Inline assembly for detecting int32 overflow on IA32 and AMD64

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

Hi all,

I've been a C programmer for about a month and I'm trying to work around
some of the deficiencies of portable C. A major one is the inability to
efficiently detect signed integer overflow.

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

Any tips for fixing this?

#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] "=R" (sum)
		       : [src] "g" (b));
  __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;
}


I chose R as the register destination for the sum because it denotes a
legacy register (equivalent to r class in i386 mode).

I'm using a char as an overflow flag because there doesn't appear to be
any way to jump to a C label from within the assembly.

Tips about faster approaches are also appreciated :-)

Many thanks,
Adam