Function DoAllocWithArena

BeefRT/gperftools/src/base/low_level_alloc.cc:434–497

allocates and returns a block of size bytes, to be freed with Free() L < arena->mu

// 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 = 0;
  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(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, false) - 1;
      if (i < arena->freelist.levels) {   // potential blocks exist
        AllocList *before = &arena->freelist;  // predecessor of s
        while ((s = Next(i, before, arena)) != 0 && s->header.size < req_rnd) {
          before = s;
        }
        if (s != 0) {       // 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;
      if ((arena->flags & LowLevelAlloc::kAsyncSignalSafe) != 0) {
        new_pages = MallocHook::UnhookedMMap(0, new_pages_size,
            PROT_WRITE|PROT_READ, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
      } else {
        new_pages = mmap(0, new_pages_size,
            PROT_WRITE|PROT_READ, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
      }
      RAW_CHECK(new_pages != MAP_FAILED, "mmap error");
      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 (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);
    RAW_CHECK(s->header.arena == arena, "");
    arena->allocation_count++;