From: Nathan Bossart <nathandbossart@gmail.com>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Joseph Koshakow <koshy44@gmail.com>
Cc: jian he <jian.universality@gmail.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Matthew Kim <matthewkmkim@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: Fri, 16 Aug 2024 13:56:05 -0500
Message-ID: <Zr-gxcOehrmppDGk@nathan> (raw)
In-Reply-To: <Zr-b1c6x-Yf31TCZ@nathan>
References: <ZrznaJTIOPgSjEJ7@nathan>
<CAAvxfHc2Smk92cJX4aUju1AdW_RpNN8csEj5rvf29b-YFHRmfQ@mail.gmail.com>
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<Zr5M3KxOZlEZRZIN@nathan>
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<CAAvxfHfkHS1n0+wMbtKwj6_GE6+aSJ9JjnsCOOdZPhTKRs-Yhw@mail.gmail.com>
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<bfff906f-300d-81ea-83b7-f2c93845e7f2@gmail.com>
<Zr-b1c6x-Yf31TCZ@nathan>
On Fri, Aug 16, 2024 at 01:35:01PM -0500, Nathan Bossart wrote:
> On Fri, Aug 16, 2024 at 09:00:00PM +0300, Alexander Lakhin wrote:
>> #6 0x00005576cf627c68 in bms_singleton_member (a=0x5576d09f7fb0) at bitmapset.c:691
>> 691 if (result >= 0 || HAS_MULTIPLE_ONES(w))
>
> At a glance, this appears to be caused by the RIGHTMOST_ONE macro:
>
> #define RIGHTMOST_ONE(x) ((signedbitmapword) (x) & -((signedbitmapword) (x)))
I believe hand-rolling the two's complement calculation should be
sufficient to avoid depending on -fwrapv here. godbolt.org indicates that
it produces roughly the same code, too.
diff --git a/src/backend/nodes/bitmapset.c b/src/backend/nodes/bitmapset.cindex cd05c642b0..d37a997c0e 100644--- a/src/backend/nodes/bitmapset.c+++ b/src/backend/nodes/bitmapset.c@@ -67,7 +67,7 @@
* we get zero.
*----------
*/
-#define RIGHTMOST_ONE(x) ((signedbitmapword) (x) & -((signedbitmapword) (x)))+#define RIGHTMOST_ONE(x) ((bitmapword) (x) & (~((bitmapword) (x)) + 1))
#define HAS_MULTIPLE_ONES(x) ((bitmapword) RIGHTMOST_ONE(x) != (x))
--
nathan
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To: pgsql-hackers@postgresql.org
Cc: nathandbossart@gmail.com, exclusion@gmail.com, koshy44@gmail.com, jian.universality@gmail.com, hlinnaka@iki.fi, matthewkmkim@gmail.com, tgl@sss.pgh.pa.us, pgsql-hackers@lists.postgresql.org, andres@anarazel.de
Subject: Re: Remove dependence on integer wrapping
In-Reply-To: <Zr-gxcOehrmppDGk@nathan>
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