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 {
 179             ctx,
 180             stack_slot_usage_map,
 181         }
 182     }
 183 }
 184
 185 pub(super) fn optimize_function(
 186     ctx: &mut Context,
 187     clif_comments: &mut crate::pretty_clif::CommentWriter,
 188 ) {
 189     combine_stack_addr_with_load_store(&mut ctx.func);
 190
 191     let mut opt_ctx = OptimizeContext::for_context(ctx);
 192
 193     // FIXME Repeat following instructions until fixpoint.
 194
 195     remove_unused_stack_addr_and_stack_load(&mut opt_ctx);
 196
 197     #[cfg(debug_assertions)]
 198     {
 199         for (&OrdStackSlot(stack_slot), usage) in &opt_ctx.stack_slot_usage_map {
 200             clif_comments.add_comment(stack_slot, format!("used by: {:?}", usage));
 201         }
 202     }
 203
 204     for (stack_slot, users) in opt_ctx.stack_slot_usage_map.iter_mut() {
 205         if users.stack_addr.is_empty().not() {
 206             // Stack addr leaked; there may be unknown loads and stores.
 207             // FIXME use stacked borrows to optimize
 208             continue;
 209         }
 210
 211         for load in users.stack_load.clone().into_iter() {
 212             let potential_stores = users.potential_stores_for_load(&opt_ctx.ctx, load);
 213
 214             #[cfg(debug_assertions)]
 215             for &store in &potential_stores {
 216                 clif_comments.add_comment(
 217                     load,
 218                     format!(
 219                         "Potential store -> load forwarding {} -> {} ({:?}, {:?})",
 220                         opt_ctx.ctx.func.dfg.display_inst(store, None),
 221                         opt_ctx.ctx.func.dfg.display_inst(load, None),
 222                         spatial_overlap(&opt_ctx.ctx.func, store, load),
 223                         temporal_order(&opt_ctx.ctx, store, load),
 224                     ),
 225                 );
 226             }
 227
 228             match *potential_stores {
 229                 [] => {
 230                     #[cfg(debug_assertions)]
 231                     clif_comments.add_comment(load, format!("[BUG?] Reading uninitialized memory"));
 232                 }
 233                 [store]
 234                     if spatial_overlap(&opt_ctx.ctx.func, store, load) == SpatialOverlap::Full
 235                         && temporal_order(&opt_ctx.ctx, store, load)
 236                             == TemporalOrder::DefinitivelyBefore =>
 237                 {
 238                     // Only one store could have been the origin of the value.
 239                     let stored_value = opt_ctx.ctx.func.dfg.inst_args(store)[0];
 240
 241                     #[cfg(debug_assertions)]
 242                     clif_comments
 243                         .add_comment(load, format!("Store to load forward {} -> {}", store, load));
 244
 245                     users.change_load_to_alias(&mut opt_ctx.ctx.func, load, stored_value);
 246                 }
 247                 _ => {} // FIXME implement this
 248             }
 249         }
 250
 251         for store in users.stack_store.clone().into_iter() {
 252             let potential_loads = users.potential_loads_of_store(&opt_ctx.ctx, store);
 253
 254             #[cfg(debug_assertions)]
 255             for &load in &potential_loads {
 256                 clif_comments.add_comment(
 257                     store,
 258                     format!(
 259                         "Potential load from store {} <- {} ({:?}, {:?})",
 260                         opt_ctx.ctx.func.dfg.display_inst(load, None),
 261                         opt_ctx.ctx.func.dfg.display_inst(store, None),
 262                         spatial_overlap(&opt_ctx.ctx.func, store, load),
 263                         temporal_order(&opt_ctx.ctx, store, load),
 264                     ),
 265                 );
 266             }
 267
 268             if potential_loads.is_empty() {
 269                 // Never loaded; can safely remove all stores and the stack slot.
 270                 // FIXME also remove stores when there is always a next store before a load.
 271
 272                 #[cfg(debug_assertions)]
 273                 clif_comments.add_comment(
 274                     store,
 275                     format!(
 276                         "Remove dead stack store {} of {}",
 277                         opt_ctx.ctx.func.dfg.display_inst(store, None),
 278                         stack_slot.0
 279                     ),
 280                 );
 281
 282                 users.remove_dead_store(&mut opt_ctx.ctx.func, store);
 283             }
 284         }
 285
 286         if users.stack_store.is_empty() && users.stack_load.is_empty() {
 287             opt_ctx.ctx.func.stack_slots[stack_slot.0].size = 0;
 288         }
 289     }
 290 }
 291
 292 fn combine_stack_addr_with_load_store(func: &mut Function) {
 293     // Turn load and store into stack_load and stack_store when possible.
 294     let mut cursor = FuncCursor::new(func);
 295     while let Some(_block) = cursor.next_block() {
 296         while let Some(inst) = cursor.next_inst() {
 297             match cursor.func.dfg[inst] {
 298                 InstructionData::Load {
 299                     opcode: Opcode::Load,
 300                     arg: addr,
 301                     flags: _,
 302                     offset,
 303                 } => {
 304                     if cursor.func.dfg.ctrl_typevar(inst) == types::I128
 305                         || cursor.func.dfg.ctrl_typevar(inst).is_vector()
 306                     {
 307                         continue; // WORKAROUD: stack_load.i128 not yet implemented
 308                     }
 309                     if let Some((stack_slot, stack_addr_offset)) =
 310                         try_get_stack_slot_and_offset_for_addr(cursor.func, addr)
 311                     {
 312                         if let Some(combined_offset) = offset.try_add_i64(stack_addr_offset.into())
 313                         {
 314                             let ty = cursor.func.dfg.ctrl_typevar(inst);
 315                             cursor.func.dfg.replace(inst).stack_load(
 316                                 ty,
 317                                 stack_slot,
 318                                 combined_offset,
 319                             );
 320                         }
 321                     }
 322                 }
 323                 InstructionData::Store {
 324                     opcode: Opcode::Store,
 325                     args: [value, addr],
 326                     flags: _,
 327                     offset,
 328                 } => {
 329                     if cursor.func.dfg.ctrl_typevar(inst) == types::I128
 330                         || cursor.func.dfg.ctrl_typevar(inst).is_vector()
 331                     {
 332                         continue; // WORKAROUND: stack_store.i128 not yet implemented
 333                     }
 334                     if let Some((stack_slot, stack_addr_offset)) =
 335                         try_get_stack_slot_and_offset_for_addr(cursor.func, addr)
 336                     {
 337                         if let Some(combined_offset) = offset.try_add_i64(stack_addr_offset.into())
 338                         {
 339                             cursor.func.dfg.replace(inst).stack_store(
 340                                 value,
 341                                 stack_slot,
 342                                 combined_offset,
 343                             );
 344                         }
 345                     }
 346                 }
 347                 _ => {}
 348             }
 349         }
 350     }
 351 }
 352
 353 fn remove_unused_stack_addr_and_stack_load(opt_ctx: &mut OptimizeContext<'_>) {
 354     // FIXME incrementally rebuild on each call?
 355     let mut stack_addr_load_insts_users = FxHashMap::<Inst, FxHashSet<Inst>>::default();
 356
 357     let mut cursor = FuncCursor::new(&mut opt_ctx.ctx.func);
 358     while let Some(_block) = cursor.next_block() {
 359         while let Some(inst) = cursor.next_inst() {
 360             for &arg in cursor.func.dfg.inst_args(inst) {
 361                 if let ValueDef::Result(arg_origin, 0) = cursor.func.dfg.value_def(arg) {
 362                     match cursor.func.dfg[arg_origin].opcode() {
 363                         Opcode::StackAddr | Opcode::StackLoad => {
 364                             stack_addr_load_insts_users
 365                                 .entry(arg_origin)
 366                                 .or_insert_with(FxHashSet::default)
 367                                 .insert(inst);
 368                         }
 369                         _ => {}
 370                     }
 371                 }
 372             }
 373         }
 374     }
 375
 376     #[cfg(debug_assertions)]
 377     for inst in stack_addr_load_insts_users.keys() {
 378         let mut is_recorded_stack_addr_or_stack_load = false;
 379         for stack_slot_users in opt_ctx.stack_slot_usage_map.values() {
 380             is_recorded_stack_addr_or_stack_load |= stack_slot_users.stack_addr.contains(inst)
 381                 || stack_slot_users.stack_load.contains(inst);
 382         }
 383         assert!(is_recorded_stack_addr_or_stack_load);
 384     }
 385
 386     // Replace all unused stack_addr and stack_load instructions with nop.
 387     let mut func = &mut opt_ctx.ctx.func;
 388
 389     for stack_slot_users in opt_ctx.stack_slot_usage_map.values_mut() {
 390         stack_slot_users
 391             .stack_addr
 392             .drain_filter(|inst| {
 393                 stack_addr_load_insts_users
 394                     .get(inst)
 395                     .map(|users| users.is_empty())
 396                     .unwrap_or(true)
 397             })
 398             .for_each(|inst| StackSlotUsage::remove_unused_stack_addr(&mut func, inst));
 399
 400         stack_slot_users
 401             .stack_load
 402             .drain_filter(|inst| {
 403                 stack_addr_load_insts_users
 404                     .get(inst)
 405                     .map(|users| users.is_empty())
 406                     .unwrap_or(true)
 407             })
 408             .for_each(|inst| StackSlotUsage::remove_unused_load(&mut func, inst));
 409     }
 410 }
 411
 412 fn try_get_stack_slot_and_offset_for_addr(
 413     func: &Function,
 414     addr: Value,
 415 ) -> Option<(StackSlot, Offset32)> {
 416     if let ValueDef::Result(addr_inst, 0) = func.dfg.value_def(addr) {
 417         if let InstructionData::StackLoad {
 418             opcode: Opcode::StackAddr,
 419             stack_slot,
 420             offset,
 421         } = func.dfg[addr_inst]
 422         {
 423             return Some((stack_slot, offset));
 424         }
 425     }
 426     None
 427 }
 428
 429 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 430 enum SpatialOverlap {
 431     No,
 432     Partial,
 433     Full,
 434 }
 435
 436 fn spatial_overlap(func: &Function, src: Inst, dest: Inst) -> SpatialOverlap {
 437     fn inst_info(func: &Function, inst: Inst) -> (StackSlot, Offset32, u32) {
 438         match func.dfg[inst] {
 439             InstructionData::StackLoad {
 440                 opcode: Opcode::StackAddr,
 441                 stack_slot,
 442                 offset,
 443             }
 444             | InstructionData::StackLoad {
 445                 opcode: Opcode::StackLoad,
 446                 stack_slot,
 447                 offset,
 448             }
 449             | InstructionData::StackStore {
 450                 opcode: Opcode::StackStore,
 451                 stack_slot,
 452                 offset,
 453                 arg: _,
 454             } => (stack_slot, offset, func.dfg.ctrl_typevar(inst).bytes()),
 455             _ => unreachable!("{:?}", func.dfg[inst]),
 456         }
 457     }
 458
 459     debug_assert_ne!(src, dest);
 460
 461     let (src_ss, src_offset, src_size) = inst_info(func, src);
 462     let (dest_ss, dest_offset, dest_size) = inst_info(func, dest);
 463
 464     if src_ss != dest_ss {
 465         return SpatialOverlap::No;
 466     }
 467
 468     if src_offset == dest_offset && src_size == dest_size {
 469         return SpatialOverlap::Full;
 470     }
 471
 472     let src_end: i64 = src_offset.try_add_i64(i64::from(src_size)).unwrap().into();
 473     let dest_end: i64 = dest_offset
 474         .try_add_i64(i64::from(dest_size))
 475         .unwrap()
 476         .into();
 477     if src_end <= dest_offset.into() || dest_end <= src_offset.into() {
 478         return SpatialOverlap::No;
 479     }
 480
 481     SpatialOverlap::Partial
 482 }
 483
 484 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 485 enum TemporalOrder {
 486     /// `src` will never be executed before `dest`.
 487     NeverBefore,
 488
 489     /// `src` may be executed before `dest`.
 490     MaybeBefore,
 491
 492     /// `src` will always be executed before `dest`.
 493     /// There may still be other instructions in between.
 494     DefinitivelyBefore,
 495 }
 496
 497 fn temporal_order(ctx: &Context, src: Inst, dest: Inst) -> TemporalOrder {
 498     debug_assert_ne!(src, dest);
 499
 500     if ctx.domtree.dominates(src, dest, &ctx.func.layout) {
 501         TemporalOrder::DefinitivelyBefore
 502     } else if ctx.domtree.dominates(src, dest, &ctx.func.layout) {
 503         TemporalOrder::NeverBefore
 504     } else {
 505         TemporalOrder::MaybeBefore
 506     }
 507 }