Function do_memalign

BeefRT/gperftools/src/tcmalloc.cc:1328–1400

For use by exported routines below that want specific alignments Note: this code can be slow for alignments > 16, and can significantly fragment memory. The expectation is that memalign/posix_memalign/valloc/pvalloc will not be invoked very often. This requirement simplifies our implementation and allows us to tune for expected allocation patterns.

// often.  This requirement simplifies our implementation and allows
// us to tune for expected allocation patterns.
void* do_memalign(size_t align, size_t size) {
  ASSERT((align & (align - 1)) == 0);
  ASSERT(align > 0);
  if (size + align < size) return NULL;         // Overflow

  // Fall back to malloc if we would already align this memory access properly.
  if (align <= AlignmentForSize(size)) {
    void* p = do_malloc(size);
    ASSERT((reinterpret_cast<uintptr_t>(p) % align) == 0);
    return p;
  }

  if (UNLIKELY(Static::pageheap() == NULL)) ThreadCache::InitModule();

  // Allocate at least one byte to avoid boundary conditions below
  if (size == 0) size = 1;

  if (size <= kMaxSize && align < kPageSize) {
    // Search through acceptable size classes looking for one with
    // enough alignment.  This depends on the fact that
    // InitSizeClasses() currently produces several size classes that
    // are aligned at powers of two.  We will waste time and space if
    // we miss in the size class array, but that is deemed acceptable
    // since memalign() should be used rarely.
    int cl = Static::sizemap()->SizeClass(size);
    while (cl < kNumClasses &&
           ((Static::sizemap()->class_to_size(cl) & (align - 1)) != 0)) {
      cl++;
    }
    if (cl < kNumClasses) {
      ThreadCache* heap = ThreadCache::GetCache();
      size = Static::sizemap()->class_to_size(cl);
      return CheckedMallocResult(heap->Allocate(size, cl));
    }
  }

  // We will allocate directly from the page heap
  SpinLockHolder h(Static::pageheap_lock());

  if (align <= kPageSize) {
    // Any page-level allocation will be fine
    // TODO: We could put the rest of this page in the appropriate
    // TODO: cache but it does not seem worth it.
    Span* span = Static::pageheap()->New(TCMALLOC_NAMESPACE::pages(size));
    return UNLIKELY(span == NULL) ? NULL : SpanToMallocResult(span);
  }

  // Allocate extra pages and carve off an aligned portion
  const Length alloc = TCMALLOC_NAMESPACE::pages(size + align);
  Span* span = Static::pageheap()->New(alloc);
  if (UNLIKELY(span == NULL)) return NULL;

  // Skip starting portion so that we end up aligned
  Length skip = 0;
  while ((((span->start+skip) << kPageShift) & (align - 1)) != 0) {
    skip++;
  }
  ASSERT(skip < alloc);