Add support for safe math operations in base/numerics

base/numerics/safe_math.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_SAFE_MATH_H_
+#define BASE_SAFE_MATH_H_
+
+#include "base/numerics/safe_math_impl.h"
+
+namespace base {
+
+namespace internal {
+
+// CheckedNumeric implements all the logic and operators for detecting integer
+// boundary conditions such as overflow, underflow, and invalid conversions.
+// The CheckedNumeric type implicitly converts from floating point and integer
+// dat types, and contains overloads for basic arithmetic operations (i.e.: +,

[Ryan Sleevi, 2014/02/10 23:42:41]

typo: data

[jschuh, 2014/02/11 00:43:28]

Done.

+// -, *, /, %).
+//
+// The following methods convert from CheckedNumeric to standard numeric values:
+// IsValid() - Returns true if the underlying numeric value is valid (i.e. has
+//             has not wrapped and is not the result of an invalid conversion).
+// ValueOrDie() - Returns the underlying value. If the state is not valid this
+//                call will crash on a CHECK.
+// ValueOrDefault() - Returns the current value, or the supplied default if the
+//                    state is not valid.
+// ValueFloating() - Returns the underlying floating point value (valid only
+//                   only for floating point CheckedNumeric types).
+//
+// Bitwise operations are explicitly not supported, because correct
+// handling of some cases (e.g. sign manipulation) is ambiguous. Comparison
+// operations are explicitly not supported because they could result in a crash
+// on a CHECK condition. You should use patterns like the following for these
+// operations:
+// Bitwise operation:
+//   int x = checked_int.ValueOrDefault(0) | kFlagValues;
+// Comparison:
+//   if (checked_size.IsValid() && checked_size.ValueOrDie() < buffer_size)
+//     Do stuff...
+template <typename T>
+class CheckedNumeric {
+ private:
+  CheckedNumericState<T> state_;
+
+ public:
+  typedef T type;
+  template <typename Src>
+  friend class CheckedNumeric;

[Ryan Sleevi, 2014/02/10 23:42:41]

Does this actually work? I didn't think generic friends like this were supported
in MSVC?

[jschuh, 2014/02/11 00:43:28]

Yep, works correct in msvc, gcc, and clang.

+
+  CheckedNumeric() {}
+
+  // Copy constructor.
+  template <typename Src>
+  CheckedNumeric(const CheckedNumeric<Src>& rhs) :
+      state_(rhs.state_.value(), rhs.state_.validity()) {}

[Ryan Sleevi, 2014/02/10 23:42:41]

git-cl-format this

CheckedNumeric(const CheckedNumeric<Src>& rhs)
    : state_(....) {}

[jschuh, 2014/02/11 00:43:28]

Done.

+
+  template <typename Src>
+  CheckedNumeric(Src value, RangeCheckId validity) : state_(value, validity) {}
+
+  // This is not an explicit constructor because we implicitly upgrade regular
+  // numerics to CheckedNumerics to make them easier to use.
+  template <typename Src>
+  CheckedNumeric(Src value) : state_(value) {
+    COMPILE_ASSERT(numeric_limits<Src>::is_specialized,
+                   argument_must_be_numeric);
+  }
+
+  // validity() - DO NOT USE THIS IN EXTERNAL CODE - It is public right now for
+  // tests and to avoid a big matrix of friend operator overloads. But the
+  // values it returns are likely to change in the future.
+  // Returns: current validity state (i.e. valid, overflow, underflow, nan).
+  // TODO(jschuh): crbug.com/332611 Figure out and implement semantics for
+  // saturation/wrapping so we can expose this state consistently and implement
+  // saturated arithmetic.
+  RangeCheckId validity() const { return state_.validity(); }

[Ryan Sleevi, 2014/02/10 23:42:41]

if it's only for tests, you can call it validity() [the internal method] and
ValidityForTesting() [public method], the latter which will yak in presubmit.py
if someone uses it outside of a test file.

[jschuh, 2014/02/11 00:43:28]

So... that doesn't solve the nasty mess of friend overloads I'd. Also, I'm
expecting some non-negligible work of foisting math into CheckedNumericState
when I have to approach the issue of consistent saturation semantics. So, for
now I'd much rather punt on this.

+
+  // IsValid() is the public API to test if a CheckedNumeric is currently valid.
+  bool IsValid() const { return validity() == TYPE_VALID; }
+
+  // ValueUnsafe() - DO NOT USE THIS IN EXTERNAL CODE - It is public right now
+  // for tests and to avoid a big matrix of friend operator overloads. But the
+  // values it returns are likely to change in the future.
+  // Returns: the raw numeric value, regardless of the current state.
+  // TODO(jschuh): crbug.com/332611 Figure out and implement semantics for
+  // saturation/wrapping so we can expose this state consistently and implement
+  // saturated arithmetic.
+  T ValueUnsafe() const { return state_.value(); }

[Ryan Sleevi, 2014/02/10 23:42:41]

Ditto

+
+  // ValueOrDie() The primary accessor for the underlying value. If the current
+  // state is not valid it will CHECK and crash.
+  T ValueOrDie() const {
+    CHECK(IsValid());
+    return state_.value();
+  }
+
+  // ValueOrDefault(T default_value) A convenience method that returns the
+  // current value if the state is valid, and the supplied default_value for
+  // any other state.
+  T ValueOrDefault(T default_value) const {
+    return IsValid() ? state_.value() : default_value;
+  }
+
+  // ValueFloating() - Since floating point values include their validity state,
+  // we provide an easy method for extracting them directly, without a risk of
+  // crashing on a CHECK.
+  T ValueFloating() {
+    COMPILE_ASSERT(numeric_limits<T>::is_iec559, argument_must_be_float);
+    return CheckedNumeric<T>::cast(*this).ValueUnsafe();
+  }
+
+  // Prototypes for the supported arithmetic operator overloads.
+  template <typename Src> CheckedNumeric& operator +=(Src rhs);
+  template <typename Src> CheckedNumeric& operator -=(Src rhs);
+  template <typename Src> CheckedNumeric& operator *=(Src rhs);
+  template <typename Src> CheckedNumeric& operator /=(Src rhs);
+  template <typename Src> CheckedNumeric& operator %=(Src rhs);
+
+  CheckedNumeric operator -() const {
+    RangeCheckId validity;
+    T value = CheckedNeg(state_.value(), &validity);
+    // Negation is always valid for floating point.
+    if (numeric_limits<T>::is_iec559)
+      return CheckedNumeric<T>(value);
+
+    validity = static_cast<RangeCheckId>(state_.validity() | validity);
+    return CheckedNumeric<T>(value, validity);
+  }
+
+  CheckedNumeric Abs() const {
+    RangeCheckId validity;
+    T value = CheckedAbs(state_.value(), &validity);
+    // Absolute value is always valid for floating point.
+    if (numeric_limits<T>::is_iec559)
+      return CheckedNumeric<T>(value);
+
+    validity = static_cast<RangeCheckId>(state_.validity() | validity);
+    return CheckedNumeric<T>(value, validity);
+  }
+
+  CheckedNumeric& operator++() {
+    *this += 1;
+    return *this;
+  }
+
+  CheckedNumeric operator++(int) {
+    CheckedNumeric value = *this;
+    *this += 1;
+    return value;
+  }
+
+  CheckedNumeric& operator--() {
+    *this -= 1;
+    return *this;
+  }
+
+  CheckedNumeric operator--(int) {
+    CheckedNumeric value = *this;
+    *this -= 1;
+    return value;
+  }
+
+  // These static methods behave like a convenience cast operator targeting
+  // the desired CheckedNumeric type. As an optimization, a reference is
+  // returned when Src is the same type as T.
+  template <typename Src>
+  static CheckedNumeric<T> cast(Src u,
+      typename enable_if<numeric_limits<Src>::is_specialized, int>::type = 0) {
+    return u;
+  }
+
+  template <typename Src>
+  static CheckedNumeric<T> cast(const CheckedNumeric<Src>& u,
+      typename enable_if<!is_same<Src, T>::value, int>::type = 0) {
+    return u;
+  }
+
+  static const CheckedNumeric<T>& cast(const CheckedNumeric<T>& u) {
+    return u;
+  }
+};
+
+// This is the boilerplate for the standard arithmetic operator overloads. A
+// macro isn't the prettiest solution, but it beats rewriting these five times.
+// Some details worth noting are:
+//  * We apply the standard arithmetic promotions.
+//  * We skip range checks for floating points.
+//  * We skip range checks for destination integers with sufficient range.
+#define BASE_NUMERIC_ARITHMETIC_OPERATORS(NAME,OP,COMPOUND_OP)                 \
+/* Binary arithmetic operator for CheckedNumerics of the same type. */         \
+template <typename T>                                                          \
+CheckedNumeric<typename ArithmeticPromotion<T>::type>                          \
+operator OP(const CheckedNumeric<T> &lhs, const CheckedNumeric<T> &rhs) {      \
+  typedef typename ArithmeticPromotion<T>::type Promotion;                     \
+  /* Floating point always takes the fast path */                              \
+  if (numeric_limits<T>::is_iec559)                                            \
+    return CheckedNumeric<T>(lhs.ValueUnsafe() OP rhs.ValueUnsafe());          \
+  if (IsIntegerArithmeticSafe<Promotion, T, T>::value)                         \
+    return CheckedNumeric<Promotion>(lhs.ValueUnsafe() OP rhs.ValueUnsafe(),   \
+           static_cast<RangeCheckId>(rhs.validity() | lhs.validity()));        \
+  RangeCheckId validity = TYPE_VALID;                                          \
+  T result =                                                                   \
+      Checked##NAME(static_cast<Promotion>(lhs.ValueUnsafe()),                 \
+                    static_cast<Promotion>(rhs.ValueUnsafe()), &validity);     \
+  return CheckedNumeric<Promotion>(result, static_cast<RangeCheckId>(          \
+             validity | lhs.validity() | rhs.validity()));                     \
+}                                                                              \
+/* Assignment arithmetic operator implementation from CheckedNumeric above. */ \
+template <typename T>                                                          \
+template <typename Src>                                                        \
+CheckedNumeric<T>&                                                             \
+CheckedNumeric<T>::operator COMPOUND_OP(Src rhs) {                             \
+  *this = CheckedNumeric<T>::cast(*this) OP CheckedNumeric<Src>::cast(rhs);    \
+  return *this;                                                                \
+}                                                                              \
+/* Binary arithmetic operator for CheckedNumeric of different type. */         \
+template <typename T, typename Src>                                            \
+CheckedNumeric<typename ArithmeticPromotion<T, Src>::type>                     \
+operator OP(const CheckedNumeric<Src> &lhs, const CheckedNumeric<T> &rhs) {    \
+  typedef typename ArithmeticPromotion<T, Src>::type Promotion;                \
+  if (IsIntegerArithmeticSafe<Promotion, T, Src>::value)                       \
+    return CheckedNumeric<Promotion>(lhs.ValueUnsafe() OP rhs.ValueUnsafe(),   \
+           static_cast<RangeCheckId>(rhs.validity() | lhs.validity()));        \
+  return CheckedNumeric<Promotion>::cast(lhs) OP                               \
+         CheckedNumeric<Promotion>::cast(rhs);                                 \
+}                                                                              \
+/* Binary arithmetic operator for left CheckedNumeric and right numeric. */    \
+template <typename T, typename Src>                                            \
+CheckedNumeric<typename ArithmeticPromotion<T, Src>::type>                     \
+operator OP(const CheckedNumeric<T> &lhs, Src rhs) {                           \
+  typedef typename ArithmeticPromotion<T, Src>::type Promotion;                \
+  if (IsIntegerArithmeticSafe<Promotion, T, Src>::value)                       \
+    return CheckedNumeric<Promotion>(lhs.ValueUnsafe() OP                      \
+           rhs, lhs.validity());                                               \
+  return CheckedNumeric<Promotion>::cast(lhs) OP                               \
+         CheckedNumeric<Promotion>::cast(rhs);                                 \
+}                                                                              \
+/* Binary arithmetic operator for right numeric and left CheckedNumeric. */    \
+template <typename T, typename Src>                                            \
+CheckedNumeric<typename ArithmeticPromotion<T, Src>::type>                     \
+operator OP(Src lhs, const CheckedNumeric<T> &rhs) {                           \
+  typedef typename ArithmeticPromotion<T, Src>::type Promotion;                \
+  if (IsIntegerArithmeticSafe<Promotion, T, Src>::value)                       \
+    return CheckedNumeric<Promotion>(lhs OP rhs.ValueUnsafe(),                 \
+                                     rhs.validity());                          \
+  return CheckedNumeric<Promotion>::cast(lhs) OP                               \
+         CheckedNumeric<Promotion>::cast(rhs);                                 \
+}
+
+BASE_NUMERIC_ARITHMETIC_OPERATORS(Add, +, +=)
+BASE_NUMERIC_ARITHMETIC_OPERATORS(Sub, -, -=)
+BASE_NUMERIC_ARITHMETIC_OPERATORS(Mul, *, *=)
+BASE_NUMERIC_ARITHMETIC_OPERATORS(Div, /, /=)
+BASE_NUMERIC_ARITHMETIC_OPERATORS(Mod, %, %=)
+
+#undef BASE_NUMERIC_ARITHMETIC_OPERATORS
+
+}  // namespace internal
+
+using internal::CheckedNumeric;
+
+}  // namespace base
+
+#endif  // BASE_SAFE_MATH_H_
+