Source/wtf/QuantizedAllocation.h

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#ifndef WTF_QuantizedAllocation_h
#define WTF_QuantizedAllocation_h

// DESCRIPTION
// QuantizedAllocation::quantizedSize() rounds up the specific size such that:
// 1) Granularity of allocation sizes is >1, and grows as a fraction of the
// allocation size. This property can be used to limit bucket fragmentation
// when sitting on top of a high-granularity allocator such as PartitionAlloc.
// 2) The sizes are likely (but not guaranteed) to correspond to exact "bucket
// sizes" of any underlying allocator that implements standard bucketing.
// This currently includes tcmalloc, Windows LFH and PartitionAlloc.

#include "wtf/WTFExport.h"
#include <limits.h>

namespace WTF {

class WTF_EXPORT QuantizedAllocation {
public:
    // Allocations above 32k are rounded to 4k (which is typically the system
    // page size).
    static const size_t kMaxAllocation = 32768;
    static const size_t kMaxRounding = 4096;

    // This is the minimum granularity of allocation, currently 8 bytes.
    static const size_t kMinRounding = 8;

    // After 128 bytes, the granularity doubles, and proceeds to double again
    // as the allocation size further doubles.
    static const size_t kMinRoundingLimit = 128;
    static const size_t kTableSize = kMaxAllocation / kMinRoundingLimit;

    // Using "unsigned" is not a limitation because Chromium's max malloc() is
    // 2GB even on 64-bit.
    static const size_t kMaxUnquantizedAllocation = UINT_MAX - kMaxRounding;

    static void init();

    static size_t quantizedSize(size_t size)
    {
        size_t roundToLessOne;
        if (UNLIKELY(size >= kMaxAllocation))
            roundToLessOne = kMaxRounding - 1;
        else
            roundToLessOne = table[size / kMinRoundingLimit];
        return (size + roundToLessOne) & ~roundToLessOne;
    }

private:
    static unsigned short table[kTableSize];
};

} // namespace WTF

#endif // WTF_QuantizedAllocation_h