compiler/rustc_mir_transform/src/normalize_array_len.rs

   1 //! This pass eliminates casting of arrays into slices when their length
   2 //! is taken using `.len()` method. Handy to preserve information in MIR for const prop
   3
   4 use crate::MirPass;
   5 use rustc_data_structures::fx::FxIndexMap;
   6 use rustc_data_structures::intern::Interned;
   7 use rustc_index::bit_set::BitSet;
   8 use rustc_index::vec::IndexVec;
   9 use rustc_middle::mir::*;
  10 use rustc_middle::ty::{self, ReErased, Region, TyCtxt};
  11
  12 const MAX_NUM_BLOCKS: usize = 800;
  13 const MAX_NUM_LOCALS: usize = 3000;
  14
  15 pub struct NormalizeArrayLen;
  16
  17 impl<'tcx> MirPass<'tcx> for NormalizeArrayLen {
  18     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
  19         sess.mir_opt_level() >= 4
  20     }
  21
  22     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  23         // early returns for edge cases of highly unrolled functions
  24         if body.basic_blocks().len() > MAX_NUM_BLOCKS {
  25             return;
  26         }
  27         if body.local_decls().len() > MAX_NUM_LOCALS {
  28             return;
  29         }
  30         normalize_array_len_calls(tcx, body)
  31     }
  32 }
  33
  34 pub fn normalize_array_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  35     let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
  36
  37     // do a preliminary analysis to see if we ever have locals of type `[T;N]` or `&[T;N]`
  38     let mut interesting_locals = BitSet::new_empty(local_decls.len());
  39     for (local, decl) in local_decls.iter_enumerated() {
  40         match decl.ty.kind() {
  41             ty::Array(..) => {
  42                 interesting_locals.insert(local);
  43             }
  44             ty::Ref(.., ty, Mutability::Not) => match ty.kind() {
  45                 ty::Array(..) => {
  46                     interesting_locals.insert(local);
  47                 }
  48                 _ => {}
  49             },
  50             _ => {}
  51         }
  52     }
  53     if interesting_locals.is_empty() {
  54         // we have found nothing to analyze
  55         return;
  56     }
  57     let num_intesting_locals = interesting_locals.count();
  58     let mut state = FxIndexMap::with_capacity_and_hasher(num_intesting_locals, Default::default());
  59     let mut patches_scratchpad =
  60         FxIndexMap::with_capacity_and_hasher(num_intesting_locals, Default::default());
  61     let mut replacements_scratchpad =
  62         FxIndexMap::with_capacity_and_hasher(num_intesting_locals, Default::default());
  63     for block in basic_blocks {
  64         // make length calls for arrays [T; N] not to decay into length calls for &[T]
  65         // that forbids constant propagation
  66         normalize_array_len_call(
  67             tcx,
  68             block,
  69             local_decls,
  70             &interesting_locals,
  71             &mut state,
  72             &mut patches_scratchpad,
  73             &mut replacements_scratchpad,
  74         );
  75         state.clear();
  76         patches_scratchpad.clear();
  77         replacements_scratchpad.clear();
  78     }
  79 }
  80
  81 struct Patcher<'a, 'tcx> {
  82     tcx: TyCtxt<'tcx>,
  83     patches_scratchpad: &'a FxIndexMap<usize, usize>,
  84     replacements_scratchpad: &'a mut FxIndexMap<usize, Local>,
  85     local_decls: &'a mut IndexVec<Local, LocalDecl<'tcx>>,
  86     statement_idx: usize,
  87 }
  88
  89 impl<'tcx> Patcher<'_, 'tcx> {
  90     fn patch_expand_statement(
  91         &mut self,
  92         statement: &mut Statement<'tcx>,
  93     ) -> Option<std::vec::IntoIter<Statement<'tcx>>> {
  94         let idx = self.statement_idx;
  95         if let Some(len_statemnt_idx) = self.patches_scratchpad.get(&idx).copied() {
  96             let mut statements = Vec::with_capacity(2);
  97
  98             // we are at statement that performs a cast. The only sound way is
  99             // to create another local that performs a similar copy without a cast and then
 100             // use this copy in the Len operation
 101
 102             match &statement.kind {
 103                 StatementKind::Assign(box (
 104                     ..,
 105                     Rvalue::Cast(
 106                         CastKind::Pointer(ty::adjustment::PointerCast::Unsize),
 107                         operand,
 108                         _,
 109                     ),
 110                 )) => {
 111                     match operand {
 112                         Operand::Copy(place) | Operand::Move(place) => {
 113                             // create new local
 114                             let ty = operand.ty(self.local_decls, self.tcx);
 115                             let local_decl = LocalDecl::with_source_info(ty, statement.source_info);
 116                             let local = self.local_decls.push(local_decl);
 117                             // make it live
 118                             let mut make_live_statement = statement.clone();
 119                             make_live_statement.kind = StatementKind::StorageLive(local);
 120                             statements.push(make_live_statement);
 121                             // copy into it
 122
 123                             let operand = Operand::Copy(*place);
 124                             let mut make_copy_statement = statement.clone();
 125                             let assign_to = Place::from(local);
 126                             let rvalue = Rvalue::Use(operand);
 127                             make_copy_statement.kind =
 128                                 StatementKind::Assign(box (assign_to, rvalue));
 129                             statements.push(make_copy_statement);
 130
 131                             // to reorder we have to copy and make NOP
 132                             statements.push(statement.clone());
 133                             statement.make_nop();
 134
 135                             self.replacements_scratchpad.insert(len_statemnt_idx, local);
 136                         }
 137                         _ => {
 138                             unreachable!("it's a bug in the implementation")
 139                         }
 140                     }
 141                 }
 142                 _ => {
 143                     unreachable!("it's a bug in the implementation")
 144                 }
 145             }
 146
 147             self.statement_idx += 1;
 148
 149             Some(statements.into_iter())
 150         } else if let Some(local) = self.replacements_scratchpad.get(&idx).copied() {
 151             let mut statements = Vec::with_capacity(2);
 152
 153             match &statement.kind {
 154                 StatementKind::Assign(box (into, Rvalue::Len(place))) => {
 155                     let add_deref = if let Some(..) = place.as_local() {
 156                         false
 157                     } else if let Some(..) = place.local_or_deref_local() {
 158                         true
 159                     } else {
 160                         unreachable!("it's a bug in the implementation")
 161                     };
 162                     // replace len statement
 163                     let mut len_statement = statement.clone();
 164                     let mut place = Place::from(local);
 165                     if add_deref {
 166                         place = self.tcx.mk_place_deref(place);
 167                     }
 168                     len_statement.kind = StatementKind::Assign(box (*into, Rvalue::Len(place)));
 169                     statements.push(len_statement);
 170
 171                     // make temporary dead
 172                     let mut make_dead_statement = statement.clone();
 173                     make_dead_statement.kind = StatementKind::StorageDead(local);
 174                     statements.push(make_dead_statement);
 175
 176                     // make original statement NOP
 177                     statement.make_nop();
 178                 }
 179                 _ => {
 180                     unreachable!("it's a bug in the implementation")
 181                 }
 182             }
 183
 184             self.statement_idx += 1;
 185
 186             Some(statements.into_iter())
 187         } else {
 188             self.statement_idx += 1;
 189             None
 190         }
 191     }
 192 }
 193
 194 fn normalize_array_len_call<'tcx>(
 195     tcx: TyCtxt<'tcx>,
 196     block: &mut BasicBlockData<'tcx>,
 197     local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
 198     interesting_locals: &BitSet<Local>,
 199     state: &mut FxIndexMap<Local, usize>,
 200     patches_scratchpad: &mut FxIndexMap<usize, usize>,
 201     replacements_scratchpad: &mut FxIndexMap<usize, Local>,
 202 ) {
 203     for (statement_idx, statement) in block.statements.iter_mut().enumerate() {
 204         match &mut statement.kind {
 205             StatementKind::Assign(box (place, rvalue)) => {
 206                 match rvalue {
 207                     Rvalue::Cast(
 208                         CastKind::Pointer(ty::adjustment::PointerCast::Unsize),
 209                         operand,
 210                         cast_ty,
 211                     ) => {
 212                         let Some(local) = place.as_local() else { return };
 213                         match operand {
 214                             Operand::Copy(place) | Operand::Move(place) => {
 215                                 let Some(operand_local) = place.local_or_deref_local() else { return; };
 216                                 if !interesting_locals.contains(operand_local) {
 217                                     return;
 218                                 }
 219                                 let operand_ty = local_decls[operand_local].ty;
 220                                 match (operand_ty.kind(), cast_ty.kind()) {
 221                                     (ty::Array(of_ty_src, ..), ty::Slice(of_ty_dst)) => {
 222                                         if of_ty_src == of_ty_dst {
 223                                             // this is a cast from [T; N] into [T], so we are good
 224                                             state.insert(local, statement_idx);
 225                                         }
 226                                     }
 227                                     // current way of patching doesn't allow to work with `mut`
 228                                     (
 229                                         ty::Ref(
 230                                             Region(Interned(ReErased, _)),
 231                                             operand_ty,
 232                                             Mutability::Not,
 233                                         ),
 234                                         ty::Ref(
 235                                             Region(Interned(ReErased, _)),
 236                                             cast_ty,
 237                                             Mutability::Not,
 238                                         ),
 239                                     ) => {
 240                                         match (operand_ty.kind(), cast_ty.kind()) {
 241                                             // current way of patching doesn't allow to work with `mut`
 242                                             (ty::Array(of_ty_src, ..), ty::Slice(of_ty_dst)) => {
 243                                                 if of_ty_src == of_ty_dst {
 244                                                     // this is a cast from [T; N] into [T], so we are good
 245                                                     state.insert(local, statement_idx);
 246                                                 }
 247                                             }
 248                                             _ => {}
 249                                         }
 250                                     }
 251                                     _ => {}
 252                                 }
 253                             }
 254                             _ => {}
 255                         }
 256                     }
 257                     Rvalue::Len(place) => {
 258                         let Some(local) = place.local_or_deref_local() else {
 259                             return;
 260                         };
 261                         if let Some(cast_statement_idx) = state.get(&local).copied() {
 262                             patches_scratchpad.insert(cast_statement_idx, statement_idx);
 263                         }
 264                     }
 265                     _ => {
 266                         // invalidate
 267                         state.remove(&place.local);
 268                     }
 269                 }
 270             }
 271             _ => {}
 272         }
 273     }
 274
 275     let mut patcher = Patcher {
 276         tcx,
 277         patches_scratchpad: &*patches_scratchpad,
 278         replacements_scratchpad,
 279         local_decls,
 280         statement_idx: 0,
 281     };
 282
 283     block.expand_statements(|st| patcher.patch_expand_statement(st));
 284 }