Re: Inline assembly for detecting int32 overflow on IA32 and AMD64

[Adam Warner <lists at consulting dot net dot nz>]

On Tue, 18 Jan 2005 17: 06:13 +1300, Adam Warner wrote:

On Tue, 18 Jan 2005 17:06:13 +1300, Adam Warner wrote:
> The code I've written below surprisingly works on IA32 and AMD64 at zero
> optimisation with Debian GNU/Linux:
> 
> sum is 2147483647
> No Overflow
> sum is -2147483648
> Overflow
> 
> And gives nonsense answers at higher levels of optimisation, e.g.:
> 
> sum is -1073745624
> No Overflow
> sum is -1789480959
> No Overflow

I've solved the corruption by forcing the addition to the variable
sum out to memory. Improvements welcome. Optimal pseudocode would look
something like this with appropriate gcc support:


   asm add
   asm jump no overflow -> C_label
   ...
   C code to handle overflow
   ...
 C_label:
   Remaining C code
   ...


#include <stdint.h>
#include <stdio.h>

inline int32_t add(int32_t a, int32_t b) {
  signed char overflow=0;
  int32_t sum=a;
  __asm__ __volatile__("add %[src], %[dest]"
		       : [dest] "=m" (sum)
		       : [src] "R" (b)
		       : "0");
  __asm__ __volatile__("jno 1f\n\t"
                       "movb $1, %[dest]\n\t"
                       "1:"
		       : [dest] "=m" (overflow));

  printf("sum is %i\n", sum);
  if (overflow) printf("Overflow\n");
  else printf("No Overflow\n");
  return sum;
}

int main(void) {
  add(0x7FFFFFFF, 0);
  add(0x7FFFFFFF, 1);
  return 0;
}

Regards,
Adam