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28
29 // safe_math.h: Helpful math functions.
30 #ifndef SAFE_MATH_H__
31 #define SAFE_MATH_H__
32
33 #include <utility>
34
35 namespace google_breakpad {
36
37 // Adds `a` and `b`, returning a pair of:
38 // - The result after any truncation.
39 // - Whether an overflow/underflow occurred.
40 template <typename T>
AddWithOverflowCheck(T a,T b)41 std::pair<T, bool> AddWithOverflowCheck(T a, T b) {
42 #ifdef _WIN32
43 // Since C++11, unsigned overflow is well-defined; do everything unsigned,
44 // assuming 2's complement.
45 if (std::is_unsigned<T>::value) {
46 T result = a + b;
47 // Since we're adding two values >= 0, having a smaller value implies
48 // overflow.
49 bool overflow = result < a;
50 return {result, overflow};
51 }
52
53 using TUnsigned = typename std::make_unsigned<T>::type;
54 T result = TUnsigned(a) + TUnsigned(b);
55 bool overflow;
56 if ((a >= 0) == (b >= 0)) {
57 if (a >= 0) {
58 overflow = result < a;
59 } else {
60 overflow = result > a;
61 }
62 } else {
63 // If signs are different, it's impossible for overflow to happen.
64 overflow = false;
65 }
66 return {result, overflow};
67 #else
68 T result;
69 bool overflow = __builtin_add_overflow(a, b, &result);
70 return {result, overflow};
71 #endif
72 }
73
74 template <typename T>
AddIgnoringOverflow(T a,T b)75 T AddIgnoringOverflow(T a, T b) {
76 return AddWithOverflowCheck(a, b).first;
77 }
78
79 } // namespace google_breakpad
80
81 #endif // SAFE_MATH_H__
82