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Subject: Re: Missed compiler optimization issue in function
 select_rtable_names_for_explain
From: Daniel Gustafsson <daniel@yesql.se>
In-Reply-To: <OS0P286MB01632009BF9AA52386BA9E4D82EB2@OS0P286MB0163.JPNP286.PROD.OUTLOOK.COM>
Date: Wed, 22 May 2024 12:55:48 +0200
Cc: pgsql-general@postgresql.org
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Message-Id: <23848714-3662-428E-8E6D-5986D87491D6@yesql.se>
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To: XChy <xxs_chy@outlook.com>
Archived-At: <https://www.postgresql.org/message-id/23848714-3662-428E-8E6D-5986D87491D6%40yesql.se>
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> On 22 May 2024, at 12:12, XChy <xxs_chy@outlook.com> wrote:
>=20
>> How is the memset in select_rtable_names_for_explain a dead-store? =
Even memset calls could be optimized away from the EXPLAIN codepath I =
have a feeling it
>> would have to be many in a tight loop for it to be measurable even?

> For the first question, I don't mean that the memset is the dead =
store.

Gotcha

> I mean that the stores with value "0" after the memset are dead:
> ```=20
>     dpns.subplans =3D NIL;
>     dpns.ctes =3D NIL;
>     dpns.appendrels =3D NULL;
> ```
> since the memset has written zeroes to the object "dpns", and these =
members are known to be zero.

They are known to be zero, but that's not entirely equivalent though is =
it?
NIL is defined as ((List *) NULL) and NULL is typically defined as =
((void *)
0), so sizeof(0) would be the size of an int and sizeof(NULL) would be =
the size
of a void pointer.

--
Daniel Gustafsson