Multiplying 2 32 bit ints without integer overflow

Question

I would like to multiple 2 positive signed 32-bit integers i and j. The worst case scenario is when i and j are both INT_MAX where their product is within the limit of a 64-bit integer, but when I perform the operation

int i = INT_MAX;
int j = INT_MAX;
long long int res = i * j;

I get garbage due to integer overflow. So I've typically solved that problem by casting i or j to a long long int

int i = INT_MAX;
int j = INT_MAX;
long long int res = (long long int)i * j;

Is this the typical workaround for this issue? Are there other ways that may be better?

Tangent: Prefer static_cast<long long>(i) over a C-style cast. — sweenish
– sweenish, Commented Jun 24, 2021 at 15:55

MSalters · Accepted Answer · 2021-06-24 16:05:21Z

3

Your solution is correct, and standard enough that quality compilers will recognize it. Some CPU's have dedicated 32x32->64 multiplication instructions, and you can reasonably expect a compiler to use such an instruction despite the cast.

answered Jun 24, 2021 at 16:05

MSalters

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