mfbt/Casting.h
author Dustin J. Mitchell <dustin@mozilla.com>
Wed, 07 Sep 2016 00:10:51 +0000
changeset 354971 1627fd341fc154d4fb771366676b7101204362df
parent 349203 89a131cc0bfdfe00262a53738a59219ad9e91389
child 398844 261772d7198542e9cf1f0577a55446785a2c189d
permissions -rw-r--r--
Bug 1286075: allow optimization of tasks whose dependencies have not been optimized; r=armenzg MikeLing initially did this in bug 1287018. The intent of this conditional was to make optimization faster by not even checking most tasks, based on the assumption that if the prerequisite to a task has changed (for example, a docker image or a build), then naturally we will want to execute that task. However, as we have developed actual optimization methods, this has proven not to be the case: we might want to optimize a test out if its inputs have not changed, even if a new installer has been built. Similarly, SETA may optimize tasks out even if their inputs have changed. MozReview-Commit-ID: LgHET3Z84GB

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* Cast operations to supplement the built-in casting operations. */

#ifndef mozilla_Casting_h
#define mozilla_Casting_h

#include "mozilla/Assertions.h"
#include "mozilla/TypeTraits.h"

#include <limits.h>

namespace mozilla {

/**
 * Sets the outparam value of type |To| with the same underlying bit pattern of
 * |aFrom|.
 *
 * |To| and |From| must be types of the same size; be careful of cross-platform
 * size differences, or this might fail to compile on some but not all
 * platforms.
 *
 * There is also a variant that returns the value directly.  In most cases, the
 * two variants should be identical.  However, in the specific case of x86
 * chips, the behavior differs: returning floating-point values directly is done
 * through the x87 stack, and x87 loads and stores turn signaling NaNs into
 * quiet NaNs... silently.  Returning floating-point values via outparam,
 * however, is done entirely within the SSE registers when SSE2 floating-point
 * is enabled in the compiler, which has semantics-preserving behavior you would
 * expect.
 *
 * If preserving the distinction between signaling NaNs and quiet NaNs is
 * important to you, you should use the outparam version.  In all other cases,
 * you should use the direct return version.
 */
template<typename To, typename From>
inline void
BitwiseCast(const From aFrom, To* aResult)
{
  static_assert(sizeof(From) == sizeof(To),
                "To and From must have the same size");
  union
  {
    From mFrom;
    To mTo;
  } u;
  u.mFrom = aFrom;
  *aResult = u.mTo;
}

template<typename To, typename From>
inline To
BitwiseCast(const From aFrom)
{
  To temp;
  BitwiseCast<To, From>(aFrom, &temp);
  return temp;
}

namespace detail {

enum ToSignedness { ToIsSigned, ToIsUnsigned };
enum FromSignedness { FromIsSigned, FromIsUnsigned };

template<typename From,
         typename To,
         FromSignedness = IsSigned<From>::value ? FromIsSigned : FromIsUnsigned,
         ToSignedness = IsSigned<To>::value ? ToIsSigned : ToIsUnsigned>
struct BoundsCheckImpl;

// Implicit conversions on operands to binary operations make this all a bit
// hard to verify.  Attempt to ease the pain below by *only* comparing values
// that are obviously the same type (and will undergo no further conversions),
// even when it's not strictly necessary, for explicitness.

enum UUComparison { FromIsBigger, FromIsNotBigger };

// Unsigned-to-unsigned range check

template<typename From, typename To,
         UUComparison = (sizeof(From) > sizeof(To))
                        ? FromIsBigger
                        : FromIsNotBigger>
struct UnsignedUnsignedCheck;

template<typename From, typename To>
struct UnsignedUnsignedCheck<From, To, FromIsBigger>
{
public:
  static bool checkBounds(const From aFrom)
  {
    return aFrom <= From(To(-1));
  }
};

template<typename From, typename To>
struct UnsignedUnsignedCheck<From, To, FromIsNotBigger>
{
public:
  static bool checkBounds(const From aFrom)
  {
    return true;
  }
};

template<typename From, typename To>
struct BoundsCheckImpl<From, To, FromIsUnsigned, ToIsUnsigned>
{
public:
  static bool checkBounds(const From aFrom)
  {
    return UnsignedUnsignedCheck<From, To>::checkBounds(aFrom);
  }
};

// Signed-to-unsigned range check

template<typename From, typename To>
struct BoundsCheckImpl<From, To, FromIsSigned, ToIsUnsigned>
{
public:
  static bool checkBounds(const From aFrom)
  {
    if (aFrom < 0) {
      return false;
    }
    if (sizeof(To) >= sizeof(From)) {
      return true;
    }
    return aFrom <= From(To(-1));
  }
};

// Unsigned-to-signed range check

enum USComparison { FromIsSmaller, FromIsNotSmaller };

template<typename From, typename To,
         USComparison = (sizeof(From) < sizeof(To))
                        ? FromIsSmaller
                        : FromIsNotSmaller>
struct UnsignedSignedCheck;

template<typename From, typename To>
struct UnsignedSignedCheck<From, To, FromIsSmaller>
{
public:
  static bool checkBounds(const From aFrom)
  {
    return true;
  }
};

template<typename From, typename To>
struct UnsignedSignedCheck<From, To, FromIsNotSmaller>
{
public:
  static bool checkBounds(const From aFrom)
  {
    const To MaxValue = To((1ULL << (CHAR_BIT * sizeof(To) - 1)) - 1);
    return aFrom <= From(MaxValue);
  }
};

template<typename From, typename To>
struct BoundsCheckImpl<From, To, FromIsUnsigned, ToIsSigned>
{
public:
  static bool checkBounds(const From aFrom)
  {
    return UnsignedSignedCheck<From, To>::checkBounds(aFrom);
  }
};

// Signed-to-signed range check

template<typename From, typename To>
struct BoundsCheckImpl<From, To, FromIsSigned, ToIsSigned>
{
public:
  static bool checkBounds(const From aFrom)
  {
    if (sizeof(From) <= sizeof(To)) {
      return true;
    }
    const To MaxValue = To((1ULL << (CHAR_BIT * sizeof(To) - 1)) - 1);
    const To MinValue = -MaxValue - To(1);
    return From(MinValue) <= aFrom &&
           From(aFrom) <= From(MaxValue);
  }
};

template<typename From, typename To,
         bool TypesAreIntegral = IsIntegral<From>::value &&
                                 IsIntegral<To>::value>
class BoundsChecker;

template<typename From>
class BoundsChecker<From, From, true>
{
public:
  static bool checkBounds(const From aFrom) { return true; }
};

template<typename From, typename To>
class BoundsChecker<From, To, true>
{
public:
  static bool checkBounds(const From aFrom)
  {
    return BoundsCheckImpl<From, To>::checkBounds(aFrom);
  }
};

template<typename From, typename To>
inline bool
IsInBounds(const From aFrom)
{
  return BoundsChecker<From, To>::checkBounds(aFrom);
}

} // namespace detail

/**
 * Cast a value of integral type |From| to a value of integral type |To|,
 * asserting that the cast will be a safe cast per C++ (that is, that |to| is in
 * the range of values permitted for the type |From|).
 */
template<typename To, typename From>
inline To
AssertedCast(const From aFrom)
{
  MOZ_ASSERT((detail::IsInBounds<From, To>(aFrom)));
  return static_cast<To>(aFrom);
}

} // namespace mozilla

#endif /* mozilla_Casting_h */