CODEWITHSUNDEEP
cassemblyx86-64att
pigsploof
pigsploof

Reputation: 35

Rotate Function in x86_64 AT&T assembly

I am implementing a rotate function in AT&T 64 bit assembly and am having trouble getting the correct output in my code.

The function I'm trying to implement is 

unsigned long rotate(unsigned long val, ul num, ul dir);

Val is value I wanted rotated, num is how many times and direction is left or right, 0 being right, 1 being left.

My assembly code: 

.global rotate

rotate: #value = %rdi
        #num = %rsi 
        #direction = %rdx 
        mov %rsi, %r10 #puts num into register
        test %rdx, %rdx 
        jz right #if rdx is 0 jump to rotate right
        #else go to loop right below which rotates left
loop: 
     test %r10, %r10 #if num is 0 I am done
     jz done 
     rol $1, %rdi #rotate left 1 bit
     dec %r10 #decrement my count
     jmp loop #jump to top of loop 

 right: #same logic as left 
      test %r10, %10 
      jz done 
      rol $1, %rdi  
      dec %r10 
      jmp loop 
 done: 
      mov %rdi, %rax
      ret

My C code: 

#include <stdio.h> 
extern unsigned long rotate(unsigned long val, unsigned long num, unsigned long direction); 

int main()
{
unsigned long v,n,d; 
v = 0xDEADBEEFDEADBEEF; 
n = 2; 
d = 1; 

printf("%x\n ", rotate(v,n,d));    
}

When I compile and run, I get the value 0x7AB6FBBF, when I am supposed to get 0x7AB6FBBF7AB6FBBF.

Is there something wrong with my instructions not sending in unsigned longs or somethings?

Upvotes: 0

Views: 666

Answers (2)

Peter Cordes
Peter Cordes

Reputation: 364180

You have a bug in your asm: the loop branch in right does jmp loop instead of jmp right. At least that's one of your bugs, IDK if there are more. (update: @zch spotted the bug in your C which explains the truncation you're mentioning.)

This would have been easier to spot if you'd used a better name than loop. e.g. left.

But you shouldn't be looping anyway. x86 has rol %cl, %rdi and ror %cl, %rdi, which you can use instead of a loop. Simply mov your shift count into %ecx, like mov %esi, %ecx.

Upvotes: 1

zch
zch

Reputation: 15278

printf("%x", a) works on unsigned int type. On unsigned long you need to use "%lx" as a format string.

Upvotes: 1

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