Official Intel ASM Opcode Reference

For reasons I have not yet been able to figure out, @tobi is making me implement a couple of very rudimentary routines in x86 GCC inline assembler because he wants them faster than possible for mere mortal C. The first was a routine to calculate $\lfloor\log_2(n)\rfloor$ for $n\in\mathbb{N}$ and the second one was to zero out a large block of memory. For instance,
unsigned inline log2int(unsigned x) {
    unsigned l;
    asm("bsrl %1, %0" : "=r" (l) : "r" (x));
    return ( 1 << l == x ) ? l : l + 1;
is about 50 times faster than the C-native Version
unsigned inline log2int(unsigned x) {
   unsigned l = 0;
   while(x > (1<<l)) l++;
   return l;
even after optimization. For some reason, I found it tricky to google up the official intel x86 opcode reference ((Opcode Reference Part 1)) ((Opcode Reference Part 2)), so I am linking these here.

Leave a Reply

Your email address will not be published. Required fields are marked *