Initial Commit

1 files changed, 598 insertions, 0 deletions

diff --git a/absl/base/internal/low_level_alloc.cc b/absl/base/internal/low_level_alloc.cc
new file mode 100644
index 000000000000..c55201b38bad
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+++ b/absl/base/internal/low_level_alloc.cc
@@ -0,0 +1,598 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// A low-level allocator that can be used by other low-level
+// modules without introducing dependency cycles.
+// This allocator is slow and wasteful of memory;
+// it should not be used when performance is key.
+
+#include "absl/base/config.h"
+
+#include "absl/base/internal/low_level_alloc.h"
+
+// LowLevelAlloc requires that the platform support low-level
+// allocation of virtual memory. Platforms lacking this cannot use
+// LowLevelAlloc.
+#ifndef ABSL_LOW_LEVEL_ALLOC_MISSING
+
+#ifndef _WIN32
+#include <pthread.h>
+#include <signal.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#else
+#include <windows.h>
+#endif
+
+#include <string.h>
+#include <algorithm>
+#include <atomic>
+#include <cstddef>
+#include <cerrno>
+#include <new>                   // for placement-new
+
+#include "absl/base/dynamic_annotations.h"
+#include "absl/base/internal/malloc_hook.h"
+#include "absl/base/internal/malloc_hook_invoke.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/internal/spinlock.h"
+
+// MAP_ANONYMOUS
+#if defined(__APPLE__)
+// For mmap, Linux defines both MAP_ANONYMOUS and MAP_ANON and says MAP_ANON is
+// deprecated. In Darwin, MAP_ANON is all there is.
+#if !defined MAP_ANONYMOUS
+#define MAP_ANONYMOUS MAP_ANON
+#endif  // !MAP_ANONYMOUS
+#endif  // __APPLE__
+
+namespace absl {
+namespace base_internal {
+
+// A first-fit allocator with amortized logarithmic free() time.
+
+// ---------------------------------------------------------------------------
+static const int kMaxLevel = 30;
+
+namespace {
+// This struct describes one allocated block, or one free block.
+struct AllocList {
+  struct Header {
+    // Size of entire region, including this field. Must be
+    // first. Valid in both allocated and unallocated blocks.
+    uintptr_t size;
+
+    // kMagicAllocated or kMagicUnallocated xor this.
+    uintptr_t magic;
+
+    // Pointer to parent arena.
+    LowLevelAlloc::Arena *arena;
+
+    // Aligns regions to 0 mod 2*sizeof(void*).
+    void *dummy_for_alignment;
+  } header;
+
+  // Next two fields: in unallocated blocks: freelist skiplist data
+  //                  in allocated blocks: overlaps with client data
+
+  // Levels in skiplist used.
+  int levels;
+
+  // Actually has levels elements. The AllocList node may not have room
+  // for all kMaxLevel entries. See max_fit in LLA_SkiplistLevels().
+  AllocList *next[kMaxLevel];
+};
+}  // namespace
+
+// ---------------------------------------------------------------------------
+// A trivial skiplist implementation.  This is used to keep the freelist
+// in address order while taking only logarithmic time per insert and delete.
+
+// An integer approximation of log2(size/base)
+// Requires size >= base.
+static int IntLog2(size_t size, size_t base) {
+  int result = 0;
+  for (size_t i = size; i > base; i >>= 1) {  // i == floor(size/2**result)
+    result++;
+  }
+  //    floor(size / 2**result) <= base < floor(size / 2**(result-1))
+  // =>     log2(size/(base+1)) <= result < 1+log2(size/base)
+  // => result ~= log2(size/base)
+  return result;
+}
+
+// Return a random integer n:  p(n)=1/(2**n) if 1 <= n; p(n)=0 if n < 1.
+static int Random(uint32_t *state) {
+  uint32_t r = *state;
+  int result = 1;
+  while ((((r = r*1103515245 + 12345) >> 30) & 1) == 0) {
+    result++;
+  }
+  *state = r;
+  return result;
+}
+
+// Return a number of skiplist levels for a node of size bytes, where
+// base is the minimum node size.  Compute level=log2(size / base)+n
+// where n is 1 if random is false and otherwise a random number generated with
+// the standard distribution for a skiplist:  See Random() above.
+// Bigger nodes tend to have more skiplist levels due to the log2(size / base)
+// term, so first-fit searches touch fewer nodes.  "level" is clipped so
+// level<kMaxLevel and next[level-1] will fit in the node.
+// 0 < LLA_SkiplistLevels(x,y,false) <= LLA_SkiplistLevels(x,y,true) < kMaxLevel
+static int LLA_SkiplistLevels(size_t size, size_t base, uint32_t *random) {
+  // max_fit is the maximum number of levels that will fit in a node for the
+  // given size.   We can't return more than max_fit, no matter what the
+  // random number generator says.
+  size_t max_fit = (size - offsetof(AllocList, next)) / sizeof(AllocList *);
+  int level = IntLog2(size, base) + (random != nullptr ? Random(random) : 1);
+  if (static_cast<size_t>(level) > max_fit) level = static_cast<int>(max_fit);
+  if (level > kMaxLevel-1) level = kMaxLevel - 1;
+  ABSL_RAW_CHECK(level >= 1, "block not big enough for even one level");
+  return level;
+}
+
+// Return "atleast", the first element of AllocList *head s.t. *atleast >= *e.
+// For 0 <= i < head->levels, set prev[i] to "no_greater", where no_greater
+// points to the last element at level i in the AllocList less than *e, or is
+// head if no such element exists.
+static AllocList *LLA_SkiplistSearch(AllocList *head,
+                                     AllocList *e, AllocList **prev) {
+  AllocList *p = head;
+  for (int level = head->levels - 1; level >= 0; level--) {
+    for (AllocList *n; (n = p->next[level]) != nullptr && n < e; p = n) {
+    }
+    prev[level] = p;
+  }
+  return (head->levels == 0) ? nullptr : prev[0]->next[0];
+}
+
+// Insert element *e into AllocList *head.  Set prev[] as LLA_SkiplistSearch.
+// Requires that e->levels be previously set by the caller (using
+// LLA_SkiplistLevels())
+static void LLA_SkiplistInsert(AllocList *head, AllocList *e,
+                               AllocList **prev) {
+  LLA_SkiplistSearch(head, e, prev);
+  for (; head->levels < e->levels; head->levels++) {  // extend prev pointers
+    prev[head->levels] = head;                        // to all *e's levels
+  }
+  for (int i = 0; i != e->levels; i++) {  // add element to list
+    e->next[i] = prev[i]->next[i];
+    prev[i]->next[i] = e;
+  }
+}
+
+// Remove element *e from AllocList *head.  Set prev[] as LLA_SkiplistSearch().
+// Requires that e->levels be previous set by the caller (using
+// LLA_SkiplistLevels())
+static void LLA_SkiplistDelete(AllocList *head, AllocList *e,
+                               AllocList **prev) {
+  AllocList *found = LLA_SkiplistSearch(head, e, prev);
+  ABSL_RAW_CHECK(e == found, "element not in freelist");
+  for (int i = 0; i != e->levels && prev[i]->next[i] == e; i++) {
+    prev[i]->next[i] = e->next[i];
+  }
+  while (head->levels > 0 && head->next[head->levels - 1] == nullptr) {
+    head->levels--;   // reduce head->levels if level unused
+  }
+}
+
+// ---------------------------------------------------------------------------
+// Arena implementation
+
+struct LowLevelAlloc::Arena {
+  // This constructor does nothing, and relies on zero-initialization to get
+  // the proper initial state.
+  Arena() : mu(base_internal::kLinkerInitialized) {}  // NOLINT
+  explicit Arena(int)  // NOLINT(readability/casting)
+      :  // Avoid recursive cooperative scheduling w/ kernel scheduling.
+        mu(base_internal::SCHEDULE_KERNEL_ONLY),
+        // Set pagesize to zero explicitly for non-static init.
+        pagesize(0),
+        random(0) {}
+
+  base_internal::SpinLock mu;   // protects freelist, allocation_count,
+                                // pagesize, roundup, min_size
+  AllocList freelist;           // head of free list; sorted by addr (under mu)
+  int32_t allocation_count;     // count of allocated blocks (under mu)
+  std::atomic<uint32_t> flags;  // flags passed to NewArena (ro after init)
+  size_t pagesize;              // ==getpagesize()  (init under mu, then ro)
+  size_t roundup;               // lowest 2^n >= max(16,sizeof (AllocList))
+                                // (init under mu, then ro)
+  size_t min_size;              // smallest allocation block size
+                                // (init under mu, then ro)
+  uint32_t random;              // PRNG state
+};
+
+// The default arena, which is used when 0 is passed instead of an Arena
+// pointer.
+static struct LowLevelAlloc::Arena default_arena;  // NOLINT
+
+// Non-malloc-hooked arenas: used only to allocate metadata for arenas that
+// do not want malloc hook reporting, so that for them there's no malloc hook
+// reporting even during arena creation.
+static struct LowLevelAlloc::Arena unhooked_arena;  // NOLINT
+
+#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
+static struct LowLevelAlloc::Arena unhooked_async_sig_safe_arena;  // NOLINT
+#endif
+
+// magic numbers to identify allocated and unallocated blocks
+static const uintptr_t kMagicAllocated = 0x4c833e95U;
+static const uintptr_t kMagicUnallocated = ~kMagicAllocated;
+
+namespace {
+class SCOPED_LOCKABLE ArenaLock {
+ public:
+  explicit ArenaLock(LowLevelAlloc::Arena *arena)
+      EXCLUSIVE_LOCK_FUNCTION(arena->mu)
+      : arena_(arena) {
+#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
+    if (arena == &unhooked_async_sig_safe_arena ||
+        (arena->flags.load(std::memory_order_relaxed) &
+         LowLevelAlloc::kAsyncSignalSafe) != 0) {
+      sigset_t all;
+      sigfillset(&all);
+      mask_valid_ = pthread_sigmask(SIG_BLOCK, &all, &mask_) == 0;
+    }
+#endif
+    arena_->mu.Lock();
+  }
+  ~ArenaLock() { ABSL_RAW_CHECK(left_, "haven't left Arena region"); }
+  void Leave() UNLOCK_FUNCTION() {
+    arena_->mu.Unlock();
+#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
+    if (mask_valid_) {
+      pthread_sigmask(SIG_SETMASK, &mask_, nullptr);
+    }
+#endif
+    left_ = true;
+  }
+
+ private:
+  bool left_ = false;  // whether left region
+#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
+  bool mask_valid_ = false;
+  sigset_t mask_;  // old mask of blocked signals
+#endif
+  LowLevelAlloc::Arena *arena_;
+  ArenaLock(const ArenaLock &) = delete;
+  ArenaLock &operator=(const ArenaLock &) = delete;
+};
+}  // namespace
+
+// create an appropriate magic number for an object at "ptr"
+// "magic" should be kMagicAllocated or kMagicUnallocated
+inline static uintptr_t Magic(uintptr_t magic, AllocList::Header *ptr) {
+  return magic ^ reinterpret_cast<uintptr_t>(ptr);
+}
+
+// Initialize the fields of an Arena
+static void ArenaInit(LowLevelAlloc::Arena *arena) {
+  if (arena->pagesize == 0) {
+#ifdef _WIN32
+    SYSTEM_INFO system_info;
+    GetSystemInfo(&system_info);
+    arena->pagesize = std::max(system_info.dwPageSize,
+                               system_info.dwAllocationGranularity);
+#else
+    arena->pagesize = getpagesize();
+#endif
+    // Round up block sizes to a power of two close to the header size.
+    arena->roundup = 16;
+    while (arena->roundup < sizeof (arena->freelist.header)) {
+      arena->roundup += arena->roundup;
+    }
+    // Don't allocate blocks less than twice the roundup size to avoid tiny
+    // free blocks.
+    arena->min_size = 2 * arena->roundup;
+    arena->freelist.header.size = 0;
+    arena->freelist.header.magic =
+        Magic(kMagicUnallocated, &arena->freelist.header);
+    arena->freelist.header.arena = arena;
+    arena->freelist.levels = 0;
+    memset(arena->freelist.next, 0, sizeof (arena->freelist.next));
+    arena->allocation_count = 0;
+    if (arena == &default_arena) {
+      // Default arena should be hooked, e.g. for heap-checker to trace
+      // pointer chains through objects in the default arena.
+      arena->flags.store(LowLevelAlloc::kCallMallocHook,
+                         std::memory_order_relaxed);
+    }
+#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
+    else if (arena ==  // NOLINT(readability/braces)
+             &unhooked_async_sig_safe_arena) {
+      arena->flags.store(LowLevelAlloc::kAsyncSignalSafe,
+                         std::memory_order_relaxed);
+    }
+#endif
+    else {  // NOLINT(readability/braces)
+      // other arenas' flags may be overridden by client,
+      // but unhooked_arena will have 0 in 'flags'.
+      arena->flags.store(0, std::memory_order_relaxed);
+    }
+  }
+}
+
+// L < meta_data_arena->mu
+LowLevelAlloc::Arena *LowLevelAlloc::NewArena(int32_t flags,
+                                              Arena *meta_data_arena) {
+  ABSL_RAW_CHECK(meta_data_arena != nullptr, "must pass a valid arena");
+  if (meta_data_arena == &default_arena) {
+#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
+    if ((flags & LowLevelAlloc::kAsyncSignalSafe) != 0) {
+      meta_data_arena = &unhooked_async_sig_safe_arena;
+    } else  // NOLINT(readability/braces)
+#endif
+        if ((flags & LowLevelAlloc::kCallMallocHook) == 0) {
+      meta_data_arena = &unhooked_arena;
+    }
+  }
+  // Arena(0) uses the constructor for non-static contexts
+  Arena *result =
+    new (AllocWithArena(sizeof (*result), meta_data_arena)) Arena(0);
+  ArenaInit(result);
+  result->flags.store(flags, std::memory_order_relaxed);
+  return result;
+}
+
+// L < arena->mu, L < arena->arena->mu
+bool LowLevelAlloc::DeleteArena(Arena *arena) {
+  ABSL_RAW_CHECK(
+      arena != nullptr && arena != &default_arena && arena != &unhooked_arena,
+      "may not delete default arena");
+  ArenaLock section(arena);
+  bool empty = (arena->allocation_count == 0);
+  section.Leave();
+  if (empty) {
+    while (arena->freelist.next[0] != nullptr) {
+      AllocList *region = arena->freelist.next[0];
+      size_t size = region->header.size;
+      arena->freelist.next[0] = region->next[0];
+      ABSL_RAW_CHECK(
+          region->header.magic == Magic(kMagicUnallocated, &region->header),
+          "bad magic number in DeleteArena()");
+      ABSL_RAW_CHECK(region->header.arena == arena,
+                     "bad arena pointer in DeleteArena()");
+      ABSL_RAW_CHECK(size % arena->pagesize == 0,
+                     "empty arena has non-page-aligned block size");
+      ABSL_RAW_CHECK(reinterpret_cast<uintptr_t>(region) % arena->pagesize == 0,
+                     "empty arena has non-page-aligned block");
+      int munmap_result;
+#ifdef _WIN32
+      munmap_result = VirtualFree(region, 0, MEM_RELEASE);
+      ABSL_RAW_CHECK(munmap_result != 0,
+                     "LowLevelAlloc::DeleteArena: VitualFree failed");
+#else
+      if ((arena->flags.load(std::memory_order_relaxed) &
+           LowLevelAlloc::kAsyncSignalSafe) == 0) {
+        munmap_result = munmap(region, size);
+      } else {
+        munmap_result = MallocHook::UnhookedMUnmap(region, size);
+      }
+      if (munmap_result != 0) {
+        ABSL_RAW_LOG(FATAL, "LowLevelAlloc::DeleteArena: munmap failed: %d",
+                     errno);
+      }
+#endif
+    }
+    Free(arena);
+  }
+  return empty;
+}
+
+// ---------------------------------------------------------------------------
+
+// Addition, checking for overflow.  The intent is to die if an external client
+// manages to push through a request that would cause arithmetic to fail.
+static inline uintptr_t CheckedAdd(uintptr_t a, uintptr_t b) {
+  uintptr_t sum = a + b;
+  ABSL_RAW_CHECK(sum >= a, "LowLevelAlloc arithmetic overflow");
+  return sum;
+}
+
+// Return value rounded up to next multiple of align.
+// align must be a power of two.
+static inline uintptr_t RoundUp(uintptr_t addr, uintptr_t align) {
+  return CheckedAdd(addr, align - 1) & ~(align - 1);
+}
+
+// Equivalent to "return prev->next[i]" but with sanity checking
+// that the freelist is in the correct order, that it
+// consists of regions marked "unallocated", and that no two regions
+// are adjacent in memory (they should have been coalesced).
+// L < arena->mu
+static AllocList *Next(int i, AllocList *prev, LowLevelAlloc::Arena *arena) {
+  ABSL_RAW_CHECK(i < prev->levels, "too few levels in Next()");
+  AllocList *next = prev->next[i];
+  if (next != nullptr) {
+    ABSL_RAW_CHECK(
+        next->header.magic == Magic(kMagicUnallocated, &next->header),
+        "bad magic number in Next()");
+    ABSL_RAW_CHECK(next->header.arena == arena, "bad arena pointer in Next()");
+    if (prev != &arena->freelist) {
+      ABSL_RAW_CHECK(prev < next, "unordered freelist");
+      ABSL_RAW_CHECK(reinterpret_cast<char *>(prev) + prev->header.size <
+                         reinterpret_cast<char *>(next),
+                     "malformed freelist");
+    }
+  }
+  return next;
+}
+
+// Coalesce list item "a" with its successor if they are adjacent.
+static void Coalesce(AllocList *a) {
+  AllocList *n = a->next[0];
+  if (n != nullptr && reinterpret_cast<char *>(a) + a->header.size ==
+                          reinterpret_cast<char *>(n)) {
+    LowLevelAlloc::Arena *arena = a->header.arena;
+    a->header.size += n->header.size;
+    n->header.magic = 0;
+    n->header.arena = nullptr;
+    AllocList *prev[kMaxLevel];
+    LLA_SkiplistDelete(&arena->freelist, n, prev);
+    LLA_SkiplistDelete(&arena->freelist, a, prev);
+    a->levels = LLA_SkiplistLevels(a->header.size, arena->min_size,
+                                   &arena->random);
+    LLA_SkiplistInsert(&arena->freelist, a, prev);
+  }
+}
+
+// Adds block at location "v" to the free list
+// L >= arena->mu
+static void AddToFreelist(void *v, LowLevelAlloc::Arena *arena) {
+  AllocList *f = reinterpret_cast<AllocList *>(
+                        reinterpret_cast<char *>(v) - sizeof (f->header));
+  ABSL_RAW_CHECK(f->header.magic == Magic(kMagicAllocated, &f->header),
+                 "bad magic number in AddToFreelist()");
+  ABSL_RAW_CHECK(f->header.arena == arena,
+                 "bad arena pointer in AddToFreelist()");
+  f->levels = LLA_SkiplistLevels(f->header.size, arena->min_size,
+                                 &arena->random);
+  AllocList *prev[kMaxLevel];
+  LLA_SkiplistInsert(&arena->freelist, f, prev);
+  f->header.magic = Magic(kMagicUnallocated, &f->header);
+  Coalesce(f);                  // maybe coalesce with successor
+  Coalesce(prev[0]);            // maybe coalesce with predecessor
+}
+
+// Frees storage allocated by LowLevelAlloc::Alloc().
+// L < arena->mu
+void LowLevelAlloc::Free(void *v) {
+  if (v != nullptr) {
+    AllocList *f = reinterpret_cast<AllocList *>(
+                        reinterpret_cast<char *>(v) - sizeof (f->header));
+    ABSL_RAW_CHECK(f->header.magic == Magic(kMagicAllocated, &f->header),
+                   "bad magic number in Free()");
+    LowLevelAlloc::Arena *arena = f->header.arena;
+    if ((arena->flags.load(std::memory_order_relaxed) & kCallMallocHook) != 0) {
+      MallocHook::InvokeDeleteHook(v);
+    }
+    ArenaLock section(arena);
+    AddToFreelist(v, arena);
+    ABSL_RAW_CHECK(arena->allocation_count > 0, "nothing in arena to free");
+    arena->allocation_count--;
+    section.Leave();
+  }
+}
+
+// allocates and returns a block of size bytes, to be freed with Free()
+// L < arena->mu
+static void *DoAllocWithArena(size_t request, LowLevelAlloc::Arena *arena) {
+  void *result = nullptr;
+  if (request != 0) {
+    AllocList *s;       // will point to region that satisfies request
+    ArenaLock section(arena);
+    ArenaInit(arena);
+    // round up with header
+    size_t req_rnd = RoundUp(CheckedAdd(request, sizeof (s->header)),
+                             arena->roundup);
+    for (;;) {      // loop until we find a suitable region
+      // find the minimum levels that a block of this size must have
+      int i = LLA_SkiplistLevels(req_rnd, arena->min_size, nullptr) - 1;
+      if (i < arena->freelist.levels) {   // potential blocks exist
+        AllocList *before = &arena->freelist;  // predecessor of s
+        while ((s = Next(i, before, arena)) != nullptr &&
+               s->header.size < req_rnd) {
+          before = s;
+        }
+        if (s != nullptr) {       // we found a region
+          break;
+        }
+      }
+      // we unlock before mmap() both because mmap() may call a callback hook,
+      // and because it may be slow.
+      arena->mu.Unlock();
+      // mmap generous 64K chunks to decrease
+      // the chances/impact of fragmentation:
+      size_t new_pages_size = RoundUp(req_rnd, arena->pagesize * 16);
+      void *new_pages;
+#ifdef _WIN32
+      new_pages = VirtualAlloc(0, new_pages_size,
+                               MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
+      ABSL_RAW_CHECK(new_pages != nullptr, "VirtualAlloc failed");
+#else
+      if ((arena->flags.load(std::memory_order_relaxed) &
+           LowLevelAlloc::kAsyncSignalSafe) != 0) {
+        new_pages = MallocHook::UnhookedMMap(nullptr, new_pages_size,
+            PROT_WRITE|PROT_READ, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
+      } else {
+        new_pages = mmap(nullptr, new_pages_size, PROT_WRITE | PROT_READ,
+                         MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+      }
+      if (new_pages == MAP_FAILED) {
+        ABSL_RAW_LOG(FATAL, "mmap error: %d", errno);
+      }
+#endif
+      arena->mu.Lock();
+      s = reinterpret_cast<AllocList *>(new_pages);
+      s->header.size = new_pages_size;
+      // Pretend the block is allocated; call AddToFreelist() to free it.
+      s->header.magic = Magic(kMagicAllocated, &s->header);
+      s->header.arena = arena;
+      AddToFreelist(&s->levels, arena);  // insert new region into free list
+    }
+    AllocList *prev[kMaxLevel];
+    LLA_SkiplistDelete(&arena->freelist, s, prev);    // remove from free list
+    // s points to the first free region that's big enough
+    if (CheckedAdd(req_rnd, arena->min_size) <= s->header.size) {
+      // big enough to split
+      AllocList *n = reinterpret_cast<AllocList *>
+                        (req_rnd + reinterpret_cast<char *>(s));
+      n->header.size = s->header.size - req_rnd;
+      n->header.magic = Magic(kMagicAllocated, &n->header);
+      n->header.arena = arena;
+      s->header.size = req_rnd;
+      AddToFreelist(&n->levels, arena);
+    }
+    s->header.magic = Magic(kMagicAllocated, &s->header);
+    ABSL_RAW_CHECK(s->header.arena == arena, "");
+    arena->allocation_count++;
+    section.Leave();
+    result = &s->levels;
+  }
+  ANNOTATE_MEMORY_IS_UNINITIALIZED(result, request);
+  return result;
+}
+
+void *LowLevelAlloc::Alloc(size_t request) {
+  void *result = DoAllocWithArena(request, &default_arena);
+  if ((default_arena.flags.load(std::memory_order_relaxed) &
+       kCallMallocHook) != 0) {
+    // this call must be directly in the user-called allocator function
+    // for MallocHook::GetCallerStackTrace to work properly
+    MallocHook::InvokeNewHook(result, request);
+  }
+  return result;
+}
+
+void *LowLevelAlloc::AllocWithArena(size_t request, Arena *arena) {
+  ABSL_RAW_CHECK(arena != nullptr, "must pass a valid arena");
+  void *result = DoAllocWithArena(request, arena);
+  if ((arena->flags.load(std::memory_order_relaxed) & kCallMallocHook) != 0) {
+    // this call must be directly in the user-called allocator function
+    // for MallocHook::GetCallerStackTrace to work properly
+    MallocHook::InvokeNewHook(result, request);
+  }
+  return result;
+}
+
+LowLevelAlloc::Arena *LowLevelAlloc::DefaultArena() {
+  return &default_arena;
+}
+
+}  // namespace base_internal
+}  // namespace absl
+
+#endif  // ABSL_LOW_LEVEL_ALLOC_MISSING