 | + &mut dead_store_analysis, |
| 73 | + ) { |
| 74 | + let statement_index = bbs[switch_bb_idx].statements.len(); |
| 75 | + let parent_end = Location { block: switch_bb_idx, statement_index }; |
| 76 | + let mut patch = MirPatch::new(body); |
| 77 | + patch.add_assign(parent_end, dest_place, Rvalue::Use(Operand::Copy(src_place))); |
| 78 | + patch.patch_terminator(switch_bb_idx, first_terminator_kind.clone()); |
| 79 | + patch.apply(body); |
| 80 | + super::simplify::remove_dead_blocks(body); |
| 81 | + // After modifying the MIR, the result of `MaybeTransitiveLiveLocals` may become invalid, |
| 82 | + // keeping it simple to process only once. |
| 83 | + break; |
| 84 | + } |
| 85 | + } |
| 86 | + } |
| 87 | +} |
| 88 | + |
| 89 | +/// The GVN simplified |
| 90 | +/// ```ignore (syntax-highlighting-only) |
| 91 | +/// match a { |
| 92 | +/// Foo::A(x) => Foo::A(*x), |
| 93 | +/// Foo::B => Foo::B |
| 94 | +/// } |
| 95 | +/// ``` |
| 96 | +/// to |
| 97 | +/// ```ignore (syntax-highlighting-only) |
| 98 | +/// match a { |
| 99 | +/// Foo::A(_x) => a, // copy a |
| 100 | +/// Foo::B => Foo::B |
| 101 | +/// } |
| 102 | +/// ``` |
| 103 | +/// This function answers whether it can be simplified to a copy statement |
| 104 | +/// by returning the copy destination. |
| 105 | +fn can_simplify_to_copy<'tcx>( |
| 106 | + tcx: TyCtxt<'tcx>, |
| 107 | + param_env: ParamEnv<'tcx>, |
| 108 | + body: &Body<'tcx>, |
| 109 | + targets: &SwitchTargets, |
| 110 | + src_place: Place<'tcx>, |
| 111 | + src_ty: tcx::PlaceTy<'tcx>, |
| 112 | + dead_store_analysis: &mut DeadStoreAnalysis<'tcx, '_, '_>, |
| 113 | +) -> Option<Place<'tcx>> { |
| 114 | + let mut targets_iter = targets.iter(); |
| 115 | + let (first_index, first_target) = targets_iter.next()?; |
| 116 | + let dest_place = find_copy_assign( |
| 117 | + tcx, |
| 118 | + param_env, |
| 119 | + body, |
| 120 | + first_index, |
| 121 | + first_target, |
| 122 | + src_place, |
| 123 | + src_ty, |
| 124 | + dead_store_analysis, |
| 125 | + )?; |
| 126 | + let dest_ty = dest_place.ty(body.local_decls(), tcx); |
| 127 | + if dest_ty.ty != src_ty.ty { |
| 128 | + return None; |
| 129 | + } |
| 130 | + for (other_index, other_target) in targets_iter { |
| 131 | + if dest_place |
| 132 | + != find_copy_assign( |
| 133 | + tcx, |
| 134 | + param_env, |
| 135 | + body, |
| 136 | + other_index, |
| 137 | + other_target, |
| 138 | + src_place, |
| 139 | + src_ty, |
| 140 | + dead_store_analysis, |
| 141 | + )? |
| 142 | + { |
| 143 | + return None; |
| 144 | + } |
| 145 | + } |
| 146 | + Some(dest_place) |
| 147 | +} |
| 148 | + |
| 149 | +// Find the single assignment statement where the source of the copy is from the source. |
| 150 | +// All other statements are dead statements or have no effect that can be eliminated. |
| 151 | +fn find_copy_assign<'tcx>( |
| 152 | + tcx: TyCtxt<'tcx>, |
| 153 | + param_env: ParamEnv<'tcx>, |
| 154 | + body: &Body<'tcx>, |
| 155 | + index: u128, |
| 156 | + target_block: BasicBlock, |
| 157 | + src_place: Place<'tcx>, |
| 158 | + src_ty: tcx::PlaceTy<'tcx>, |
| 159 | + dead_store_analysis: &mut DeadStoreAnalysis<'tcx, '_, '_>, |
| 160 | +) -> Option<Place<'tcx>> { |
| 161 | + let statements = &body.basic_blocks[target_block].statements; |
| 162 | + if statements.is_empty() { |
| 163 | + return None; |
| 164 | + } |
| 165 | + let assign_stmt = if statements.len() == 1 { |
| 166 | + 0 |
| 167 | + } else { |
| 168 | + let mut lived_stmts: BitSet<usize> = BitSet::new_filled(statements.len()); |
| 169 | + let mut expected_assign_stmt = None; |
| 170 | + for (statement_index, statement) in statements.iter().enumerate().rev() { |
| 171 | + let loc = Location { block: target_block, statement_index }; |
| 172 | + if dead_store_analysis.is_dead_store(loc, &statement.kind) { |
| 173 | + lived_stmts.remove(statement_index); |
| 174 | + } else if matches!( |
| 175 | + statement.kind, |
| 176 | + StatementKind::StorageLive(_) | StatementKind::StorageDead(_) |
| 177 | + ) { |
| 178 | + } else if matches!(statement.kind, StatementKind::Assign(_)) |
| 179 | + && expected_assign_stmt.is_none() |
| 180 | + { |
| 181 | + // There is only one assign statement that cannot be ignored |
| 182 | + // that can be used as an expected copy statement. |
| 183 | + expected_assign_stmt = Some(statement_index); |
| 184 | + lived_stmts.remove(statement_index); |
| 185 | + } else { |
| 186 | + return None; |
| 187 | + } |
| 188 | + } |
| 189 | + let expected_assign = expected_assign_stmt?; |
| 190 | + if !lived_stmts.is_empty() { |
| 191 | + // We can ignore the paired StorageLive and StorageDead. |
| 192 | + let mut storage_live_locals: BitSet<Local> = BitSet::new_empty(body.local_decls.len()); |
| 193 | + for stmt_index in lived_stmts.iter() { |
| 194 | + let statement = &statements[stmt_index]; |
| 195 | + match &statement.kind { |
| 196 | + StatementKind::StorageLive(local) if storage_live_locals.insert(*local) => {} |
| 197 | + StatementKind::StorageDead(local) if storage_live_locals.remove(*local) => {} |
| 198 | + _ => return None, |
| 199 | + } |
| 200 | + } |
| 201 | + if !storage_live_locals.is_empty() { |
| 202 | + return None; |
| 203 | + } |
| 204 | + } |
| 205 | + expected_assign |
| 206 | + }; |
| 207 | + let &(dest_place, ref rvalue) = statements[assign_stmt].kind.as_assign()?; |
| 208 | + let dest_ty = dest_place.ty(body.local_decls(), tcx); |
| 209 | + if dest_ty.ty != src_ty.ty { |
| 210 | + return None; |
| 211 | + } |
| 212 | + let ty::Adt(def, _) = dest_ty.ty.kind() else { |
| 213 | + return None; |
| 214 | + }; |
| 215 | + match rvalue { |
| 216 | + // Check if `_3 = const Foo::B` can be transformed to `_3 = copy *_1`. |
| 217 | + Rvalue::Use(Operand::Constant(box constant)) |
| 218 | + if let Const::Val(const_, ty) = constant.const_ => |
| 219 | + { |
| 220 | + let (ecx, op) = mk_eval_cx_for_const_val(tcx.at(constant.span), param_env, const_, ty)?; |
| 221 | + let variant = ecx.read_discriminant(&op).discard_err()?; |
| 222 | + if !def.variants()[variant].fields.is_empty() { |
| 223 | + return None; |
| 224 | + } |
| 225 | + let Discr { val, .. } = ty.discriminant_for_variant(tcx, variant)?; |
| 226 | + if val != index { |
| 227 | + return None; |
| 228 | + } |
| 229 | + } |
| 230 | + Rvalue::Use(Operand::Copy(place)) if *place == src_place => {} |
| 231 | + // Check if `_3 = Foo::B` can be transformed to `_3 = copy *_1`. |
| 232 | + Rvalue::Aggregate(box AggregateKind::Adt(_, variant_index, _, _, None), fields) |
| 233 | + if fields.is_empty() |
| 234 | + && let Some(Discr { val, .. }) = |
| 235 | + src_ty.ty.discriminant_for_variant(tcx, *variant_index) |
| 236 | + && val == index => {} |
| 237 | + _ => return None, |
| 238 | + } |
| 239 | + Some(dest_place) |
| 240 | +} |
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