Efficient computation of the high order bits of a 32 bit integer multiplication

Question

Many CPUs have single assembly opcodes for returning the high order bits of a 32 bit integer multiplication. Normally multiplying two 32 bit integers produc

Answer 1

gcc 4.3.2, with -O1 optimisation or higher, translated your function exactly as you showed it to IA32 assembly like this:

umulhi32:
        pushl   %ebp
        movl    %esp, %ebp
        movl    12(%ebp), %eax
        mull    8(%ebp)
        movl    %edx, %eax
        popl    %ebp
        ret

Which is just doing a single 32 bit mull and putting the high 32 bits of the result (from %edx) into the return value.

That's what you wanted, right? Sounds like you just need to turn up the optimisation on your compiler ;) It's possible you could push the compiler in the right direction by eliminating the intermediate variable:

unsigned int umulhi32(unsigned int x, unsigned int y)
{
  return (unsigned int)(((unsigned long long)x * y)>>32);
}