Yeah that div check works I guess, but as you say is horribly expensive. For the latter it looks like at least on recent versions of gcc, you can do something

Yeah that div check works I guess, but as you say is horribly expensive. For the latter it looks like at least on recent versions of gcc, you can do something like (WARNING: Just a vague proof of concept, please don't use it. It might kill kittens): uint64_t func(uint64_t a, uint64_t b) { unsigned __int128 res = (unsigned __int128)a * (unsigned __int128)b; if (res >> 64) { asm volatile ("nop;" ::); /* handle here */ } return (uint64_t)res; } Which compiles to this in gcc -Os and -O3: 0: 48 89 f8 mov %rdi,%rax 3: 48 f7 e6 mul %rsi 6: 48 85 d2 test %rdx,%rdx 9: 74 01 je c <func+0xc> b: 90 nop c: f3 c3 repz retq Which I'm pretty sure could be optimized further by using the overflow flag, but the compiler doesn't seem to want to ignore the upper word. edit: and it probably suffers the same problem as described here: https://news.ycombinator.com/item?id=7848771 https://news.ycombinator.com/item?id=7848771 -- and also probably has general issues with signedness that would need to be further checked. edit2: changed it to uints to simplify and avoid a whole class of problem. I still don't think you should go use this, though. (inspiration from SO: http://stackoverflow.com/a/13187798 http://stackoverflow.com/a/13187798)