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From: Nathan Bossart <nathandbossart@gmail.com>
To: Matthew Kim <matthewkmkim@gmail.com>
Cc: Joseph Koshakow <koshy44@gmail.com>
Cc: jian he <jian.universality@gmail.com>
Cc: Alexander Lakhin <exclusion@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Cc: Andres Freund <andres@anarazel.de>
Subject: Re: Remove dependence on integer wrapping
Date: Wed, 7 Aug 2024 10:08:04 -0500
Message-ID: <ZrON1CT7Lk9_EJOG@nathan> (raw)
In-Reply-To: <ZrKY8PJwGg23eZ3R@nathan>
References: <Zp54I4WMbWW6AQnP@nathan>
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	<ZrKY8PJwGg23eZ3R@nathan>

I started looking at 0001 again with the intent of committing it, and this
caught my eye:

-        /* make the amount positive for digit-reconstruction loop */
-        value = -value;
+        /*
+         * make the amount positive for digit-reconstruction loop, we can
+         * leave INT64_MIN unchanged
+         */
+        pg_neg_s64_overflow(value, &value);

The comment mentions that we can leave the minimum value unchanged, but it
doesn't explain why.  Can we explain why?

+static inline bool
+pg_neg_s64_overflow(int64 a, int64 *result)
+{
+    if (unlikely(a == PG_INT64_MIN))
+    {
+        return true;
+    }
+    else
+    {
+        *result = -a;
+        return false;
+    }
+}

Can we add a comment that these routines do not set "result" when true is
returned?

-- 
nathan





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