Division and modulus using single divl instruction (i386, amd64)

gcc assembly division x86-64 i386
user405725 picture user405725 · Apr 10, 2011 · Viewed 13.3k times · Source

I was trying to come up with inline assembly for gcc to get both division and modulus using single divl instruction. Unfortunately, I am not that good at assembly. Could someone please help me on this? Thank you.

Answer

D0SBoots picture D0SBoots · May 28, 2012

You're looking for something like this:

__asm__("divl %2\n"
       : "=d" (remainder), "=a" (quotient)
       : "g" (modulus), "d" (high), "a" (low));

Although I agree with the other commenters that usually GCC will do this for you and you should avoid inline assembly when possible, sometimes you need this construct.

For instance, if the high word is less than the modulus, then it is safe to perform the division like this. However, GCC isn't smart enough to realize this, because in the general case dividing a 64 bit number by a 32 bit number can lead to overflow, and so it calls to a library routine to do extra work. (Replace with 128 bit/64 bit for 64 bit ISAs.)

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