This is the mail archive of the
gcc-prs@gcc.gnu.org
mailing list for the GCC project.
Re: c/7696: Spurious shift warning
- From: "Christian Ehrhardt" <ehrhardt at mathematik dot uni-ulm dot de>
- To: nobody at gcc dot gnu dot org
- Cc: gcc-prs at gcc dot gnu dot org,
- Date: 6 Apr 2003 12:26:01 -0000
- Subject: Re: c/7696: Spurious shift warning
- Reply-to: "Christian Ehrhardt" <ehrhardt at mathematik dot uni-ulm dot de>
The following reply was made to PR c/7696; it has been noted by GNATS.
From: "Christian Ehrhardt" <ehrhardt at mathematik dot uni-ulm dot de>
To: gcc-gnats at gcc dot gnu dot org, schwab at suse dot de, gcc-bugs at gcc dot gnu dot org,
nobody at gcc dot gnu dot org, gcc-prs at gcc dot gnu dot org
Cc:
Subject: Re: c/7696: Spurious shift warning
Date: Sun, 6 Apr 2003 14:17:56 +0200
http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7696
PR 7696 started out as a spurious warning on ia64 but it turns out that
this is probably a wrong code generation bug on any 64bit arch. The
source code in question is this:
$ cat 7696.c
struct ia64_psr {
unsigned long cpl : 2;
} x = {1};
main ()
{
unsigned long y = ((unsigned long) x.cpl) << 40;
printf ("%lx %d\n", y, sizeof (y));
return 0;
}
$ gcc -m64 7696.c
7696.c: In function `main':
7696.c:7: warning: left shift count >= width of type
This program will print
$ a.out
100 8
According to gdb the cast is thrown away early (build_binary_op doesn't see
it at all), probably because the bitfield is of type unsigned long. But
later on default_conversion sees that the precision is only two and converts
x.cpl to an integer (instead of a long). This oviouly hurts on arches where
int has 32 bit but long has 64.
Two different work arounds can be used to cure the problem: either
declare the bitfield as int or cast x.cpl to long long instead of long.
Obviously none of these is a fix. I'll see if I can come up with a
patch.
regards Christian
--
THAT'S ALL FOLKS!