Re: 64-bit integer subtraction bug on some platforms

On Wed, 2023-11-08 at 11:58 +0000, Dean Rasheed wrote:
> One of the new tests in the infinite interval patch has revealed a bug
> in our 64-bit integer subtraction code. Consider the following:
>
> select 0::int8 - '-9223372036854775808'::int8;
>
> This should overflow, since the correct result (+9223372036854775808)
> is out of range. However, on platforms without integer overflow
> builtins or 128-bit integers, pg_sub_s64_overflow() does the
> following:
>
> if ((a < 0 && b > 0 && a < PG_INT64_MIN + b) ||
> (a > 0 && b < 0 && a > PG_INT64_MAX + b))
> {
> *result = 0x5EED; /* to avoid spurious warnings */
> return true;
> }
> *result = a - b;
> return false;
>
> which fails to spot the fact that overflow is also possible when a ==
> 0. So on such platforms, it returns the wrong result.
>
> Patch attached.

The patch looks good to me.

Yours,
Laurenz Albe