compiler/rustc_codegen_cranelift/src/optimize/stack2reg.rs

   1 //! This optimization replaces stack accesses with SSA variables and removes dead stores when possible.
   2 //!
   3 //! # Undefined behaviour
   4 //!
   5 //! This optimization is based on the assumption that stack slots which don't have their address
   6 //! leaked through `stack_addr` are only accessed using `stack_load` and `stack_store` in the
   7 //! function which has the stack slots. This optimization also assumes that stack slot accesses
   8 //! are never out of bounds. If these assumptions are not correct, then this optimization may remove
   9 //! `stack_store` instruction incorrectly, or incorrectly use a previously stored value as the value
  10 //! being loaded by a `stack_load`.
  11
  12 use std::collections::BTreeMap;
  13 use std::fmt;
  14 use std::ops::Not;
  15
  16 use rustc_data_structures::fx::FxHashSet;
  17
  18 use cranelift_codegen::cursor::{Cursor, FuncCursor};
  19 use cranelift_codegen::ir::immediates::Offset32;
  20 use cranelift_codegen::ir::{InstructionData, Opcode, ValueDef};
  21
  22 use crate::prelude::*;
  23
  24 /// Workaround for `StackSlot` not implementing `Ord`.
  25 #[derive(Copy, Clone, PartialEq, Eq)]
  26 struct OrdStackSlot(StackSlot);
  27
  28 impl fmt::Debug for OrdStackSlot {
  29     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  30         write!(f, "{:?}", self.0)
  31     }
  32 }
  33
  34 impl PartialOrd for OrdStackSlot {
  35     fn partial_cmp(&self, rhs: &Self) -> Option<std::cmp::Ordering> {
  36         self.0.as_u32().partial_cmp(&rhs.0.as_u32())
  37     }
  38 }
  39
  40 impl Ord for OrdStackSlot {
  41     fn cmp(&self, rhs: &Self) -> std::cmp::Ordering {
  42         self.0.as_u32().cmp(&rhs.0.as_u32())
  43     }
  44 }
  45
  46 #[derive(Debug, Default)]
  47 struct StackSlotUsage {
  48     stack_addr: FxHashSet<Inst>,
  49     stack_load: FxHashSet<Inst>,
  50     stack_store: FxHashSet<Inst>,
  51 }
  52
  53 impl StackSlotUsage {
  54     fn potential_stores_for_load(&self, ctx: &Context, load: Inst) -> Vec<Inst> {
  55         self.stack_store
  56             .iter()
  57             .cloned()
  58             .filter(|&store| {
  59                 match spatial_overlap(&ctx.func, store, load) {
  60                     SpatialOverlap::No => false, // Can never be the source of the loaded value.
  61                     SpatialOverlap::Partial | SpatialOverlap::Full => true,
  62                 }
  63             })
  64             .filter(|&store| {
  65                 match temporal_order(ctx, store, load) {
  66                     TemporalOrder::NeverBefore => false, // Can never be the source of the loaded value.
  67                     TemporalOrder::MaybeBefore | TemporalOrder::DefinitivelyBefore => true,
  68                 }
  69             })
  70             .collect::<Vec<Inst>>()
  71     }
  72
  73     fn potential_loads_of_store(&self, ctx: &Context, store: Inst) -> Vec<Inst> {
  74         self.stack_load
  75             .iter()
  76             .cloned()
  77             .filter(|&load| {
  78                 match spatial_overlap(&ctx.func, store, load) {
  79                     SpatialOverlap::No => false, // Can never be the source of the loaded value.
  80                     SpatialOverlap::Partial | SpatialOverlap::Full => true,
  81                 }
  82             })
  83             .filter(|&load| {
  84                 match temporal_order(ctx, store, load) {
  85                     TemporalOrder::NeverBefore => false, // Can never be the source of the loaded value.
  86                     TemporalOrder::MaybeBefore | TemporalOrder::DefinitivelyBefore => true,
  87                 }
  88             })
  89             .collect::<Vec<Inst>>()
  90     }
  91
  92     fn remove_unused_stack_addr(func: &mut Function, inst: Inst) {
  93         func.dfg.detach_results(inst);
  94         func.dfg.replace(inst).nop();
  95     }
  96
  97     fn remove_unused_load(func: &mut Function, load: Inst) {
  98         func.dfg.detach_results(load);
  99         func.dfg.replace(load).nop();
 100     }
 101
 102     fn remove_dead_store(&mut self, func: &mut Function, store: Inst) {
 103         func.dfg.replace(store).nop();
 104         self.stack_store.remove(&store);
 105     }
 106
 107     fn change_load_to_alias(&mut self, func: &mut Function, load: Inst, value: Value) {
 108         let loaded_value = func.dfg.inst_results(load)[0];
 109         let loaded_type = func.dfg.value_type(loaded_value);
 110
 111         if func.dfg.value_type(value) == loaded_type {
 112             func.dfg.detach_results(load);
 113             func.dfg.replace(load).nop();
 114             func.dfg.change_to_alias(loaded_value, value);
 115         } else {
 116             func.dfg.replace(load).bitcast(loaded_type, value);
 117         }
 118
 119         self.stack_load.remove(&load);
 120     }
 121 }
 122
 123 struct OptimizeContext<'a> {
 124     ctx: &'a mut Context,
 125     stack_slot_usage_map: BTreeMap<OrdStackSlot, StackSlotUsage>,
 126 }
 127
 128 impl<'a> OptimizeContext<'a> {
 129     fn for_context(ctx: &'a mut Context) -> Self {
 130         ctx.flowgraph(); // Compute cfg and domtree.
 131
 132         // Record all stack_addr, stack_load and stack_store instructions.
 133         let mut stack_slot_usage_map = BTreeMap::<OrdStackSlot, StackSlotUsage>::new();
 134
 135         let mut cursor = FuncCursor::new(&mut ctx.func);
 136         while let Some(_block) = cursor.next_block() {
 137             while let Some(inst) = cursor.next_inst() {
 138                 match cursor.func.dfg[inst] {
 139                     InstructionData::StackLoad {
 140                         opcode: Opcode::StackAddr,
 141                         stack_slot,
 142                         offset: _,
 143                     } => {
 144                         stack_slot_usage_map
 145                             .entry(OrdStackSlot(stack_slot))
 146                             .or_insert_with(StackSlotUsage::default)
 147                             .stack_addr
 148                             .insert(inst);
 149                     }
 150                     InstructionData::StackLoad {
 151                         opcode: Opcode::StackLoad,
 152                         stack_slot,
 153                         offset: _,
 154                     } => {
 155                         stack_slot_usage_map
 156                             .entry(OrdStackSlot(stack_slot))
 157                             .or_insert_with(StackSlotUsage::default)
 158                             .stack_load
 159                             .insert(inst);
 160                     }
 161                     InstructionData::StackStore {
 162                         opcode: Opcode::StackStore,
 163                         arg: _,
 164                         stack_slot,
 165                         offset: _,
 166                     } => {
 167                         stack_slot_usage_map
 168                             .entry(OrdStackSlot(stack_slot))
 169                             .or_insert_with(StackSlotUsage::default)
 170                             .stack_store
 171                             .insert(inst);
 172                     }
 173                     _ => {}
 174                 }
 175             }
 176         }
 177
 178         OptimizeContext { ctx, stack_slot_usage_map }
 179     }
 180 }
 181
 182 pub(super) fn optimize_function(
 183     ctx: &mut Context,
 184     #[cfg_attr(not(debug_assertions), allow(unused_variables))]
 185     clif_comments: &mut crate::pretty_clif::CommentWriter,
 186 ) {
 187     combine_stack_addr_with_load_store(&mut ctx.func);
 188
 189     let mut opt_ctx = OptimizeContext::for_context(ctx);
 190
 191     // FIXME Repeat following instructions until fixpoint.
 192
 193     remove_unused_stack_addr_and_stack_load(&mut opt_ctx);
 194
 195     #[cfg(debug_assertions)]
 196     {
 197         for (&OrdStackSlot(stack_slot), usage) in &opt_ctx.stack_slot_usage_map {
 198             clif_comments.add_comment(stack_slot, format!("used by: {:?}", usage));
 199         }
 200     }
 201
 202     for (stack_slot, users) in opt_ctx.stack_slot_usage_map.iter_mut() {
 203         if users.stack_addr.is_empty().not() {
 204             // Stack addr leaked; there may be unknown loads and stores.
 205             // FIXME use stacked borrows to optimize
 206             continue;
 207         }
 208
 209         for load in users.stack_load.clone().into_iter() {
 210             let potential_stores = users.potential_stores_for_load(&opt_ctx.ctx, load);
 211
 212             #[cfg(debug_assertions)]
 213             for &store in &potential_stores {
 214                 clif_comments.add_comment(
 215                     load,
 216                     format!(
 217                         "Potential store -> load forwarding {} -> {} ({:?}, {:?})",
 218                         opt_ctx.ctx.func.dfg.display_inst(store, None),
 219                         opt_ctx.ctx.func.dfg.display_inst(load, None),
 220                         spatial_overlap(&opt_ctx.ctx.func, store, load),
 221                         temporal_order(&opt_ctx.ctx, store, load),
 222                     ),
 223                 );
 224             }
 225
 226             match *potential_stores {
 227                 [] => {
 228                     #[cfg(debug_assertions)]
 229                     clif_comments
 230                         .add_comment(load, "[BUG?] Reading uninitialized memory".to_string());
 231                 }
 232                 [store]
 233                     if spatial_overlap(&opt_ctx.ctx.func, store, load) == SpatialOverlap::Full
 234                         && temporal_order(&opt_ctx.ctx, store, load)
 235                             == TemporalOrder::DefinitivelyBefore =>
 236                 {
 237                     // Only one store could have been the origin of the value.
 238                     let stored_value = opt_ctx.ctx.func.dfg.inst_args(store)[0];
 239
 240                     #[cfg(debug_assertions)]
 241                     clif_comments
 242                         .add_comment(load, format!("Store to load forward {} -> {}", store, load));
 243
 244                     users.change_load_to_alias(&mut opt_ctx.ctx.func, load, stored_value);
 245                 }
 246                 _ => {} // FIXME implement this
 247             }
 248         }
 249
 250         for store in users.stack_store.clone().into_iter() {
 251             let potential_loads = users.potential_loads_of_store(&opt_ctx.ctx, store);
 252
 253             #[cfg(debug_assertions)]
 254             for &load in &potential_loads {
 255                 clif_comments.add_comment(
 256                     store,
 257                     format!(
 258                         "Potential load from store {} <- {} ({:?}, {:?})",
 259                         opt_ctx.ctx.func.dfg.display_inst(load, None),
 260                         opt_ctx.ctx.func.dfg.display_inst(store, None),
 261                         spatial_overlap(&opt_ctx.ctx.func, store, load),
 262                         temporal_order(&opt_ctx.ctx, store, load),
 263                     ),
 264                 );
 265             }
 266
 267             if potential_loads.is_empty() {
 268                 // Never loaded; can safely remove all stores and the stack slot.
 269                 // FIXME also remove stores when there is always a next store before a load.
 270
 271                 #[cfg(debug_assertions)]
 272                 clif_comments.add_comment(
 273                     store,
 274                     format!(
 275                         "Remove dead stack store {} of {}",
 276                         opt_ctx.ctx.func.dfg.display_inst(store, None),
 277                         stack_slot.0
 278                     ),
 279                 );
 280
 281                 users.remove_dead_store(&mut opt_ctx.ctx.func, store);
 282             }
 283         }
 284
 285         if users.stack_store.is_empty() && users.stack_load.is_empty() {
 286             opt_ctx.ctx.func.stack_slots[stack_slot.0].size = 0;
 287         }
 288     }
 289 }
 290
 291 fn combine_stack_addr_with_load_store(func: &mut Function) {
 292     // Turn load and store into stack_load and stack_store when possible.
 293     let mut cursor = FuncCursor::new(func);
 294     while let Some(_block) = cursor.next_block() {
 295         while let Some(inst) = cursor.next_inst() {
 296             match cursor.func.dfg[inst] {
 297                 InstructionData::Load { opcode: Opcode::Load, arg: addr, flags: _, offset } => {
 298                     if cursor.func.dfg.ctrl_typevar(inst) == types::I128
 299                         || cursor.func.dfg.ctrl_typevar(inst).is_vector()
 300                     {
 301                         continue; // WORKAROUD: stack_load.i128 not yet implemented
 302                     }
 303                     if let Some((stack_slot, stack_addr_offset)) =
 304                         try_get_stack_slot_and_offset_for_addr(cursor.func, addr)
 305                     {
 306                         if let Some(combined_offset) = offset.try_add_i64(stack_addr_offset.into())
 307                         {
 308                             let ty = cursor.func.dfg.ctrl_typevar(inst);
 309                             cursor.func.dfg.replace(inst).stack_load(
 310                                 ty,
 311                                 stack_slot,
 312                                 combined_offset,
 313                             );
 314                         }
 315                     }
 316                 }
 317                 InstructionData::Store {
 318                     opcode: Opcode::Store,
 319                     args: [value, addr],
 320                     flags: _,
 321                     offset,
 322                 } => {
 323                     if cursor.func.dfg.ctrl_typevar(inst) == types::I128
 324                         || cursor.func.dfg.ctrl_typevar(inst).is_vector()
 325                     {
 326                         continue; // WORKAROUND: stack_store.i128 not yet implemented
 327                     }
 328                     if let Some((stack_slot, stack_addr_offset)) =
 329                         try_get_stack_slot_and_offset_for_addr(cursor.func, addr)
 330                     {
 331                         if let Some(combined_offset) = offset.try_add_i64(stack_addr_offset.into())
 332                         {
 333                             cursor.func.dfg.replace(inst).stack_store(
 334                                 value,
 335                                 stack_slot,
 336                                 combined_offset,
 337                             );
 338                         }
 339                     }
 340                 }
 341                 _ => {}
 342             }
 343         }
 344     }
 345 }
 346
 347 fn remove_unused_stack_addr_and_stack_load(opt_ctx: &mut OptimizeContext<'_>) {
 348     // FIXME incrementally rebuild on each call?
 349     let mut stack_addr_load_insts_users = FxHashMap::<Inst, FxHashSet<Inst>>::default();
 350
 351     let mut cursor = FuncCursor::new(&mut opt_ctx.ctx.func);
 352     while let Some(_block) = cursor.next_block() {
 353         while let Some(inst) = cursor.next_inst() {
 354             for &arg in cursor.func.dfg.inst_args(inst) {
 355                 if let ValueDef::Result(arg_origin, 0) = cursor.func.dfg.value_def(arg) {
 356                     match cursor.func.dfg[arg_origin].opcode() {
 357                         Opcode::StackAddr | Opcode::StackLoad => {
 358                             stack_addr_load_insts_users
 359                                 .entry(arg_origin)
 360                                 .or_insert_with(FxHashSet::default)
 361                                 .insert(inst);
 362                         }
 363                         _ => {}
 364                     }
 365                 }
 366             }
 367         }
 368     }
 369
 370     #[cfg(debug_assertions)]
 371     for inst in stack_addr_load_insts_users.keys() {
 372         let mut is_recorded_stack_addr_or_stack_load = false;
 373         for stack_slot_users in opt_ctx.stack_slot_usage_map.values() {
 374             is_recorded_stack_addr_or_stack_load |= stack_slot_users.stack_addr.contains(inst)
 375                 || stack_slot_users.stack_load.contains(inst);
 376         }
 377         assert!(is_recorded_stack_addr_or_stack_load);
 378     }
 379
 380     // Replace all unused stack_addr and stack_load instructions with nop.
 381     let mut func = &mut opt_ctx.ctx.func;
 382
 383     for stack_slot_users in opt_ctx.stack_slot_usage_map.values_mut() {
 384         stack_slot_users
 385             .stack_addr
 386             .drain_filter(|inst| {
 387                 stack_addr_load_insts_users.get(inst).map(|users| users.is_empty()).unwrap_or(true)
 388             })
 389             .for_each(|inst| StackSlotUsage::remove_unused_stack_addr(&mut func, inst));
 390
 391         stack_slot_users
 392             .stack_load
 393             .drain_filter(|inst| {
 394                 stack_addr_load_insts_users.get(inst).map(|users| users.is_empty()).unwrap_or(true)
 395             })
 396             .for_each(|inst| StackSlotUsage::remove_unused_load(&mut func, inst));
 397     }
 398 }
 399
 400 fn try_get_stack_slot_and_offset_for_addr(
 401     func: &Function,
 402     addr: Value,
 403 ) -> Option<(StackSlot, Offset32)> {
 404     if let ValueDef::Result(addr_inst, 0) = func.dfg.value_def(addr) {
 405         if let InstructionData::StackLoad { opcode: Opcode::StackAddr, stack_slot, offset } =
 406             func.dfg[addr_inst]
 407         {
 408             return Some((stack_slot, offset));
 409         }
 410     }
 411     None
 412 }
 413
 414 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 415 enum SpatialOverlap {
 416     No,
 417     Partial,
 418     Full,
 419 }
 420
 421 fn spatial_overlap(func: &Function, src: Inst, dest: Inst) -> SpatialOverlap {
 422     fn inst_info(func: &Function, inst: Inst) -> (StackSlot, Offset32, u32) {
 423         match func.dfg[inst] {
 424             InstructionData::StackLoad { opcode: Opcode::StackAddr, stack_slot, offset }
 425             | InstructionData::StackLoad { opcode: Opcode::StackLoad, stack_slot, offset }
 426             | InstructionData::StackStore {
 427                 opcode: Opcode::StackStore,
 428                 stack_slot,
 429                 offset,
 430                 arg: _,
 431             } => (stack_slot, offset, func.dfg.ctrl_typevar(inst).bytes()),
 432             _ => unreachable!("{:?}", func.dfg[inst]),
 433         }
 434     }
 435
 436     debug_assert_ne!(src, dest);
 437
 438     let (src_ss, src_offset, src_size) = inst_info(func, src);
 439     let (dest_ss, dest_offset, dest_size) = inst_info(func, dest);
 440
 441     if src_ss != dest_ss {
 442         return SpatialOverlap::No;
 443     }
 444
 445     if src_offset == dest_offset && src_size == dest_size {
 446         return SpatialOverlap::Full;
 447     }
 448
 449     let src_end: i64 = src_offset.try_add_i64(i64::from(src_size)).unwrap().into();
 450     let dest_end: i64 = dest_offset.try_add_i64(i64::from(dest_size)).unwrap().into();
 451     if src_end <= dest_offset.into() || dest_end <= src_offset.into() {
 452         return SpatialOverlap::No;
 453     }
 454
 455     SpatialOverlap::Partial
 456 }
 457
 458 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 459 enum TemporalOrder {
 460     /// `src` will never be executed before `dest`.
 461     NeverBefore,
 462
 463     /// `src` may be executed before `dest`.
 464     MaybeBefore,
 465
 466     /// `src` will always be executed before `dest`.
 467     /// There may still be other instructions in between.
 468     DefinitivelyBefore,
 469 }
 470
 471 fn temporal_order(ctx: &Context, src: Inst, dest: Inst) -> TemporalOrder {
 472     debug_assert_ne!(src, dest);
 473
 474     if ctx.domtree.dominates(src, dest, &ctx.func.layout) {
 475         TemporalOrder::DefinitivelyBefore
 476     } else if ctx.domtree.dominates(src, dest, &ctx.func.layout) {
 477         TemporalOrder::NeverBefore
 478     } else {
 479         TemporalOrder::MaybeBefore
 480     }
 481 }