[Bug c/66077] New: Right shift calculation error
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66077
Bug ID: 66077
Summary: Right shift calculation error
Product: gcc
Version: 4.4.7
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: mike.jost at hp dot com
Target Milestone: ---
Curious right shift calculation error. I have used this sequence many times in
de Bruijn sequence calc to count trailing zeros. simplified to basic steps. Not
sure if this is a linux VM issue or compiler issue.
This same code works
using Quincy 2205 v1.3 05-Feb-2008
MinGW GCC version 4.2.1 on windows platform
fails with.
version:
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enax
Thread model: posix
gcc version 4.4.7 20120313 (Red Hat 4.4.7-11) (GCC)
Linux VM
$ uname -a
Linux hpnsw019.rose.hp.com 2.6.32-504.12.2.el6.centos.plus.x86_64 #1 SMP Thu
Mar 12 18:39:03 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
#include
int main(int argc, char **argv)
{
unsigned long w = 64UL;
unsigned long x = 0xF12B3740UL;
unsigned long y = 0UL;
unsigned long z = 0UL;
unsigned long a = 0UL;
y = (w * 0x07C4ACDDUL); /* = 0xF12B3740UL */
z = y >> 27UL;
a = x >> 27UL;
printf("w = %lu\n",w);
printf("x = 0x%X\n", x);
printf("y = 0x%X\n", y);
printf("z = %lu\n", z);
printf("a = %lu\n", a);
}
output:
w = 64
x = 0xF12B3740
y = 0xF12B3740
z = 62
a = 30
z and a should be the same (30).
[Bug c/66077] Right shift calculation error
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66077
--- Comment #2 from Mike Jost ---
Thanks for the explanation! Bit by the 64 bit. That will teach me to use
uint32_t instead of assuming 32 for unsigned long. I've been running on 32 bit
embedded targets for too long!
Regards,
Michael Jost
mike.j...@hp.com
Software Engineer
Object Technology Solutions, Inc. (OTSI) contractor
HP Roseville - R3U Q8
(916) 785-2815
-Original Message-
From: pinskia at gcc dot gnu.org [mailto:gcc-bugzi...@gcc.gnu.org]
Sent: Friday, May 08, 2015 11:40 AM
To: Jost, Mike (OTSI Contractor)
Subject: [Bug c/66077] Right shift calculation error
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66077
Andrew Pinski changed:
What|Removed |Added
Status|UNCONFIRMED |RESOLVED
Resolution|--- |INVALID
--- Comment #1 from Andrew Pinski --- Your X and Y
are not correctly displaying.
The correct values are:
x = 0xF12B3740
y = 0x1F12B3740
You should be using %lX rather than %X to display unsigned long.
Like:
printf("x = 0x%lX\n", x);
printf("y = 0x%lX\n", y);
As you can see there is another bit set in the upper 32bits which causes the
difference.
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