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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::{MaybeTransitiveLiveLocals, borrowed_locals}; |
| 13 | +use rustc_mir_dataflow::{Analysis, ResultsCursor}; |
| 14 | + |
| 15 | +pub(super) struct MergeBranchSimplification; |
| 16 | + |
| 17 | +impl<'tcx> crate::MirPass<'tcx> for MergeBranchSimplification { |
| 18 | + fn is_enabled(&self, sess: &rustc_session::Session) -> bool { |
| 19 | + sess.mir_opt_level() >= 2 |
| 20 | + } |
| 21 | + |
| 22 | + fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { |
| 23 | + let def_id = body.source.def_id(); |
| 24 | + let param_env = tcx.param_env_reveal_all_normalized(def_id); |
| 25 | + |
| 26 | + let borrowed_locals = borrowed_locals(body); |
| 27 | + let mut stmt_live_result = StatementLiveResult::new(tcx, body, &borrowed_locals); |
| 28 | + for i in 0..body.basic_blocks.len() { |
| 29 | + let bbs = &*body.basic_blocks; |
| 30 | + let switch_bb_idx = BasicBlock::from_usize(i); |
| 31 | + let Some((switch_discr, targets)) = bbs[switch_bb_idx].terminator().kind.as_switch() |
| 32 | + else { |
| 33 | + continue; |
| 34 | + }; |
| 35 | + // Check if the copy source matches the following pattern. |
| 36 | + // _2 = discriminant(*_1); // "*_1" is the expected the copy source. |
| 37 | + // switchInt(move _2) -> [0: bb3, 1: bb2, otherwise: bb1]; |
| 38 | + let Some(&Statement { |
| 39 | + kind: StatementKind::Assign(box (discr_place, Rvalue::Discriminant(src_place))), |
| 40 | + .. |
| 41 | + }) = bbs[switch_bb_idx].statements.last() |
| 42 | + else { |
| 43 | + continue; |
| 44 | + }; |
| 45 | + if switch_discr.place() != Some(discr_place) { |
| 46 | + continue; |
| 47 | + } |
| 48 | + let src_ty = src_place.ty(body.local_decls(), tcx); |
| 49 | + if !src_ty.ty.is_enum() || src_ty.variant_index.is_some() { |
| 50 | + continue; |
| 51 | + } |
| 52 | + // We require that the possible target blocks all be distinct. |
| 53 | + if !targets.is_distinct() { |
| 54 | + continue; |
| 55 | + } |
| 56 | + if !bbs[targets.otherwise()].is_empty_unreachable() { |
| 57 | + continue; |
| 58 | + } |
| 59 | + // Check that destinations are identical, and if not, then don't optimize this block. |
| 60 | + let mut targets_iter = targets.iter(); |
| 61 | + let first_terminator_kind = &bbs[targets_iter.next().unwrap().1].terminator().kind; |
| 62 | + if !targets_iter.all(|(_, other_target)| { |
| 63 | + first_terminator_kind == &bbs[other_target].terminator().kind |
| 64 | + }) { |
| 65 | + continue; |
| 66 | + } |
| 67 | + if let Some(dest_place) = can_simplify_to_copy( |
| 68 | + tcx, |
| 69 | + param_env, |
| 70 | + body, |
| 71 | + targets, |
| 72 | + src_place, |
| 73 | + src_ty, |
| 74 | + &mut stmt_live_result, |
| 75 | + ) { |
| 76 | + let statement_index = bbs[switch_bb_idx].statements.len(); |
| 77 | + let parent_end = Location { block: switch_bb_idx, statement_index }; |
| 78 | + let mut patch = MirPatch::new(body); |
| 79 | + patch.add_assign(parent_end, dest_place, Rvalue::Use(Operand::Copy(src_place))); |
| 80 | + patch.patch_terminator(switch_bb_idx, first_terminator_kind.clone()); |
| 81 | + patch.apply(body); |
| 82 | + super::simplify::remove_dead_blocks(body); |
| 83 | + // After modifying the MIR, the result of `MaybeTransitiveLiveLocals` may become invalid, |
| 84 | + // keeping it simple to process only once. |
| 85 | + break; |
| 86 | + } |
| 87 | + } |
| 88 | + } |
| 89 | +} |
| 90 | + |
| 91 | +struct StatementLiveResult<'tcx, 'mir, 'a> { |
| 92 | + tcx: TyCtxt<'tcx>, |
| 93 | + body: &'mir Body<'tcx>, |
| 94 | + result: Option<ResultsCursor<'mir, 'tcx, MaybeTransitiveLiveLocals<'a>>>, |
| 95 | + borrowed_locals: &'a BitSet<Local>, |
| 96 | +} |
| 97 | + |
| 98 | +impl<'tcx, 'mir, 'a> StatementLiveResult<'tcx, 'mir, 'a> { |
| 99 | + fn new(tcx: TyCtxt<'tcx>, body: &'mir Body<'tcx>, borrowed_locals: &'a BitSet<Local>) -> Self { |
| 100 | + Self { tcx, body, result: None, borrowed_locals } |
| 101 | + } |
| 102 | + |
| 103 | + fn is_live(&mut self, loc: Location, local: Local) -> bool { |
| 104 | + if self.borrowed_locals.contains(local) { |
| 105 | + return true; |
| 106 | + } |
| 107 | + let maybe_live = self.result.get_or_insert_with(|| { |
| 108 | + MaybeTransitiveLiveLocals::new(&self.borrowed_locals) |
| 109 | + .into_engine(self.tcx, self.body) |
| 110 | + .iterate_to_fixpoint() |
| 111 | + .into_results_cursor(self.body) |
| 112 | + }); |
| 113 | + maybe_live.seek_before_primary_effect(loc); |
| 114 | + maybe_live.get().contains(local) |
| 115 | + } |
| 116 | +} |
| 117 | + |
| 118 | +/// The GVN simplified |
| 119 | +/// ```ignore (syntax-highlighting-only) |
| 120 | +/// match a { |
| 121 | +/// Foo::A(x) => Foo::A(*x), |
| 122 | +/// Foo::B => Foo::B |
| 123 | +/// } |
| 124 | +/// ``` |
| 125 | +/// to |
| 126 | +/// ```ignore (syntax-highlighting-only) |
| 127 | +/// match a { |
| 128 | +/// Foo::A(_x) => a, // copy a |
| 129 | +/// Foo::B => Foo::B |
| 130 | +/// } |
| 131 | +/// ``` |
| 132 | +/// This function answers whether it can be simplified to a copy statement |
| 133 | +/// by returning the copy destination. |
| 134 | +fn can_simplify_to_copy<'tcx>( |
| 135 | + tcx: TyCtxt<'tcx>, |
| 136 | + param_env: ParamEnv<'tcx>, |
| 137 | + body: &Body<'tcx>, |
| 138 | + targets: &SwitchTargets, |
| 139 | + src_place: Place<'tcx>, |
| 140 | + src_ty: tcx::PlaceTy<'tcx>, |
| 141 | + stmt_live_result: &mut StatementLiveResult<'tcx, '_, '_>, |
| 142 | +) -> Option<Place<'tcx>> { |
| 143 | + let mut targets_iter = targets.iter(); |
| 144 | + let dest_place = targets_iter.next().and_then(|(index, target)| { |
| 145 | + find_copy_assign(tcx, param_env, body, index, target, src_place, src_ty, stmt_live_result) |
| 146 | + })?; |
| 147 | + let dest_ty = dest_place.ty(body.local_decls(), tcx); |
| 148 | + if dest_ty.ty != src_ty.ty || dest_ty.variant_index.is_some() { |
| 149 | + return None; |
| 150 | + } |
| 151 | + if targets_iter.any(|(other_index, other_target)| { |
| 152 | + Some(dest_place) |
| 153 | + != find_copy_assign( |
| 154 | + tcx, |
| 155 | + param_env, |
| 156 | + body, |
| 157 | + other_index, |
| 158 | + other_target, |
| 159 | + src_place, |
| 160 | + src_ty, |
| 161 | + stmt_live_result, |
| 162 | + ) |
| 163 | + }) { |
| 164 | + return None; |
| 165 | + } |
| 166 | + Some(dest_place) |
| 167 | +} |
| 168 | + |
| 169 | +fn find_copy_assign<'tcx>( |
| 170 | + tcx: TyCtxt<'tcx>, |
| 171 | + param_env: ParamEnv<'tcx>, |
| 172 | + body: &Body<'tcx>, |
| 173 | + index: u128, |
| 174 | + target_block: BasicBlock, |
| 175 | + src_place: Place<'tcx>, |
| 176 | + src_ty: tcx::PlaceTy<'tcx>, |
| 177 | + stmt_live_result: &mut StatementLiveResult<'tcx, '_, '_>, |
| 178 | +) -> Option<Place<'tcx>> { |
| 179 | + let statements = &body.basic_blocks[target_block].statements; |
| 180 | + if statements.is_empty() { |
| 181 | + return None; |
| 182 | + } |
| 183 | + let assign_stmt = if statements.len() == 1 { |
| 184 | + 0 |
| 185 | + } else { |
| 186 | + // We are matching a statement copied from the source to the same destination from the BB, |
| 187 | + // and dead statements can be ignored. |
| 188 | + // We can treat the rvalue is the source if it's equal to the source. |
| 189 | + let mut lived_stmts: BitSet<usize> = BitSet::new_filled(statements.len()); |
| 190 | + let mut expected_assign_stmt = None; |
| 191 | + for (statement_index, statement) in statements.iter().enumerate().rev() { |
| 192 | + let loc = Location { block: target_block, statement_index }; |
| 193 | + if let StatementKind::Assign(assign) = &statement.kind { |
| 194 | + if !assign.1.is_safe_to_remove() { |
| 195 | + return None; |
| 196 | + } |
| 197 | + } |
| 198 | + match &statement.kind { |
| 199 | + StatementKind::Assign(box (dest_place, _)) |
| 200 | + | StatementKind::SetDiscriminant { place: box dest_place, .. } |
| 201 | + | StatementKind::Deinit(box dest_place) => { |
| 202 | + if dest_place.is_indirect() { |
| 203 | + return None; |
| 204 | + } |
| 205 | + if !stmt_live_result.is_live(loc, dest_place.local) { |
| 206 | + lived_stmts.remove(statement_index); |
| 207 | + } else if matches!(statement.kind, StatementKind::Assign(_)) |
| 208 | + && expected_assign_stmt.is_none() |
| 209 | + { |
| 210 | + // There is only one statement that cannot be ignored |
| 211 | + // that can be used as an expected copy statement. |
| 212 | + expected_assign_stmt = Some(statement_index); |
| 213 | + lived_stmts.remove(statement_index); |
| 214 | + } else { |
| 215 | + return None; |
| 216 | + } |
| 217 | + } |
| 218 | + StatementKind::StorageLive(_) |
| 219 | + | StatementKind::StorageDead(_) |
| 220 | + | StatementKind::Nop => (), |
| 221 | + |
| 222 | + StatementKind::Retag(_, _) |
| 223 | + | StatementKind::Coverage(_) |
| 224 | + | StatementKind::Intrinsic(_) |
| 225 | + | StatementKind::ConstEvalCounter |
| 226 | + | StatementKind::PlaceMention(_) |
| 227 | + | StatementKind::FakeRead(_) |
| 228 | + | StatementKind::AscribeUserType(_, _) => { |
| 229 | + return None; |
| 230 | + } |
| 231 | + } |
| 232 | + } |
| 233 | + let expected_assign = expected_assign_stmt?; |
| 234 | + // We can ignore the paired StorageLive and StorageDead. |
| 235 | + let mut storage_live_locals: BitSet<Local> = BitSet::new_empty(body.local_decls.len()); |
| 236 | + for stmt_index in lived_stmts.iter() { |
| 237 | + let statement = &statements[stmt_index]; |
| 238 | + match &statement.kind { |
| 239 | + StatementKind::StorageLive(local) if storage_live_locals.insert(*local) => {} |
| 240 | + StatementKind::StorageDead(local) if storage_live_locals.remove(*local) => {} |
| 241 | + StatementKind::Nop => {} |
| 242 | + _ => return None, |
| 243 | + } |
| 244 | + } |
| 245 | + if !storage_live_locals.is_empty() { |
| 246 | + return None; |
| 247 | + } |
| 248 | + expected_assign |
| 249 | + }; |
| 250 | + let Statement { kind: StatementKind::Assign(box (dest_place, ref rvalue)), .. } = |
| 251 | + statements[assign_stmt] |
| 252 | + else { |
| 253 | + return None; |
| 254 | + }; |
| 255 | + let dest_ty = dest_place.ty(body.local_decls(), tcx); |
| 256 | + if dest_ty.ty != src_ty.ty || dest_ty.variant_index.is_some() { |
| 257 | + return None; |
| 258 | + } |
| 259 | + let ty::Adt(def, _) = dest_ty.ty.kind() else { |
| 260 | + return None; |
| 261 | + }; |
| 262 | + match rvalue { |
| 263 | + // Check if `_3 = const Foo::B` can be transformed to `_3 = copy *_1`. |
| 264 | + Rvalue::Use(Operand::Constant(box constant)) |
| 265 | + if let Const::Val(const_, ty) = constant.const_ => |
| 266 | + { |
| 267 | + let (ecx, op) = mk_eval_cx_for_const_val(tcx.at(constant.span), param_env, const_, ty)?; |
| 268 | + let variant = ecx.read_discriminant(&op).discard_err()?; |
| 269 | + if !def.variants()[variant].fields.is_empty() { |
| 270 | + return None; |
| 271 | + } |
| 272 | + let Discr { val, .. } = ty.discriminant_for_variant(tcx, variant)?; |
| 273 | + if val != index { |
| 274 | + return None; |
| 275 | + } |
| 276 | + } |
| 277 | + Rvalue::Use(Operand::Copy(place)) if *place == src_place => {} |
| 278 | + // Check if `_3 = Foo::B` can be transformed to `_3 = copy *_1`. |
| 279 | + Rvalue::Aggregate(box AggregateKind::Adt(_, variant_index, _, _, None), fields) |
| 280 | + if fields.is_empty() |
| 281 | + && let Some(Discr { val, .. }) = |
| 282 | + src_ty.ty.discriminant_for_variant(tcx, *variant_index) |
| 283 | + && val == index => {} |
| 284 | + _ => return None, |
| 285 | + } |
| 286 | + Some(dest_place) |
| 287 | +} |
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