Date: Sun, 9 Jun 2024 20:48:12 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Joseph Koshakow <koshy44@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	Andres Freund <andres@anarazel.de>
Subject: Re: Remove dependence on integer wrapping
Message-ID: <ZmZbXK1CNMbkty0W@nathan>
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On Sun, Jun 09, 2024 at 04:59:22PM -0400, Joseph Koshakow wrote:
> I originally added the helper functions to int.h thinking I'd find
> many more instances of overflow due to integer negation, however I
> didn't find that many. So let me know if you think we'd be better
> off without the functions.

I'd vote for the functions, even if it's just future-proofing at the
moment.  IMHO it helps with readability, too.

> The following comment was in the code for parsing timestamps:
> 
>     /* check for just-barely overflow (okay except time-of-day wraps) */
>     /* caution: we want to allow 1999-12-31 24:00:00 */
> 
> I wasn't able to fully understand it even after staring at it for
> a while. Is the comment suggesting that it is ok for the months field,
> for example, to wrap around? That doesn't sound right to me I tested
> the supplied timestamp, 1999-12-31 24:00:00, and it behaves the same
> before and after the patch.

I haven't stared at this for a while like you, but I am likewise confused
at first glance.  This dates back to commit 84df54b, and it looks like this
comment was present in the first version of the patch in the thread [0].  I
CTRL+F'd for any discussion about this but couldn't immediately find
anything.

>  		/* check the negative equivalent will fit without overflowing */
>  		if (unlikely(tmp > (uint16) (-(PG_INT16_MIN + 1)) + 1))
>  			goto out_of_range;
> +
> +		/*
> +		 * special case the minimum integer because its negation cannot be
> +		 * represented
> +		 */
> +		if (tmp == ((uint16) PG_INT16_MAX) + 1)
> +			return PG_INT16_MIN;
>  		return -((int16) tmp);

My first impression is that there appears to be two overflow checks, one of
which sends us to out_of_range, and another that just returns a special
result.  Why shouldn't we add a pg_neg_s16_overflow() and replace this
whole chunk with something like this?

	if (unlikely(pg_neg_s16_overflow(tmp, &tmp)))
		goto out_of_range;
	else
		return tmp;

> +		return ((uint32) INT32_MAX) + 1;

> +		return ((uint64) INT64_MAX) + 1;

nitpick: Any reason not to use PG_INT32_MAX/PG_INT64_MAX for these?

[0] https://postgr.es/m/flat/CAFj8pRBwqALkzc%3D1WV%2Bh5e%2BDcALY2EizjHCvFi9vHbs%2Bz1OhjA%40mail.gmail.com

-- 
nathan