Re: checking against signed integer overflow

[Paul Eggert]

On 12/5/20 10:07 AM, Bruno Haible wrote:
> Jeffrey Walton wrote:
> > For the C language, safe_iop is available.
>
> Gnulib's INT_*_WRAPV macros are similar. [1]

Similar, but more portable, as safe-iop requires things like ({...}) and typeof 
that the Gnulib macros don't assume.

Also, safe-iop generates considerably worse code. For example, on Fedora x86-64 
with gcc -O2 the following program generates 33 instructions (with several 
conditional branches) for the safe-iop function 'mul_ok1' while generating only 
3 instructions (branch-free) for the equivalent intprops.h function 'mul_ok2'.

#include <safe_iop.h>
#include <intprops.h>

_Bool
mul_ok1 (long int a, long int b)
{
  long c;
  return safe_mul (&c, a, b);
}

_Bool
mul_ok2 (long int a, long int b)
{
  long c;
  return !INT_MULTIPLY_WRAPV (a, b, &c);
}