util/base/casts.h

/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * 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.
 */

#ifndef LIBTEXTCLASSIFIER_UTIL_BASE_CASTS_H_
#define LIBTEXTCLASSIFIER_UTIL_BASE_CASTS_H_

#include <string.h>  // for memcpy

namespace libtextclassifier {

// bit_cast<Dest, Source> is a template function that implements the equivalent
// of "*reinterpret_cast<Dest*>(&source)".  We need this in very low-level
// functions like fast math support.
//
//   float f = 3.14159265358979;
//   int i = bit_cast<int32>(f);
//   // i = 0x40490fdb
//
// The classical address-casting method is:
//
//   // WRONG
//   float f = 3.14159265358979;            // WRONG
//   int i = * reinterpret_cast<int*>(&f);  // WRONG
//
// The address-casting method actually produces undefined behavior
// according to ISO C++ specification section 3.10 -15 -.  Roughly, this
// section says: if an object in memory has one type, and a program
// accesses it with a different type, then the result is undefined
// behavior for most values of "different type".
//
// This is true for any cast syntax, either *(int*)&f or
// *reinterpret_cast<int*>(&f).  And it is particularly true for
// conversions between integral lvalues and floating-point lvalues.
//
// The purpose of 3.10 -15- is to allow optimizing compilers to assume
// that expressions with different types refer to different memory.  gcc
// 4.0.1 has an optimizer that takes advantage of this.  So a
// non-conforming program quietly produces wildly incorrect output.
//
// The problem is not the use of reinterpret_cast.  The problem is type
// punning: holding an object in memory of one type and reading its bits
// back using a different type.
//
// The C++ standard is more subtle and complex than this, but that
// is the basic idea.
//
// Anyways ...
//
// bit_cast<> calls memcpy() which is blessed by the standard, especially by the
// example in section 3.9 .  Also, of course, bit_cast<> wraps up the nasty
// logic in one place.
//
// Fortunately memcpy() is very fast.  In optimized mode, with a
// constant size, gcc 2.95.3, gcc 4.0.1, and msvc 7.1 produce inline
// code with the minimal amount of data movement.  On a 32-bit system,
// memcpy(d,s,4) compiles to one load and one store, and memcpy(d,s,8)
// compiles to two loads and two stores.
//
// Mike Chastain tested this code with gcc 2.95.3, gcc 4.0.1, icc 8.1, and msvc
// 7.1.
//
// WARNING: if Dest or Source is a non-POD type, the result of the memcpy
// is likely to surprise you.
//
// Props to Bill Gibbons for the compile time assertion technique and
// Art Komninos and Igor Tandetnik for the msvc experiments.

template <class Dest, class Source>
inline Dest bit_cast(const Source &source) {
  static_assert(sizeof(Dest) == sizeof(Source), "Sizes do not match");

  Dest dest;
  memcpy(&dest, &source, sizeof(dest));
  return dest;
}

}  // namespace libtextclassifier

#endif  // LIBTEXTCLASSIFIER_UTIL_BASE_CASTS_H_