1 // Copyright 2009 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 // Garbage collector: marking and scanning 6 7 package runtime 8 9 import ( 10 "runtime/internal/atomic" 11 "runtime/internal/sys" 12 "unsafe" 13 ) 14 15 const ( 16 fixedRootFinalizers = iota 17 fixedRootFreeGStacks 18 fixedRootCount 19 20 // rootBlockBytes is the number of bytes to scan per data or 21 // BSS root. 22 rootBlockBytes = 256 << 10 23 24 // rootBlockSpans is the number of spans to scan per span 25 // root. 26 rootBlockSpans = 8 * 1024 // 64MB worth of spans 27 28 // maxObletBytes is the maximum bytes of an object to scan at 29 // once. Larger objects will be split up into "oblets" of at 30 // most this size. Since we can scan 1–2 MB/ms, 128 KB bounds 31 // scan preemption at ~100 µs. 32 // 33 // This must be > _MaxSmallSize so that the object base is the 34 // span base. 35 maxObletBytes = 128 << 10 36 37 // drainCheckThreshold specifies how many units of work to do 38 // between self-preemption checks in gcDrain. Assuming a scan 39 // rate of 1 MB/ms, this is ~100 µs. Lower values have higher 40 // overhead in the scan loop (the scheduler check may perform 41 // a syscall, so its overhead is nontrivial). Higher values 42 // make the system less responsive to incoming work. 43 drainCheckThreshold = 100000 44 ) 45 46 // gcMarkRootPrepare queues root scanning jobs (stacks, globals, and 47 // some miscellany) and initializes scanning-related state. 48 // 49 // The caller must have call gcCopySpans(). 50 // 51 // The world must be stopped. 52 // 53 //go:nowritebarrier 54 func gcMarkRootPrepare() { 55 work.nFlushCacheRoots = 0 56 57 // Compute how many data and BSS root blocks there are. 58 nBlocks := func(bytes uintptr) int { 59 return int((bytes + rootBlockBytes - 1) / rootBlockBytes) 60 } 61 62 work.nDataRoots = 0 63 work.nBSSRoots = 0 64 65 // Scan globals. 66 for _, datap := range activeModules() { 67 nDataRoots := nBlocks(datap.edata - datap.data) 68 if nDataRoots > work.nDataRoots { 69 work.nDataRoots = nDataRoots 70 } 71 } 72 73 for _, datap := range activeModules() { 74 nBSSRoots := nBlocks(datap.ebss - datap.bss) 75 if nBSSRoots > work.nBSSRoots { 76 work.nBSSRoots = nBSSRoots 77 } 78 } 79 80 // Scan span roots for finalizer specials. 81 // 82 // We depend on addfinalizer to mark objects that get 83 // finalizers after root marking. 84 // 85 // We're only interested in scanning the in-use spans, 86 // which will all be swept at this point. More spans 87 // may be added to this list during concurrent GC, but 88 // we only care about spans that were allocated before 89 // this mark phase. 90 work.nSpanRoots = mheap_.sweepSpans[mheap_.sweepgen/2%2].numBlocks() 91 92 // Scan stacks. 93 // 94 // Gs may be created after this point, but it's okay that we 95 // ignore them because they begin life without any roots, so 96 // there's nothing to scan, and any roots they create during 97 // the concurrent phase will be scanned during mark 98 // termination. 99 work.nStackRoots = int(atomic.Loaduintptr(&allglen)) 100 101 work.markrootNext = 0 102 work.markrootJobs = uint32(fixedRootCount + work.nFlushCacheRoots + work.nDataRoots + work.nBSSRoots + work.nSpanRoots + work.nStackRoots) 103 } 104 105 // gcMarkRootCheck checks that all roots have been scanned. It is 106 // purely for debugging. 107 func gcMarkRootCheck() { 108 if work.markrootNext < work.markrootJobs { 109 print(work.markrootNext, " of ", work.markrootJobs, " markroot jobs done\n") 110 throw("left over markroot jobs") 111 } 112 113 lock(&allglock) 114 // Check that stacks have been scanned. 115 var gp *g 116 for i := 0; i < work.nStackRoots; i++ { 117 gp = allgs[i] 118 if !gp.gcscandone { 119 goto fail 120 } 121 } 122 unlock(&allglock) 123 return 124 125 fail: 126 println("gp", gp, "goid", gp.goid, 127 "status", readgstatus(gp), 128 "gcscandone", gp.gcscandone, 129 "gcscanvalid", gp.gcscanvalid) 130 unlock(&allglock) // Avoid self-deadlock with traceback. 131 throw("scan missed a g") 132 } 133 134 // ptrmask for an allocation containing a single pointer. 135 var oneptrmask = [...]uint8{1} 136 137 // markroot scans the i'th root. 138 // 139 // Preemption must be disabled (because this uses a gcWork). 140 // 141 // nowritebarrier is only advisory here. 142 // 143 //go:nowritebarrier 144 func markroot(gcw *gcWork, i uint32) { 145 // TODO(austin): This is a bit ridiculous. Compute and store 146 // the bases in gcMarkRootPrepare instead of the counts. 147 baseFlushCache := uint32(fixedRootCount) 148 baseData := baseFlushCache + uint32(work.nFlushCacheRoots) 149 baseBSS := baseData + uint32(work.nDataRoots) 150 baseSpans := baseBSS + uint32(work.nBSSRoots) 151 baseStacks := baseSpans + uint32(work.nSpanRoots) 152 end := baseStacks + uint32(work.nStackRoots) 153 154 // Note: if you add a case here, please also update heapdump.go:dumproots. 155 switch { 156 case baseFlushCache <= i && i < baseData: 157 flushmcache(int(i - baseFlushCache)) 158 159 case baseData <= i && i < baseBSS: 160 for _, datap := range activeModules() { 161 markrootBlock(datap.data, datap.edata-datap.data, datap.gcdatamask.bytedata, gcw, int(i-baseData)) 162 } 163 164 case baseBSS <= i && i < baseSpans: 165 for _, datap := range activeModules() { 166 markrootBlock(datap.bss, datap.ebss-datap.bss, datap.gcbssmask.bytedata, gcw, int(i-baseBSS)) 167 } 168 169 case i == fixedRootFinalizers: 170 for fb := allfin; fb != nil; fb = fb.alllink { 171 cnt := uintptr(atomic.Load(&fb.cnt)) 172 scanblock(uintptr(unsafe.Pointer(&fb.fin[0])), cnt*unsafe.Sizeof(fb.fin[0]), &finptrmask[0], gcw, nil) 173 } 174 175 case i == fixedRootFreeGStacks: 176 // Switch to the system stack so we can call 177 // stackfree. 178 systemstack(markrootFreeGStacks) 179 180 case baseSpans <= i && i < baseStacks: 181 // mark mspan.specials 182 markrootSpans(gcw, int(i-baseSpans)) 183 184 default: 185 // the rest is scanning goroutine stacks 186 var gp *g 187 if baseStacks <= i && i < end { 188 gp = allgs[i-baseStacks] 189 } else { 190 throw("markroot: bad index") 191 } 192 193 // remember when we've first observed the G blocked 194 // needed only to output in traceback 195 status := readgstatus(gp) // We are not in a scan state 196 if (status == _Gwaiting || status == _Gsyscall) && gp.waitsince == 0 { 197 gp.waitsince = work.tstart 198 } 199 200 // scang must be done on the system stack in case 201 // we're trying to scan our own stack. 202 systemstack(func() { 203 // If this is a self-scan, put the user G in 204 // _Gwaiting to prevent self-deadlock. It may 205 // already be in _Gwaiting if this is a mark 206 // worker or we're in mark termination. 207 userG := getg().m.curg 208 selfScan := gp == userG && readgstatus(userG) == _Grunning 209 if selfScan { 210 casgstatus(userG, _Grunning, _Gwaiting) 211 userG.waitreason = waitReasonGarbageCollectionScan 212 } 213 214 // TODO: scang blocks until gp's stack has 215 // been scanned, which may take a while for 216 // running goroutines. Consider doing this in 217 // two phases where the first is non-blocking: 218 // we scan the stacks we can and ask running 219 // goroutines to scan themselves; and the 220 // second blocks. 221 scang(gp, gcw) 222 223 if selfScan { 224 casgstatus(userG, _Gwaiting, _Grunning) 225 } 226 }) 227 } 228 } 229 230 // markrootBlock scans the shard'th shard of the block of memory [b0, 231 // b0+n0), with the given pointer mask. 232 // 233 //go:nowritebarrier 234 func markrootBlock(b0, n0 uintptr, ptrmask0 *uint8, gcw *gcWork, shard int) { 235 if rootBlockBytes%(8*sys.PtrSize) != 0 { 236 // This is necessary to pick byte offsets in ptrmask0. 237 throw("rootBlockBytes must be a multiple of 8*ptrSize") 238 } 239 240 b := b0 + uintptr(shard)*rootBlockBytes 241 if b >= b0+n0 { 242 return 243 } 244 ptrmask := (*uint8)(add(unsafe.Pointer(ptrmask0), uintptr(shard)*(rootBlockBytes/(8*sys.PtrSize)))) 245 n := uintptr(rootBlockBytes) 246 if b+n > b0+n0 { 247 n = b0 + n0 - b 248 } 249 250 // Scan this shard. 251 scanblock(b, n, ptrmask, gcw, nil) 252 } 253 254 // markrootFreeGStacks frees stacks of dead Gs. 255 // 256 // This does not free stacks of dead Gs cached on Ps, but having a few 257 // cached stacks around isn't a problem. 258 func markrootFreeGStacks() { 259 // Take list of dead Gs with stacks. 260 lock(&sched.gFree.lock) 261 list := sched.gFree.stack 262 sched.gFree.stack = gList{} 263 unlock(&sched.gFree.lock) 264 if list.empty() { 265 return 266 } 267 268 // Free stacks. 269 q := gQueue{list.head, list.head} 270 for gp := list.head.ptr(); gp != nil; gp = gp.schedlink.ptr() { 271 stackfree(gp.stack) 272 gp.stack.lo = 0 273 gp.stack.hi = 0 274 // Manipulate the queue directly since the Gs are 275 // already all linked the right way. 276 q.tail.set(gp) 277 } 278 279 // Put Gs back on the free list. 280 lock(&sched.gFree.lock) 281 sched.gFree.noStack.pushAll(q) 282 unlock(&sched.gFree.lock) 283 } 284 285 // markrootSpans marks roots for one shard of work.spans. 286 // 287 //go:nowritebarrier 288 func markrootSpans(gcw *gcWork, shard int) { 289 // Objects with finalizers have two GC-related invariants: 290 // 291 // 1) Everything reachable from the object must be marked. 292 // This ensures that when we pass the object to its finalizer, 293 // everything the finalizer can reach will be retained. 294 // 295 // 2) Finalizer specials (which are not in the garbage 296 // collected heap) are roots. In practice, this means the fn 297 // field must be scanned. 298 // 299 // TODO(austin): There are several ideas for making this more 300 // efficient in issue #11485. 301 302 sg := mheap_.sweepgen 303 spans := mheap_.sweepSpans[mheap_.sweepgen/2%2].block(shard) 304 // Note that work.spans may not include spans that were 305 // allocated between entering the scan phase and now. This is 306 // okay because any objects with finalizers in those spans 307 // must have been allocated and given finalizers after we 308 // entered the scan phase, so addfinalizer will have ensured 309 // the above invariants for them. 310 for _, s := range spans { 311 if s.state != mSpanInUse { 312 continue 313 } 314 // Check that this span was swept (it may be cached or uncached). 315 if !useCheckmark && !(s.sweepgen == sg || s.sweepgen == sg+3) { 316 // sweepgen was updated (+2) during non-checkmark GC pass 317 print("sweep ", s.sweepgen, " ", sg, "\n") 318 throw("gc: unswept span") 319 } 320 321 // Speculatively check if there are any specials 322 // without acquiring the span lock. This may race with 323 // adding the first special to a span, but in that 324 // case addfinalizer will observe that the GC is 325 // active (which is globally synchronized) and ensure 326 // the above invariants. We may also ensure the 327 // invariants, but it's okay to scan an object twice. 328 if s.specials == nil { 329 continue 330 } 331 332 // Lock the specials to prevent a special from being 333 // removed from the list while we're traversing it. 334 lock(&s.speciallock) 335 336 for sp := s.specials; sp != nil; sp = sp.next { 337 if sp.kind != _KindSpecialFinalizer { 338 continue 339 } 340 // don't mark finalized object, but scan it so we 341 // retain everything it points to. 342 spf := (*specialfinalizer)(unsafe.Pointer(sp)) 343 // A finalizer can be set for an inner byte of an object, find object beginning. 344 p := s.base() + uintptr(spf.special.offset)/s.elemsize*s.elemsize 345 346 // Mark everything that can be reached from 347 // the object (but *not* the object itself or 348 // we'll never collect it). 349 scanobject(p, gcw) 350 351 // The special itself is a root. 352 scanblock(uintptr(unsafe.Pointer(&spf.fn)), sys.PtrSize, &oneptrmask[0], gcw, nil) 353 } 354 355 unlock(&s.speciallock) 356 } 357 } 358 359 // gcAssistAlloc performs GC work to make gp's assist debt positive. 360 // gp must be the calling user gorountine. 361 // 362 // This must be called with preemption enabled. 363 func gcAssistAlloc(gp *g) { 364 // Don't assist in non-preemptible contexts. These are 365 // generally fragile and won't allow the assist to block. 366 if getg() == gp.m.g0 { 367 return 368 } 369 if mp := getg().m; mp.locks > 0 || mp.preemptoff != "" { 370 return 371 } 372 373 traced := false 374 retry: 375 // Compute the amount of scan work we need to do to make the 376 // balance positive. When the required amount of work is low, 377 // we over-assist to build up credit for future allocations 378 // and amortize the cost of assisting. 379 debtBytes := -gp.gcAssistBytes 380 scanWork := int64(gcController.assistWorkPerByte * float64(debtBytes)) 381 if scanWork < gcOverAssistWork { 382 scanWork = gcOverAssistWork 383 debtBytes = int64(gcController.assistBytesPerWork * float64(scanWork)) 384 } 385 386 // Steal as much credit as we can from the background GC's 387 // scan credit. This is racy and may drop the background 388 // credit below 0 if two mutators steal at the same time. This 389 // will just cause steals to fail until credit is accumulated 390 // again, so in the long run it doesn't really matter, but we 391 // do have to handle the negative credit case. 392 bgScanCredit := atomic.Loadint64(&gcController.bgScanCredit) 393 stolen := int64(0) 394 if bgScanCredit > 0 { 395 if bgScanCredit < scanWork { 396 stolen = bgScanCredit 397 gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(stolen)) 398 } else { 399 stolen = scanWork 400 gp.gcAssistBytes += debtBytes 401 } 402 atomic.Xaddint64(&gcController.bgScanCredit, -stolen) 403 404 scanWork -= stolen 405 406 if scanWork == 0 { 407 // We were able to steal all of the credit we 408 // needed. 409 if traced { 410 traceGCMarkAssistDone() 411 } 412 return 413 } 414 } 415 416 if trace.enabled && !traced { 417 traced = true 418 traceGCMarkAssistStart() 419 } 420 421 // Perform assist work 422 systemstack(func() { 423 gcAssistAlloc1(gp, scanWork) 424 // The user stack may have moved, so this can't touch 425 // anything on it until it returns from systemstack. 426 }) 427 428 completed := gp.param != nil 429 gp.param = nil 430 if completed { 431 gcMarkDone() 432 } 433 434 if gp.gcAssistBytes < 0 { 435 // We were unable steal enough credit or perform 436 // enough work to pay off the assist debt. We need to 437 // do one of these before letting the mutator allocate 438 // more to prevent over-allocation. 439 // 440 // If this is because we were preempted, reschedule 441 // and try some more. 442 if gp.preempt { 443 Gosched() 444 goto retry 445 } 446 447 // Add this G to an assist queue and park. When the GC 448 // has more background credit, it will satisfy queued 449 // assists before flushing to the global credit pool. 450 // 451 // Note that this does *not* get woken up when more 452 // work is added to the work list. The theory is that 453 // there wasn't enough work to do anyway, so we might 454 // as well let background marking take care of the 455 // work that is available. 456 if !gcParkAssist() { 457 goto retry 458 } 459 460 // At this point either background GC has satisfied 461 // this G's assist debt, or the GC cycle is over. 462 } 463 if traced { 464 traceGCMarkAssistDone() 465 } 466 } 467 468 // gcAssistAlloc1 is the part of gcAssistAlloc that runs on the system 469 // stack. This is a separate function to make it easier to see that 470 // we're not capturing anything from the user stack, since the user 471 // stack may move while we're in this function. 472 // 473 // gcAssistAlloc1 indicates whether this assist completed the mark 474 // phase by setting gp.param to non-nil. This can't be communicated on 475 // the stack since it may move. 476 // 477 //go:systemstack 478 func gcAssistAlloc1(gp *g, scanWork int64) { 479 // Clear the flag indicating that this assist completed the 480 // mark phase. 481 gp.param = nil 482 483 if atomic.Load(&gcBlackenEnabled) == 0 { 484 // The gcBlackenEnabled check in malloc races with the 485 // store that clears it but an atomic check in every malloc 486 // would be a performance hit. 487 // Instead we recheck it here on the non-preemptable system 488 // stack to determine if we should perform an assist. 489 490 // GC is done, so ignore any remaining debt. 491 gp.gcAssistBytes = 0 492 return 493 } 494 // Track time spent in this assist. Since we're on the 495 // system stack, this is non-preemptible, so we can 496 // just measure start and end time. 497 startTime := nanotime() 498 499 decnwait := atomic.Xadd(&work.nwait, -1) 500 if decnwait == work.nproc { 501 println("runtime: work.nwait =", decnwait, "work.nproc=", work.nproc) 502 throw("nwait > work.nprocs") 503 } 504 505 // gcDrainN requires the caller to be preemptible. 506 casgstatus(gp, _Grunning, _Gwaiting) 507 gp.waitreason = waitReasonGCAssistMarking 508 509 // drain own cached work first in the hopes that it 510 // will be more cache friendly. 511 gcw := &getg().m.p.ptr().gcw 512 workDone := gcDrainN(gcw, scanWork) 513 514 casgstatus(gp, _Gwaiting, _Grunning) 515 516 // Record that we did this much scan work. 517 // 518 // Back out the number of bytes of assist credit that 519 // this scan work counts for. The "1+" is a poor man's 520 // round-up, to ensure this adds credit even if 521 // assistBytesPerWork is very low. 522 gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(workDone)) 523 524 // If this is the last worker and we ran out of work, 525 // signal a completion point. 526 incnwait := atomic.Xadd(&work.nwait, +1) 527 if incnwait > work.nproc { 528 println("runtime: work.nwait=", incnwait, 529 "work.nproc=", work.nproc) 530 throw("work.nwait > work.nproc") 531 } 532 533 if incnwait == work.nproc && !gcMarkWorkAvailable(nil) { 534 // This has reached a background completion point. Set 535 // gp.param to a non-nil value to indicate this. It 536 // doesn't matter what we set it to (it just has to be 537 // a valid pointer). 538 gp.param = unsafe.Pointer(gp) 539 } 540 duration := nanotime() - startTime 541 _p_ := gp.m.p.ptr() 542 _p_.gcAssistTime += duration 543 if _p_.gcAssistTime > gcAssistTimeSlack { 544 atomic.Xaddint64(&gcController.assistTime, _p_.gcAssistTime) 545 _p_.gcAssistTime = 0 546 } 547 } 548 549 // gcWakeAllAssists wakes all currently blocked assists. This is used 550 // at the end of a GC cycle. gcBlackenEnabled must be false to prevent 551 // new assists from going to sleep after this point. 552 func gcWakeAllAssists() { 553 lock(&work.assistQueue.lock) 554 list := work.assistQueue.q.popList() 555 injectglist(&list) 556 unlock(&work.assistQueue.lock) 557 } 558 559 // gcParkAssist puts the current goroutine on the assist queue and parks. 560 // 561 // gcParkAssist reports whether the assist is now satisfied. If it 562 // returns false, the caller must retry the assist. 563 // 564 //go:nowritebarrier 565 func gcParkAssist() bool { 566 lock(&work.assistQueue.lock) 567 // If the GC cycle finished while we were getting the lock, 568 // exit the assist. The cycle can't finish while we hold the 569 // lock. 570 if atomic.Load(&gcBlackenEnabled) == 0 { 571 unlock(&work.assistQueue.lock) 572 return true 573 } 574 575 gp := getg() 576 oldList := work.assistQueue.q 577 work.assistQueue.q.pushBack(gp) 578 579 // Recheck for background credit now that this G is in 580 // the queue, but can still back out. This avoids a 581 // race in case background marking has flushed more 582 // credit since we checked above. 583 if atomic.Loadint64(&gcController.bgScanCredit) > 0 { 584 work.assistQueue.q = oldList 585 if oldList.tail != 0 { 586 oldList.tail.ptr().schedlink.set(nil) 587 } 588 unlock(&work.assistQueue.lock) 589 return false 590 } 591 // Park. 592 goparkunlock(&work.assistQueue.lock, waitReasonGCAssistWait, traceEvGoBlockGC, 2) 593 return true 594 } 595 596 // gcFlushBgCredit flushes scanWork units of background scan work 597 // credit. This first satisfies blocked assists on the 598 // work.assistQueue and then flushes any remaining credit to 599 // gcController.bgScanCredit. 600 // 601 // Write barriers are disallowed because this is used by gcDrain after 602 // it has ensured that all work is drained and this must preserve that 603 // condition. 604 // 605 //go:nowritebarrierrec 606 func gcFlushBgCredit(scanWork int64) { 607 if work.assistQueue.q.empty() { 608 // Fast path; there are no blocked assists. There's a 609 // small window here where an assist may add itself to 610 // the blocked queue and park. If that happens, we'll 611 // just get it on the next flush. 612 atomic.Xaddint64(&gcController.bgScanCredit, scanWork) 613 return 614 } 615 616 scanBytes := int64(float64(scanWork) * gcController.assistBytesPerWork) 617 618 lock(&work.assistQueue.lock) 619 for !work.assistQueue.q.empty() && scanBytes > 0 { 620 gp := work.assistQueue.q.pop() 621 // Note that gp.gcAssistBytes is negative because gp 622 // is in debt. Think carefully about the signs below. 623 if scanBytes+gp.gcAssistBytes >= 0 { 624 // Satisfy this entire assist debt. 625 scanBytes += gp.gcAssistBytes 626 gp.gcAssistBytes = 0 627 // It's important that we *not* put gp in 628 // runnext. Otherwise, it's possible for user 629 // code to exploit the GC worker's high 630 // scheduler priority to get itself always run 631 // before other goroutines and always in the 632 // fresh quantum started by GC. 633 ready(gp, 0, false) 634 } else { 635 // Partially satisfy this assist. 636 gp.gcAssistBytes += scanBytes 637 scanBytes = 0 638 // As a heuristic, we move this assist to the 639 // back of the queue so that large assists 640 // can't clog up the assist queue and 641 // substantially delay small assists. 642 work.assistQueue.q.pushBack(gp) 643 break 644 } 645 } 646 647 if scanBytes > 0 { 648 // Convert from scan bytes back to work. 649 scanWork = int64(float64(scanBytes) * gcController.assistWorkPerByte) 650 atomic.Xaddint64(&gcController.bgScanCredit, scanWork) 651 } 652 unlock(&work.assistQueue.lock) 653 } 654 655 // scanstack scans gp's stack, greying all pointers found on the stack. 656 // 657 // scanstack is marked go:systemstack because it must not be preempted 658 // while using a workbuf. 659 // 660 //go:nowritebarrier 661 //go:systemstack 662 func scanstack(gp *g, gcw *gcWork) { 663 if gp.gcscanvalid { 664 return 665 } 666 667 if readgstatus(gp)&_Gscan == 0 { 668 print("runtime:scanstack: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", hex(readgstatus(gp)), "\n") 669 throw("scanstack - bad status") 670 } 671 672 switch readgstatus(gp) &^ _Gscan { 673 default: 674 print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n") 675 throw("mark - bad status") 676 case _Gdead: 677 return 678 case _Grunning: 679 print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n") 680 throw("scanstack: goroutine not stopped") 681 case _Grunnable, _Gsyscall, _Gwaiting: 682 // ok 683 } 684 685 if gp == getg() { 686 throw("can't scan our own stack") 687 } 688 689 // Shrink the stack if not much of it is being used. 690 shrinkstack(gp) 691 692 var state stackScanState 693 state.stack = gp.stack 694 695 if stackTraceDebug { 696 println("stack trace goroutine", gp.goid) 697 } 698 699 // Scan the saved context register. This is effectively a live 700 // register that gets moved back and forth between the 701 // register and sched.ctxt without a write barrier. 702 if gp.sched.ctxt != nil { 703 scanblock(uintptr(unsafe.Pointer(&gp.sched.ctxt)), sys.PtrSize, &oneptrmask[0], gcw, &state) 704 } 705 706 // Scan the stack. Accumulate a list of stack objects. 707 scanframe := func(frame *stkframe, unused unsafe.Pointer) bool { 708 scanframeworker(frame, &state, gcw) 709 return true 710 } 711 gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, scanframe, nil, 0) 712 713 // Find additional pointers that point into the stack from the heap. 714 // Currently this includes defers and panics. See also function copystack. 715 716 // Find and trace all defer arguments. 717 tracebackdefers(gp, scanframe, nil) 718 719 // Find and trace other pointers in defer records. 720 for d := gp._defer; d != nil; d = d.link { 721 if d.fn != nil { 722 // tracebackdefers above does not scan the func value, which could 723 // be a stack allocated closure. See issue 30453. 724 scanblock(uintptr(unsafe.Pointer(&d.fn)), sys.PtrSize, &oneptrmask[0], gcw, &state) 725 } 726 if d.link != nil { 727 // The link field of a stack-allocated defer record might point 728 // to a heap-allocated defer record. Keep that heap record live. 729 scanblock(uintptr(unsafe.Pointer(&d.link)), sys.PtrSize, &oneptrmask[0], gcw, &state) 730 } 731 // Retain defers records themselves. 732 // Defer records might not be reachable from the G through regular heap 733 // tracing because the defer linked list might weave between the stack and the heap. 734 if d.heap { 735 scanblock(uintptr(unsafe.Pointer(&d)), sys.PtrSize, &oneptrmask[0], gcw, &state) 736 } 737 } 738 if gp._panic != nil { 739 // Panics are always stack allocated. 740 state.putPtr(uintptr(unsafe.Pointer(gp._panic))) 741 } 742 743 // Find and scan all reachable stack objects. 744 state.buildIndex() 745 for { 746 p := state.getPtr() 747 if p == 0 { 748 break 749 } 750 obj := state.findObject(p) 751 if obj == nil { 752 continue 753 } 754 t := obj.typ 755 if t == nil { 756 // We've already scanned this object. 757 continue 758 } 759 obj.setType(nil) // Don't scan it again. 760 if stackTraceDebug { 761 println(" live stkobj at", hex(state.stack.lo+uintptr(obj.off)), "of type", t.string()) 762 } 763 gcdata := t.gcdata 764 var s *mspan 765 if t.kind&kindGCProg != 0 { 766 // This path is pretty unlikely, an object large enough 767 // to have a GC program allocated on the stack. 768 // We need some space to unpack the program into a straight 769 // bitmask, which we allocate/free here. 770 // TODO: it would be nice if there were a way to run a GC 771 // program without having to store all its bits. We'd have 772 // to change from a Lempel-Ziv style program to something else. 773 // Or we can forbid putting objects on stacks if they require 774 // a gc program (see issue 27447). 775 s = materializeGCProg(t.ptrdata, gcdata) 776 gcdata = (*byte)(unsafe.Pointer(s.startAddr)) 777 } 778 779 scanblock(state.stack.lo+uintptr(obj.off), t.ptrdata, gcdata, gcw, &state) 780 781 if s != nil { 782 dematerializeGCProg(s) 783 } 784 } 785 786 // Deallocate object buffers. 787 // (Pointer buffers were all deallocated in the loop above.) 788 for state.head != nil { 789 x := state.head 790 state.head = x.next 791 if stackTraceDebug { 792 for _, obj := range x.obj[:x.nobj] { 793 if obj.typ == nil { // reachable 794 continue 795 } 796 println(" dead stkobj at", hex(gp.stack.lo+uintptr(obj.off)), "of type", obj.typ.string()) 797 // Note: not necessarily really dead - only reachable-from-ptr dead. 798 } 799 } 800 x.nobj = 0 801 putempty((*workbuf)(unsafe.Pointer(x))) 802 } 803 if state.buf != nil || state.freeBuf != nil { 804 throw("remaining pointer buffers") 805 } 806 807 gp.gcscanvalid = true 808 } 809 810 // Scan a stack frame: local variables and function arguments/results. 811 //go:nowritebarrier 812 func scanframeworker(frame *stkframe, state *stackScanState, gcw *gcWork) { 813 if _DebugGC > 1 && frame.continpc != 0 { 814 print("scanframe ", funcname(frame.fn), "\n") 815 } 816 817 locals, args, objs := getStackMap(frame, &state.cache, false) 818 819 // Scan local variables if stack frame has been allocated. 820 if locals.n > 0 { 821 size := uintptr(locals.n) * sys.PtrSize 822 scanblock(frame.varp-size, size, locals.bytedata, gcw, state) 823 } 824 825 // Scan arguments. 826 if args.n > 0 { 827 scanblock(frame.argp, uintptr(args.n)*sys.PtrSize, args.bytedata, gcw, state) 828 } 829 830 // Add all stack objects to the stack object list. 831 if frame.varp != 0 { 832 // varp is 0 for defers, where there are no locals. 833 // In that case, there can't be a pointer to its args, either. 834 // (And all args would be scanned above anyway.) 835 for _, obj := range objs { 836 off := obj.off 837 base := frame.varp // locals base pointer 838 if off >= 0 { 839 base = frame.argp // arguments and return values base pointer 840 } 841 ptr := base + uintptr(off) 842 if ptr < frame.sp { 843 // object hasn't been allocated in the frame yet. 844 continue 845 } 846 if stackTraceDebug { 847 println("stkobj at", hex(ptr), "of type", obj.typ.string()) 848 } 849 state.addObject(ptr, obj.typ) 850 } 851 } 852 } 853 854 type gcDrainFlags int 855 856 const ( 857 gcDrainUntilPreempt gcDrainFlags = 1 << iota 858 gcDrainFlushBgCredit 859 gcDrainIdle 860 gcDrainFractional 861 ) 862 863 // gcDrain scans roots and objects in work buffers, blackening grey 864 // objects until it is unable to get more work. It may return before 865 // GC is done; it's the caller's responsibility to balance work from 866 // other Ps. 867 // 868 // If flags&gcDrainUntilPreempt != 0, gcDrain returns when g.preempt 869 // is set. 870 // 871 // If flags&gcDrainIdle != 0, gcDrain returns when there is other work 872 // to do. 873 // 874 // If flags&gcDrainFractional != 0, gcDrain self-preempts when 875 // pollFractionalWorkerExit() returns true. This implies 876 // gcDrainNoBlock. 877 // 878 // If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work 879 // credit to gcController.bgScanCredit every gcCreditSlack units of 880 // scan work. 881 // 882 //go:nowritebarrier 883 func gcDrain(gcw *gcWork, flags gcDrainFlags) { 884 if !writeBarrier.needed { 885 throw("gcDrain phase incorrect") 886 } 887 888 gp := getg().m.curg 889 preemptible := flags&gcDrainUntilPreempt != 0 890 flushBgCredit := flags&gcDrainFlushBgCredit != 0 891 idle := flags&gcDrainIdle != 0 892 893 initScanWork := gcw.scanWork 894 895 // checkWork is the scan work before performing the next 896 // self-preempt check. 897 checkWork := int64(1<<63 - 1) 898 var check func() bool 899 if flags&(gcDrainIdle|gcDrainFractional) != 0 { 900 checkWork = initScanWork + drainCheckThreshold 901 if idle { 902 check = pollWork 903 } else if flags&gcDrainFractional != 0 { 904 check = pollFractionalWorkerExit 905 } 906 } 907 908 // Drain root marking jobs. 909 if work.markrootNext < work.markrootJobs { 910 for !(preemptible && gp.preempt) { 911 job := atomic.Xadd(&work.markrootNext, +1) - 1 912 if job >= work.markrootJobs { 913 break 914 } 915 markroot(gcw, job) 916 if check != nil && check() { 917 goto done 918 } 919 } 920 } 921 922 // Drain heap marking jobs. 923 for !(preemptible && gp.preempt) { 924 // Try to keep work available on the global queue. We used to 925 // check if there were waiting workers, but it's better to 926 // just keep work available than to make workers wait. In the 927 // worst case, we'll do O(log(_WorkbufSize)) unnecessary 928 // balances. 929 if work.full == 0 { 930 gcw.balance() 931 } 932 933 b := gcw.tryGetFast() 934 if b == 0 { 935 b = gcw.tryGet() 936 if b == 0 { 937 // Flush the write barrier 938 // buffer; this may create 939 // more work. 940 wbBufFlush(nil, 0) 941 b = gcw.tryGet() 942 } 943 } 944 if b == 0 { 945 // Unable to get work. 946 break 947 } 948 scanobject(b, gcw) 949 950 // Flush background scan work credit to the global 951 // account if we've accumulated enough locally so 952 // mutator assists can draw on it. 953 if gcw.scanWork >= gcCreditSlack { 954 atomic.Xaddint64(&gcController.scanWork, gcw.scanWork) 955 if flushBgCredit { 956 gcFlushBgCredit(gcw.scanWork - initScanWork) 957 initScanWork = 0 958 } 959 checkWork -= gcw.scanWork 960 gcw.scanWork = 0 961 962 if checkWork <= 0 { 963 checkWork += drainCheckThreshold 964 if check != nil && check() { 965 break 966 } 967 } 968 } 969 } 970 971 done: 972 // Flush remaining scan work credit. 973 if gcw.scanWork > 0 { 974 atomic.Xaddint64(&gcController.scanWork, gcw.scanWork) 975 if flushBgCredit { 976 gcFlushBgCredit(gcw.scanWork - initScanWork) 977 } 978 gcw.scanWork = 0 979 } 980 } 981 982 // gcDrainN blackens grey objects until it has performed roughly 983 // scanWork units of scan work or the G is preempted. This is 984 // best-effort, so it may perform less work if it fails to get a work 985 // buffer. Otherwise, it will perform at least n units of work, but 986 // may perform more because scanning is always done in whole object 987 // increments. It returns the amount of scan work performed. 988 // 989 // The caller goroutine must be in a preemptible state (e.g., 990 // _Gwaiting) to prevent deadlocks during stack scanning. As a 991 // consequence, this must be called on the system stack. 992 // 993 //go:nowritebarrier 994 //go:systemstack 995 func gcDrainN(gcw *gcWork, scanWork int64) int64 { 996 if !writeBarrier.needed { 997 throw("gcDrainN phase incorrect") 998 } 999 1000 // There may already be scan work on the gcw, which we don't 1001 // want to claim was done by this call. 1002 workFlushed := -gcw.scanWork 1003 1004 gp := getg().m.curg 1005 for !gp.preempt && workFlushed+gcw.scanWork < scanWork { 1006 // See gcDrain comment. 1007 if work.full == 0 { 1008 gcw.balance() 1009 } 1010 1011 // This might be a good place to add prefetch code... 1012 // if(wbuf.nobj > 4) { 1013 // PREFETCH(wbuf->obj[wbuf.nobj - 3]; 1014 // } 1015 // 1016 b := gcw.tryGetFast() 1017 if b == 0 { 1018 b = gcw.tryGet() 1019 if b == 0 { 1020 // Flush the write barrier buffer; 1021 // this may create more work. 1022 wbBufFlush(nil, 0) 1023 b = gcw.tryGet() 1024 } 1025 } 1026 1027 if b == 0 { 1028 // Try to do a root job. 1029 // 1030 // TODO: Assists should get credit for this 1031 // work. 1032 if work.markrootNext < work.markrootJobs { 1033 job := atomic.Xadd(&work.markrootNext, +1) - 1 1034 if job < work.markrootJobs { 1035 markroot(gcw, job) 1036 continue 1037 } 1038 } 1039 // No heap or root jobs. 1040 break 1041 } 1042 scanobject(b, gcw) 1043 1044 // Flush background scan work credit. 1045 if gcw.scanWork >= gcCreditSlack { 1046 atomic.Xaddint64(&gcController.scanWork, gcw.scanWork) 1047 workFlushed += gcw.scanWork 1048 gcw.scanWork = 0 1049 } 1050 } 1051 1052 // Unlike gcDrain, there's no need to flush remaining work 1053 // here because this never flushes to bgScanCredit and 1054 // gcw.dispose will flush any remaining work to scanWork. 1055 1056 return workFlushed + gcw.scanWork 1057 } 1058 1059 // scanblock scans b as scanobject would, but using an explicit 1060 // pointer bitmap instead of the heap bitmap. 1061 // 1062 // This is used to scan non-heap roots, so it does not update 1063 // gcw.bytesMarked or gcw.scanWork. 1064 // 1065 // If stk != nil, possible stack pointers are also reported to stk.putPtr. 1066 //go:nowritebarrier 1067 func scanblock(b0, n0 uintptr, ptrmask *uint8, gcw *gcWork, stk *stackScanState) { 1068 // Use local copies of original parameters, so that a stack trace 1069 // due to one of the throws below shows the original block 1070 // base and extent. 1071 b := b0 1072 n := n0 1073 1074 for i := uintptr(0); i < n; { 1075 // Find bits for the next word. 1076 bits := uint32(*addb(ptrmask, i/(sys.PtrSize*8))) 1077 if bits == 0 { 1078 i += sys.PtrSize * 8 1079 continue 1080 } 1081 for j := 0; j < 8 && i < n; j++ { 1082 if bits&1 != 0 { 1083 // Same work as in scanobject; see comments there. 1084 p := *(*uintptr)(unsafe.Pointer(b + i)) 1085 if p != 0 { 1086 if obj, span, objIndex := findObject(p, b, i); obj != 0 { 1087 greyobject(obj, b, i, span, gcw, objIndex) 1088 } else if stk != nil && p >= stk.stack.lo && p < stk.stack.hi { 1089 stk.putPtr(p) 1090 } 1091 } 1092 } 1093 bits >>= 1 1094 i += sys.PtrSize 1095 } 1096 } 1097 } 1098 1099 // scanobject scans the object starting at b, adding pointers to gcw. 1100 // b must point to the beginning of a heap object or an oblet. 1101 // scanobject consults the GC bitmap for the pointer mask and the 1102 // spans for the size of the object. 1103 // 1104 //go:nowritebarrier 1105 func scanobject(b uintptr, gcw *gcWork) { 1106 // Find the bits for b and the size of the object at b. 1107 // 1108 // b is either the beginning of an object, in which case this 1109 // is the size of the object to scan, or it points to an 1110 // oblet, in which case we compute the size to scan below. 1111 hbits := heapBitsForAddr(b) 1112 s := spanOfUnchecked(b) 1113 n := s.elemsize 1114 if n == 0 { 1115 throw("scanobject n == 0") 1116 } 1117 1118 if n > maxObletBytes { 1119 // Large object. Break into oblets for better 1120 // parallelism and lower latency. 1121 if b == s.base() { 1122 // It's possible this is a noscan object (not 1123 // from greyobject, but from other code 1124 // paths), in which case we must *not* enqueue 1125 // oblets since their bitmaps will be 1126 // uninitialized. 1127 if s.spanclass.noscan() { 1128 // Bypass the whole scan. 1129 gcw.bytesMarked += uint64(n) 1130 return 1131 } 1132 1133 // Enqueue the other oblets to scan later. 1134 // Some oblets may be in b's scalar tail, but 1135 // these will be marked as "no more pointers", 1136 // so we'll drop out immediately when we go to 1137 // scan those. 1138 for oblet := b + maxObletBytes; oblet < s.base()+s.elemsize; oblet += maxObletBytes { 1139 if !gcw.putFast(oblet) { 1140 gcw.put(oblet) 1141 } 1142 } 1143 } 1144 1145 // Compute the size of the oblet. Since this object 1146 // must be a large object, s.base() is the beginning 1147 // of the object. 1148 n = s.base() + s.elemsize - b 1149 if n > maxObletBytes { 1150 n = maxObletBytes 1151 } 1152 } 1153 1154 var i uintptr 1155 for i = 0; i < n; i += sys.PtrSize { 1156 // Find bits for this word. 1157 if i != 0 { 1158 // Avoid needless hbits.next() on last iteration. 1159 hbits = hbits.next() 1160 } 1161 // Load bits once. See CL 22712 and issue 16973 for discussion. 1162 bits := hbits.bits() 1163 // During checkmarking, 1-word objects store the checkmark 1164 // in the type bit for the one word. The only one-word objects 1165 // are pointers, or else they'd be merged with other non-pointer 1166 // data into larger allocations. 1167 if i != 1*sys.PtrSize && bits&bitScan == 0 { 1168 break // no more pointers in this object 1169 } 1170 if bits&bitPointer == 0 { 1171 continue // not a pointer 1172 } 1173 1174 // Work here is duplicated in scanblock and above. 1175 // If you make changes here, make changes there too. 1176 obj := *(*uintptr)(unsafe.Pointer(b + i)) 1177 1178 // At this point we have extracted the next potential pointer. 1179 // Quickly filter out nil and pointers back to the current object. 1180 if obj != 0 && obj-b >= n { 1181 // Test if obj points into the Go heap and, if so, 1182 // mark the object. 1183 // 1184 // Note that it's possible for findObject to 1185 // fail if obj points to a just-allocated heap 1186 // object because of a race with growing the 1187 // heap. In this case, we know the object was 1188 // just allocated and hence will be marked by 1189 // allocation itself. 1190 if obj, span, objIndex := findObject(obj, b, i); obj != 0 { 1191 greyobject(obj, b, i, span, gcw, objIndex) 1192 } 1193 } 1194 } 1195 gcw.bytesMarked += uint64(n) 1196 gcw.scanWork += int64(i) 1197 } 1198 1199 // Shade the object if it isn't already. 1200 // The object is not nil and known to be in the heap. 1201 // Preemption must be disabled. 1202 //go:nowritebarrier 1203 func shade(b uintptr) { 1204 if obj, span, objIndex := findObject(b, 0, 0); obj != 0 { 1205 gcw := &getg().m.p.ptr().gcw 1206 greyobject(obj, 0, 0, span, gcw, objIndex) 1207 } 1208 } 1209 1210 // obj is the start of an object with mark mbits. 1211 // If it isn't already marked, mark it and enqueue into gcw. 1212 // base and off are for debugging only and could be removed. 1213 // 1214 // See also wbBufFlush1, which partially duplicates this logic. 1215 // 1216 //go:nowritebarrierrec 1217 func greyobject(obj, base, off uintptr, span *mspan, gcw *gcWork, objIndex uintptr) { 1218 // obj should be start of allocation, and so must be at least pointer-aligned. 1219 if obj&(sys.PtrSize-1) != 0 { 1220 throw("greyobject: obj not pointer-aligned") 1221 } 1222 mbits := span.markBitsForIndex(objIndex) 1223 1224 if useCheckmark { 1225 if !mbits.isMarked() { 1226 printlock() 1227 print("runtime:greyobject: checkmarks finds unexpected unmarked object obj=", hex(obj), "\n") 1228 print("runtime: found obj at *(", hex(base), "+", hex(off), ")\n") 1229 1230 // Dump the source (base) object 1231 gcDumpObject("base", base, off) 1232 1233 // Dump the object 1234 gcDumpObject("obj", obj, ^uintptr(0)) 1235 1236 getg().m.traceback = 2 1237 throw("checkmark found unmarked object") 1238 } 1239 hbits := heapBitsForAddr(obj) 1240 if hbits.isCheckmarked(span.elemsize) { 1241 return 1242 } 1243 hbits.setCheckmarked(span.elemsize) 1244 if !hbits.isCheckmarked(span.elemsize) { 1245 throw("setCheckmarked and isCheckmarked disagree") 1246 } 1247 } else { 1248 if debug.gccheckmark > 0 && span.isFree(objIndex) { 1249 print("runtime: marking free object ", hex(obj), " found at *(", hex(base), "+", hex(off), ")\n") 1250 gcDumpObject("base", base, off) 1251 gcDumpObject("obj", obj, ^uintptr(0)) 1252 getg().m.traceback = 2 1253 throw("marking free object") 1254 } 1255 1256 // If marked we have nothing to do. 1257 if mbits.isMarked() { 1258 return 1259 } 1260 mbits.setMarked() 1261 1262 // Mark span. 1263 arena, pageIdx, pageMask := pageIndexOf(span.base()) 1264 if arena.pageMarks[pageIdx]&pageMask == 0 { 1265 atomic.Or8(&arena.pageMarks[pageIdx], pageMask) 1266 } 1267 1268 // If this is a noscan object, fast-track it to black 1269 // instead of greying it. 1270 if span.spanclass.noscan() { 1271 gcw.bytesMarked += uint64(span.elemsize) 1272 return 1273 } 1274 } 1275 1276 // Queue the obj for scanning. The PREFETCH(obj) logic has been removed but 1277 // seems like a nice optimization that can be added back in. 1278 // There needs to be time between the PREFETCH and the use. 1279 // Previously we put the obj in an 8 element buffer that is drained at a rate 1280 // to give the PREFETCH time to do its work. 1281 // Use of PREFETCHNTA might be more appropriate than PREFETCH 1282 if !gcw.putFast(obj) { 1283 gcw.put(obj) 1284 } 1285 } 1286 1287 // gcDumpObject dumps the contents of obj for debugging and marks the 1288 // field at byte offset off in obj. 1289 func gcDumpObject(label string, obj, off uintptr) { 1290 s := spanOf(obj) 1291 print(label, "=", hex(obj)) 1292 if s == nil { 1293 print(" s=nil\n") 1294 return 1295 } 1296 print(" s.base()=", hex(s.base()), " s.limit=", hex(s.limit), " s.spanclass=", s.spanclass, " s.elemsize=", s.elemsize, " s.state=") 1297 if 0 <= s.state && int(s.state) < len(mSpanStateNames) { 1298 print(mSpanStateNames[s.state], "\n") 1299 } else { 1300 print("unknown(", s.state, ")\n") 1301 } 1302 1303 skipped := false 1304 size := s.elemsize 1305 if s.state == mSpanManual && size == 0 { 1306 // We're printing something from a stack frame. We 1307 // don't know how big it is, so just show up to an 1308 // including off. 1309 size = off + sys.PtrSize 1310 } 1311 for i := uintptr(0); i < size; i += sys.PtrSize { 1312 // For big objects, just print the beginning (because 1313 // that usually hints at the object's type) and the 1314 // fields around off. 1315 if !(i < 128*sys.PtrSize || off-16*sys.PtrSize < i && i < off+16*sys.PtrSize) { 1316 skipped = true 1317 continue 1318 } 1319 if skipped { 1320 print(" ...\n") 1321 skipped = false 1322 } 1323 print(" *(", label, "+", i, ") = ", hex(*(*uintptr)(unsafe.Pointer(obj + i)))) 1324 if i == off { 1325 print(" <==") 1326 } 1327 print("\n") 1328 } 1329 if skipped { 1330 print(" ...\n") 1331 } 1332 } 1333 1334 // gcmarknewobject marks a newly allocated object black. obj must 1335 // not contain any non-nil pointers. 1336 // 1337 // This is nosplit so it can manipulate a gcWork without preemption. 1338 // 1339 //go:nowritebarrier 1340 //go:nosplit 1341 func gcmarknewobject(obj, size, scanSize uintptr) { 1342 if useCheckmark { // The world should be stopped so this should not happen. 1343 throw("gcmarknewobject called while doing checkmark") 1344 } 1345 markBitsForAddr(obj).setMarked() 1346 gcw := &getg().m.p.ptr().gcw 1347 gcw.bytesMarked += uint64(size) 1348 gcw.scanWork += int64(scanSize) 1349 } 1350 1351 // gcMarkTinyAllocs greys all active tiny alloc blocks. 1352 // 1353 // The world must be stopped. 1354 func gcMarkTinyAllocs() { 1355 for _, p := range allp { 1356 c := p.mcache 1357 if c == nil || c.tiny == 0 { 1358 continue 1359 } 1360 _, span, objIndex := findObject(c.tiny, 0, 0) 1361 gcw := &p.gcw 1362 greyobject(c.tiny, 0, 0, span, gcw, objIndex) 1363 } 1364 } 1365 1366 // Checkmarking 1367 1368 // To help debug the concurrent GC we remark with the world 1369 // stopped ensuring that any object encountered has their normal 1370 // mark bit set. To do this we use an orthogonal bit 1371 // pattern to indicate the object is marked. The following pattern 1372 // uses the upper two bits in the object's boundary nibble. 1373 // 01: scalar not marked 1374 // 10: pointer not marked 1375 // 11: pointer marked 1376 // 00: scalar marked 1377 // Xoring with 01 will flip the pattern from marked to unmarked and vica versa. 1378 // The higher bit is 1 for pointers and 0 for scalars, whether the object 1379 // is marked or not. 1380 // The first nibble no longer holds the typeDead pattern indicating that the 1381 // there are no more pointers in the object. This information is held 1382 // in the second nibble. 1383 1384 // If useCheckmark is true, marking of an object uses the 1385 // checkmark bits (encoding above) instead of the standard 1386 // mark bits. 1387 var useCheckmark = false 1388 1389 //go:nowritebarrier 1390 func initCheckmarks() { 1391 useCheckmark = true 1392 for _, s := range mheap_.allspans { 1393 if s.state == mSpanInUse { 1394 heapBitsForAddr(s.base()).initCheckmarkSpan(s.layout()) 1395 } 1396 } 1397 } 1398 1399 func clearCheckmarks() { 1400 useCheckmark = false 1401 for _, s := range mheap_.allspans { 1402 if s.state == mSpanInUse { 1403 heapBitsForAddr(s.base()).clearCheckmarkSpan(s.layout()) 1404 } 1405 } 1406 } 1407
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