include/swift/ABI/Enum.h

//===--- Enum.h - Enum implementation runtime declarations ------*- C++ -*-===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// Enum implementation details declarations relevant to the ABI.
//
//===----------------------------------------------------------------------===//

#ifndef SWIFT_ABI_ENUM_H
#define SWIFT_ABI_ENUM_H

#include <stdlib.h>

namespace swift {

struct EnumTagCounts {
  unsigned numTags, numTagBytes;
};

inline EnumTagCounts
getEnumTagCounts(size_t size, unsigned emptyCases, unsigned payloadCases) {
  // We can use the payload area with a tag bit set somewhere outside of the
  // payload area to represent cases. See how many bytes we need to cover
  // all the empty cases.
  unsigned numTags = payloadCases;
  if (emptyCases > 0) {
    if (size >= 4)
      // Assume that one tag bit is enough if the precise calculation overflows
      // an int32.
      numTags += 1;
    else {
      unsigned bits = size * 8U;
      unsigned casesPerTagBitValue = 1U << bits;
      numTags += ((emptyCases + (casesPerTagBitValue-1U)) >> bits);
    }
  }
  unsigned numTagBytes = (numTags <=    1 ? 0 :
                          numTags <   256 ? 1 :
                          numTags < 65536 ? 2 : 4);
  return {numTags, numTagBytes};
}

} // namespace swift

#endif // SWIFT_ABI_ENUM_H