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6.54 Built-in Functions to Perform Arithmetic with Overflow Checking

The following built-in functions allow performing simple arithmetic operations together with checking whether the operations overflowed.

— Built-in Function: bool __builtin_add_overflow (type1 a, type2 b, type3 *res)
— Built-in Function: bool __builtin_sadd_overflow (int a, int b, int *res)
— Built-in Function: bool __builtin_saddl_overflow (long int a, long int b, long int *res)
— Built-in Function: bool __builtin_saddll_overflow (long long int a, long long int b, long long int *res)
— Built-in Function: bool __builtin_uadd_overflow (unsigned int a, unsigned int b, unsigned int *res)
— Built-in Function: bool __builtin_uaddl_overflow (unsigned long int a, unsigned long int b, unsigned long int *res)
— Built-in Function: bool __builtin_uaddll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res)

These built-in functions promote the first two operands into infinite precision signed type and perform addition on those promoted operands. The result is then cast to the type the third pointer argument points to and stored there. If the stored result is equal to the infinite precision result, the built-in functions return false, otherwise they return true. As the addition is performed in infinite signed precision, these built-in functions have fully defined behavior for all argument values.

The first built-in function allows arbitrary integral types for operands and the result type must be pointer to some integral type other than enumerated or boolean type, the rest of the built-in functions have explicit integer types.

The compiler will attempt to use hardware instructions to implement these built-in functions where possible, like conditional jump on overflow after addition, conditional jump on carry etc.

— Built-in Function: bool __builtin_sub_overflow (type1 a, type2 b, type3 *res)
— Built-in Function: bool __builtin_ssub_overflow (int a, int b, int *res)
— Built-in Function: bool __builtin_ssubl_overflow (long int a, long int b, long int *res)
— Built-in Function: bool __builtin_ssubll_overflow (long long int a, long long int b, long long int *res)
— Built-in Function: bool __builtin_usub_overflow (unsigned int a, unsigned int b, unsigned int *res)
— Built-in Function: bool __builtin_usubl_overflow (unsigned long int a, unsigned long int b, unsigned long int *res)
— Built-in Function: bool __builtin_usubll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res)

These built-in functions are similar to the add overflow checking built-in functions above, except they perform subtraction, subtract the second argument from the first one, instead of addition.

— Built-in Function: bool __builtin_mul_overflow (type1 a, type2 b, type3 *res)
— Built-in Function: bool __builtin_smul_overflow (int a, int b, int *res)
— Built-in Function: bool __builtin_smull_overflow (long int a, long int b, long int *res)
— Built-in Function: bool __builtin_smulll_overflow (long long int a, long long int b, long long int *res)
— Built-in Function: bool __builtin_umul_overflow (unsigned int a, unsigned int b, unsigned int *res)
— Built-in Function: bool __builtin_umull_overflow (unsigned long int a, unsigned long int b, unsigned long int *res)
— Built-in Function: bool __builtin_umulll_overflow (unsigned long long int a, unsigned long long int b, unsigned long long int *res)

These built-in functions are similar to the add overflow checking built-in functions above, except they perform multiplication, instead of addition.

The following built-in functions allow checking if simple arithmetic operation would overflow.

— Built-in Function: bool __builtin_add_overflow_p (type1 a, type2 b, type3 c)
— Built-in Function: bool __builtin_sub_overflow_p (type1 a, type2 b, type3 c)
— Built-in Function: bool __builtin_mul_overflow_p (type1 a, type2 b, type3 c)

These built-in functions are similar to __builtin_add_overflow, __builtin_sub_overflow, or __builtin_mul_overflow, except that they don't store the result of the arithmetic operation anywhere and the last argument is not a pointer, but some expression with integral type other than enumerated or boolean type.

The built-in functions promote the first two operands into infinite precision signed type and perform addition on those promoted operands. The result is then cast to the type of the third argument. If the cast result is equal to the infinite precision result, the built-in functions return false, otherwise they return true. The value of the third argument is ignored, just the side-effects in the third argument are evaluated, and no integral argument promotions are performed on the last argument. If the third argument is a bit-field, the type used for the result cast has the precision and signedness of the given bit-field, rather than precision and signedness of the underlying type.

For example, the following macro can be used to portably check, at compile-time, whether or not adding two constant integers will overflow, and perform the addition only when it is known to be safe and not to trigger a -Woverflow warning.

          #define INT_ADD_OVERFLOW_P(a, b) \
             __builtin_add_overflow_p (a, b, (__typeof__ ((a) + (b))) 0)
          
          enum {
              A = INT_MAX, B = 3,
              C = INT_ADD_OVERFLOW_P (A, B) ? 0 : A + B,
              D = __builtin_add_overflow_p (1, SCHAR_MAX, (signed char) 0)
          };

The compiler will attempt to use hardware instructions to implement these built-in functions where possible, like conditional jump on overflow after addition, conditional jump on carry etc.