From: PG Bug reporting form <noreply@postgresql.org>
To: pgsql-bugs@lists.postgresql.org
Cc: ssubhrajyoti@google.com
Subject: BUG #17167: UndefinedBehaviorSanitizer: invalid-shift-exponent while running int4shr/int4shl
Date: Mon, 30 Aug 2021 09:02:36 +0000
Message-ID: <17167-028026e4ca333817@postgresql.org> (raw)
The following bug has been logged on the website:
Bug reference: 17167
Logged by: Subhrajyoti Senapati
Email address: ssubhrajyoti@google.com
PostgreSQL version: 12.8
Operating system: Linux
Description:
Hi,
When executing in an asan instrumented build,
```
postgres=> SELECT int4shr(10, -1);
```
throws an error `runtime error: shift exponent 32 is too large for 32-bit
type 'int32' (aka 'int')`
This seems to be coming from this line:
https://github.com/postgres/postgres/blob/c30f54ad732ca5c8762bb68bbe0f51de9137dd72/src/backend/utils...
While browsing, i found this link:
https://wiki.sei.cmu.edu/confluence/display/c/INT34-C.+Do+not+shift+an+expression+by+a+negative+numb...,
according to which shifting by negative number of bits or >= bits in the
operand is classified as a undefined behaviour.
Few experiments i tried:
```
postgres=> SELECT int4shr(10, 1);
int4shr
---------
5
(1 row)
postgres=> SELECT int4shr(10, -1);
int4shr
---------
0
(1 row)
postgres=> SELECT int4shr(10, -2);
int4shr
---------
0
(1 row)
```
I am not sure if it's a bug or expected behaviour.
Thanks,
Subhrajyoti
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Subject: Re: BUG #17167: UndefinedBehaviorSanitizer: invalid-shift-exponent while running int4shr/int4shl
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