Source file src/internal/runtime/maps/map.go
1 // Copyright 2024 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 // Package maps implements Go's builtin map type. 6 package maps 7 8 import ( 9 "internal/abi" 10 "internal/goarch" 11 "internal/runtime/math" 12 "internal/runtime/sys" 13 "unsafe" 14 ) 15 16 // This package contains the implementation of Go's builtin map type. 17 // 18 // The map design is based on Abseil's "Swiss Table" map design 19 // (https://abseil.io/about/design/swisstables), with additional modifications 20 // to cover Go's additional requirements, discussed below. 21 // 22 // Terminology: 23 // - Slot: A storage location of a single key/element pair. 24 // - Group: A group of abi.MapGroupSlots (8) slots, plus a control word. 25 // - Control word: An 8-byte word which denotes whether each slot is empty, 26 // deleted, or used. If a slot is used, its control byte also contains the 27 // lower 7 bits of the hash (H2). 28 // - H1: Upper 57 bits of a hash. 29 // - H2: Lower 7 bits of a hash. 30 // - Table: A complete "Swiss Table" hash table. A table consists of one or 31 // more groups for storage plus metadata to handle operation and determining 32 // when to grow. 33 // - Map: The top-level Map type consists of zero or more tables for storage. 34 // The upper bits of the hash select which table a key belongs to. 35 // - Directory: Array of the tables used by the map. 36 // 37 // At its core, the table design is similar to a traditional open-addressed 38 // hash table. Storage consists of an array of groups, which effectively means 39 // an array of key/elem slots with some control words interspersed. Lookup uses 40 // the hash to determine an initial group to check. If, due to collisions, this 41 // group contains no match, the probe sequence selects the next group to check 42 // (see below for more detail about the probe sequence). 43 // 44 // The key difference occurs within a group. In a standard open-addressed 45 // linear probed hash table, we would check each slot one at a time to find a 46 // match. A swiss table utilizes the extra control word to check all 8 slots in 47 // parallel. 48 // 49 // Each byte in the control word corresponds to one of the slots in the group. 50 // In each byte, 1 bit is used to indicate whether the slot is in use, or if it 51 // is empty/deleted. The other 7 bits contain the lower 7 bits of the hash for 52 // the key in that slot. See [ctrl] for the exact encoding. 53 // 54 // During lookup, we can use some clever bitwise manipulation to compare all 8 55 // 7-bit hashes against the input hash in parallel (see [ctrlGroup.matchH2]). 56 // That is, we effectively perform 8 steps of probing in a single operation. 57 // With SIMD instructions, this could be extended to 16 slots with a 16-byte 58 // control word. 59 // 60 // Since we only use 7 bits of the 64 bit hash, there is a 1 in 128 (~0.7%) 61 // probability of false positive on each slot, but that's fine: we always need 62 // double check each match with a standard key comparison regardless. 63 // 64 // Probing 65 // 66 // Probing is done using the upper 57 bits (H1) of the hash as an index into 67 // the groups array. Probing walks through the groups using quadratic probing 68 // until it finds a group with a match or a group with an empty slot. See 69 // [probeSeq] for specifics about the probe sequence. Note the probe 70 // invariants: the number of groups must be a power of two, and the end of a 71 // probe sequence must be a group with an empty slot (the table can never be 72 // 100% full). 73 // 74 // Deletion 75 // 76 // Probing stops when it finds a group with an empty slot. This affects 77 // deletion: when deleting from a completely full group, we must not mark the 78 // slot as empty, as there could be more slots used later in a probe sequence 79 // and this deletion would cause probing to stop too early. Instead, we mark 80 // such slots as "deleted" with a tombstone. If the group still has an empty 81 // slot, we don't need a tombstone and directly mark the slot empty. Insert 82 // prioritizes reuse of tombstones over filling an empty slots. Otherwise, 83 // tombstones are only completely cleared during grow, as an in-place cleanup 84 // complicates iteration. 85 // 86 // Growth 87 // 88 // The probe sequence depends on the number of groups. Thus, when growing the 89 // group count all slots must be reordered to match the new probe sequence. In 90 // other words, an entire table must be grown at once. 91 // 92 // In order to support incremental growth, the map splits its contents across 93 // multiple tables. Each table is still a full hash table, but an individual 94 // table may only service a subset of the hash space. Growth occurs on 95 // individual tables, so while an entire table must grow at once, each of these 96 // grows is only a small portion of a map. The maximum size of a single grow is 97 // limited by limiting the maximum size of a table before it is split into 98 // multiple tables. 99 // 100 // A map starts with a single table. Up to [maxTableCapacity], growth simply 101 // replaces this table with a replacement with double capacity. Beyond this 102 // limit, growth splits the table into two. 103 // 104 // The map uses "extendible hashing" to select which table to use. In 105 // extendible hashing, we use the upper bits of the hash as an index into an 106 // array of tables (called the "directory"). The number of bits uses increases 107 // as the number of tables increases. For example, when there is only 1 table, 108 // we use 0 bits (no selection necessary). When there are 2 tables, we use 1 109 // bit to select either the 0th or 1st table. [Map.globalDepth] is the number 110 // of bits currently used for table selection, and by extension (1 << 111 // globalDepth), the size of the directory. 112 // 113 // Note that each table has its own load factor and grows independently. If the 114 // 1st bucket grows, it will split. We'll need 2 bits to select tables, though 115 // we'll have 3 tables total rather than 4. We support this by allowing 116 // multiple indices to point to the same table. This example: 117 // 118 // directory (globalDepth=2) 119 // +----+ 120 // | 00 | --\ 121 // +----+ +--> table (localDepth=1) 122 // | 01 | --/ 123 // +----+ 124 // | 10 | ------> table (localDepth=2) 125 // +----+ 126 // | 11 | ------> table (localDepth=2) 127 // +----+ 128 // 129 // Tables track the depth they were created at (localDepth). It is necessary to 130 // grow the directory when splitting a table where globalDepth == localDepth. 131 // 132 // Iteration 133 // 134 // Iteration is the most complex part of the map due to Go's generous iteration 135 // semantics. A summary of semantics from the spec: 136 // 1. Adding and/or deleting entries during iteration MUST NOT cause iteration 137 // to return the same entry more than once. 138 // 2. Entries added during iteration MAY be returned by iteration. 139 // 3. Entries modified during iteration MUST return their latest value. 140 // 4. Entries deleted during iteration MUST NOT be returned by iteration. 141 // 5. Iteration order is unspecified. In the implementation, it is explicitly 142 // randomized. 143 // 144 // If the map never grows, these semantics are straightforward: just iterate 145 // over every table in the directory and every group and slot in each table. 146 // These semantics all land as expected. 147 // 148 // If the map grows during iteration, things complicate significantly. First 149 // and foremost, we need to track which entries we already returned to satisfy 150 // (1). There are three types of grow: 151 // a. A table replaced by a single larger table. 152 // b. A table split into two replacement tables. 153 // c. Growing the directory (occurs as part of (b) if necessary). 154 // 155 // For all of these cases, the replacement table(s) will have a different probe 156 // sequence, so simply tracking the current group and slot indices is not 157 // sufficient. 158 // 159 // For (a) and (b), note that grows of tables other than the one we are 160 // currently iterating over are irrelevant. 161 // 162 // We handle (a) and (b) by having the iterator keep a reference to the table 163 // it is currently iterating over, even after the table is replaced. We keep 164 // iterating over the original table to maintain the iteration order and avoid 165 // violating (1). Any new entries added only to the replacement table(s) will 166 // be skipped (allowed by (2)). To avoid violating (3) or (4), while we use the 167 // original table to select the keys, we must look them up again in the new 168 // table(s) to determine if they have been modified or deleted. There is yet 169 // another layer of complexity if the key does not compare equal itself. See 170 // [Iter.Next] for the gory details. 171 // 172 // Note that for (b) once we finish iterating over the old table we'll need to 173 // skip the next entry in the directory, as that contains the second split of 174 // the old table. We can use the old table's localDepth to determine the next 175 // logical index to use. 176 // 177 // For (b), we must adjust the current directory index when the directory 178 // grows. This is more straightforward, as the directory orders remains the 179 // same after grow, so we just double the index if the directory size doubles. 180 181 // Extracts the H1 portion of a hash: the 57 upper bits. 182 // TODO(prattmic): what about 32-bit systems? 183 func h1(h uintptr) uintptr { 184 return h >> 7 185 } 186 187 // Extracts the H2 portion of a hash: the 7 bits not used for h1. 188 // 189 // These are used as an occupied control byte. 190 func h2(h uintptr) uintptr { 191 return h & 0x7f 192 } 193 194 // Note: changes here must be reflected in cmd/compile/internal/reflectdata/map.go:MapType. 195 type Map struct { 196 // The number of filled slots (i.e. the number of elements in all 197 // tables). Excludes deleted slots. 198 // Must be first (known by the compiler, for len() builtin). 199 used uint64 200 201 // seed is the hash seed, computed as a unique random number per map. 202 seed uintptr 203 204 // The directory of tables. 205 // 206 // Normally dirPtr points to an array of table pointers 207 // 208 // dirPtr *[dirLen]*table 209 // 210 // The length (dirLen) of this array is `1 << globalDepth`. Multiple 211 // entries may point to the same table. See top-level comment for more 212 // details. 213 // 214 // Small map optimization: if the map always contained 215 // abi.MapGroupSlots or fewer entries, it fits entirely in a 216 // single group. In that case dirPtr points directly to a single group. 217 // 218 // dirPtr *group 219 // 220 // In this case, dirLen is 0. used counts the number of used slots in 221 // the group. Note that small maps never have deleted slots (as there 222 // is no probe sequence to maintain). 223 dirPtr unsafe.Pointer 224 dirLen int 225 226 // The number of bits to use in table directory lookups. 227 globalDepth uint8 228 229 // The number of bits to shift out of the hash for directory lookups. 230 // On 64-bit systems, this is 64 - globalDepth. 231 globalShift uint8 232 233 // writing is a flag that is toggled (XOR 1) while the map is being 234 // written. Normally it is set to 1 when writing, but if there are 235 // multiple concurrent writers, then toggling increases the probability 236 // that both sides will detect the race. 237 writing uint8 238 239 // tombstonePossible is false if we know that no table in this map 240 // contains a tombstone. 241 tombstonePossible bool 242 243 // clearSeq is a sequence counter of calls to Clear. It is used to 244 // detect map clears during iteration. 245 clearSeq uint64 246 } 247 248 func depthToShift(depth uint8) uint8 { 249 if goarch.PtrSize == 4 { 250 return 32 - depth 251 } 252 return 64 - depth 253 } 254 255 // If m is non-nil, it should be used rather than allocating. 256 // 257 // maxAlloc should be runtime.maxAlloc. 258 // 259 // TODO(prattmic): Put maxAlloc somewhere accessible. 260 func NewMap(mt *abi.MapType, hint uintptr, m *Map, maxAlloc uintptr) *Map { 261 if m == nil { 262 m = new(Map) 263 } 264 265 m.seed = uintptr(rand()) 266 267 if hint <= abi.MapGroupSlots { 268 // A small map can fill all 8 slots, so no need to increase 269 // target capacity. 270 // 271 // In fact, since an 8 slot group is what the first assignment 272 // to an empty map would allocate anyway, it doesn't matter if 273 // we allocate here or on the first assignment. 274 // 275 // Thus we just return without allocating. (We'll save the 276 // allocation completely if no assignment comes.) 277 278 // Note that the compiler may have initialized m.dirPtr with a 279 // pointer to a stack-allocated group, in which case we already 280 // have a group. The control word is already initialized. 281 282 return m 283 } 284 285 // Full size map. 286 287 // Set initial capacity to hold hint entries without growing in the 288 // average case. 289 targetCapacity := (hint * abi.MapGroupSlots) / maxAvgGroupLoad 290 if targetCapacity < hint { // overflow 291 return m // return an empty map. 292 } 293 294 dirSize := (uint64(targetCapacity) + maxTableCapacity - 1) / maxTableCapacity 295 dirSize, overflow := alignUpPow2(dirSize) 296 if overflow || dirSize > uint64(math.MaxUintptr) { 297 return m // return an empty map. 298 } 299 300 // Reject hints that are obviously too large. 301 groups, overflow := math.MulUintptr(uintptr(dirSize), maxTableCapacity) 302 if overflow { 303 return m // return an empty map. 304 } else { 305 mem, overflow := math.MulUintptr(groups, mt.GroupSize) 306 if overflow || mem > maxAlloc { 307 return m // return an empty map. 308 } 309 } 310 311 m.globalDepth = uint8(sys.TrailingZeros64(dirSize)) 312 m.globalShift = depthToShift(m.globalDepth) 313 314 directory := make([]*table, dirSize) 315 316 for i := range directory { 317 // TODO: Think more about initial table capacity. 318 directory[i] = newTable(mt, uint64(targetCapacity)/dirSize, i, m.globalDepth) 319 } 320 321 m.dirPtr = unsafe.Pointer(&directory[0]) 322 m.dirLen = len(directory) 323 324 return m 325 } 326 327 func NewEmptyMap() *Map { 328 m := new(Map) 329 m.seed = uintptr(rand()) 330 // See comment in NewMap. No need to eager allocate a group. 331 return m 332 } 333 334 func (m *Map) directoryIndex(hash uintptr) uintptr { 335 if m.dirLen == 1 { 336 return 0 337 } 338 return hash >> (m.globalShift & 63) 339 } 340 341 func (m *Map) directoryAt(i uintptr) *table { 342 return *(**table)(unsafe.Pointer(uintptr(m.dirPtr) + goarch.PtrSize*i)) 343 } 344 345 func (m *Map) directorySet(i uintptr, nt *table) { 346 *(**table)(unsafe.Pointer(uintptr(m.dirPtr) + goarch.PtrSize*i)) = nt 347 } 348 349 func (m *Map) replaceTable(nt *table) { 350 // The number of entries that reference the same table doubles for each 351 // time the globalDepth grows without the table splitting. 352 entries := 1 << (m.globalDepth - nt.localDepth) 353 for i := 0; i < entries; i++ { 354 //m.directory[nt.index+i] = nt 355 m.directorySet(uintptr(nt.index+i), nt) 356 } 357 } 358 359 func (m *Map) installTableSplit(old, left, right *table) { 360 if old.localDepth == m.globalDepth { 361 // No room for another level in the directory. Grow the 362 // directory. 363 newDir := make([]*table, m.dirLen*2) 364 for i := range m.dirLen { 365 t := m.directoryAt(uintptr(i)) 366 newDir[2*i] = t 367 newDir[2*i+1] = t 368 // t may already exist in multiple indices. We should 369 // only update t.index once. Since the index must 370 // increase, seeing the original index means this must 371 // be the first time we've encountered this table. 372 if t.index == i { 373 t.index = 2 * i 374 } 375 } 376 m.globalDepth++ 377 m.globalShift-- 378 //m.directory = newDir 379 m.dirPtr = unsafe.Pointer(&newDir[0]) 380 m.dirLen = len(newDir) 381 } 382 383 // N.B. left and right may still consume multiple indices if the 384 // directory has grown multiple times since old was last split. 385 left.index = old.index 386 m.replaceTable(left) 387 388 entries := 1 << (m.globalDepth - left.localDepth) 389 right.index = left.index + entries 390 m.replaceTable(right) 391 } 392 393 func (m *Map) Used() uint64 { 394 return m.used 395 } 396 397 // Get performs a lookup of the key that key points to. It returns a pointer to 398 // the element, or false if the key doesn't exist. 399 func (m *Map) Get(typ *abi.MapType, key unsafe.Pointer) (unsafe.Pointer, bool) { 400 return m.getWithoutKey(typ, key) 401 } 402 403 func (m *Map) getWithKey(typ *abi.MapType, key unsafe.Pointer) (unsafe.Pointer, unsafe.Pointer, bool) { 404 if m.Used() == 0 { 405 return nil, nil, false 406 } 407 408 if m.writing != 0 { 409 fatal("concurrent map read and map write") 410 } 411 412 hash := typ.Hasher(key, m.seed) 413 414 if m.dirLen == 0 { 415 return m.getWithKeySmall(typ, hash, key) 416 } 417 418 idx := m.directoryIndex(hash) 419 return m.directoryAt(idx).getWithKey(typ, hash, key) 420 } 421 422 func (m *Map) getWithoutKey(typ *abi.MapType, key unsafe.Pointer) (unsafe.Pointer, bool) { 423 if m.Used() == 0 { 424 return nil, false 425 } 426 427 if m.writing != 0 { 428 fatal("concurrent map read and map write") 429 } 430 431 hash := typ.Hasher(key, m.seed) 432 433 if m.dirLen == 0 { 434 _, elem, ok := m.getWithKeySmall(typ, hash, key) 435 return elem, ok 436 } 437 438 idx := m.directoryIndex(hash) 439 return m.directoryAt(idx).getWithoutKey(typ, hash, key) 440 } 441 442 func (m *Map) getWithKeySmall(typ *abi.MapType, hash uintptr, key unsafe.Pointer) (unsafe.Pointer, unsafe.Pointer, bool) { 443 g := groupReference{ 444 data: m.dirPtr, 445 } 446 447 match := g.ctrls().matchH2(h2(hash)) 448 449 for match != 0 { 450 i := match.first() 451 452 slotKey := g.key(typ, i) 453 if typ.IndirectKey() { 454 slotKey = *((*unsafe.Pointer)(slotKey)) 455 } 456 457 if typ.Key.Equal(key, slotKey) { 458 slotElem := g.elem(typ, i) 459 if typ.IndirectElem() { 460 slotElem = *((*unsafe.Pointer)(slotElem)) 461 } 462 return slotKey, slotElem, true 463 } 464 465 match = match.removeFirst() 466 } 467 468 // No match here means key is not in the map. 469 // (A single group means no need to probe or check for empty). 470 return nil, nil, false 471 } 472 473 func (m *Map) Put(typ *abi.MapType, key, elem unsafe.Pointer) { 474 slotElem := m.PutSlot(typ, key) 475 typedmemmove(typ.Elem, slotElem, elem) 476 } 477 478 // PutSlot returns a pointer to the element slot where an inserted element 479 // should be written. 480 // 481 // PutSlot never returns nil. 482 func (m *Map) PutSlot(typ *abi.MapType, key unsafe.Pointer) unsafe.Pointer { 483 if m.writing != 0 { 484 fatal("concurrent map writes") 485 } 486 487 hash := typ.Hasher(key, m.seed) 488 489 // Set writing after calling Hasher, since Hasher may panic, in which 490 // case we have not actually done a write. 491 m.writing ^= 1 // toggle, see comment on writing 492 493 if m.dirPtr == nil { 494 m.growToSmall(typ) 495 } 496 497 if m.dirLen == 0 { 498 if m.used < abi.MapGroupSlots { 499 elem := m.putSlotSmall(typ, hash, key) 500 501 if m.writing == 0 { 502 fatal("concurrent map writes") 503 } 504 m.writing ^= 1 505 506 return elem 507 } 508 509 // Can't fit another entry, grow to full size map. 510 // 511 // TODO(prattmic): If this is an update to an existing key then 512 // we actually don't need to grow. 513 m.growToTable(typ) 514 } 515 516 for { 517 idx := m.directoryIndex(hash) 518 elem, ok := m.directoryAt(idx).PutSlot(typ, m, hash, key) 519 if !ok { 520 continue 521 } 522 523 if m.writing == 0 { 524 fatal("concurrent map writes") 525 } 526 m.writing ^= 1 527 528 return elem 529 } 530 } 531 532 func (m *Map) putSlotSmall(typ *abi.MapType, hash uintptr, key unsafe.Pointer) unsafe.Pointer { 533 g := groupReference{ 534 data: m.dirPtr, 535 } 536 537 match := g.ctrls().matchH2(h2(hash)) 538 539 // Look for an existing slot containing this key. 540 for match != 0 { 541 i := match.first() 542 543 slotKey := g.key(typ, i) 544 if typ.IndirectKey() { 545 slotKey = *((*unsafe.Pointer)(slotKey)) 546 } 547 if typ.Key.Equal(key, slotKey) { 548 if typ.NeedKeyUpdate() { 549 typedmemmove(typ.Key, slotKey, key) 550 } 551 552 slotElem := g.elem(typ, i) 553 if typ.IndirectElem() { 554 slotElem = *((*unsafe.Pointer)(slotElem)) 555 } 556 557 return slotElem 558 } 559 match = match.removeFirst() 560 } 561 562 // There can't be deleted slots, small maps can't have them 563 // (see deleteSmall). Use matchEmptyOrDeleted as it is a bit 564 // more efficient than matchEmpty. 565 match = g.ctrls().matchEmptyOrDeleted() 566 if match == 0 { 567 fatal("small map with no empty slot (concurrent map writes?)") 568 return nil 569 } 570 571 i := match.first() 572 573 slotKey := g.key(typ, i) 574 if typ.IndirectKey() { 575 kmem := newobject(typ.Key) 576 *(*unsafe.Pointer)(slotKey) = kmem 577 slotKey = kmem 578 } 579 typedmemmove(typ.Key, slotKey, key) 580 581 slotElem := g.elem(typ, i) 582 if typ.IndirectElem() { 583 emem := newobject(typ.Elem) 584 *(*unsafe.Pointer)(slotElem) = emem 585 slotElem = emem 586 } 587 588 g.ctrls().set(i, ctrl(h2(hash))) 589 m.used++ 590 591 return slotElem 592 } 593 594 func (m *Map) growToSmall(typ *abi.MapType) { 595 grp := newGroups(typ, 1) 596 m.dirPtr = grp.data 597 598 g := groupReference{ 599 data: m.dirPtr, 600 } 601 g.ctrls().setEmpty() 602 } 603 604 func (m *Map) growToTable(typ *abi.MapType) { 605 tab := newTable(typ, 2*abi.MapGroupSlots, 0, 0) 606 607 g := groupReference{ 608 data: m.dirPtr, 609 } 610 611 for i := uintptr(0); i < abi.MapGroupSlots; i++ { 612 if (g.ctrls().get(i) & ctrlEmpty) == ctrlEmpty { 613 // Empty 614 continue 615 } 616 617 key := g.key(typ, i) 618 if typ.IndirectKey() { 619 key = *((*unsafe.Pointer)(key)) 620 } 621 622 elem := g.elem(typ, i) 623 if typ.IndirectElem() { 624 elem = *((*unsafe.Pointer)(elem)) 625 } 626 627 hash := typ.Hasher(key, m.seed) 628 629 tab.uncheckedPutSlot(typ, hash, key, elem) 630 } 631 632 directory := make([]*table, 1) 633 634 directory[0] = tab 635 636 m.dirPtr = unsafe.Pointer(&directory[0]) 637 m.dirLen = len(directory) 638 639 m.globalDepth = 0 640 m.globalShift = depthToShift(m.globalDepth) 641 } 642 643 func (m *Map) Delete(typ *abi.MapType, key unsafe.Pointer) { 644 if m == nil || m.Used() == 0 { 645 if err := mapKeyError(typ, key); err != nil { 646 panic(err) // see issue 23734 647 } 648 return 649 } 650 651 if m.writing != 0 { 652 fatal("concurrent map writes") 653 } 654 655 hash := typ.Hasher(key, m.seed) 656 657 // Set writing after calling Hasher, since Hasher may panic, in which 658 // case we have not actually done a write. 659 m.writing ^= 1 // toggle, see comment on writing 660 661 if m.dirLen == 0 { 662 m.deleteSmall(typ, hash, key) 663 } else { 664 idx := m.directoryIndex(hash) 665 if m.directoryAt(idx).Delete(typ, m, hash, key) { 666 m.tombstonePossible = true 667 } 668 } 669 670 if m.used == 0 { 671 // Reset the hash seed to make it more difficult for attackers 672 // to repeatedly trigger hash collisions. See 673 // https://go.dev/issue/25237. 674 m.seed = uintptr(rand()) 675 } 676 677 if m.writing == 0 { 678 fatal("concurrent map writes") 679 } 680 m.writing ^= 1 681 } 682 683 func (m *Map) deleteSmall(typ *abi.MapType, hash uintptr, key unsafe.Pointer) { 684 g := groupReference{ 685 data: m.dirPtr, 686 } 687 688 match := g.ctrls().matchH2(h2(hash)) 689 690 for match != 0 { 691 i := match.first() 692 slotKey := g.key(typ, i) 693 origSlotKey := slotKey 694 if typ.IndirectKey() { 695 slotKey = *((*unsafe.Pointer)(slotKey)) 696 } 697 if typ.Key.Equal(key, slotKey) { 698 m.used-- 699 700 if typ.IndirectKey() { 701 // Clearing the pointer is sufficient. 702 *(*unsafe.Pointer)(origSlotKey) = nil 703 } else if typ.Key.Pointers() { 704 // Only bother clearing if there are pointers. 705 typedmemclr(typ.Key, slotKey) 706 } 707 708 slotElem := g.elem(typ, i) 709 if typ.IndirectElem() { 710 // Clearing the pointer is sufficient. 711 *(*unsafe.Pointer)(slotElem) = nil 712 } else { 713 // Unlike keys, always clear the elem (even if 714 // it contains no pointers), as compound 715 // assignment operations depend on cleared 716 // deleted values. See 717 // https://go.dev/issue/25936. 718 typedmemclr(typ.Elem, slotElem) 719 } 720 721 // We only have 1 group, so it is OK to immediately 722 // reuse deleted slots. 723 g.ctrls().set(i, ctrlEmpty) 724 return 725 } 726 match = match.removeFirst() 727 } 728 } 729 730 // Clear deletes all entries from the map resulting in an empty map. 731 func (m *Map) Clear(typ *abi.MapType) { 732 if m == nil || m.Used() == 0 && !m.tombstonePossible { 733 return 734 } 735 736 if m.writing != 0 { 737 fatal("concurrent map writes") 738 } 739 m.writing ^= 1 // toggle, see comment on writing 740 741 if m.dirLen == 0 { 742 m.clearSmall(typ) 743 } else { 744 var lastTab *table 745 for i := range m.dirLen { 746 t := m.directoryAt(uintptr(i)) 747 if t == lastTab { 748 continue 749 } 750 t.Clear(typ) 751 lastTab = t 752 } 753 m.used = 0 754 m.tombstonePossible = false 755 // TODO: shrink directory? 756 } 757 m.clearSeq++ 758 759 // Reset the hash seed to make it more difficult for attackers to 760 // repeatedly trigger hash collisions. See https://go.dev/issue/25237. 761 m.seed = uintptr(rand()) 762 763 if m.writing == 0 { 764 fatal("concurrent map writes") 765 } 766 m.writing ^= 1 767 } 768 769 func (m *Map) clearSmall(typ *abi.MapType) { 770 g := groupReference{ 771 data: m.dirPtr, 772 } 773 774 typedmemclr(typ.Group, g.data) 775 g.ctrls().setEmpty() 776 777 m.used = 0 778 } 779 780 func (m *Map) Clone(typ *abi.MapType) *Map { 781 // Note: this should never be called with a nil map. 782 if m.writing != 0 { 783 fatal("concurrent map clone and map write") 784 } 785 786 // Shallow copy the Map structure. 787 m2 := new(Map) 788 *m2 = *m 789 m = m2 790 791 // We need to just deep copy the dirPtr field. 792 if m.dirPtr == nil { 793 // delayed group allocation, nothing to do. 794 } else if m.dirLen == 0 { 795 // Clone one group. 796 oldGroup := groupReference{data: m.dirPtr} 797 newGroup := groupReference{data: newGroups(typ, 1).data} 798 cloneGroup(typ, newGroup, oldGroup) 799 m.dirPtr = newGroup.data 800 } else { 801 // Clone each (different) table. 802 oldDir := unsafe.Slice((**table)(m.dirPtr), m.dirLen) 803 newDir := make([]*table, m.dirLen) 804 for i, t := range oldDir { 805 if i > 0 && t == oldDir[i-1] { 806 newDir[i] = newDir[i-1] 807 continue 808 } 809 newDir[i] = t.clone(typ) 810 } 811 m.dirPtr = unsafe.Pointer(&newDir[0]) 812 } 813 814 return m 815 } 816 817 func mapKeyError(t *abi.MapType, p unsafe.Pointer) error { 818 if !t.HashMightPanic() { 819 return nil 820 } 821 return mapKeyError2(t.Key, p) 822 } 823 824 func mapKeyError2(t *abi.Type, p unsafe.Pointer) error { 825 if t.TFlag&abi.TFlagRegularMemory != 0 { 826 return nil 827 } 828 switch t.Kind() { 829 case abi.Float32, abi.Float64, abi.Complex64, abi.Complex128, abi.String: 830 return nil 831 case abi.Interface: 832 i := (*abi.InterfaceType)(unsafe.Pointer(t)) 833 var t *abi.Type 834 var pdata *unsafe.Pointer 835 if len(i.Methods) == 0 { 836 a := (*abi.EmptyInterface)(p) 837 t = a.Type 838 if t == nil { 839 return nil 840 } 841 pdata = &a.Data 842 } else { 843 a := (*abi.NonEmptyInterface)(p) 844 if a.ITab == nil { 845 return nil 846 } 847 t = a.ITab.Type 848 pdata = &a.Data 849 } 850 851 if t.Equal == nil { 852 return unhashableTypeError{t} 853 } 854 855 if t.IsDirectIface() { 856 return mapKeyError2(t, unsafe.Pointer(pdata)) 857 } else { 858 return mapKeyError2(t, *pdata) 859 } 860 case abi.Array: 861 a := (*abi.ArrayType)(unsafe.Pointer(t)) 862 for i := uintptr(0); i < a.Len; i++ { 863 if err := mapKeyError2(a.Elem, unsafe.Pointer(uintptr(p)+i*a.Elem.Size_)); err != nil { 864 return err 865 } 866 } 867 return nil 868 case abi.Struct: 869 s := (*abi.StructType)(unsafe.Pointer(t)) 870 for _, f := range s.Fields { 871 if f.Name.IsBlank() { 872 continue 873 } 874 if err := mapKeyError2(f.Typ, unsafe.Pointer(uintptr(p)+f.Offset)); err != nil { 875 return err 876 } 877 } 878 return nil 879 default: 880 // Should never happen, keep this case for robustness. 881 return unhashableTypeError{t} 882 } 883 } 884 885 type unhashableTypeError struct{ typ *abi.Type } 886 887 func (unhashableTypeError) RuntimeError() {} 888 889 func (e unhashableTypeError) Error() string { return "hash of unhashable type: " + typeString(e.typ) } 890 891 // Pushed from runtime 892 // 893 //go:linkname typeString 894 func typeString(typ *abi.Type) string 895