Re: pgsql: Use new overflow aware integer operations.

On 2017-12-29 12:21:54 -0800, Andres Freund wrote:
> On 2017-12-27 17:59:26 -0500, Tom Lane wrote:
> >  #if defined(HAVE__BUILTIN_OP_OVERFLOW)
> >     return __builtin_add_overflow(a, b, result);
> >  #else
> >     int32        res = (int32) a + (int32) b;
> > 
> >     if (res > PG_INT16_MAX || res < PG_INT16_MIN)
> > +    {
> > +        *result = 0;        /* just to keep compiler quiet */
> >         return true;
> > +    }
> >     *result = (int16) res;
> >     return false;
> >  #endif
> > 
> > I do not think this would cause any performance loss in our expected
> > usage, because reasonably bright compilers would detect that the store
> > is dead code and remove it.  But less-bright compilers would not be
> > issuing warnings.
> 
> Yea, that works for me. I wonder if we should choose an absurd sentinel
> value to prevent code from relying on one? 0x0000beef or such. Unless
> somebody protests soon-ish I'll make it so.

Pushed that way (with 0x5EED as the value, seems more appropriate ;)).

I can't convince any of my compilers to actual emit warnings in this
case, so we'll have to see whether prairiedog like this...

Greetings,

Andres Freund